gclient_eval.py

# Copyright 2017 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.

import ast
import cStringIO
import collections
import tokenize

from third_party import schema


class _NodeDict(collections.MutableMapping):
  """Dict-like type that also stores information on AST nodes and tokens."""
  def __init__(self, data, tokens=None):
    self.data = collections.OrderedDict(data)
    self.tokens = tokens

  def __str__(self):
    return str({k: v[0] for k, v in self.data.iteritems()})

  def __getitem__(self, key):
    return self.data[key][0]

  def __setitem__(self, key, value):
    self.data[key] = (value, None)

  def __delitem__(self, key):
    del self.data[key]

  def __iter__(self):
    return iter(self.data)

  def __len__(self):
    return len(self.data)

  def MoveTokens(self, origin, delta):
    if self.tokens:
      new_tokens = {}
      for pos, token in self.tokens.iteritems():
        if pos[0] >= origin:
          pos = (pos[0] + delta, pos[1])
          token = token[:2] + (pos,) + token[3:]
        new_tokens[pos] = token

    for value, node in self.data.values():
      if node.lineno >= origin:
        node.lineno += delta
        if isinstance(value, _NodeDict):
          value.MoveTokens(origin, delta)

  def GetNode(self, key):
    return self.data[key][1]

  def SetNode(self, key, value, node):
    self.data[key] = (value, node)


def _NodeDictSchema(dict_schema):
  """Validate dict_schema after converting _NodeDict to a regular dict."""
  def validate(d):
    schema.Schema(dict_schema).validate(dict(d))
    return True
  return validate


# See https://github.com/keleshev/schema for docs how to configure schema.
_GCLIENT_DEPS_SCHEMA = _NodeDictSchema({
    schema.Optional(basestring): schema.Or(
        None,
        basestring,
        _NodeDictSchema({
            # Repo and revision to check out under the path
            # (same as if no dict was used).
            'url': schema.Or(None, basestring),

            # Optional condition string. The dep will only be processed
            # if the condition evaluates to True.
            schema.Optional('condition'): basestring,

            schema.Optional('dep_type', default='git'): basestring,
        }),
        # CIPD package.
        _NodeDictSchema({
            'packages': [
                _NodeDictSchema({
                    'package': basestring,

                    'version': basestring,
                })
            ],

            schema.Optional('condition'): basestring,

            schema.Optional('dep_type', default='cipd'): basestring,
        }),
    ),
})

_GCLIENT_HOOKS_SCHEMA = [_NodeDictSchema({
    # Hook action: list of command-line arguments to invoke.
    'action': [basestring],

    # Name of the hook. Doesn't affect operation.
    schema.Optional('name'): basestring,

    # Hook pattern (regex). Originally intended to limit some hooks to run
    # only when files matching the pattern have changed. In practice, with git,
    # gclient runs all the hooks regardless of this field.
    schema.Optional('pattern'): basestring,

    # Working directory where to execute the hook.
    schema.Optional('cwd'): basestring,

    # Optional condition string. The hook will only be run
    # if the condition evaluates to True.
    schema.Optional('condition'): basestring,
})]

_GCLIENT_SCHEMA = schema.Schema(_NodeDictSchema({
    # List of host names from which dependencies are allowed (whitelist).
    # NOTE: when not present, all hosts are allowed.
    # NOTE: scoped to current DEPS file, not recursive.
    schema.Optional('allowed_hosts'): [schema.Optional(basestring)],

    # Mapping from paths to repo and revision to check out under that path.
    # Applying this mapping to the on-disk checkout is the main purpose
    # of gclient, and also why the config file is called DEPS.
    #
    # The following functions are allowed:
    #
    #   Var(): allows variable substitution (either from 'vars' dict below,
    #          or command-line override)
    schema.Optional('deps'): _GCLIENT_DEPS_SCHEMA,

    # Similar to 'deps' (see above) - also keyed by OS (e.g. 'linux').
    # Also see 'target_os'.
    schema.Optional('deps_os'): _NodeDictSchema({
        schema.Optional(basestring): _GCLIENT_DEPS_SCHEMA,
    }),

    # Dependency to get gclient_gn_args* settings from. This allows these values
    # to be set in a recursedeps file, rather than requiring that they exist in
    # the top-level solution.
    schema.Optional('gclient_gn_args_from'): basestring,

