webrtc/base/schanneladapter.cc

/*
 *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#include "webrtc/base/win32.h"
#define SECURITY_WIN32
#include <security.h>
#include <schannel.h>

#include <iomanip>
#include <vector>

#include "webrtc/base/common.h"
#include "webrtc/base/logging.h"
#include "webrtc/base/schanneladapter.h"
#include "webrtc/base/sec_buffer.h"
#include "webrtc/base/thread.h"

namespace rtc {

/////////////////////////////////////////////////////////////////////////////
// SChannelAdapter
/////////////////////////////////////////////////////////////////////////////

extern const ConstantLabel SECURITY_ERRORS[];

const ConstantLabel SCHANNEL_BUFFER_TYPES[] = {
  KLABEL(SECBUFFER_EMPTY),              //  0
  KLABEL(SECBUFFER_DATA),               //  1
  KLABEL(SECBUFFER_TOKEN),              //  2
  KLABEL(SECBUFFER_PKG_PARAMS),         //  3
  KLABEL(SECBUFFER_MISSING),            //  4
  KLABEL(SECBUFFER_EXTRA),              //  5
  KLABEL(SECBUFFER_STREAM_TRAILER),     //  6
  KLABEL(SECBUFFER_STREAM_HEADER),      //  7
  KLABEL(SECBUFFER_MECHLIST),           // 11
  KLABEL(SECBUFFER_MECHLIST_SIGNATURE), // 12
  KLABEL(SECBUFFER_TARGET),             // 13
  KLABEL(SECBUFFER_CHANNEL_BINDINGS),   // 14
  LASTLABEL
};

void DescribeBuffer(LoggingSeverity severity, const char* prefix,
                    const SecBuffer& sb) {
  LOG_V(severity)
    << prefix
    << "(" << sb.cbBuffer
    << ", " << FindLabel(sb.BufferType & ~SECBUFFER_ATTRMASK,
                          SCHANNEL_BUFFER_TYPES)
    << ", " << sb.pvBuffer << ")";
}

void DescribeBuffers(LoggingSeverity severity, const char* prefix,
                     const SecBufferDesc* sbd) {
  if (!LOG_CHECK_LEVEL_V(severity))
    return;
  LOG_V(severity) << prefix << "(";
  for (size_t i=0; i<sbd->cBuffers; ++i) {
    DescribeBuffer(severity, "  ", sbd->pBuffers[i]);
  }
  LOG_V(severity) << ")";
}

const ULONG SSL_FLAGS_DEFAULT = ISC_REQ_ALLOCATE_MEMORY
                              | ISC_REQ_CONFIDENTIALITY
                              | ISC_REQ_EXTENDED_ERROR
                              | ISC_REQ_INTEGRITY
                              | ISC_REQ_REPLAY_DETECT
                              | ISC_REQ_SEQUENCE_DETECT
                              | ISC_REQ_STREAM;
                              //| ISC_REQ_USE_SUPPLIED_CREDS;

typedef std::vector<char> SChannelBuffer;

struct SChannelAdapter::SSLImpl {
  CredHandle cred;
  CtxtHandle ctx;
  bool cred_init, ctx_init;
  SChannelBuffer inbuf, outbuf, readable;
  SecPkgContext_StreamSizes sizes;

  SSLImpl() : cred_init(false), ctx_init(false) { }
};

SChannelAdapter::SChannelAdapter(AsyncSocket* socket)
  : SSLAdapter(socket), state_(SSL_NONE),
    restartable_(false), signal_close_(false), message_pending_(false),
    impl_(new SSLImpl) {
}

SChannelAdapter::~SChannelAdapter() {
  Cleanup();
}

int
SChannelAdapter::StartSSL(const char* hostname, bool restartable) {
  if (state_ != SSL_NONE)
    return ERROR_ALREADY_INITIALIZED;

  ssl_host_name_ = hostname;
  restartable_ = restartable;

  if (socket_->GetState() != Socket::CS_CONNECTED) {
    state_ = SSL_WAIT;
    return 0;
  }

  state_ = SSL_CONNECTING;
  if (int err = BeginSSL()) {
    Error("BeginSSL", err, false);
    return err;
  }

  return 0;
}

int
SChannelAdapter::BeginSSL() {
  LOG(LS_VERBOSE) << "BeginSSL: " << ssl_host_name_;
  ASSERT(state_ == SSL_CONNECTING);

  SECURITY_STATUS ret;

