trace_inputs.py

#!/usr/bin/env python
# coding=utf-8
# Copyright (c) 2012 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Traces an executable and its child processes and extract the files accessed
by them.

The implementation uses OS-specific API. The native Kernel logger and the ETL
interface is used on Windows. Dtrace is used on OSX. Strace is used otherwise.
The OS-specific implementation is hidden in an 'API' interface.

The results are embedded in a Results instance. The tracing is done in two
phases, the first is to do the actual trace and generate an
implementation-specific log file. Then the log file is parsed to extract the
information, including the individual child processes and the files accessed
from the log.
"""

import codecs
import csv
import getpass
import glob
import json
import logging
import optparse
import os
import re
import subprocess
import sys
import tempfile
import threading
import time
import weakref

## OS-specific imports

if sys.platform == 'win32':
  from ctypes.wintypes import byref, create_unicode_buffer, c_int, c_wchar_p
  from ctypes.wintypes import windll, FormatError  # pylint: disable=E0611
  from ctypes.wintypes import GetLastError  # pylint: disable=E0611
elif sys.platform == 'darwin':
  import Carbon.File  #  pylint: disable=F0401
  import MacOS  # pylint: disable=F0401


BASE_DIR = os.path.dirname(os.path.abspath(__file__))
ROOT_DIR = os.path.dirname(os.path.dirname(BASE_DIR))


class TracingFailure(Exception):
  """An exception occured during tracing."""
  def __init__(self, description, pid, line_number, line, *args):
    super(TracingFailure, self).__init__(
        description, pid, line_number, line, *args)
    self.description = description
    self.pid = pid
    self.line_number = line_number
    self.line = line
    self.extra = args

  def __str__(self):
    out = self.description
    if self.pid:
      out += '\npid: %d' % self.pid
    if self.line_number:
      out += '\nline: %d' % self.line_number
    if self.line:
      out += '\n%s' % self.line
    if self.extra:
      out += '\n' + ', '.join(map(str, filter(None, self.extra)))
    return out


## OS-specific functions

if sys.platform == 'win32':
  def QueryDosDevice(drive_letter):
    """Returns the Windows 'native' path for a DOS drive letter."""
    assert re.match(r'^[a-zA-Z]:$', drive_letter), drive_letter
    # Guesswork. QueryDosDeviceW never returns the required number of bytes.
    chars = 1024
    drive_letter = unicode(drive_letter)
    p = create_unicode_buffer(chars)
    if 0 == windll.kernel32.QueryDosDeviceW(drive_letter, p, chars):
      err = GetLastError()
      if err:
        # pylint: disable=E0602
        raise WindowsError(
            err,
            'QueryDosDevice(%s): %s (%d)' % (
              str(drive_letter), FormatError(err), err))
    return p.value


  def GetShortPathName(long_path):
    """Returns the Windows short path equivalent for a 'long' path."""
    long_path = unicode(long_path)
    # Adds '\\\\?\\' when given an absolute path so the MAX_PATH (260) limit is
    # not enforced.
    if os.path.isabs(long_path) and not long_path.startswith('\\\\?\\'):
      long_path = '\\\\?\\' + long_path
    chars = windll.kernel32.GetShortPathNameW(long_path, None, 0)
    if chars:
      p = create_unicode_buffer(chars)
      if windll.kernel32.GetShortPathNameW(long_path, p, chars):
        return p.value

    err = GetLastError()
    if err:
      # pylint: disable=E0602
      raise WindowsError(
          err,
          'GetShortPathName(%s): %s (%d)' % (
            str(long_path), FormatError(err), err))


  def GetLongPathName(short_path):
    """Returns the Windows long path equivalent for a 'short' path."""
    short_path = unicode(short_path)
    # Adds '\\\\?\\' when given an absolute path so the MAX_PATH (260) limit is
    # not enforced.
    if os.path.isabs(short_path) and not short_path.startswith('\\\\?\\'):
      short_path = '\\\\?\\' + short_path
    chars = windll.kernel32.GetLongPathNameW(short_path, None, 0)
    if chars:
      p = create_unicode_buffer(chars)
      if windll.kernel32.GetLongPathNameW(short_path, p, chars):
        return p.value

    err = GetLastError()
    if err:
      # pylint: disable=E0602
      raise WindowsError(
          err,
          'GetLongPathName(%s): %s (%d)' % (
            str(short_path), FormatError(err), err))


  def get_current_encoding():
    """Returns the 'ANSI' code page associated to the process."""
    return 'cp%d' % int(windll.kernel32.GetACP())


  class DosDriveMap(object):
    """Maps \Device\HarddiskVolumeN to N: on Windows."""
    # Keep one global cache.
    _MAPPING = {}

    def __init__(self):
      """Lazy loads the cache."""
      if not self._MAPPING:
        # This is related to UNC resolver on windows. Ignore that.
        self._MAPPING['\\Device\\Mup'] = None
        self._MAPPING['\\SystemRoot'] = os.environ['SystemRoot']

        for letter in (chr(l) for l in xrange(ord('C'), ord('Z')+1)):
          try:
            letter = '%s:' % letter
            mapped = QueryDosDevice(letter)
            if mapped in self._MAPPING:
              logging.warn(
                  ('Two drives: \'%s\' and \'%s\', are mapped to the same disk'
                   '. Drive letters are a user-mode concept and the kernel '
                   'traces only have NT path, so all accesses will be '
                   'associated with the first drive letter, independent of the '
                   'actual letter used by the code') % (
                     self._MAPPING[mapped], letter))
            else:
              self._MAPPING[mapped] = letter
          except WindowsError:  # pylint: disable=E0602
            pass

    def to_win32(self, path):
      """Converts a native NT path to Win32/DOS compatible path."""
      match = re.match(r'(^\\Device\\[a-zA-Z0-9]+)(\\.*)?$', path)
      if not match:
        raise ValueError(
            'Can\'t convert %s into a Win32 compatible path' % path,
            path)
      if not match.group(1) in self._MAPPING:
        # Unmapped partitions may be accessed by windows for the
        # fun of it while the test is running. Discard these.
        return None
      drive = self._MAPPING[match.group(1)]
      if not drive or not match.group(2):
        return drive
      return drive + match.group(2)


  def isabs(path):
    """Accepts X: as an absolute path, unlike python's os.path.isabs()."""
    return os.path.isabs(path) or len(path) == 2 and path[1] == ':'


  def get_native_path_case(p):
    """Returns the native path case for an existing file.

    On Windows, removes any leading '\\?\'.
    """
    if not isabs(p):
      raise ValueError(
          'Can\'t get native path case for a non-absolute path: %s' % p,
          p)
    # Windows used to have an option to turn on case sensitivity on non Win32
    # subsystem but that's out of scope here and isn't supported anymore.
    # Go figure why GetShortPathName() is needed.
    try:
      out = GetLongPathName(GetShortPathName(p))
    except OSError, e:
      if e.args[0] in (2, 3, 5):
        # The path does not exist. Try to recurse and reconstruct the path.
        base = os.path.dirname(p)
        rest = os.path.basename(p)
        return os.path.join(get_native_path_case(base), rest)
      raise
    if out.startswith('\\\\?\\'):
      out = out[4:]
    # Always upper case the first letter since GetLongPathName() will return the
    # drive letter in the case it was given.
    return out[0].upper() + out[1:]


  def CommandLineToArgvW(command_line):
    """Splits a commandline into argv using CommandLineToArgvW()."""
    # http://msdn.microsoft.com/library/windows/desktop/bb776391.aspx
    size = c_int()
    ptr = windll.shell32.CommandLineToArgvW(unicode(command_line), byref(size))
    try:
      return [arg for arg in (c_wchar_p * size.value).from_address(ptr)]
    finally:
      windll.kernel32.LocalFree(ptr)


elif sys.platform == 'darwin':


  # On non-windows, keep the stdlib behavior.
  isabs = os.path.isabs


  def _find_item_native_case(root_path, item):
    """Gets the native path case of a single item based at root_path.

    There is no API to get the native path case of symlinks on OSX. So it
    needs to be done the slow way.
    """
    item = item.lower()
    for element in os.listdir(root_path):
      if element.lower() == item:
        return element


  def _native_case(p):
    """Gets the native path case. Warning: this function resolves symlinks."""
    logging.debug('native_case(%s)' % p)
    try:
      rel_ref, _ = Carbon.File.FSPathMakeRef(p)
      out = rel_ref.FSRefMakePath()
      if p.endswith(os.path.sep) and not out.endswith(os.path.sep):
        return out + os.path.sep
      return out
    except MacOS.Error, e:
      if e.args[0] in (-43, -120):
        # The path does not exist. Try to recurse and reconstruct the path.
        # -43 means file not found.
        # -120 means directory not found.
        base = os.path.dirname(p)
        rest = os.path.basename(p)
        return os.path.join(_native_case(base), rest)
      raise OSError(
          e.args[0], 'Failed to get native path for %s' % p, p, e.args[1])


  def _split_at_symlink_native(base_path, rest):
    """Returns the native path for a symlink."""
    base, symlink, rest = split_at_symlink(base_path, rest)
    if symlink:
      if not base_path:
        base_path = base
      else:
        base_path = safe_join(base_path, base)
      symlink = _find_item_native_case(base_path, symlink)
    return base, symlink, rest


  def get_native_path_case(path):
    """Returns the native path case for an existing file.

    Technically, it's only HFS+ on OSX that is case preserving and
    insensitive. It's the default setting on HFS+ but can be changed.
    """
    if not isabs(path):
      raise ValueError(
          'Can\'t get native path case for a non-absolute path: %s' % path,
          path)
    if path.startswith('/dev'):
      # /dev is not visible from Carbon, causing an exception.
      return path

    # Starts assuming there is no symlink along the path.
    resolved = _native_case(path)
    if resolved.lower() == path.lower():
      # This code path is incredibly faster.
      return resolved

    # There was a symlink, process it.
    base, symlink, rest = _split_at_symlink_native(None, path)
    assert symlink, (path, base, symlink, rest, resolved)
    prev = base
    base = safe_join(_native_case(base), symlink)
    assert len(base) > len(prev)
    while rest:
      prev = base
      relbase, symlink, rest = _split_at_symlink_native(base, rest)
      base = safe_join(base, relbase)
      assert len(base) > len(prev), (prev, base, symlink)
      if symlink:
        base = safe_join(base, symlink)
      assert len(base) > len(prev), (prev, base, symlink)
    # Make sure no symlink was resolved.
    assert base.lower() == path.lower(), (base, path)
    return base


else:  # OSes other than Windows and OSX.


  # On non-windows, keep the stdlib behavior.
  isabs = os.path.isabs


  def get_native_path_case(path):
    """Returns the native path case for an existing file.

    On OSes other than OSX and Windows, assume the file system is
    case-sensitive.

    TODO(maruel): This is not strictly true. Implement if necessary.
    """
    if not isabs(path):
      raise ValueError(
          'Can\'t get native path case for a non-absolute path: %s' % path,
          path)
    # Give up on cygwin, as GetLongPathName() can't be called.
    # Linux traces tends to not be normalized so use this occasion to normalize
    # it. This function implementation already normalizes the path on the other
    # OS so this needs to be done here to be coherent between OSes.
    out = os.path.normpath(path)
    if path.endswith(os.path.sep) and not out.endswith(os.path.sep):
      return out + os.path.sep
    return out


if sys.platform != 'win32':  # All non-Windows OSes.


  def safe_join(*args):
    """Joins path elements like os.path.join() but doesn't abort on absolute
    path.

    os.path.join('foo', '/bar') == '/bar'
    but safe_join('foo', '/bar') == 'foo/bar'.
    """
    out = ''
    for element in args:
      if element.startswith(os.path.sep):
        if out.endswith(os.path.sep):
          out += element[1:]
        else:
          out += element
      else:
        if out.endswith(os.path.sep):
          out += element
        else:
          out += os.path.sep + element
    return out


  def split_at_symlink(base_dir, relfile):
    """Scans each component of relfile and cut the string at the symlink if
    there is any.

    Returns a tuple (base_path, symlink, rest), with symlink == rest == None if
    not symlink was found.
    """
    if base_dir:
      assert relfile
      assert os.path.isabs(base_dir)
      index = 0
    else:
      assert os.path.isabs(relfile)
      index = 1

    def at_root(rest):
      if base_dir:
        return safe_join(base_dir, rest)
      return rest

    while True:
      try:
        index = relfile.index(os.path.sep, index)
      except ValueError:
        index = len(relfile)
      full = at_root(relfile[:index])
      if os.path.islink(full):
        # A symlink!
        base = os.path.dirname(relfile[:index])
        symlink = os.path.basename(relfile[:index])
        rest = relfile[index:]
        logging.debug(
            'split_at_symlink(%s, %s) -> (%s, %s, %s)' %
            (base_dir, relfile, base, symlink, rest))
        return base, symlink, rest
      if index == len(relfile):
        break
      index += 1
    return relfile, None, None


def fix_python_path(cmd):
  """Returns the fixed command line to call the right python executable."""
  out = cmd[:]
  if out[0] == 'python':
    out[0] = sys.executable
  elif out[0].endswith('.py'):
    out.insert(0, sys.executable)
  return out


def create_thunk():
  handle, name = tempfile.mkstemp(prefix='trace_inputs_thunk', suffix='.py')
  os.write(
      handle,
      (
        'import subprocess\n'
        'import sys\n'
        'sys.exit(subprocess.call(sys.argv[2:]))\n'
      ))
  os.close(handle)
  return name


def strace_process_quoted_arguments(text):
  """Extracts quoted arguments on a string and return the arguments as a list.