    # Path to GN args file to write selected variables.
    schema.Optional('gclient_gn_args_file'): basestring,

    # Subset of variables to write to the GN args file (see above).
    schema.Optional('gclient_gn_args'): [schema.Optional(basestring)],

    # Hooks executed after gclient sync (unless suppressed), or explicitly
    # on gclient hooks. See _GCLIENT_HOOKS_SCHEMA for details.
    # Also see 'pre_deps_hooks'.
    schema.Optional('hooks'): _GCLIENT_HOOKS_SCHEMA,

    # Similar to 'hooks', also keyed by OS.
    schema.Optional('hooks_os'): _NodeDictSchema({
        schema.Optional(basestring): _GCLIENT_HOOKS_SCHEMA
    }),

    # Rules which #includes are allowed in the directory.
    # Also see 'skip_child_includes' and 'specific_include_rules'.
    schema.Optional('include_rules'): [schema.Optional(basestring)],

    # Hooks executed before processing DEPS. See 'hooks' for more details.
    schema.Optional('pre_deps_hooks'): _GCLIENT_HOOKS_SCHEMA,

    # Recursion limit for nested DEPS.
    schema.Optional('recursion'): int,

    # Whitelists deps for which recursion should be enabled.
    schema.Optional('recursedeps'): [
        schema.Optional(schema.Or(
            basestring,
            (basestring, basestring),
            [basestring, basestring]
        )),
    ],

    # Blacklists directories for checking 'include_rules'.
    schema.Optional('skip_child_includes'): [schema.Optional(basestring)],

    # Mapping from paths to include rules specific for that path.
    # See 'include_rules' for more details.
    schema.Optional('specific_include_rules'): _NodeDictSchema({
        schema.Optional(basestring): [basestring]
    }),

    # List of additional OS names to consider when selecting dependencies
    # from deps_os.
    schema.Optional('target_os'): [schema.Optional(basestring)],

    # For recursed-upon sub-dependencies, check out their own dependencies
    # relative to the paren't path, rather than relative to the .gclient file.
    schema.Optional('use_relative_paths'): bool,

    # Variables that can be referenced using Var() - see 'deps'.
    schema.Optional('vars'): _NodeDictSchema({
        schema.Optional(basestring): schema.Or(basestring, bool),
    }),
}))


def _gclient_eval(node_or_string, vars_dict=None, expand_vars=False,
                  filename='<unknown>'):
  """Safely evaluates a single expression. Returns the result."""
  _allowed_names = {'None': None, 'True': True, 'False': False}
  if isinstance(node_or_string, basestring):
    node_or_string = ast.parse(node_or_string, filename=filename, mode='eval')
  if isinstance(node_or_string, ast.Expression):
    node_or_string = node_or_string.body
  def _convert(node):
    if isinstance(node, ast.Str):
      if not expand_vars:
        return node.s
      try:
        return node.s.format(**vars_dict)
      except KeyError as e:
        raise ValueError(
            '%s was used as a variable, but was not declared in the vars dict '
            '(file %r, line %s)' % (
                e.message, filename, getattr(node, 'lineno', '<unknown>')))
    elif isinstance(node, ast.Num):
      return node.n
    elif isinstance(node, ast.Tuple):
      return tuple(map(_convert, node.elts))
    elif isinstance(node, ast.List):
      return list(map(_convert, node.elts))
    elif isinstance(node, ast.Dict):
      return _NodeDict((_convert(k), (_convert(v), v))
          for k, v in zip(node.keys, node.values))
    elif isinstance(node, ast.Name):
      if node.id not in _allowed_names:
        raise ValueError(
            'invalid name %r (file %r, line %s)' % (
                node.id, filename, getattr(node, 'lineno', '<unknown>')))
      return _allowed_names[node.id]
    elif isinstance(node, ast.Call):
      if not isinstance(node.func, ast.Name) or node.func.id != 'Var':
        raise ValueError(
            'Var is the only allowed function (file %r, line %s)' % (
                filename, getattr(node, 'lineno', '<unknown>')))
      if node.keywords or node.starargs or node.kwargs or len(node.args) != 1:
        raise ValueError(
            'Var takes exactly one argument (file %r, line %s)' % (
                filename, getattr(node, 'lineno', '<unknown>')))
      arg = _convert(node.args[0])
      if not isinstance(arg, basestring):
        raise ValueError(
            'Var\'s argument must be a variable name (file %r, line %s)' % (
                filename, getattr(node, 'lineno', '<unknown>')))
      if not expand_vars:
        return '{%s}' % arg
      if vars_dict is None:
        raise ValueError(
            'vars must be declared before Var can be used (file %r, line %s)'
            % (filename, getattr(node, 'lineno', '<unknown>')))
      if arg not in vars_dict:
        raise ValueError(
            '%s was used as a variable, but was not declared in the vars dict '
            '(file %r, line %s)' % (
                arg, filename, getattr(node, 'lineno', '<unknown>')))
      return vars_dict[arg]
    elif isinstance(node, ast.BinOp) and isinstance(node.op, ast.Add):
      return _convert(node.left) + _convert(node.right)
    elif isinstance(node, ast.BinOp) and isinstance(node.op, ast.Mod):
      return _convert(node.left) % _convert(node.right)
    else:
      raise ValueError(
          'unexpected AST node: %s %s (file %r, line %s)' % (
              node, ast.dump(node), filename,
              getattr(node, 'lineno', '<unknown>')))
  return _convert(node_or_string)