  SCHANNEL_CRED sc_cred = { 0 };
  sc_cred.dwVersion = SCHANNEL_CRED_VERSION;
  //sc_cred.dwMinimumCipherStrength = 128; // Note: use system default
  sc_cred.dwFlags = SCH_CRED_NO_DEFAULT_CREDS | SCH_CRED_AUTO_CRED_VALIDATION;

  ret = AcquireCredentialsHandle(NULL, UNISP_NAME, SECPKG_CRED_OUTBOUND, NULL,
                                 &sc_cred, NULL, NULL, &impl_->cred, NULL);
  if (ret != SEC_E_OK) {
    LOG(LS_ERROR) << "AcquireCredentialsHandle error: "
                  << ErrorName(ret, SECURITY_ERRORS);
    return ret;
  }
  impl_->cred_init = true;

  if (LOG_CHECK_LEVEL(LS_VERBOSE)) {
    SecPkgCred_CipherStrengths cipher_strengths = { 0 };
    ret = QueryCredentialsAttributes(&impl_->cred,
                                     SECPKG_ATTR_CIPHER_STRENGTHS,
                                     &cipher_strengths);
    if (SUCCEEDED(ret)) {
      LOG(LS_VERBOSE) << "SChannel cipher strength: "
                  << cipher_strengths.dwMinimumCipherStrength << " - "
                  << cipher_strengths.dwMaximumCipherStrength;
    }

    SecPkgCred_SupportedAlgs supported_algs = { 0 };
    ret = QueryCredentialsAttributes(&impl_->cred,
                                     SECPKG_ATTR_SUPPORTED_ALGS,
                                     &supported_algs);
    if (SUCCEEDED(ret)) {
      LOG(LS_VERBOSE) << "SChannel supported algorithms:";
      for (DWORD i=0; i<supported_algs.cSupportedAlgs; ++i) {
        ALG_ID alg_id = supported_algs.palgSupportedAlgs[i];
        PCCRYPT_OID_INFO oinfo = CryptFindOIDInfo(CRYPT_OID_INFO_ALGID_KEY,
                                                  &alg_id, 0);
        LPCWSTR alg_name = (NULL != oinfo) ? oinfo->pwszName : L"Unknown";
        LOG(LS_VERBOSE) << "  " << ToUtf8(alg_name) << " (" << alg_id << ")";
      }
      CSecBufferBase::FreeSSPI(supported_algs.palgSupportedAlgs);
    }
  }

  ULONG flags = SSL_FLAGS_DEFAULT, ret_flags = 0;
  if (ignore_bad_cert())
    flags |= ISC_REQ_MANUAL_CRED_VALIDATION;

  CSecBufferBundle<2, CSecBufferBase::FreeSSPI> sb_out;
  ret = InitializeSecurityContextA(&impl_->cred, NULL,
                                   const_cast<char*>(ssl_host_name_.c_str()),
                                   flags, 0, 0, NULL, 0,
                                   &impl_->ctx, sb_out.desc(),
                                   &ret_flags, NULL);
  if (SUCCEEDED(ret))
    impl_->ctx_init = true;
  return ProcessContext(ret, NULL, sb_out.desc());
}

int
SChannelAdapter::ContinueSSL() {
  LOG(LS_VERBOSE) << "ContinueSSL";
  ASSERT(state_ == SSL_CONNECTING);

  SECURITY_STATUS ret;

  CSecBufferBundle<2> sb_in;
  sb_in[0].BufferType = SECBUFFER_TOKEN;
  sb_in[0].cbBuffer = static_cast<unsigned long>(impl_->inbuf.size());
  sb_in[0].pvBuffer = &impl_->inbuf[0];
  //DescribeBuffers(LS_VERBOSE, "Input Buffer ", sb_in.desc());

  ULONG flags = SSL_FLAGS_DEFAULT, ret_flags = 0;
  if (ignore_bad_cert())
    flags |= ISC_REQ_MANUAL_CRED_VALIDATION;

  CSecBufferBundle<2, CSecBufferBase::FreeSSPI> sb_out;
  ret = InitializeSecurityContextA(&impl_->cred, &impl_->ctx,
                                   const_cast<char*>(ssl_host_name_.c_str()),
                                   flags, 0, 0, sb_in.desc(), 0,
                                   NULL, sb_out.desc(),
                                   &ret_flags, NULL);
  return ProcessContext(ret, sb_in.desc(), sb_out.desc());
}

int
SChannelAdapter::ProcessContext(long int status, _SecBufferDesc* sbd_in,
                                _SecBufferDesc* sbd_out) {
  if (status != SEC_E_OK && status != SEC_I_CONTINUE_NEEDED &&
      status != SEC_E_INCOMPLETE_MESSAGE) {
    LOG(LS_ERROR)
      << "InitializeSecurityContext error: "
      << ErrorName(status, SECURITY_ERRORS);
  }
  //if (sbd_in)
  //  DescribeBuffers(LS_VERBOSE, "Input Buffer ", sbd_in);
  //if (sbd_out)
  //  DescribeBuffers(LS_VERBOSE, "Output Buffer ", sbd_out);

  if (status == SEC_E_INCOMPLETE_MESSAGE) {
    // Wait for more input from server.
    return Flush();
  }

  if (FAILED(status)) {
    // We can't continue.  Common errors:
    // SEC_E_CERT_EXPIRED - Typically, this means the computer clock is wrong.
    return status;
  }