  Implemented as an automaton. Supports incomplete strings in the form
  '"foo"...'.

  Example:
    With text = '"foo", "bar"', the function will return ['foo', 'bar']

  TODO(maruel): Implement escaping.
  """
  # All the possible states of the DFA.
  ( NEED_QUOTE,         # Begining of a new arguments.
    INSIDE_STRING,      # Inside an argument.
    ESCAPED,            # Found a '\' inside a quote. Treat the next char as-is.
    NEED_COMMA_OR_DOT,  # Right after the closing quote of an argument. Could be
                        # a serie of 3 dots or a comma.
    NEED_SPACE,         # Right after a comma
    NEED_DOT_2,         # Found a dot, need a second one.
    NEED_DOT_3,         # Found second dot, need a third one.
    NEED_COMMA,         # Found third dot, need a comma.
    ) = range(8)

  state = NEED_QUOTE
  out = []
  for index, char in enumerate(text):
    if char == '"':
      if state == NEED_QUOTE:
        state = INSIDE_STRING
        # A new argument was found.
        out.append('')
      elif state == INSIDE_STRING:
        # The argument is now closed.
        state = NEED_COMMA_OR_DOT
      elif state == ESCAPED:
        out[-1] += char
        state = INSIDE_STRING
      else:
        raise ValueError(
            'Can\'t process char at column %d for: %r' % (index, text),
            index,
            text)
    elif char == ',':
      if state in (NEED_COMMA_OR_DOT, NEED_COMMA):
        state = NEED_SPACE
      elif state == INSIDE_STRING:
        out[-1] += char
      elif state == ESCAPED:
        out[-1] += char
        state = INSIDE_STRING
      else:
        raise ValueError(
            'Can\'t process char at column %d for: %r' % (index, text),
            index,
            text)
    elif char == ' ':
      if state == NEED_SPACE:
        state = NEED_QUOTE
      elif state == INSIDE_STRING:
        out[-1] += char
      elif state == ESCAPED:
        out[-1] += char
        state = INSIDE_STRING
      else:
        raise ValueError(
            'Can\'t process char at column %d for: %r' % (index, text),
            index,
            text)
    elif char == '.':
      if state == NEED_COMMA_OR_DOT:
        # The string is incomplete, this mean the strace -s flag should be
        # increased.
        state = NEED_DOT_2
      elif state == NEED_DOT_2:
        state = NEED_DOT_3
      elif state == NEED_DOT_3:
        state = NEED_COMMA
      elif state == INSIDE_STRING:
        out[-1] += char
      elif state == ESCAPED:
        out[-1] += char
        state = INSIDE_STRING
      else:
        raise ValueError(
            'Can\'t process char at column %d for: %r' % (index, text),
            index,
            text)
    elif char == '\\':
      if state == ESCAPED:
        out[-1] += char
        state = INSIDE_STRING
      elif state == INSIDE_STRING:
        state = ESCAPED
      else:
        raise ValueError(
            'Can\'t process char at column %d for: %r' % (index, text),
            index,
            text)
    else:
      if state == INSIDE_STRING:
        out[-1] += char
      else:
        raise ValueError(
            'Can\'t process char at column %d for: %r' % (index, text),
            index,
            text)
  if state not in (NEED_COMMA, NEED_COMMA_OR_DOT):
    raise ValueError(
        'String is incorrectly terminated: %r' % text,
        text)
  return out


def read_json(filepath):
  with open(filepath, 'r') as f:
    return json.load(f)


def write_json(filepath_or_handle, data, dense):
  """Writes data into filepath or file handle encoded as json.

  If dense is True, the json is packed. Otherwise, it is human readable.
  """
  if hasattr(filepath_or_handle, 'write'):
    if dense:
      filepath_or_handle.write(
          json.dumps(data, sort_keys=True, separators=(',',':')))
    else:
      filepath_or_handle.write(json.dumps(data, sort_keys=True, indent=2))
  else:
    with open(filepath_or_handle, 'wb') as f:
      if dense:
        json.dump(data, f, sort_keys=True, separators=(',',':'))
      else:
        json.dump(data, f, sort_keys=True, indent=2)


class Results(object):
  """Results of a trace session."""

  class _TouchedObject(object):
    """Something, a file or a directory, that was accessed."""
    def __init__(self, root, path, tainted, size, nb_files):
      logging.debug(
          '%s(%s, %s, %s, %s, %s)' %
          (self.__class__.__name__, root, path, tainted, size, nb_files))
      self.root = root
      self.path = path
      self.tainted = tainted
      self.nb_files = nb_files
      # Can be used as a cache or a default value, depending on context.
      self._size = size
      # These are cache only.
      self._real_path = None

      # Check internal consistency.
      assert path, path
      assert tainted or bool(root) != bool(isabs(path)), (root, path)
      assert tainted or (
          not os.path.exists(self.full_path) or
          (self.full_path == get_native_path_case(self.full_path))), (
              tainted, self.full_path, get_native_path_case(self.full_path))

    @property
    def existent(self):
      return self.size != -1

    @property
    def full_path(self):
      if self.root:
        return os.path.join(self.root, self.path)
      return self.path

    @property
    def real_path(self):
      """Returns the path with symlinks resolved."""
      if not self._real_path:
        self._real_path = os.path.realpath(self.full_path)
      return self._real_path

    @property
    def size(self):
      """File's size. -1 is not existent."""
      if self._size is None and not self.tainted:
        try:
          self._size = os.stat(self.full_path).st_size
        except OSError:
          self._size = -1
      return self._size

    def flatten(self):
      """Returns a dict representing this object.

      A 'size' of 0 means the file was only touched and not read.
      """
      return {
        'path': self.path,
        'size': self.size,
      }

    def replace_variables(self, variables):
      """Replaces the root of this File with one of the variables if it matches.

      If a variable replacement occurs, the cloned object becomes tainted.
      """
      for variable, root_path in variables.iteritems():
        if self.path.startswith(root_path):
          return self._clone(
              self.root, variable + self.path[len(root_path):], True)
      # No need to clone, returns ourself.
      return self

    def strip_root(self, root):
      """Returns a clone of itself with 'root' stripped off."""
      # Check internal consistency.
      assert self.tainted or (isabs(root) and root.endswith(os.path.sep)), root
      if not self.full_path.startswith(root):
        # Now try to resolve the symlinks to see if it can be reached this way.
        # Only try *after* trying without resolving symlink.
        if not self.real_path.startswith(root):
          return None
        path = self.real_path
      else:
        path = self.full_path
      return self._clone(root, path[len(root):], self.tainted)

    def _clone(self, new_root, new_path, tainted):
      raise NotImplementedError(self.__class__.__name__)

  class File(_TouchedObject):
    """A file that was accessed. May not be present anymore.

    If tainted is true, it means it is not a real path anymore as a variable
    replacement occured.

    If touched_only is True, this means the file was probed for existence, and
    it is existent, but was never _opened_. If touched_only is True, the file
    must have existed.
    """
    def __init__(self, root, path, tainted, size):
      super(Results.File, self).__init__(root, path, tainted, size, 1)

    def _clone(self, new_root, new_path, tainted):
      """Clones itself keeping meta-data."""
      # Keep the self.size and self._real_path caches for performance reason. It
      # is also important when the file becomes tainted (with a variable instead
      # of the real path) since self.path is not an on-disk path anymore so
      # out._size cannot be updated.
      out = self.__class__(new_root, new_path, tainted, self.size)
      out._real_path = self._real_path
      return out

  class Directory(_TouchedObject):
    """A directory of files. Must exist."""
    def __init__(self, root, path, tainted, size, nb_files):
      """path='.' is a valid value and must be handled appropriately."""
      assert not path.endswith(os.path.sep), path
      super(Results.Directory, self).__init__(
          root, path + os.path.sep, tainted, size, nb_files)
      # For a Directory instance, self.size is not a cache, it's an actual value
      # that is never modified and represents the total size of the files
      # contained in this directory. It is possible that the directory is empty
      # so that size == 0; this happens if there's only an invalid symlink in
      # it.

    def flatten(self):
      out = super(Results.Directory, self).flatten()
      out['nb_files'] = self.nb_files
      return out

    def _clone(self, new_root, new_path, tainted):
      """Clones itself keeping meta-data."""
      out = self.__class__(
          new_root,
          new_path.rstrip(os.path.sep),
          tainted,
          self.size,
          self.nb_files)
      out._real_path = self._real_path
      return out

  class Process(object):
    """A process that was traced.

    Contains references to the files accessed by this process and its children.
    """
    def __init__(self, pid, files, executable, command, initial_cwd, children):
      logging.debug('Process(%s, %d, ...)' % (pid, len(files)))
      self.pid = pid
      self.files = sorted(files, key=lambda x: x.path)
      self.children = children
      self.executable = executable
      self.command = command
      self.initial_cwd = initial_cwd

      # Check internal consistency.
      assert len(set(f.path for f in self.files)) == len(self.files), sorted(
          f.path for f in self.files)
      assert isinstance(self.children, list)
      assert isinstance(self.files, list)

    @property
    def all(self):
      for child in self.children:
        for i in child.all:
          yield i
      yield self

    def flatten(self):
      return {
        'children': [c.flatten() for c in self.children],
        'command': self.command,
        'executable': self.executable,
        'files': [f.flatten() for f in self.files],
        'initial_cwd': self.initial_cwd,
        'pid': self.pid,
      }

    def strip_root(self, root):
      assert isabs(root) and root.endswith(os.path.sep), root
      # Loads the files after since they are constructed as objects.
      out = self.__class__(
          self.pid,
          filter(None, (f.strip_root(root) for f in self.files)),
          self.executable,
          self.command,
          self.initial_cwd,
          [c.strip_root(root) for c in self.children])
      logging.debug(
          'strip_root(%s) %d -> %d' % (root, len(self.files), len(out.files)))
      return out

  def __init__(self, process):
    self.process = process
    # Cache.
    self._files = None

  def flatten(self):
    return {
      'root': self.process.flatten(),
    }

  @property
  def files(self):
    if self._files is None:
      self._files = sorted(
          sum((p.files for p in self.process.all), []),
          key=lambda x: x.path)
    return self._files

  @property
  def existent(self):
    return [f for f in self.files if f.existent]

  @property
  def non_existent(self):
    return [f for f in self.files if not f.existent]

  def strip_root(self, root):
    """Returns a clone with all the files outside the directory |root| removed
    and converts all the path to be relative paths.
    """
    # Resolve any symlink
    root = os.path.realpath(root)
    root = get_native_path_case(root).rstrip(os.path.sep) + os.path.sep
    logging.debug('strip_root(%s)' % root)
    return Results(self.process.strip_root(root))


class ApiBase(object):
  """OS-agnostic API to trace a process and its children."""
  class Context(object):
    """Processes one log line at a time and keeps the list of traced processes.

    The parsing is complicated by the fact that logs are traced out of order for
    strace but in-order for dtrace and logman. In addition, on Windows it is
    very frequent that processids are reused so a flat list cannot be used. But
    at the same time, it is impossible to faithfully construct a graph when the
    logs are processed out of order. So both a tree and a flat mapping are used,
    the tree is the real process tree, while the flat mapping stores the last
    valid process for the corresponding processid. For the strace case, the
    tree's head is guessed at the last moment.
    """
    class Process(object):
      """Keeps context for one traced child process.

      Logs all the files this process touched. Ignores directories.
      """
      def __init__(self, blacklist, pid, initial_cwd):
        # Check internal consistency.
        assert isinstance(pid, int), repr(pid)
        self.pid = pid
        # children are Process instances.
        self.children = []
        self.initial_cwd = initial_cwd
        self.cwd = None
        self.files = set()
        self.only_touched = set()
        self.executable = None
        self.command = None
        self._blacklist = blacklist

      def to_results_process(self):
        """Resolves file case sensitivity and or late-bound strings."""
        # When resolving files, it's normal to get dupe because a file could be
        # opened multiple times with different case. Resolve the deduplication
        # here.
        def fix_path(x):
          """Returns the native file path case.

          Converts late-bound strings.
          """
          if not x:
            # Do not convert None instance to 'None'.
            return x
          # TODO(maruel): Do not upconvert to unicode here, on linux we don't
          # know the file path encoding so they must be treated as bytes.
          x = unicode(x)
          if os.path.isabs(x):
            # If the path is not absolute, which tends to happen occasionally on
            # Windows, it is not possible to get the native path case so ignore
            # that trace. It mostly happens for 'executable' value.
            x = get_native_path_case(x)
          return x

        def fix_and_blacklist_path(x):
          x = fix_path(x)
          if not x:
            return
          # The blacklist needs to be reapplied, since path casing could
          # influence blacklisting.
          if self._blacklist(x):
            return
          return x

        # Filters out directories. Some may have passed through.
        files = set(f for f in map(fix_and_blacklist_path, self.files) if f)
        only_touched = set(
            f for f in map(fix_and_blacklist_path, self.only_touched) if f)
        only_touched -= files

        files = [
          Results.File(None, f, False, None) for f in files
          if not os.path.isdir(f)
        ]
        # Using 0 as size means the file's content is ignored since the file was
        # never opened for I/O.
        files.extend(
          Results.File(None, f, False, 0) for f in only_touched
          if not os.path.isdir(f)
        )
        return Results.Process(
            self.pid,
            files,
            fix_path(self.executable),
            self.command,
            fix_path(self.initial_cwd),
            [c.to_results_process() for c in self.children])

      def add_file(self, filepath, touch_only):
        """Adds a file if it passes the blacklist."""
        if self._blacklist(unicode(filepath)):
          return
        logging.debug('add_file(%d, %s, %s)' % (self.pid, filepath, touch_only))
        # Note that filepath and not unicode(filepath) is added. It is because
        # filepath could be something else than a string, like a RelativePath
        # instance for dtrace logs.
        if touch_only:
          self.only_touched.add(filepath)
        else:
          self.files.add(filepath)

    def __init__(self, blacklist):
      self.blacklist = blacklist
      # Initial process.
      self.root_process = None
      # dict to accelerate process lookup, to not have to lookup the whole graph
      # each time.
      self._process_lookup = {}

  class Tracer(object):
    """During it's lifetime, the tracing subsystem is enabled."""
    def __init__(self, logname):
      self._logname = logname
      self._lock = threading.Lock()
      self._traces = []
      self._initialized = True

    def trace(self, cmd, cwd, tracename, output):
      """Runs the OS-specific trace program on an executable.