def Exec(content, expand_vars=True, filename='<unknown>', vars_override=None):
  """Safely execs a set of assignments."""
  def _validate_statement(node, local_scope):
    if not isinstance(node, ast.Assign):
      raise ValueError(
          'unexpected AST node: %s %s (file %r, line %s)' % (
              node, ast.dump(node), filename,
              getattr(node, 'lineno', '<unknown>')))

    if len(node.targets) != 1:
      raise ValueError(
          'invalid assignment: use exactly one target (file %r, line %s)' % (
              filename, getattr(node, 'lineno', '<unknown>')))

    target = node.targets[0]
    if not isinstance(target, ast.Name):
      raise ValueError(
          'invalid assignment: target should be a name (file %r, line %s)' % (
              filename, getattr(node, 'lineno', '<unknown>')))
    if target.id in local_scope:
      raise ValueError(
          'invalid assignment: overrides var %r (file %r, line %s)' % (
              target.id, filename, getattr(node, 'lineno', '<unknown>')))

  node_or_string = ast.parse(content, filename=filename, mode='exec')
  if isinstance(node_or_string, ast.Expression):
    node_or_string = node_or_string.body

  if not isinstance(node_or_string, ast.Module):
    raise ValueError(
        'unexpected AST node: %s %s (file %r, line %s)' % (
            node_or_string,
            ast.dump(node_or_string),
            filename,
            getattr(node_or_string, 'lineno', '<unknown>')))

  statements = {}
  for statement in node_or_string.body:
    _validate_statement(statement, statements)
    statements[statement.targets[0].id] = statement.value

  tokens = {
      token[2]: list(token)
      for token in tokenize.generate_tokens(
          cStringIO.StringIO(content).readline)
  }
  local_scope = _NodeDict({}, tokens)

  # Process vars first, so we can expand variables in the rest of the DEPS file.
  vars_dict = {}
  if 'vars' in statements:
    vars_statement = statements['vars']
    value = _gclient_eval(vars_statement, None, False, filename)
    local_scope.SetNode('vars', value, vars_statement)
    # Update the parsed vars with the overrides, but only if they are already
    # present (overrides do not introduce new variables).
    vars_dict.update(value)
    if vars_override:
      vars_dict.update({
        k: v
        for k, v in vars_override.iteritems()
        if k in vars_dict})

  for name, node in statements.iteritems():
    value = _gclient_eval(node, vars_dict, expand_vars, filename)
    local_scope.SetNode(name, value, node)

  return _GCLIENT_SCHEMA.validate(local_scope)


def Parse(content, expand_vars, validate_syntax, filename, vars_override=None):
  """Parses DEPS strings.

  Executes the Python-like string stored in content, resulting in a Python
  dictionary specifyied by the schema above. Supports syntax validation and
  variable expansion.

  Args:
    content: str. DEPS file stored as a string.
    expand_vars: bool. Whether variables should be expanded to their values.
    validate_syntax: bool. Whether syntax should be validated using the schema
      defined above.
    filename: str. The name of the DEPS file, or a string describing the source
      of the content, e.g. '<string>', '<unknown>'.
    vars_override: dict, optional. A dictionary with overrides for the variables
      defined by the DEPS file.