  // Note: we check both input and output buffers for SECBUFFER_EXTRA.
  // Experience shows it appearing in the input, but the documentation claims
  // it should appear in the output.
  size_t extra = 0;
  if (sbd_in) {
    for (size_t i=0; i<sbd_in->cBuffers; ++i) {
      SecBuffer& buffer = sbd_in->pBuffers[i];
      if (buffer.BufferType == SECBUFFER_EXTRA) {
        extra += buffer.cbBuffer;
      }
    }
  }
  if (sbd_out) {
    for (size_t i=0; i<sbd_out->cBuffers; ++i) {
      SecBuffer& buffer = sbd_out->pBuffers[i];
      if (buffer.BufferType == SECBUFFER_EXTRA) {
        extra += buffer.cbBuffer;
      } else if (buffer.BufferType == SECBUFFER_TOKEN) {
        impl_->outbuf.insert(impl_->outbuf.end(),
          reinterpret_cast<char*>(buffer.pvBuffer),
          reinterpret_cast<char*>(buffer.pvBuffer) + buffer.cbBuffer);
      }
    }
  }

  if (extra) {
    ASSERT(extra <= impl_->inbuf.size());
    size_t consumed = impl_->inbuf.size() - extra;
    memmove(&impl_->inbuf[0], &impl_->inbuf[consumed], extra);
    impl_->inbuf.resize(extra);
  } else {
    impl_->inbuf.clear();
  }

  if (SEC_I_CONTINUE_NEEDED == status) {
    // Send data to server and wait for response.
    // Note: ContinueSSL will result in a Flush, anyway.
    return impl_->inbuf.empty() ? Flush() : ContinueSSL();
  }

  if (SEC_E_OK == status) {
    LOG(LS_VERBOSE) << "QueryContextAttributes";
    status = QueryContextAttributes(&impl_->ctx, SECPKG_ATTR_STREAM_SIZES,
                                    &impl_->sizes);
    if (FAILED(status)) {
      LOG(LS_ERROR) << "QueryContextAttributes error: "
                    << ErrorName(status, SECURITY_ERRORS);
      return status;
    }

    state_ = SSL_CONNECTED;

    if (int err = DecryptData()) {
      return err;
    } else if (int err = Flush()) {
      return err;
    } else {
      // If we decrypted any data, queue up a notification here
      PostEvent();
      // Signal our connectedness
      AsyncSocketAdapter::OnConnectEvent(this);
    }
    return 0;
  }

  if (SEC_I_INCOMPLETE_CREDENTIALS == status) {
    // We don't support client authentication in schannel.
    return status;
  }

  // We don't expect any other codes
  ASSERT(false);
  return status;
}

int
SChannelAdapter::DecryptData() {
  SChannelBuffer& inbuf = impl_->inbuf;
  SChannelBuffer& readable = impl_->readable;

  while (!inbuf.empty()) {
    CSecBufferBundle<4> in_buf;
    in_buf[0].BufferType = SECBUFFER_DATA;
    in_buf[0].cbBuffer = static_cast<unsigned long>(inbuf.size());
    in_buf[0].pvBuffer = &inbuf[0];

    //DescribeBuffers(LS_VERBOSE, "Decrypt In ", in_buf.desc());
    SECURITY_STATUS status = DecryptMessage(&impl_->ctx, in_buf.desc(), 0, 0);
    //DescribeBuffers(LS_VERBOSE, "Decrypt Out ", in_buf.desc());

    // Note: We are explicitly treating SEC_E_OK, SEC_I_CONTEXT_EXPIRED, and
    // any other successful results as continue.
    if (SUCCEEDED(status)) {
      size_t data_len = 0, extra_len = 0;
      for (size_t i=0; i<in_buf.desc()->cBuffers; ++i) {
        if (in_buf[i].BufferType == SECBUFFER_DATA) {
          data_len += in_buf[i].cbBuffer;
          readable.insert(readable.end(),
            reinterpret_cast<char*>(in_buf[i].pvBuffer),
            reinterpret_cast<char*>(in_buf[i].pvBuffer) + in_buf[i].cbBuffer);
        } else if (in_buf[i].BufferType == SECBUFFER_EXTRA) {
          extra_len += in_buf[i].cbBuffer;
        }
      }
      // There is a bug on Win2K where SEC_I_CONTEXT_EXPIRED is misclassified.
      if ((data_len == 0) && (inbuf[0] == 0x15)) {
        status = SEC_I_CONTEXT_EXPIRED;
      }
      if (extra_len) {
        size_t consumed = inbuf.size() - extra_len;
        memmove(&inbuf[0], &inbuf[consumed], extra_len);
        inbuf.resize(extra_len);
      } else {
        inbuf.clear();
      }
      // TODO: Handle SEC_I_CONTEXT_EXPIRED to do clean shutdown
      if (status != SEC_E_OK) {
        LOG(LS_INFO) << "DecryptMessage returned continuation code: "
                      << ErrorName(status, SECURITY_ERRORS);
      }
      continue;
    }