      Arguments:
      - cmd: The command (a list) to run.
      - cwd: Current directory to start the child process in.
      - tracename: Name of the trace in the logname file.
      - output: If False, redirects output to PIPEs.

      Returns a tuple (resultcode, output) and updates the internal trace
      entries.
      """
      # The implementation adds an item to self._traces.
      raise NotImplementedError(self.__class__.__name__)

    def close(self, _timeout=None):
      """Saves the meta-data in the logname file.

      For kernel-based tracing, stops the tracing subsystem.

      Must not be used manually when using 'with' construct.
      """
      with self._lock:
        assert self._initialized
        try:
          data = {
            'traces': self._traces,
          }
          write_json(self._logname, data, False)
        finally:
          self._initialized = False

    def post_process_log(self):
      """Post-processes the log so it becomes faster to load afterward.

      Must not be used manually when using 'with' construct.
      """
      assert not self._initialized, 'Must stop tracing first.'

    def __enter__(self):
      """Enables 'with' statement."""
      return self

    def __exit__(self, exc_type, exc_value, traceback):
      """Enables 'with' statement."""
      self.close()
      # If an exception was thrown, do not process logs.
      if not exc_type:
        self.post_process_log()

  def get_tracer(self, logname):
    """Returns an ApiBase.Tracer instance.

    Initializes the tracing subsystem, which is a requirement for kernel-based
    tracers. Only one tracer instance should be live at a time!

    logname is the filepath to the json file that will contain the meta-data
    about the logs.
    """
    return self.Tracer(logname)

  @staticmethod
  def clean_trace(logname):
    """Deletes an old log."""
    raise NotImplementedError()

  @classmethod
  def parse_log(cls, logname, blacklist):
    """Processes trace logs and returns the files opened and the files that do
    not exist.

    It does not track directories.

    Most of the time, files that do not exist are temporary test files that
    should be put in /tmp instead. See http://crbug.com/116251.

    Returns a list of dict with keys:
      - results: A Results instance.
      - trace: The corresponding tracename parameter provided to
               get_tracer().trace().
      - output: Output gathered during execution, if get_tracer().trace(...,
                output=False) was used.
    """
    raise NotImplementedError(cls.__class__.__name__)


class Strace(ApiBase):
  """strace implies linux."""
  class Context(ApiBase.Context):
    """Processes a strace log line and keeps the list of existent and non
    existent files accessed.

    Ignores directories.

    Uses late-binding to processes the cwd of each process. The problem is that
    strace generates one log file per process it traced but doesn't give any
    information about which process was started when and by who. So we don't
    even know which process is the initial one. So process the logs out of
    order and use late binding with RelativePath to be able to deduce the
    initial directory of each process once all the logs are parsed.
    """
    class Process(ApiBase.Context.Process):
      """Represents the state of a process.

      Contains all the information retrieved from the pid-specific log.
      """
      # Function names are using ([a-z_0-9]+)
      # This is the most common format. function(args) = result
      RE_HEADER = re.compile(r'^([a-z_0-9]+)\((.+?)\)\s+= (.+)$')
      # An interrupted function call, only grab the minimal header.
      RE_UNFINISHED = re.compile(r'^([^\(]+)(.*) \<unfinished \.\.\.\>$')
      # A resumed function call.
      RE_RESUMED = re.compile(r'^<\.\.\. ([^ ]+) resumed> (.+)$')
      # A process received a signal.
      RE_SIGNAL = re.compile(r'^--- SIG[A-Z]+ .+ ---')
      # A process didn't handle a signal. Ignore any junk appearing before,
      # because the process was forcibly killed so it won't open any new file.
      RE_KILLED = re.compile(
          r'^.*\+\+\+ killed by ([A-Z]+)( \(core dumped\))? \+\+\+$')
      # The process has exited.
      RE_PROCESS_EXITED = re.compile(r'^\+\+\+ exited with (\d+) \+\+\+')
      # A call was canceled. Ignore any prefix.
      RE_UNAVAILABLE = re.compile(r'^.*\)\s*= \? <unavailable>$')
      # Happens when strace fails to even get the function name.
      UNNAMED_FUNCTION = '????'

      # Corner-case in python, a class member function decorator must not be
      # @staticmethod.
      def parse_args(regexp, expect_zero):  # pylint: disable=E0213
        """Automatically convert the str 'args' into a list of processed
        arguments.

        Arguments:
        - regexp is used to parse args.
        - expect_zero: one of True, False or None.
          - True: will check for result.startswith('0') first and will ignore
            the trace line completely otherwise. This is important because for
            many functions, the regexp will not process if the call failed.
          - False: will check for not result.startswith(('?', '-1')) for the
            same reason than with True.
          - None: ignore result.
        """
        def meta_hook(function):
          assert function.__name__.startswith('handle_')
          def hook(self, args, result):
            if expect_zero is True and not result.startswith('0'):
              return
            if expect_zero is False and result.startswith(('?', '-1')):
              return
            match = re.match(regexp, args)
            if not match:
              raise TracingFailure(
                  'Failed to parse %s(%s) = %s' %
                  (function.__name__[len('handle_'):], args, result),
                  None, None, None)
            return function(self, match.groups(), result)
          return hook
        return meta_hook

      class RelativePath(object):
        """A late-bound relative path."""
        def __init__(self, parent, value):
          self.parent = parent
          self.value = value

        def render(self):
          """Returns the current directory this instance is representing.

          This function is used to return the late-bound value.
          """
          if self.value and self.value.startswith(u'/'):
            # An absolute path.
            return self.value
          parent = self.parent.render() if self.parent else u'<None>'
          if self.value:
            return os.path.normpath(os.path.join(parent, self.value))
          return parent

        def __unicode__(self):
          """Acts as a string whenever needed."""
          return unicode(self.render())

        def __str__(self):
          """Acts as a string whenever needed."""
          return str(self.render())

      def __init__(self, root, pid):
        """Keeps enough information to be able to guess the original process
        root.

        strace doesn't store which process was the initial process. So more
        information needs to be kept so the graph can be reconstructed from the
        flat map.
        """
        logging.info('%s(%d)' % (self.__class__.__name__, pid))
        super(Strace.Context.Process, self).__init__(root.blacklist, pid, None)
        assert isinstance(root, ApiBase.Context)
        self._root = weakref.ref(root)
        # The dict key is the function name of the pending call, like 'open'
        # or 'execve'.
        self._pending_calls = {}
        self._line_number = 0
        # Current directory when the process started.
        self.initial_cwd = self.RelativePath(self._root(), None)
        self.parentid = None

      def get_cwd(self):
        """Returns the best known value of cwd."""
        return self.cwd or self.initial_cwd

      def render(self):
        """Returns the string value of the RelativePath() object.

        Used by RelativePath. Returns the initial directory and not the
        current one since the current directory 'cwd' validity is time-limited.

        The validity is only guaranteed once all the logs are processed.
        """
        return self.initial_cwd.render()

      def on_line(self, line):
        self._line_number += 1
        if self.RE_SIGNAL.match(line):
          # Ignore signals.
          return

        try:
          match = self.RE_KILLED.match(line)
          if match:
            # Converts a '+++ killed by Foo +++' trace into an exit_group().
            self.handle_exit_group(match.group(1), None)
            return

          match = self.RE_PROCESS_EXITED.match(line)
          if match:
            # Converts a '+++ exited with 1 +++' trace into an exit_group()
            self.handle_exit_group(match.group(1), None)
            return

          match = self.RE_UNFINISHED.match(line)
          if match:
            if match.group(1) in self._pending_calls:
              raise TracingFailure(
                  'Found two unfinished calls for the same function',
                  None, None, None,
                  self._pending_calls)
            self._pending_calls[match.group(1)] = (
                match.group(1) + match.group(2))
            return

          match = self.RE_UNAVAILABLE.match(line)
          if match:
            # This usually means a process was killed and a pending call was
            # canceled.
            # TODO(maruel): Look up the last exit_group() trace just above and
            # make sure any self._pending_calls[anything] is properly flushed.
            return

          match = self.RE_RESUMED.match(line)
          if match:
            if match.group(1) not in self._pending_calls:
              raise TracingFailure(
                  'Found a resumed call that was not logged as unfinished',
                  None, None, None,
                  self._pending_calls)
            pending = self._pending_calls.pop(match.group(1))
            # Reconstruct the line.
            line = pending + match.group(2)

          match = self.RE_HEADER.match(line)
          if not match:
            raise TracingFailure(
                'Found an invalid line: %s' % line,
                None, None, None)
          if match.group(1) == self.UNNAMED_FUNCTION:
            return

          # It's a valid line, handle it.
          handler = getattr(self, 'handle_%s' % match.group(1), None)
          if not handler:
            self._handle_unknown(match.group(1), match.group(2), match.group(3))
          return handler(match.group(2), match.group(3))
        except TracingFailure, e:
          # Hack in the values since the handler could be a static function.
          e.pid = self.pid
          e.line = line
          e.line_number = self._line_number
          # Re-raise the modified exception.
          raise
        except (KeyError, NotImplementedError, ValueError), e:
          raise TracingFailure(
              'Trace generated a %s exception: %s' % (
                  e.__class__.__name__, str(e)),
              self.pid,
              self._line_number,
              line,
              e)

      @parse_args(r'^\"(.+?)\", [FKORWX_|]+$', True)
      def handle_access(self, args, _result):
        self._handle_file(args[0], True)

      @parse_args(r'^\"(.+?)\"$', True)
      def handle_chdir(self, args, _result):
        """Updates cwd."""
        self.cwd = self.RelativePath(self, args[0])
        logging.debug('handle_chdir(%d, %s)' % (self.pid, self.cwd))

      def handle_clone(self, _args, result):
        """Transfers cwd."""
        if result.startswith(('?', '-1')):
          # The call failed.
          return
        # Update the other process right away.
        childpid = int(result)
        child = self._root().get_or_set_proc(childpid)
        if child.parentid is not None or childpid in self.children:
          raise TracingFailure(
              'Found internal inconsitency in process lifetime detection '
              'during a clone() call',
              None, None, None)

        # Copy the cwd object.
        child.initial_cwd = self.get_cwd()
        child.parentid = self.pid
        # It is necessary because the logs are processed out of order.
        self.children.append(child)

      def handle_close(self, _args, _result):
        pass

      def handle_chmod(self, _args, _result):
        pass

      def handle_creat(self, _args, _result):
        # Ignore files created, since they didn't need to exist.
        pass

      @parse_args(r'^\"(.+?)\", \[(.+)\], \[\/\* \d+ vars? \*\/\]$', True)
      def handle_execve(self, args, _result):
        # Even if in practice execve() doesn't returns when it succeeds, strace
        # still prints '0' as the result.
        filepath = args[0]
        self._handle_file(filepath, False)
        self.executable = self.RelativePath(self.get_cwd(), filepath)
        self.command = strace_process_quoted_arguments(args[1])

      def handle_exit_group(self, _args, _result):
        """Removes cwd."""
        self.cwd = None

      def handle_fork(self, args, result):
        self._handle_unknown('fork', args, result)

      def handle_getcwd(self, _args, _result):
        pass

      @parse_args(r'^\"(.+?)\", \"(.+?)\"$', True)
      def handle_link(self, args, _result):
        self._handle_file(args[0], False)
        self._handle_file(args[1], False)

      @parse_args(r'\"(.+?)\", \{.+?, \.\.\.\}', True)
      def handle_lstat(self, args, _result):
        self._handle_file(args[0], True)

      def handle_mkdir(self, _args, _result):
        pass

      @parse_args(r'^\"(.*?)\", ([A-Z\_\|]+)(|, \d+)$', False)
      def handle_open(self, args, _result):
        if 'O_DIRECTORY' in args[1]:
          return
        self._handle_file(args[0], False)

      @parse_args(r'^(\d+|AT_FDCWD), \"(.*?)\", ([A-Z\_\|]+)(|, \d+)$', False)
      def handle_openat(self, args, _result):
        if 'O_DIRECTORY' in args[2]:
          return
        if args[1] == 'AT_FDCWD':
          self._handle_file(args[1], False)
        else:
          # TODO(maruel): Implement relative open if necessary instead of the
          # AT_FDCWD flag, let's hope not since this means tracking all active
          # directory handles.
          raise Exception('Relative open via openat not implemented.')