  Returns:
    A Python dict with the parsed contents of the DEPS file, as specified by the
    schema above.
  """
  # TODO(ehmaldonado): Make validate_syntax = True the only case
  if validate_syntax:
    return Exec(content, expand_vars, filename, vars_override)

  local_scope = {}
  global_scope = {'Var': lambda var_name: '{%s}' % var_name}

  # If we use 'exec' directly, it complains that 'Parse' contains a nested
  # function with free variables.
  # This is because on versions of Python < 2.7.9, "exec(a, b, c)" not the same
  # as "exec a in b, c" (See https://bugs.python.org/issue21591).
  eval(compile(content, filename, 'exec'), global_scope, local_scope)

  if 'vars' not in local_scope or not expand_vars:
    return local_scope

  vars_dict = {}
  vars_dict.update(local_scope['vars'])
  if vars_override:
    vars_dict.update({
        k: v
        for k, v in vars_override.iteritems()
        if k in vars_dict
    })

  def _DeepFormat(node):
    if isinstance(node, basestring):
      return node.format(**vars_dict)
    elif isinstance(node, dict):
      return {
          k.format(**vars_dict): _DeepFormat(v)
          for k, v in node.iteritems()
      }
    elif isinstance(node, list):
      return [_DeepFormat(elem) for elem in node]
    elif isinstance(node, tuple):
      return tuple(_DeepFormat(elem) for elem in node)
    else:
      return node

  return _DeepFormat(local_scope)


def EvaluateCondition(condition, variables, referenced_variables=None):
  """Safely evaluates a boolean condition. Returns the result."""
  if not referenced_variables:
    referenced_variables = set()
  _allowed_names = {'None': None, 'True': True, 'False': False}
  main_node = ast.parse(condition, mode='eval')
  if isinstance(main_node, ast.Expression):
    main_node = main_node.body
  def _convert(node):
    if isinstance(node, ast.Str):
      return node.s
    elif isinstance(node, ast.Name):
      if node.id in referenced_variables:
        raise ValueError(
            'invalid cyclic reference to %r (inside %r)' % (
                node.id, condition))
      elif node.id in _allowed_names:
        return _allowed_names[node.id]
      elif node.id in variables:
        value = variables[node.id]

        # Allow using "native" types, without wrapping everything in strings.
        # Note that schema constraints still apply to variables.
        if not isinstance(value, basestring):
          return value

        # Recursively evaluate the variable reference.
        return EvaluateCondition(
            variables[node.id],
            variables,
            referenced_variables.union([node.id]))
      else:
        # Implicitly convert unrecognized names to strings.
        # If we want to change this, we'll need to explicitly distinguish
        # between arguments for GN to be passed verbatim, and ones to
        # be evaluated.
        return node.id
    elif isinstance(node, ast.BoolOp) and isinstance(node.op, ast.Or):
      if len(node.values) != 2:
        raise ValueError(
            'invalid "or": exactly 2 operands required (inside %r)' % (
                condition))
      left = _convert(node.values[0])
      right = _convert(node.values[1])
      if not isinstance(left, bool):
        raise ValueError(
            'invalid "or" operand %r (inside %r)' % (left, condition))
      if not isinstance(right, bool):
        raise ValueError(
            'invalid "or" operand %r (inside %r)' % (right, condition))
      return left or right
    elif isinstance(node, ast.BoolOp) and isinstance(node.op, ast.And):
      if len(node.values) != 2:
        raise ValueError(
            'invalid "and": exactly 2 operands required (inside %r)' % (
                condition))
      left = _convert(node.values[0])
      right = _convert(node.values[1])
      if not isinstance(left, bool):
        raise ValueError(
            'invalid "and" operand %r (inside %r)' % (left, condition))
      if not isinstance(right, bool):
        raise ValueError(
            'invalid "and" operand %r (inside %r)' % (right, condition))
      return left and right
    elif isinstance(node, ast.UnaryOp) and isinstance(node.op, ast.Not):
      value = _convert(node.operand)
      if not isinstance(value, bool):
        raise ValueError(
            'invalid "not" operand %r (inside %r)' % (value, condition))
      return not value
    elif isinstance(node, ast.Compare):
      if len(node.ops) != 1:
        raise ValueError(
            'invalid compare: exactly 1 operator required (inside %r)' % (
                condition))
      if len(node.comparators) != 1:
        raise ValueError(
            'invalid compare: exactly 1 comparator required (inside %r)' % (
                condition))