      @parse_args(r'^\"(.+?)\", \".+?\"(\.\.\.)?, \d+$', False)
      def handle_readlink(self, args, _result):
        self._handle_file(args[0], False)

      @parse_args(r'^\"(.+?)\", \"(.+?)\"$', True)
      def handle_rename(self, args, _result):
        self._handle_file(args[0], False)
        self._handle_file(args[1], False)

      def handle_rmdir(self, _args, _result):
        pass

      def handle_setxattr(self, _args, _result):
        pass

      @parse_args(r'\"(.+?)\", \{.+?, \.\.\.\}', True)
      def handle_stat(self, args, _result):
        self._handle_file(args[0], True)

      def handle_symlink(self, _args, _result):
        pass

      def handle_unlink(self, _args, _result):
        # In theory, the file had to be created anyway.
        pass

      def handle_statfs(self, _args, _result):
        pass

      def handle_vfork(self, args, result):
        self._handle_unknown('vfork', args, result)

      @staticmethod
      def _handle_unknown(function, args, result):
        raise TracingFailure(
            'Unexpected/unimplemented trace %s(%s)= %s' %
            (function, args, result),
            None, None, None)

      def _handle_file(self, filepath, touch_only):
        filepath = self.RelativePath(self.get_cwd(), filepath)
        #assert not touch_only, unicode(filepath)
        self.add_file(filepath, touch_only)

    def __init__(self, blacklist, initial_cwd):
      super(Strace.Context, self).__init__(blacklist)
      self.initial_cwd = initial_cwd

    def render(self):
      """Returns the string value of the initial cwd of the root process.

      Used by RelativePath.
      """
      return self.initial_cwd

    def on_line(self, pid, line):
      """Transfers control into the Process.on_line() function."""
      self.get_or_set_proc(pid).on_line(line.strip())

    def to_results(self):
      """Finds back the root process and verify consistency."""
      # TODO(maruel): Absolutely unecessary, fix me.
      root = [p for p in self._process_lookup.itervalues() if not p.parentid]
      if len(root) != 1:
        raise TracingFailure(
            'Found internal inconsitency in process lifetime detection '
            'while finding the root process',
            None,
            None,
            None,
            sorted(p.pid for p in root))
      self.root_process = root[0]
      process = self.root_process.to_results_process()
      if sorted(self._process_lookup) != sorted(p.pid for p in process.all):
        raise TracingFailure(
            'Found internal inconsitency in process lifetime detection '
            'while looking for len(tree) == len(list)',
            None,
            None,
            None,
            sorted(self._process_lookup),
            sorted(p.pid for p in process.all))
      return Results(process)

    def get_or_set_proc(self, pid):
      """Returns the Context.Process instance for this pid or creates a new one.
      """
      if not pid or not isinstance(pid, int):
        raise TracingFailure(
            'Unpexpected value for pid: %r' % pid,
            pid,
            None,
            None,
            pid)
      if pid not in self._process_lookup:
        self._process_lookup[pid] = self.Process(self, pid)
      return self._process_lookup[pid]

    @classmethod
    def traces(cls):
      """Returns the list of all handled traces to pass this as an argument to
      strace.
      """
      prefix = 'handle_'
      return [i[len(prefix):] for i in dir(cls.Process) if i.startswith(prefix)]

  class Tracer(ApiBase.Tracer):
    MAX_LEN = 256

    def trace(self, cmd, cwd, tracename, output):
      """Runs strace on an executable."""
      logging.info('trace(%s, %s, %s, %s)' % (cmd, cwd, tracename, output))
      assert os.path.isabs(cmd[0]), cmd[0]
      assert os.path.isabs(cwd), cwd
      assert os.path.normpath(cwd) == cwd, cwd
      with self._lock:
        if not self._initialized:
          raise TracingFailure(
              'Called Tracer.trace() on an unitialized object',
              None, None, None, tracename)
        assert tracename not in (i['trace'] for i in self._traces)
      stdout = stderr = None
      if output:
        stdout = subprocess.PIPE
        stderr = subprocess.STDOUT
      # Ensure all file related APIs are hooked.
      traces = ','.join(Strace.Context.traces() + ['file'])
      trace_cmd = [
        'strace',
        '-ff',
        '-s', '%d' % self.MAX_LEN,
        '-e', 'trace=%s' % traces,
        '-o', self._logname + '.' + tracename,
      ]
      child = subprocess.Popen(
          trace_cmd + cmd,
          cwd=cwd,
          stdin=subprocess.PIPE,
          stdout=stdout,
          stderr=stderr)
      out = child.communicate()[0]
      # TODO(maruel): Walk the logs and figure out the root process would
      # simplify parsing the logs a *lot*.
      with self._lock:
        assert tracename not in (i['trace'] for i in self._traces)
        self._traces.append(
          {
            'cmd': cmd,
            'cwd': cwd,
            # The pid of strace process, not very useful.
            'pid': child.pid,
            'trace': tracename,
            'output': out,
          })
      return child.returncode, out

  @staticmethod
  def clean_trace(logname):
    if os.path.isfile(logname):
      os.remove(logname)
    # Also delete any pid specific file from previous traces.
    for i in glob.iglob(logname + '.*'):
      if i.rsplit('.', 1)[1].isdigit():
        os.remove(i)

  @classmethod
  def parse_log(cls, logname, blacklist):
    logging.info('parse_log(%s, %s)' % (logname, blacklist))
    data = read_json(logname)
    out = []
    for item in data['traces']:
      result = {
        'trace': item['trace'],
        'output': item['output'],
      }
      try:
        context = cls.Context(blacklist, item['cwd'])
        for pidfile in glob.iglob('%s.%s.*' % (logname, item['trace'])):
          pid = pidfile.rsplit('.', 1)[1]
          if pid.isdigit():
            pid = int(pid)
            # TODO(maruel): Load as utf-8
            for line in open(pidfile, 'rb'):
              context.on_line(pid, line)
        result['results'] = context.to_results()
      except TracingFailure, e:
        result['exception'] = e
      out.append(result)
    return out


class Dtrace(ApiBase):
  """Uses DTrace framework through dtrace. Requires root access.

  Implies Mac OSX.

  dtruss can't be used because it has compatibility issues with python.

  Also, the pid->cwd handling needs to be done manually since OSX has no way to
  get the absolute path of the 'cwd' dtrace variable from the probe.

  Also, OSX doesn't populate curpsinfo->pr_psargs properly, see
  https://discussions.apple.com/thread/1980539. So resort to handling execve()
  manually.

  errno is not printed in the log since this implementation currently only cares
  about files that were successfully opened.
  """
  class Context(ApiBase.Context):
    # Format: index pid function(args)
    RE_HEADER = re.compile(r'^\d+ (\d+) ([a-zA-Z_\-]+)\((.*?)\)$')

    # Arguments parsing.
    RE_DTRACE_BEGIN = re.compile(r'^\"(.+?)\"$')
    RE_CHDIR = re.compile(r'^\"(.+?)\"$')
    RE_EXECVE = re.compile(r'^\"(.+?)\", \[(\d+), (.+)\]$')
    RE_OPEN = re.compile(r'^\"(.+?)\", (0x[0-9a-z]+), (0x[0-9a-z]+)$')
    RE_PROC_START = re.compile(r'^(\d+), \"(.+?)\", (\d+)$')
    RE_RENAME = re.compile(r'^\"(.+?)\", \"(.+?)\"$')

    O_DIRECTORY = 0x100000

    class Process(ApiBase.Context.Process):
      def __init__(self, *args):
        super(Dtrace.Context.Process, self).__init__(*args)
        self.cwd = self.initial_cwd

    def __init__(self, blacklist, tracer_pid, initial_cwd):
      logging.info(
          '%s(%d, %s)' % (self.__class__.__name__, tracer_pid, initial_cwd))
      super(Dtrace.Context, self).__init__(blacklist)
      # Process ID of the temporary script created by create_thunk().
      self._tracer_pid = tracer_pid
      self._initial_cwd = initial_cwd
      self._line_number = 0

    def on_line(self, line):
      self._line_number += 1
      match = self.RE_HEADER.match(line)
      if not match:
        raise TracingFailure(
            'Found malformed line: %s' % line,
            None,
            self._line_number,
            line)
      fn = getattr(
          self,
          'handle_%s' % match.group(2).replace('-', '_'),
          self._handle_ignored)
      # It is guaranteed to succeed because of the regexp. Or at least I thought
      # it would.
      pid = int(match.group(1))
      try:
        return fn(pid, match.group(3))
      except TracingFailure, e:
        # Hack in the values since the handler could be a static function.
        e.pid = pid
        e.line = line
        e.line_number = self._line_number
        # Re-raise the modified exception.
        raise
      except (KeyError, NotImplementedError, ValueError), e:
        raise TracingFailure(
            'Trace generated a %s exception: %s' % (
                e.__class__.__name__, str(e)),
            pid,
            self._line_number,
            line,
            e)

    def to_results(self):
      process = self.root_process.to_results_process()
      # Internal concistency check.
      if sorted(self._process_lookup) != sorted(p.pid for p in process.all):
        raise TracingFailure(
            'Found internal inconsitency in process lifetime detection '
            'while looking for len(tree) == len(list)',
            None,
            None,
            None,
            sorted(self._process_lookup),
            sorted(p.pid for p in process.all))
      return Results(process)

    def handle_dtrace_BEGIN(self, _pid, args):
      if not self.RE_DTRACE_BEGIN.match(args):
        raise TracingFailure(
            'Found internal inconsitency in dtrace_BEGIN log line',
            None, None, None)

    def handle_proc_start(self, pid, args):
      """Transfers cwd.

      The dtrace script already takes care of only tracing the processes that
      are child of the traced processes so there is no need to verify the
      process hierarchy.
      """
      if pid in self._process_lookup:
        raise TracingFailure(
            'Found internal inconsitency in proc_start: %d started two times' %
                pid,
            None, None, None)
      match = self.RE_PROC_START.match(args)
      if not match:
        raise TracingFailure(
            'Failed to parse arguments: %s' % args,
            None, None, None)
      ppid = int(match.group(1))
      if ppid == self._tracer_pid and not self.root_process:
        proc = self.root_process = self.Process(
            self.blacklist, pid, self._initial_cwd)
      elif ppid in self._process_lookup:
        proc = self.Process(self.blacklist, pid, self._process_lookup[ppid].cwd)
        self._process_lookup[ppid].children.append(proc)
      else:
        # Another process tree, ignore.
        return
      self._process_lookup[pid] = proc
      logging.debug(
          'New child: %s -> %d  cwd:%s' %
          (ppid, pid, unicode(proc.initial_cwd)))

    def handle_proc_exit(self, pid, _args):
      """Removes cwd."""
      if pid in self._process_lookup:
        # self._tracer_pid is not traced itself and other traces run neither.
        self._process_lookup[pid].cwd = None

    def handle_execve(self, pid, args):
      """Sets the process' executable.

      TODO(maruel): Read command line arguments.  See
      https://discussions.apple.com/thread/1980539 for an example.
      https://gist.github.com/1242279

      Will have to put the answer at http://stackoverflow.com/questions/7556249.
      :)
      """
      if not pid in self._process_lookup:
        # Another process tree, ignore.
        return
      match = self.RE_EXECVE.match(args)
      if not match:
        raise TracingFailure(
            'Failed to parse arguments: %r' % args,
            None, None, None)
      proc = self._process_lookup[pid]
      proc.executable = match.group(1)
      proc.command = self.process_escaped_arguments(match.group(3))
      if int(match.group(2)) != len(proc.command):
        raise TracingFailure(
            'Failed to parse execve() arguments: %s' % args,
            None, None, None)

    def handle_chdir(self, pid, args):
      """Updates cwd."""
      if pid not in self._process_lookup:
        # Another process tree, ignore.
        return
      cwd = self.RE_CHDIR.match(args).group(1)
      if not cwd.startswith('/'):
        cwd2 = os.path.join(self._process_lookup[pid].cwd, cwd)
        logging.debug('handle_chdir(%d, %s) -> %s' % (pid, cwd, cwd2))
      else:
        logging.debug('handle_chdir(%d, %s)' % (pid, cwd))
        cwd2 = cwd
      self._process_lookup[pid].cwd = cwd2

    def handle_open_nocancel(self, pid, args):
      """Redirects to handle_open()."""
      return self.handle_open(pid, args)

    def handle_open(self, pid, args):
      if pid not in self._process_lookup:
        # Another process tree, ignore.
        return
      match = self.RE_OPEN.match(args)
      if not match:
        raise TracingFailure(
            'Failed to parse arguments: %s' % args,
            None, None, None)
      flag = int(match.group(2), 16)
      if self.O_DIRECTORY & flag == self.O_DIRECTORY:
        # Ignore directories.
        return
      self._handle_file(pid, match.group(1))

    def handle_rename(self, pid, args):
      if pid not in self._process_lookup:
        # Another process tree, ignore.
        return
      match = self.RE_RENAME.match(args)
      if not match:
        raise TracingFailure(
            'Failed to parse arguments: %s' % args,
            None, None, None)
      self._handle_file(pid, match.group(1))
      self._handle_file(pid, match.group(2))

    def _handle_file(self, pid, filepath):
      if not filepath.startswith('/'):
        filepath = os.path.join(self._process_lookup[pid].cwd, filepath)
      # We can get '..' in the path.
      filepath = os.path.normpath(filepath)
      # Sadly, still need to filter out directories here;
      # saw open_nocancel(".", 0, 0) = 0 lines.
      if os.path.isdir(filepath):
        return
      self._process_lookup[pid].add_file(filepath, False)

    def handle_ftruncate(self, pid, args):
      """Just used as a signal to kill dtrace, ignoring."""
      pass

    @staticmethod
    def _handle_ignored(pid, args):
      """Is called for all the event traces that are not handled."""
      raise NotImplementedError('Please implement me')

    @staticmethod
    def process_escaped_arguments(text):
      """Extracts escaped arguments on a string and return the arguments as a
      list.

      Implemented as an automaton.

      Example:
        With text = '\\001python2.7\\001-c\\001print(\\"hi\\")\\0', the
        function will return ['python2.7', '-c', 'print("hi")]
      """
      if not text.endswith('\\0'):
        raise ValueError('String is not null terminated: %r' % text, text)
      text = text[:-2]

      def unescape(x):
        """Replaces '\\' with '\' and '\?' (where ? is anything) with ?."""
        out = []
        escaped = False
        for i in x:
          if i == '\\' and not escaped:
            escaped = True
            continue
          escaped = False
          out.append(i)
        return ''.join(out)

      return [unescape(i) for i in text.split('\\001')]

  class Tracer(ApiBase.Tracer):
    # pylint: disable=C0301
    #
    # To understand the following code, you'll want to take a look at:
    # http://developers.sun.com/solaris/articles/dtrace_quickref/dtrace_quickref.html
    # https://wikis.oracle.com/display/DTrace/Variables
    # http://docs.oracle.com/cd/E19205-01/820-4221/
    #
    # 0. Dump all the valid probes into a text file. It is important, you
    #    want to redirect into a file and you don't want to constantly 'sudo'.
    # $ sudo dtrace -l > probes.txt
    #
    # 1. Count the number of probes:
    # $ wc -l probes.txt
    # 81823  # On OSX 10.7, including 1 header line.
    #
    # 2. List providers, intentionally skipping all the 'syspolicy10925' and the
    #    likes and skipping the header with NR>1:
    # $ awk 'NR>1 { print $2 }' probes.txt | sort | uniq | grep -v '[[:digit:]]'
    # dtrace
    # fbt
    # io
    # ip
    # lockstat
    # mach_trap
    # proc
    # profile
    # sched
    # syscall
    # tcp
    # vminfo
    #
    # 3. List of valid probes:
    # $ grep syscall probes.txt | less
    #    or use dtrace directly:
    # $ sudo dtrace -l -P syscall | less
    #
    # trackedpid is an associative array where its value can be 0, 1 or 2.
    # 0 is for untracked processes and is the default value for items not
    #   in the associative array.
    # 1 is for tracked processes.
    # 2 is for the script created by create_thunk() only. It is not tracked
    #   itself but all its decendants are.
    #
    # The script will kill itself only once waiting_to_die == 1 and
    # current_processes == 0, so that both getlogin() was called and that
    # all traced processes exited.
    #
    # TODO(maruel): Use cacheable predicates. See
    # https://wikis.oracle.com/display/DTrace/Performance+Considerations
    D_CODE = """
      dtrace:::BEGIN {
        waiting_to_die = 0;
        current_processes = 0;
        logindex = 0;
        printf("%d %d %s_%s(\\"%s\\")\\n",
               logindex, PID, probeprov, probename, SCRIPT);
        logindex++;
      }

      proc:::start /trackedpid[ppid]/ {
        trackedpid[pid] = 1;
        current_processes += 1;
        printf("%d %d %s_%s(%d, \\"%s\\", %d)\\n",
               logindex, pid, probeprov, probename,
               ppid,
               execname,
               current_processes);
        logindex++;
      }
      /* Should use SCRIPT but there is no access to this variable at that
       * point. */
      proc:::start /ppid == PID && execname == "Python"/ {
        trackedpid[pid] = 2;
        current_processes += 1;
        printf("%d %d %s_%s(%d, \\"%s\\", %d)\\n",
               logindex, pid, probeprov, probename,
               ppid,
               execname,
               current_processes);
        logindex++;
      }
      proc:::exit /trackedpid[pid] &&
                  current_processes == 1 &&
                  waiting_to_die == 1/ {
        trackedpid[pid] = 0;
        current_processes -= 1;
        printf("%d %d %s_%s(%d)\\n",
               logindex, pid, probeprov, probename,
               current_processes);
        logindex++;
        exit(0);
      }
      proc:::exit /trackedpid[pid]/ {
        trackedpid[pid] = 0;
        current_processes -= 1;
        printf("%d %d %s_%s(%d)\\n",
               logindex, pid, probeprov, probename,
               current_processes);
        logindex++;
      }

      /* Use an arcane function to detect when we need to die */
      syscall::ftruncate:entry /pid == PID && arg0 == FILE_ID/ {
        waiting_to_die = 1;
        printf("%d %d %s()\\n", logindex, pid, probefunc);
        logindex++;
      }
      syscall::ftruncate:entry /
          pid == PID && arg0 == FILE_ID && current_processes == 0/ {
        exit(0);
      }

      syscall::open*:entry /trackedpid[pid] == 1/ {
        self->open_arg0 = arg0;
        self->open_arg1 = arg1;
        self->open_arg2 = arg2;
      }
      syscall::open*:return /trackedpid[pid] == 1 && errno == 0/ {
        this->open_arg0 = copyinstr(self->open_arg0);
        printf("%d %d %s(\\"%s\\", 0x%x, 0x%x)\\n",
               logindex, pid, probefunc,
               this->open_arg0,
               self->open_arg1,
               self->open_arg2);
        logindex++;
        this->open_arg0 = 0;
      }
      syscall::open*:return /trackedpid[pid] == 1/ {
        self->open_arg0 = 0;
        self->open_arg1 = 0;
        self->open_arg2 = 0;
      }

      syscall::rename:entry /trackedpid[pid] == 1/ {
        self->rename_arg0 = arg0;
        self->rename_arg1 = arg1;
      }
      syscall::rename:return /trackedpid[pid] == 1 && errno == 0/ {
        this->rename_arg0 = copyinstr(self->rename_arg0);
        this->rename_arg1 = copyinstr(self->rename_arg1);
        printf("%d %d %s(\\"%s\\", \\"%s\\")\\n",
               logindex, pid, probefunc,
               this->rename_arg0,
               this->rename_arg1);
        logindex++;
        this->rename_arg0 = 0;
        this->rename_arg1 = 0;
      }
      syscall::rename:return /trackedpid[pid] == 1/ {
        self->rename_arg0 = 0;
        self->rename_arg1 = 0;
      }

      /* Track chdir, it's painful because it is only receiving relative path.
       */
      syscall::chdir:entry /trackedpid[pid] == 1/ {
        self->chdir_arg0 = arg0;
      }
      syscall::chdir:return /trackedpid[pid] == 1 && errno == 0/ {
        this->chdir_arg0 = copyinstr(self->chdir_arg0);
        printf("%d %d %s(\\"%s\\")\\n",
               logindex, pid, probefunc,
               this->chdir_arg0);
        logindex++;
        this->chdir_arg0 = 0;
      }
      syscall::chdir:return /trackedpid[pid] == 1/ {
        self->chdir_arg0 = 0;
      }
      """

    # execve-specific code, tends to throw a lot of exceptions.
    D_CODE_EXECVE = """
      /* Finally what we care about! */
      syscall::exec*:entry /trackedpid[pid]/ {
        self->exec_arg0 = copyinstr(arg0);
        /* Incrementally probe for a NULL in the argv parameter of execve() to
         * figure out argc. */
        /* TODO(maruel): Skip the remaining copyin() when a NULL pointer was
         * found. */
        self->exec_argc = 0;
        /* Probe for argc==1 */
        this->exec_argv = (user_addr_t*)copyin(
             arg1, sizeof(user_addr_t) * (self->exec_argc + 1));
        self->exec_argc = this->exec_argv[self->exec_argc] ?
            (self->exec_argc + 1) : self->exec_argc;

        /* Probe for argc==2 */
        this->exec_argv = (user_addr_t*)copyin(
             arg1, sizeof(user_addr_t) * (self->exec_argc + 1));
        self->exec_argc = this->exec_argv[self->exec_argc] ?
            (self->exec_argc + 1) : self->exec_argc;

        /* Probe for argc==3 */
        this->exec_argv = (user_addr_t*)copyin(
             arg1, sizeof(user_addr_t) * (self->exec_argc + 1));
        self->exec_argc = this->exec_argv[self->exec_argc] ?
            (self->exec_argc + 1) : self->exec_argc;

        /* Probe for argc==4 */
        this->exec_argv = (user_addr_t*)copyin(
             arg1, sizeof(user_addr_t) * (self->exec_argc + 1));
        self->exec_argc = this->exec_argv[self->exec_argc] ?
            (self->exec_argc + 1) : self->exec_argc;

        /* Copy the inputs strings since there is no guarantee they'll be
         * present after the call completed. */
        self->exec_argv0 = (self->exec_argc > 0) ?
            copyinstr(this->exec_argv[0]) : "";
        self->exec_argv1 = (self->exec_argc > 1) ?
            copyinstr(this->exec_argv[1]) : "";
        self->exec_argv2 = (self->exec_argc > 2) ?
            copyinstr(this->exec_argv[2]) : "";
        self->exec_argv3 = (self->exec_argc > 3) ?
            copyinstr(this->exec_argv[3]) : "";
        this->exec_argv = 0;
      }
      syscall::exec*:return /trackedpid[pid] && errno == 0/ {
        /* We need to join strings here, as using multiple printf() would
         * cause tearing when multiple threads/processes are traced.
         * Since it is impossible to escape a string and join it to another one,
         * like sprintf("%s%S", previous, more), use hackery.
         * Each of the elements are split with a \\1. \\0 cannot be used because
         * it is simply ignored. This will conflict with any program putting a
         * \\1 in their execve() string but this should be "rare enough" */
        this->args = "";
        /* Process exec_argv[0] */
        this->args = strjoin(
            this->args, (self->exec_argc > 0) ? self->exec_argv0 : "");

        /* Process exec_argv[1] */
        this->args = strjoin(
            this->args, (self->exec_argc > 1) ? "\\1" : "");
        this->args = strjoin(
            this->args, (self->exec_argc > 1) ? self->exec_argv1 : "");

        /* Process exec_argv[2] */
        this->args = strjoin(
            this->args, (self->exec_argc > 2) ? "\\1" : "");
        this->args = strjoin(
            this->args, (self->exec_argc > 2) ? self->exec_argv2 : "");

        /* Process exec_argv[3] */
        this->args = strjoin(
            this->args, (self->exec_argc > 3) ? "\\1" : "");
        this->args = strjoin(
            this->args, (self->exec_argc > 3) ? self->exec_argv3 : "");

        /* Prints self->exec_argc to permits verifying the internal
         * consistency since this code is quite fishy. */
        printf("%d %d %s(\\"%s\\", [%d, %S])\\n",
               logindex, pid, probefunc,
               self->exec_arg0,
               self->exec_argc,
               this->args);
        logindex++;
        this->args = 0;
      }
      syscall::exec*:return /trackedpid[pid]/ {
        self->exec_arg0 = 0;
        self->exec_argc = 0;
        self->exec_argv0 = 0;
        self->exec_argv1 = 0;
        self->exec_argv2 = 0;
        self->exec_argv3 = 0;
      }
      """

    # Code currently not used.
    D_EXTRANEOUS = """
      /* This is a good learning experience, since it traces a lot of things
       * related to the process and child processes.
       * Warning: it generates a gigantic log. For example, tracing
       * "data/trace_inputs/child1.py --child" generates a 2mb log and takes
       * several minutes to execute.
       */
      /*
      mach_trap::: /trackedpid[pid] == 1 || trackedpid[ppid]/ {
        printf("%d %d %s_%s() = %d\\n",
               logindex, pid, probeprov, probefunc, errno);
        logindex++;
      }
      proc::: /trackedpid[pid] == 1 || trackedpid[ppid]/ {
        printf("%d %d %s_%s() = %d\\n",
               logindex, pid, probeprov, probefunc, errno);
        logindex++;
      }
      sched::: /trackedpid[pid] == 1 || trackedpid[ppid]/ {
        printf("%d %d %s_%s() = %d\\n",
               logindex, pid, probeprov, probefunc, errno);
        logindex++;
      }
      syscall::: /trackedpid[pid] == 1 || trackedpid[ppid]/ {
        printf("%d %d %s_%s() = %d\\n",
               logindex, pid, probeprov, probefunc, errno);
        logindex++;
      }
      vminfo::: /trackedpid[pid] == 1 || trackedpid[ppid]/ {
        printf("%d %d %s_%s() = %d\\n",
               logindex, pid, probeprov, probefunc, errno);
        logindex++;
      }
      */
      /* TODO(maruel): *stat* functions and friends
        syscall::access:return,
        syscall::chdir:return,
        syscall::chflags:return,
        syscall::chown:return,
        syscall::chroot:return,
        syscall::getattrlist:return,
        syscall::getxattr:return,
        syscall::lchown:return,
        syscall::lstat64:return,
        syscall::lstat:return,
        syscall::mkdir:return,
        syscall::pathconf:return,
        syscall::readlink:return,
        syscall::removexattr:return,
        syscall::setxattr:return,
        syscall::stat64:return,
        syscall::stat:return,
        syscall::truncate:return,
        syscall::unlink:return,
        syscall::utimes:return,
      */
      """

    def __init__(self, logname):
      """Starts the log collection with dtrace.