      left = _convert(node.left)
      right = _convert(node.comparators[0])

      if isinstance(node.ops[0], ast.Eq):
        return left == right
      if isinstance(node.ops[0], ast.NotEq):
        return left != right

      raise ValueError(
          'unexpected operator: %s %s (inside %r)' % (
              node.ops[0], ast.dump(node), condition))
    else:
      raise ValueError(
          'unexpected AST node: %s %s (inside %r)' % (
              node, ast.dump(node), condition))
  return _convert(main_node)


def RenderDEPSFile(gclient_dict):
  contents = sorted(gclient_dict.tokens.values(), key=lambda token: token[2])
  return tokenize.untokenize(contents)


def _UpdateAstString(tokens, node, value):
  position = node.lineno, node.col_offset
  quote_char = tokens[position][1][0]
  tokens[position][1] = quote_char + value + quote_char
  node.s = value


def _ShiftLinesInTokens(tokens, delta, start):
  new_tokens = {}
  for token in tokens.values():
    if token[2][0] >= start:
      token[2] = token[2][0] + delta, token[2][1]
      token[3] = token[3][0] + delta, token[3][1]
    new_tokens[token[2]] = token
  return new_tokens


def AddVar(gclient_dict, var_name, value):
  if not isinstance(gclient_dict, _NodeDict) or gclient_dict.tokens is None:
    raise ValueError(
        "Can't use SetVar for the given gclient dict. It contains no "
        "formatting information.")

  if 'vars' not in gclient_dict:
    raise KeyError("vars dict is not defined.")

  if var_name in gclient_dict['vars']:
    raise ValueError(
        "%s has already been declared in the vars dict. Consider using SetVar "
        "instead." % var_name)

  if not gclient_dict['vars']:
    raise ValueError('vars dict is empty. This is not yet supported.')

  # We will attempt to add the var right after 'vars = {'.
  node = gclient_dict.GetNode('vars')
  if node is None:
    raise ValueError(
        "The vars dict has no formatting information." % var_name)
  line = node.lineno + 1

  # We will try to match the new var's indentation to the next variable.
  col = node.keys[0].col_offset

  # We use a minimal Python dictionary, so that ast can parse it.
  var_content = '{\n%s"%s": "%s",\n}' % (' ' * col, var_name, value)
  var_ast = ast.parse(var_content).body[0].value

  # Set the ast nodes for the key and value.
  vars_node = gclient_dict.GetNode('vars')

  var_name_node = var_ast.keys[0]
  var_name_node.lineno += line - 2
  vars_node.keys.insert(0, var_name_node)

  value_node = var_ast.values[0]
  value_node.lineno += line - 2
  vars_node.values.insert(0, value_node)

  # Update the tokens.
  var_tokens = list(tokenize.generate_tokens(
      cStringIO.StringIO(var_content).readline))
  var_tokens = {
      token[2]: list(token)
      # Ignore the tokens corresponding to braces and new lines.
      for token in var_tokens[2:-2]
  }

  gclient_dict.tokens = _ShiftLinesInTokens(gclient_dict.tokens, 1, line)
  gclient_dict.tokens.update(_ShiftLinesInTokens(var_tokens, line - 2, 0))


def SetVar(gclient_dict, var_name, value):
  if not isinstance(gclient_dict, _NodeDict) or gclient_dict.tokens is None:
    raise ValueError(
        "Can't use SetVar for the given gclient dict. It contains no "
        "formatting information.")
  tokens = gclient_dict.tokens

  if 'vars' not in gclient_dict:
    raise KeyError("vars dict is not defined.")

  if var_name not in gclient_dict['vars']:
    raise ValueError(
        "%s has not been declared in the vars dict. Consider using AddVar "
        "instead." % var_name)

  node = gclient_dict['vars'].GetNode(var_name)
  if node is None:
    raise ValueError(
        "The vars entry for %s has no formatting information." % var_name)

  _UpdateAstString(tokens, node, value)
  gclient_dict['vars'].SetNode(var_name, value, node)


def SetCIPD(gclient_dict, dep_name, package_name, new_version):
  if not isinstance(gclient_dict, _NodeDict) or gclient_dict.tokens is None:
    raise ValueError(
        "Can't use SetCIPD for the given gclient dict. It contains no "
        "formatting information.")
  tokens = gclient_dict.tokens

  if 'deps' not in gclient_dict or dep_name not in gclient_dict['deps']:
    raise KeyError(
        "Could not find any dependency called %s." % dep_name)