      Requires root access or chmod 4555 on dtrace. dtrace is asynchronous so
      this needs to wait for dtrace to be "warmed up".
      """
      super(Dtrace.Tracer, self).__init__(logname)
      self._script = create_thunk()
      # This unique dummy temp file is used to signal the dtrace script that it
      # should stop as soon as all the child processes are done. A bit hackish
      # but works fine enough.
      self._dummy_file_id, self._dummy_file_name = tempfile.mkstemp(
          prefix='trace_signal_file')

      # Note: do not use the -p flag. It's useless if the initial process quits
      # too fast, resulting in missing traces from the grand-children. The D
      # code manages the dtrace lifetime itself.
      trace_cmd = [
        'sudo',
        'dtrace',
        # Use a larger buffer if getting 'out of scratch space' errors.
        # Ref: https://wikis.oracle.com/display/DTrace/Options+and+Tunables
        '-b', '10m',
        '-x', 'dynvarsize=10m',
        #'-x', 'dtrace_global_maxsize=1m',
        '-x', 'evaltime=exec',
        '-o', '/dev/stderr',
        '-q',
        '-n', self._get_dtrace_code(),
      ]
      with open(self._logname + '.log', 'wb') as logfile:
        self._dtrace = subprocess.Popen(
            trace_cmd, stdout=logfile, stderr=subprocess.STDOUT)
      logging.debug('Started dtrace pid: %d' % self._dtrace.pid)

      # Reads until one line is printed, which signifies dtrace is up and ready.
      with open(self._logname + '.log', 'rb') as logfile:
        while 'dtrace_BEGIN' not in logfile.readline():
          if self._dtrace.poll() is not None:
            # Do a busy wait. :/
            break
      logging.debug('dtrace started')

    def _get_dtrace_code(self):
      """Setups the D code to implement child process tracking.

      Injects the cookie in the script so it knows when to stop.

      The script will detect any instance of the script created with
      create_thunk() and will start tracing it.
      """
      return (
          'inline int PID = %d;\n'
          'inline string SCRIPT = "%s";\n'
          'inline int FILE_ID = %d;\n'
          '\n'
          '%s') % (
              os.getpid(),
              self._script,
              self._dummy_file_id,
              self.D_CODE) + self.D_CODE_EXECVE

    def trace(self, cmd, cwd, tracename, output):
      """Runs dtrace on an executable.

      This dtruss is broken when it starts the process itself or when tracing
      child processes, this code starts a wrapper process
      generated with create_thunk() which starts the executable to trace.
      """
      logging.info('trace(%s, %s, %s, %s)' % (cmd, cwd, tracename, output))
      assert os.path.isabs(cmd[0]), cmd[0]
      assert os.path.isabs(cwd), cwd
      assert os.path.normpath(cwd) == cwd, cwd
      with self._lock:
        if not self._initialized:
          raise TracingFailure(
              'Called Tracer.trace() on an unitialized object',
              None, None, None, tracename)
        assert tracename not in (i['trace'] for i in self._traces)

      # Starts the script wrapper to start the child process. This signals the
      # dtrace script that this process is to be traced.
      stdout = stderr = None
      if output:
        stdout = subprocess.PIPE
        stderr = subprocess.STDOUT
      child_cmd = [
        sys.executable,
        self._script,
        tracename,
      ]
      # Call a dummy function so that dtrace knows I'm about to launch a process
      # that needs to be traced.
      # Yummy.
      child = subprocess.Popen(
          child_cmd + fix_python_path(cmd),
          stdin=subprocess.PIPE,
          stdout=stdout,
          stderr=stderr,
          cwd=cwd)
      logging.debug('Started child pid: %d' % child.pid)

      out = child.communicate()[0]
      # This doesn't mean tracing is done, one of the grand-child process may
      # still be alive. It will be tracked with the dtrace script.

      with self._lock:
        assert tracename not in (i['trace'] for i in self._traces)
        self._traces.append(
          {
            'cmd': cmd,
            'cwd': cwd,
            # The pid of strace process, not very useful.
            'pid': child.pid,
            'trace': tracename,
            'output': out,
          })
      return child.returncode, out

    def close(self, timeout=None):
      """Terminates dtrace."""
      logging.debug('close(%s)' % timeout)
      try:
        try:
          super(Dtrace.Tracer, self).close(timeout)
          # Signal dtrace that it should stop now.
          os.ftruncate(self._dummy_file_id, 0)
          if timeout:
            start = time.time()
            # Use polling. :/
            while (self._dtrace.poll() is None and
                   (time.time() - start) < timeout):
              time.sleep(0.1)
            self._dtrace.kill()
          self._dtrace.wait()
        finally:
          # Make sure to kill it in any case.
          if self._dtrace.poll() is None:
            try:
              self._dtrace.kill()
              self._dtrace.wait()
            except OSError:
              pass

        if self._dtrace.returncode != 0:
          # Warn about any dtrace failure but basically ignore it.
          print 'dtrace failure: %s' % self._dtrace.returncode
      finally:
        os.close(self._dummy_file_id)
        os.remove(self._dummy_file_name)
        os.remove(self._script)

    def post_process_log(self):
      """Sorts the log back in order when each call occured.

      dtrace doesn't save the buffer in strict order since it keeps one buffer
      per CPU.
      """
      super(Dtrace.Tracer, self).post_process_log()
      logname = self._logname + '.log'
      with open(logname, 'rb') as logfile:
        lines = [l for l in logfile if l.strip()]
      errors = [l for l in lines if l.startswith('dtrace:')]
      if errors:
        raise TracingFailure(
            'Found errors in the trace: %s' % '\n'.join(errors),
            None, None, None, logname)
      try:
        lines = sorted(lines, key=lambda l: int(l.split(' ', 1)[0]))
      except ValueError:
        raise TracingFailure(
            'Found errors in the trace: %s' % '\n'.join(
                l for l in lines if l.split(' ', 1)[0].isdigit()),
            None, None, None, logname)
      with open(logname, 'wb') as logfile:
        logfile.write(''.join(lines))

  @staticmethod
  def clean_trace(logname):
    for ext in ('', '.log'):
      if os.path.isfile(logname + ext):
        os.remove(logname + ext)

  @classmethod
  def parse_log(cls, logname, blacklist):
    logging.info('parse_log(%s, ...)' % logname)

    def blacklist_more(filepath):
      # All the HFS metadata is in the form /.vol/...
      return blacklist(filepath) or re.match(r'^\/\.vol\/.+$', filepath)

    data = read_json(logname)
    out = []
    for item in data['traces']:
      context = cls.Context(blacklist_more, item['pid'], item['cwd'])
      for line in open(logname + '.log', 'rb'):
        context.on_line(line)
      out.append(
          {
            'results': context.to_results(),
            'trace': item['trace'],
            'output': item['output'],
          })
    return out


class LogmanTrace(ApiBase):
  """Uses the native Windows ETW based tracing functionality to trace a child
  process.

  Caveat: this implementations doesn't track cwd or initial_cwd. It is because
  the Windows Kernel doesn't have a concept of 'current working directory' at
  all. A Win32 process has a map of current directories, one per drive letter
  and it is managed by the user mode kernel32.dll. In kernel, a file is always
  opened relative to another file_object or as an absolute path. All the current
  working directory logic is done in user mode.
  """
  class Context(ApiBase.Context):
    """Processes a ETW log line and keeps the list of existent and non
    existent files accessed.

    Ignores directories.
    """
    # These indexes are for the stripped version in json.
    EVENT_NAME = 0
    TYPE = 1
    PID = 2
    TID = 3
    PROCESSOR_ID = 4
    TIMESTAMP = 5
    USER_DATA = 6

    class Process(ApiBase.Context.Process):
      def __init__(self, *args):
        super(LogmanTrace.Context.Process, self).__init__(*args)
        # Handle file objects that succeeded.
        self.file_objects = {}

    def __init__(self, blacklist, tracer_pid):
      logging.info('%s(%d)' % (self.__class__.__name__, tracer_pid))
      super(LogmanTrace.Context, self).__init__(blacklist)
      self._drive_map = DosDriveMap()
      # Threads mapping to the corresponding process id.
      self._threads_active = {}
      # Process ID of the tracer, e.g. tracer_inputs.py
      self._tracer_pid = tracer_pid
      self._line_number = 0

    def on_line(self, line):
      """Processes a json Event line."""
      self._line_number += 1
      try:
        # By Opcode
        handler = getattr(
            self,
            'handle_%s_%s' % (line[self.EVENT_NAME], line[self.TYPE]),
            None)
        if not handler:
          raise TracingFailure(
              'Unexpected event %s_%s' % (
                  line[self.EVENT_NAME], line[self.TYPE]),
              None, None, None)
        handler(line)
      except TracingFailure, e:
        # Hack in the values since the handler could be a static function.
        e.pid = line[self.PID]
        e.line = line
        e.line_number = self._line_number
        # Re-raise the modified exception.
        raise
      except (KeyError, NotImplementedError, ValueError), e:
        raise TracingFailure(
            'Trace generated a %s exception: %s' % (
                e.__class__.__name__, str(e)),
            line[self.PID],
            self._line_number,
            line,
            e)

    def to_results(self):
      if not self.root_process:
        raise TracingFailure(
            'Failed to detect the initial process',
            None, None, None)
      process = self.root_process.to_results_process()
      return Results(process)

    def _thread_to_process(self, tid):
      """Finds the process from the thread id."""
      tid = int(tid, 16)
      pid = self._threads_active.get(tid)
      if not pid or not self._process_lookup.get(pid):
        return
      return self._process_lookup[pid]

    @classmethod
    def handle_EventTrace_Header(cls, line):
      """Verifies no event was dropped, e.g. no buffer overrun occured."""
      BUFFER_SIZE = cls.USER_DATA
      #VERSION = cls.USER_DATA + 1
      #PROVIDER_VERSION = cls.USER_DATA + 2
      #NUMBER_OF_PROCESSORS = cls.USER_DATA + 3
      #END_TIME = cls.USER_DATA + 4
      #TIMER_RESOLUTION = cls.USER_DATA + 5
      #MAX_FILE_SIZE = cls.USER_DATA + 6
      #LOG_FILE_MODE = cls.USER_DATA + 7
      #BUFFERS_WRITTEN = cls.USER_DATA + 8
      #START_BUFFERS = cls.USER_DATA + 9
      #POINTER_SIZE = cls.USER_DATA + 10
      EVENTS_LOST = cls.USER_DATA + 11
      #CPU_SPEED = cls.USER_DATA + 12
      #LOGGER_NAME = cls.USER_DATA + 13
      #LOG_FILE_NAME = cls.USER_DATA + 14
      #BOOT_TIME = cls.USER_DATA + 15
      #PERF_FREQ = cls.USER_DATA + 16
      #START_TIME = cls.USER_DATA + 17
      #RESERVED_FLAGS = cls.USER_DATA + 18
      #BUFFERS_LOST = cls.USER_DATA + 19
      #SESSION_NAME_STRING = cls.USER_DATA + 20
      #LOG_FILE_NAME_STRING = cls.USER_DATA + 21
      if line[EVENTS_LOST] != '0':
        raise TracingFailure(
            ( '%s events were lost during trace, please increase the buffer '
              'size from %s') % (line[EVENTS_LOST], line[BUFFER_SIZE]),
            None, None, None)

    def handle_FileIo_Cleanup(self, line):
      """General wisdom: if a file is closed, it's because it was opened.

      Note that FileIo_Close is not used since if a file was opened properly but
      not closed before the process exits, only Cleanup will be logged.
      """
      #IRP = self.USER_DATA
      TTID = self.USER_DATA + 1  # Thread ID, that's what we want.
      FILE_OBJECT = self.USER_DATA + 2
      #FILE_KEY = self.USER_DATA + 3
      proc = self._thread_to_process(line[TTID])
      if not proc:
        # Not a process we care about.
        return
      file_object = line[FILE_OBJECT]
      if file_object in proc.file_objects:
        proc.add_file(proc.file_objects.pop(file_object), False)

    def handle_FileIo_Create(self, line):
      """Handles a file open.

      All FileIo events are described at
      http://msdn.microsoft.com/library/windows/desktop/aa363884.aspx
      for some value of 'description'.

      " (..) process and thread id values of the IO events (..) are not valid "
      http://msdn.microsoft.com/magazine/ee358703.aspx

      The FileIo.Create event doesn't return if the CreateFile() call
      succeeded, so keep track of the file_object and check that it is
      eventually closed with FileIo_Cleanup.
      """
      #IRP = self.USER_DATA
      TTID = self.USER_DATA + 1  # Thread ID, that's what we want.
      FILE_OBJECT = self.USER_DATA + 2
      #CREATE_OPTIONS = self.USER_DATA + 3
      #FILE_ATTRIBUTES = self.USER_DATA + 4
      #self.USER_DATA + SHARE_ACCESS = 5
      OPEN_PATH = self.USER_DATA + 6

      proc = self._thread_to_process(line[TTID])
      if not proc:
        # Not a process we care about.
        return

      match = re.match(r'^\"(.+)\"$', line[OPEN_PATH])
      raw_path = match.group(1)
      # Ignore directories and bare drive right away.
      if raw_path.endswith(os.path.sep):
        return
      filepath = self._drive_map.to_win32(raw_path)
      # Ignore bare drive right away. Some may still fall through with format
      # like '\\?\X:'
      if len(filepath) == 2:
        return
      file_object = line[FILE_OBJECT]
      if os.path.isdir(filepath):
        # There is no O_DIRECTORY equivalent on Windows. The closed is
        # FILE_FLAG_BACKUP_SEMANTICS but it's not exactly right either. So
        # simply discard directories are they are found.
        return
      # Override any stale file object
      proc.file_objects[file_object] = filepath

    def handle_FileIo_Rename(self, line):
      # TODO(maruel): Handle?
      pass

    def handle_Process_End(self, line):
      pid = line[self.PID]
      if self._process_lookup.get(pid):
        logging.info('Terminated: %d' % pid)
        self._process_lookup[pid] = None
      else:
        logging.debug('Terminated: %d' % pid)

    def handle_Process_Start(self, line):
      """Handles a new child process started by PID."""
      #UNIQUE_PROCESS_KEY = self.USER_DATA
      PROCESS_ID = self.USER_DATA + 1
      #PARENT_PID = self.USER_DATA + 2
      #SESSION_ID = self.USER_DATA + 3
      #EXIT_STATUS = self.USER_DATA + 4
      #DIRECTORY_TABLE_BASE = self.USER_DATA + 5
      #USER_SID = self.USER_DATA + 6
      IMAGE_FILE_NAME = self.USER_DATA + 7
      COMMAND_LINE = self.USER_DATA + 8

      ppid = line[self.PID]
      pid = int(line[PROCESS_ID], 16)
      logging.debug(
          'New process %d->%d (%s) %s' %
            (ppid, pid, line[IMAGE_FILE_NAME], line[COMMAND_LINE]))

      if ppid == self._tracer_pid:
        # Need to ignore processes we don't know about because the log is
        # system-wide. self._tracer_pid shall start only one process.
        if self.root_process:
          raise TracingFailure(
              ( 'Parent process is _tracer_pid(%d) but root_process(%d) is '
                'already set') % (self._tracer_pid, self.root_process.pid),
              None, None, None)
        proc = self.Process(self.blacklist, pid, None)
        self.root_process = proc
        ppid = None
      elif self._process_lookup.get(ppid):
        proc = self.Process(self.blacklist, pid, None)
        self._process_lookup[ppid].children.append(proc)
      else:
        # Ignore
        return
      self._process_lookup[pid] = proc

      if (not line[IMAGE_FILE_NAME].startswith('"') or
          not line[IMAGE_FILE_NAME].endswith('"')):
        raise TracingFailure(
            'Command line is not properly quoted: %s' % line[IMAGE_FILE_NAME],
            None, None, None)

      # TODO(maruel): Process escapes.
      if (not line[COMMAND_LINE].startswith('"') or
          not line[COMMAND_LINE].endswith('"')):
        raise TracingFailure(
            'Command line is not properly quoted: %s' % line[COMMAND_LINE],
            None, None, None)
      proc.command = CommandLineToArgvW(line[COMMAND_LINE][1:-1])
      proc.executable = line[IMAGE_FILE_NAME][1:-1]
      # proc.command[0] may be the absolute path of 'executable' but it may be
      # anything else too. If it happens that command[0] ends with executable,
      # use it, otherwise defaults to the base name.
      cmd0 = proc.command[0].lower()
      if not cmd0.endswith('.exe'):
        # TODO(maruel): That's not strictly true either.
        cmd0 += '.exe'
      if cmd0.endswith(proc.executable) and os.path.isfile(cmd0):
        # Fix the path.
        cmd0 = cmd0.replace('/', os.path.sep)
        cmd0 = os.path.normpath(cmd0)
        proc.executable = get_native_path_case(cmd0)
      logging.info(
          'New child: %s -> %d %s' % (ppid, pid, proc.executable))

    def handle_Thread_End(self, line):
      """Has the same parameters as Thread_Start."""
      tid = int(line[self.TID], 16)
      self._threads_active.pop(tid, None)

    def handle_Thread_Start(self, line):
      """Handles a new thread created.

      Do not use self.PID here since a process' initial thread is created by
      the parent process.
      """
      PROCESS_ID = self.USER_DATA
      TTHREAD_ID = self.USER_DATA + 1
      #STACK_BASE = self.USER_DATA + 2
      #STACK_LIMIT = self.USER_DATA + 3
      #USER_STACK_BASE = self.USER_DATA + 4
      #USER_STACK_LIMIT = self.USER_DATA + 5
      #AFFINITY = self.USER_DATA + 6
      #WIN32_START_ADDR = self.USER_DATA + 7
      #TEB_BASE = self.USER_DATA + 8
      #SUB_PROCESS_TAG = self.USER_DATA + 9
      #BASE_PRIORITY = self.USER_DATA + 10
      #PAGE_PRIORITY = self.USER_DATA + 11
      #IO_PRIORITY = self.USER_DATA + 12
      #THREAD_FLAGS = self.USER_DATA + 13
      # Do not use self.PID here since a process' initial thread is created by
      # the parent process.
      pid = int(line[PROCESS_ID], 16)
      tid = int(line[TTHREAD_ID], 16)
      logging.debug('New thread pid:%d, tid:%d' % (pid, tid))
      self._threads_active[tid] = pid

    @classmethod
    def supported_events(cls):
      """Returns all the procesed events."""
      out = []
      for member in dir(cls):
        match = re.match(r'^handle_([A-Za-z]+)_([A-Za-z]+)$', member)
        if match:
          out.append(match.groups())
      return out

  class Tracer(ApiBase.Tracer):
    # The basic headers.
    EXPECTED_HEADER = [
      u'Event Name',
      u'Type',
      u'Event ID',
      u'Version',
      u'Channel',
      u'Level',  # 5
      u'Opcode',
      u'Task',
      u'Keyword',
      u'PID',
      u'TID',  # 10
      u'Processor Number',
      u'Instance ID',
      u'Parent Instance ID',
      u'Activity ID',
      u'Related Activity ID',  # 15
      u'Clock-Time',
      u'Kernel(ms)',  # Both have a resolution of ~15ms which makes them
      u'User(ms)',    # pretty much useless.
      u'User Data',   # Extra arguments that are event-specific.
    ]
    # Only the useful headers common to all entries are listed there. Any column
    # at 19 or higher is dependent on the specific event.
    EVENT_NAME = 0
    TYPE = 1
    PID = 9
    TID = 10
    PROCESSOR_ID = 11
    TIMESTAMP = 16
    NULL_GUID = '{00000000-0000-0000-0000-000000000000}'
    USER_DATA = 19

    def __init__(self, logname):
      """Starts the log collection.

      Requires administrative access. logman.exe is synchronous so no need for a
      "warmup" call.  'Windows Kernel Trace' is *localized* so use its GUID
      instead.  The GUID constant name is SystemTraceControlGuid. Lovely.

      One can get the list of potentially interesting providers with:
      "logman query providers | findstr /i file"
      """
      super(LogmanTrace.Tracer, self).__init__(logname)
      self._script = create_thunk()
      cmd_start = [
        'logman.exe',
        'start',
        'NT Kernel Logger',
        '-p', '{9e814aad-3204-11d2-9a82-006008a86939}',
        # splitio,fileiocompletion,syscall,file,cswitch,img
        '(process,fileio,thread)',
        '-o', self._logname + '.etl',
        '-ets',  # Send directly to kernel
        # Values extracted out of thin air.
        # Event Trace Session buffer size in kb.
        '-bs', '10240',
        # Number of Event Trace Session buffers.
        '-nb', '16', '256',
      ]
      logging.debug('Running: %s' % cmd_start)
      try:
        subprocess.check_call(
            cmd_start,
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE,
            stderr=subprocess.STDOUT)
      except subprocess.CalledProcessError, e:
        if e.returncode == -2147024891:
          print >> sys.stderr, 'Please restart with an elevated admin prompt'
        elif e.returncode == -2144337737:
          print >> sys.stderr, (
              'A kernel trace was already running, stop it and try again')
        raise

    def trace(self, cmd, cwd, tracename, output):
      logging.info('trace(%s, %s, %s, %s)' % (cmd, cwd, tracename, output))
      assert os.path.isabs(cmd[0]), cmd[0]
      assert os.path.isabs(cwd), cwd
      assert os.path.normpath(cwd) == cwd, cwd
      with self._lock:
        if not self._initialized:
          raise TracingFailure(
              'Called Tracer.trace() on an unitialized object',
              None, None, None, tracename)
        assert tracename not in (i['trace'] for i in self._traces)

      # Use "logman -?" for help.

      stdout = stderr = None
      if output:
        stdout = subprocess.PIPE
        stderr = subprocess.STDOUT

      # Run the child process.
      logging.debug('Running: %s' % cmd)
      # Use the temporary script generated with create_thunk() so we have a
      # clear pid owner. Since trace_inputs.py can be used as a library and
      # could trace multiple processes simultaneously, it makes it more complex
      # if the executable to be traced is executed directly here. It also solves
      # issues related to logman.exe that needs to be executed to control the
      # kernel trace.
      child_cmd = [
        sys.executable,
        self._script,
        tracename,
      ]
      child = subprocess.Popen(
          child_cmd + fix_python_path(cmd),
          cwd=cwd,
          stdin=subprocess.PIPE,
          stdout=stdout,
          stderr=stderr)
      logging.debug('Started child pid: %d' % child.pid)
      out = child.communicate()[0]
      # This doesn't mean all the grand-children are done. Sadly, we don't have
      # a good way to determine that.

      with self._lock:
        assert tracename not in (i['trace'] for i in self._traces)
        self._traces.append({
          'command': cmd,
          'cwd': cwd,
          'pid': child.pid,
          'trace': tracename,
          'output': out,
        })

      return child.returncode, out

    def close(self, _timeout=None):
      """Stops the kernel log collection and converts the traces to text
      representation.
      """
      with self._lock:
        if not self._initialized:
          raise TracingFailure(
              'Called Tracer.close() on an unitialized object',
              None, None, None)
        os.remove(self._script)
        # Save metadata, add 'format' key..
        data = {
          'format': 'csv',
          'traces': self._traces,
        }
        write_json(self._logname, data, False)

        cmd_stop = [
          'logman.exe',
          'stop',
          'NT Kernel Logger',
          '-ets',  # Sends the command directly to the kernel.
        ]
        logging.debug('Running: %s' % cmd_stop)
        subprocess.check_call(
            cmd_stop,
            stdin=subprocess.PIPE,
            stdout=subprocess.PIPE,
            stderr=subprocess.STDOUT)
        self._initialized = False

    def post_process_log(self):
      """Converts the .etl file into .csv then into .json."""
      super(LogmanTrace.Tracer, self).post_process_log()
      logformat = 'csv'
      self._convert_log(logformat)

      if logformat == 'csv_utf16':
        def load_file():
          def utf_8_encoder(unicode_csv_data):
            """Encodes the unicode object as utf-8 encoded str instance"""
            for line in unicode_csv_data:
              yield line.encode('utf-8')

          def unicode_csv_reader(unicode_csv_data, **kwargs):
            """Encodes temporarily as UTF-8 since csv module doesn't do unicode.
            """
            csv_reader = csv.reader(utf_8_encoder(unicode_csv_data), **kwargs)
            for row in csv_reader:
              # Decode str utf-8 instances back to unicode instances, cell by
              # cell:
              yield [cell.decode('utf-8') for cell in row]

          # The CSV file is UTF-16 so use codecs.open() to load the file into
          # the python internal unicode format (utf-8). Then explicitly
          # re-encode as utf8 as str instances so csv can parse it fine. Then
          # decode the utf-8 str back into python unicode instances. This
          # sounds about right.
          for line in unicode_csv_reader(
              codecs.open(self._logname + '.' + logformat, 'r', 'utf-16')):
            # line is a list of unicode objects
            # So much white space!
            yield [i.strip() for i in line]

      elif logformat == 'csv':
        def load_file():
          def ansi_csv_reader(ansi_csv_data, **kwargs):
            """Loads an 'ANSI' code page and returns unicode() objects."""
            assert sys.getfilesystemencoding() == 'mbcs'
            encoding = get_current_encoding()
            for row in csv.reader(ansi_csv_data, **kwargs):
              # Decode str 'ansi' instances to unicode instances, cell by cell:
              yield [cell.decode(encoding) for cell in row]

          # The fastest and smallest format but only supports 'ANSI' file paths.
          # E.g. the filenames are encoding in the 'current' encoding.
          for line in ansi_csv_reader(open(self._logname + '.' + logformat)):
            # line is a list of unicode objects.
            yield [i.strip() for i in line]

      supported_events = LogmanTrace.Context.supported_events()

      def trim(generator):
        for index, line in enumerate(generator):
          if not index:
            if line != self.EXPECTED_HEADER:
              raise TracingFailure(
                  'Found malformed header: %s' % ' '.join(line),
                  None, None, None)
            continue
          # As you can see, the CSV is full of useful non-redundant information:
          if (line[2] != '0' or  # Event ID
              line[3] not in ('2', '3') or  # Version
              line[4] != '0' or  # Channel
              line[5] != '0' or  # Level
              line[7] != '0' or  # Task
              line[8] != '0x0000000000000000' or  # Keyword
              line[12] != '' or  # Instance ID
              line[13] != '' or  # Parent Instance ID
              line[14] != self.NULL_GUID or  # Activity ID
              line[15] != ''):  # Related Activity ID
            raise TracingFailure(
                'Found unexpected values in line: %s' % ' '.join(line),
                  None, None, None)

          if (line[self.EVENT_NAME], line[self.TYPE]) not in supported_events:
            continue

          # Convert the PID in-place from hex.
          line[self.PID] = int(line[self.PID], 16)

          yield [
              line[self.EVENT_NAME],
              line[self.TYPE],
              line[self.PID],
              line[self.TID],
              line[self.PROCESSOR_ID],
              line[self.TIMESTAMP],
          ] + line[self.USER_DATA:]

      write_json('%s.json' % self._logname, list(trim(load_file())), True)

    def _convert_log(self, logformat):
      """Converts the ETL trace to text representation.