  # Find the package with the given name
  packages = [
      package
      for package in gclient_dict['deps'][dep_name]['packages']
      if package['package'] == package_name
  ]
  if len(packages) != 1:
    raise ValueError(
        "There must be exactly one package with the given name (%s), "
        "%s were found." % (package_name, len(packages)))

  # TODO(ehmaldonado): Support Var in package's version.
  node = packages[0].GetNode('version')
  if node is None:
    raise ValueError(
        "The deps entry for %s:%s has no formatting information." %
        (dep_name, package_name))

  new_version = 'version:' + new_version
  _UpdateAstString(tokens, node, new_version)
  packages[0].SetNode('version', new_version, node)


def SetRevision(gclient_dict, dep_name, new_revision):
  def _GetVarName(node):
    if isinstance(node, ast.Call):
      return node.args[0].s
    elif node.s.endswith('}'):
      last_brace = node.s.rfind('{')
      return node.s[last_brace+1:-1]
    return None

  def _UpdateRevision(dep_dict, dep_key, new_revision):
    dep_node = dep_dict.GetNode(dep_key)
    if dep_node is None:
      raise ValueError(
          "The deps entry for %s has no formatting information." % dep_name)

    node = dep_node
    if isinstance(node, ast.BinOp):
      node = node.right

    if not isinstance(node, ast.Call) and not isinstance(node, ast.Str):
      raise ValueError(
          "Unsupported dependency revision format. Please file a bug.")

    var_name = _GetVarName(node)
    if var_name is not None:
      SetVar(gclient_dict, var_name, new_revision)
    else:
      if '@' in node.s:
        # '@' is part of the last string, which we want to modify. Discard
        # whatever was after the '@' and put the new revision in its place.
        new_revision = node.s.split('@')[0] + '@' + new_revision
      elif '@' not in dep_dict[dep_key]:
        # '@' is not part of the URL at all. This mean the dependency is
        # unpinned and we should pin it.
        new_revision = node.s + '@' + new_revision
      _UpdateAstString(tokens, node, new_revision)
      dep_dict.SetNode(dep_key, new_revision, node)

  if not isinstance(gclient_dict, _NodeDict) or gclient_dict.tokens is None:
    raise ValueError(
        "Can't use SetRevision for the given gclient dict. It contains no "
        "formatting information.")
  tokens = gclient_dict.tokens

  if 'deps' not in gclient_dict or dep_name not in gclient_dict['deps']:
    raise KeyError(
        "Could not find any dependency called %s." % dep_name)

  if isinstance(gclient_dict['deps'][dep_name], _NodeDict):
    _UpdateRevision(gclient_dict['deps'][dep_name], 'url', new_revision)
  else:
    _UpdateRevision(gclient_dict['deps'], dep_name, new_revision)


def GetVar(gclient_dict, var_name):
  if 'vars' not in gclient_dict or var_name not in gclient_dict['vars']:
    raise KeyError(
        "Could not find any variable called %s." % var_name)

  return gclient_dict['vars'][var_name]


def GetCIPD(gclient_dict, dep_name, package_name):
  if 'deps' not in gclient_dict or dep_name not in gclient_dict['deps']:
    raise KeyError(
        "Could not find any dependency called %s." % dep_name)

  # Find the package with the given name
  packages = [
      package
      for package in gclient_dict['deps'][dep_name]['packages']
      if package['package'] == package_name
  ]
  if len(packages) != 1:
    raise ValueError(
        "There must be exactly one package with the given name (%s), "
        "%s were found." % (package_name, len(packages)))

  return packages[0]['version'][len('version:'):]


def GetRevision(gclient_dict, dep_name):
  if 'deps' not in gclient_dict or dep_name not in gclient_dict['deps']:
    raise KeyError(
        "Could not find any dependency called %s." % dep_name)

  dep = gclient_dict['deps'][dep_name]
  if dep is None:
    return None
  elif isinstance(dep, basestring):
    _, _, revision = dep.partition('@')
    return revision or None
  elif isinstance(dep, collections.Mapping) and 'url' in dep:
    _, _, revision = dep['url'].partition('@')
    return revision or None
  else:
    raise ValueError(
        '%s is not a valid git dependency.' % dep_name)