      Normally, 'csv' is sufficient. If complex scripts are used (like eastern
      languages), use 'csv_utf16'. If localization gets in the way, use 'xml'.

      Arguments:
        - logformat: Text format to be generated, csv, csv_utf16 or xml.

      Use "tracerpt -?" for help.
      """
      LOCALE_INVARIANT = 0x7F
      windll.kernel32.SetThreadLocale(LOCALE_INVARIANT)
      cmd_convert = [
        'tracerpt.exe',
        '-l', self._logname + '.etl',
        '-o', self._logname + '.' + logformat,
        '-gmt',  # Use UTC
        '-y',  # No prompt
        # Use -of XML to get the header of each items after column 19, e.g. all
        # the actual headers of 'User Data'.
      ]

      if logformat == 'csv':
        # tracerpt localizes the 'Type' column, for major brainfuck
        # entertainment. I can't imagine any sane reason to do that.
        cmd_convert.extend(['-of', 'CSV'])
      elif logformat == 'csv_utf16':
        # This causes it to use UTF-16, which doubles the log size but ensures
        # the log is readable for non-ASCII characters.
        cmd_convert.extend(['-of', 'CSV', '-en', 'Unicode'])
      elif logformat == 'xml':
        cmd_convert.extend(['-of', 'XML'])
      else:
        raise ValueError('Unexpected log format \'%s\'' % logformat)
      logging.debug('Running: %s' % cmd_convert)
      # This can takes tens of minutes for large logs.
      # Redirects all output to stderr.
      subprocess.check_call(
          cmd_convert,
          stdin=subprocess.PIPE,
          stdout=sys.stderr,
          stderr=sys.stderr)

  @staticmethod
  def clean_trace(logname):
    for ext in ('', '.csv', '.etl', '.json', '.xml'):
      if os.path.isfile(logname + ext):
        os.remove(logname + ext)

  @classmethod
  def parse_log(cls, logname, blacklist):
    logging.info('parse_log(%s, %s)' % (logname, blacklist))

    def blacklist_more(filepath):
      # All the NTFS metadata is in the form x:\$EXTEND or stuff like that.
      return blacklist(filepath) or re.match(r'[A-Z]\:\\\$EXTEND', filepath)

    data = read_json(logname)
    lines = read_json(logname + '.json')
    out = []
    for item in data['traces']:
      context = cls.Context(blacklist_more, item['pid'])
      for line in lines:
        context.on_line(line)
      out.append(
          {
            'results': context.to_results(),
            'trace': item['trace'],
            'output': item['output'],
          })
    return out


def get_api():
  """Returns the correct implementation for the current OS."""
  if sys.platform == 'cygwin':
    raise NotImplementedError(
        'Not implemented for cygwin, start the script from Win32 python')
  flavors = {
    'win32': LogmanTrace,
    'darwin': Dtrace,
    'sunos5': Dtrace,
    'freebsd7': Dtrace,
    'freebsd8': Dtrace,
  }
  # Defaults to strace.
  return flavors.get(sys.platform, Strace)()


def extract_directories(root_dir, files, blacklist):
  """Detects if all the files in a directory are in |files| and if so, replace
  the individual files by a Results.Directory instance.

  Takes a list of Results.File instances and returns a shorter list of
  Results.File and Results.Directory instances.

  Arguments:
    - root_dir: Optional base directory that shouldn't be search further.
    - files: list of Results.File instances.
    - blacklist: regexp of files to ignore, for example r'.+\.pyc'.
  """
  logging.info(
      'extract_directories(%s, %d files, ...)' % (root_dir, len(files)))
  assert not (root_dir or '').endswith(os.path.sep), root_dir
  assert not root_dir or (get_native_path_case(root_dir) == root_dir)
  assert not any(isinstance(f, Results.Directory) for f in files)
  # Remove non existent files.
  files = [f for f in files if f.existent]
  if not files:
    return files
  # All files must share the same root, which can be None.
  assert len(set(f.root for f in files)) == 1, set(f.root for f in files)

  # Creates a {directory: {filename: File}} mapping, up to root.
  buckets = {}
  if root_dir:
    buckets[root_dir] = {}
  for fileobj in files:
    path = fileobj.full_path
    directory = os.path.dirname(path)
    assert directory
    # Do not use os.path.basename() so trailing os.path.sep is kept.
    basename = path[len(directory)+1:]
    files_in_directory = buckets.setdefault(directory, {})
    files_in_directory[basename] = fileobj
    # Add all the directories recursively up to root.
    while True:
      old_d = directory
      directory = os.path.dirname(directory)
      if directory + os.path.sep == root_dir or directory == old_d:
        break
      buckets.setdefault(directory, {})

  root_prefix = len(root_dir) + 1 if root_dir else 0
  for directory in sorted(buckets, reverse=True):
    actual = set(f for f in os.listdir(directory) if not blacklist(f))
    expected = set(buckets[directory])
    if not (actual - expected):
      parent = os.path.dirname(directory)
      buckets[parent][os.path.basename(directory)] = Results.Directory(
        root_dir,
        directory[root_prefix:],
        False,
        sum(f.size for f in buckets[directory].itervalues()),
        sum(f.nb_files for f in buckets[directory].itervalues()))
      # Remove the whole bucket.
      del buckets[directory]

  # Reverse the mapping with what remains. The original instances are returned,
  # so the cached meta data is kept.
  files = sum((x.values() for x in buckets.itervalues()), [])
  return sorted(files, key=lambda x: x.path)


def trace(logfile, cmd, cwd, api, output):
  """Traces an executable. Returns (returncode, output) from api.

  Arguments:
  - logfile: file to write to.
  - cmd: command to run.
  - cwd: current directory to start the process in.
  - api: a tracing api instance.
  - output: if True, returns output, otherwise prints it at the console.
  """
  cmd = fix_python_path(cmd)
  api.clean_trace(logfile)
  with api.get_tracer(logfile) as tracer:
    return tracer.trace(cmd, cwd, 'default', output)


def load_trace(logfile, root_dir, api, blacklist):
  """Loads a trace file and returns the Results instance.

  Arguments:
  - logfile: File to load.
  - root_dir: Root directory to use to determine if a file is relevant to the
              trace or not.
  - api: A tracing api instance.
  - blacklist: Optional blacklist function to filter out unimportant files.
  """
  data = api.parse_log(logfile, (blacklist or (lambda _: False)))
  assert len(data) == 1, 'More than one trace was detected!'
  if 'exception' in data[0]:
    # It got an exception, raise it.
    raise data[0]['exception']
  results = data[0]['results']
  if root_dir:
    results = results.strip_root(root_dir)
  return results


def CMDclean(args):
  """Cleans up traces."""
  parser = OptionParserTraceInputs(command='clean')
  options, args = parser.parse_args(args)
  api = get_api()
  api.clean_trace(options.log)
  return 0


def CMDtrace(args):
  """Traces an executable."""
  parser = OptionParserTraceInputs(command='trace')
  parser.allow_interspersed_args = False
  parser.add_option(
      '-q', '--quiet', action='store_true',
      help='Redirects traced executable output to /dev/null')
  options, args = parser.parse_args(args)

  if not args:
    parser.error('Please provide a command to run')

  if not os.path.isabs(args[0]) and os.access(args[0], os.X_OK):
    args[0] = os.path.abspath(args[0])

  api = get_api()
  return trace(options.log, args, os.getcwd(), api, options.quiet)[0]


def CMDread(args):
  """Reads the logs and prints the result."""
  parser = OptionParserTraceInputs(command='read')
  parser.add_option(
      '-V', '--variable',
      nargs=2,
      action='append',
      dest='variables',
      metavar='VAR_NAME directory',
      default=[],
      help=('Variables to replace relative directories against. Example: '
            '"-v \'$HOME\' \'/home/%s\'" will replace all occurence of your '
            'home dir with $HOME') % getpass.getuser())
  parser.add_option(
      '--root-dir',
      help='Root directory to base everything off it. Anything outside of this '
           'this directory will not be reported')
  parser.add_option(
      '-j', '--json', action='store_true',
      help='Outputs raw result data as json')
  parser.add_option(
      '-b', '--blacklist', action='append', default=[],
      help='List of regexp to use as blacklist filter')
  options, args = parser.parse_args(args)

  if options.root_dir:
    options.root_dir = os.path.abspath(options.root_dir)

  variables = dict(options.variables)
  api = get_api()
  def blacklist(f):
    return any(re.match(b, f) for b in options.blacklist)
  data = api.parse_log(options.log, blacklist)
  # Process each trace.
  output_as_json = []
  for item in data:
    if 'exception' in item:
      print >> sys.stderr, (
          'Trace %s: Got an exception: %s' % (item['trace'], item['exception']))
      continue
    results = item['results']
    if options.root_dir:
      results = results.strip_root(options.root_dir)

    if options.json:
      output_as_json.append(results.flatten())
    else:
      simplified = extract_directories(
          options.root_dir, results.files, blacklist)
      simplified = [f.replace_variables(variables) for f in simplified]
      if len(data) > 1:
        print('Trace: %s' % item['trace'])
      print('Total: %d' % len(results.files))
      print('Non existent: %d' % len(results.non_existent))
      for f in results.non_existent:
        print('  %s' % f.path)
      print(
          'Interesting: %d reduced to %d' % (
              len(results.existent), len(simplified)))
      for f in simplified:
        print('  %s' % f.path)

  if options.json:
    write_json(sys.stdout, output_as_json, False)
  return 0


class OptionParserWithLogging(optparse.OptionParser):
  """Adds --verbose option."""
  def __init__(self, verbose=0, **kwargs):
    optparse.OptionParser.__init__(self, **kwargs)
    self.add_option(
        '-v', '--verbose',
        action='count',
        default=verbose,
        help='Use multiple times to increase verbosity')

  def parse_args(self, *args, **kwargs):
    options, args = optparse.OptionParser.parse_args(self, *args, **kwargs)
    levels = [logging.ERROR, logging.INFO, logging.DEBUG]
    logging.basicConfig(
        level=levels[min(len(levels)-1, options.verbose)],
        format='%(levelname)5s %(module)15s(%(lineno)3d): %(message)s')
    return options, args


class OptionParserWithNiceDescription(OptionParserWithLogging):
  """Generates the description with the command's docstring."""
  def __init__(self, **kwargs):
    """Sets 'description' and 'usage' if not already specified."""
    command = kwargs.pop('command', 'help')
    kwargs.setdefault(
        'description',
        re.sub('[\r\n ]{2,}', ' ', get_command_handler(command).__doc__))
    kwargs.setdefault('usage', '%%prog %s [options]' % command)
    OptionParserWithLogging.__init__(self, **kwargs)


class OptionParserTraceInputs(OptionParserWithNiceDescription):
  """Adds automatic --log handling."""
  def __init__(self, **kwargs):
    OptionParserWithNiceDescription.__init__(self, **kwargs)
    self.add_option(
        '-l', '--log', help='Log file to generate or read, required')

  def parse_args(self, *args, **kwargs):
    """Makes sure the paths make sense.

    On Windows, / and \ are often mixed together in a path.
    """
    options, args = OptionParserWithNiceDescription.parse_args(
        self, *args, **kwargs)
    if not options.log:
      self.error('Must supply a log file with -l')
    options.log = os.path.abspath(options.log)
    return options, args


def extract_documentation():
  """Returns a dict {command: description} for each of documented command."""
  commands = (
      fn[3:]
      for fn in dir(sys.modules['__main__'])
      if fn.startswith('CMD') and get_command_handler(fn[3:]).__doc__)
  return dict((fn, get_command_handler(fn).__doc__) for fn in commands)


def CMDhelp(args):
  """Prints list of commands or help for a specific command."""
  doc = extract_documentation()
  # Calculates the optimal offset.
  offset = max(len(cmd) for cmd in doc)
  format_str = '  %-' + str(offset + 2) + 's %s'
  # Generate a one-liner documentation of each commands.
  commands_description = '\n'.join(
       format_str % (cmd, doc[cmd].split('\n')[0]) for cmd in sorted(doc))

  parser = OptionParserWithNiceDescription(
      usage='%prog <command> [options]',
      description='Commands are:\n%s\n' % commands_description)
  parser.format_description = lambda _: parser.description

  # Strip out any -h or --help argument.
  _, args = parser.parse_args([i for i in args if not i in ('-h', '--help')])
  if len(args) == 1:
    if not get_command_handler(args[0]):
      parser.error('Unknown command %s' % args[0])
    # The command was "%prog help command", replaces ourself with
    # "%prog command --help" so help is correctly printed out.
    return main(args + ['--help'])
  elif args:
    parser.error('Unknown argument "%s"' % ' '.join(args))
  parser.print_help()
  return 0


def get_command_handler(name):
  """Returns the command handler or CMDhelp if it doesn't exist."""
  return getattr(sys.modules['__main__'], 'CMD%s' % name, None)


def main_impl(argv):
  command = get_command_handler(argv[0] if argv else 'help')
  if not command:
    return CMDhelp(argv)
  return command(argv[1:])

def main(argv):
  try:
    main_impl(argv)
  except TracingFailure, e:
    sys.stderr.write('\nError: ')
    sys.stderr.write(str(e))
    sys.stderr.write('\n')
    return 1


if __name__ == '__main__':
  sys.exit(main(sys.argv[1:]))