    if (status == SEC_E_INCOMPLETE_MESSAGE) {
      break;
    } else {
      return status;
    }
  }

  return 0;
}

void
SChannelAdapter::Cleanup() {
  if (impl_->ctx_init)
    DeleteSecurityContext(&impl_->ctx);
  if (impl_->cred_init)
    FreeCredentialsHandle(&impl_->cred);
  delete impl_;
}

void
SChannelAdapter::PostEvent() {
  // Check if there's anything notable to signal
  if (impl_->readable.empty() && !signal_close_)
    return;

  // Only one post in the queue at a time
  if (message_pending_)
    return;

  if (Thread* thread = Thread::Current()) {
    message_pending_ = true;
    thread->Post(this);
  } else {
    LOG(LS_ERROR) << "No thread context available for SChannelAdapter";
    ASSERT(false);
  }
}

void
SChannelAdapter::Error(const char* context, int err, bool signal) {
  LOG(LS_WARNING) << "SChannelAdapter::Error("
                  << context << ", "
                  << ErrorName(err, SECURITY_ERRORS) << ")";
  state_ = SSL_ERROR;
  SetError(err);
  if (signal)
    AsyncSocketAdapter::OnCloseEvent(this, err);
}

int
SChannelAdapter::Read() {
  char buffer[4096];
  SChannelBuffer& inbuf = impl_->inbuf;
  while (true) {
    int ret = AsyncSocketAdapter::Recv(buffer, sizeof(buffer));
    if (ret > 0) {
      inbuf.insert(inbuf.end(), buffer, buffer + ret);
    } else if (GetError() == EWOULDBLOCK) {
      return 0;  // Blocking
    } else {
      return GetError();
    }
  }
}

int
SChannelAdapter::Flush() {
  int result = 0;
  size_t pos = 0;
  SChannelBuffer& outbuf = impl_->outbuf;
  while (pos < outbuf.size()) {
    int sent = AsyncSocketAdapter::Send(&outbuf[pos], outbuf.size() - pos);
    if (sent > 0) {
      pos += sent;
    } else if (GetError() == EWOULDBLOCK) {
      break;  // Blocking
    } else {
      result = GetError();
      break;
    }
  }
  if (int remainder = static_cast<int>(outbuf.size() - pos)) {
    memmove(&outbuf[0], &outbuf[pos], remainder);
    outbuf.resize(remainder);
  } else {
    outbuf.clear();
  }
  return result;
}

//
// AsyncSocket Implementation
//

int
SChannelAdapter::Send(const void* pv, size_t cb) {
  switch (state_) {
  case SSL_NONE:
    return AsyncSocketAdapter::Send(pv, cb);

  case SSL_WAIT:
  case SSL_CONNECTING:
    SetError(EWOULDBLOCK);
    return SOCKET_ERROR;

  case SSL_CONNECTED:
    break;

  case SSL_ERROR:
  default:
    return SOCKET_ERROR;
  }

  size_t written = 0;
  SChannelBuffer& outbuf = impl_->outbuf;
  while (written < cb) {
    const size_t encrypt_len = std::min<size_t>(cb - written,
                                                impl_->sizes.cbMaximumMessage);

    CSecBufferBundle<4> out_buf;
    out_buf[0].BufferType = SECBUFFER_STREAM_HEADER;
    out_buf[0].cbBuffer = impl_->sizes.cbHeader;
    out_buf[1].BufferType = SECBUFFER_DATA;
    out_buf[1].cbBuffer = static_cast<unsigned long>(encrypt_len);
    out_buf[2].BufferType = SECBUFFER_STREAM_TRAILER;
    out_buf[2].cbBuffer = impl_->sizes.cbTrailer;

    size_t packet_len = out_buf[0].cbBuffer
                      + out_buf[1].cbBuffer
                      + out_buf[2].cbBuffer;

    SChannelBuffer message;
    message.resize(packet_len);
    out_buf[0].pvBuffer = &message[0];
    out_buf[1].pvBuffer = &message[out_buf[0].cbBuffer];
    out_buf[2].pvBuffer = &message[out_buf[0].cbBuffer + out_buf[1].cbBuffer];

    memcpy(out_buf[1].pvBuffer,
           static_cast<const char*>(pv) + written,
           encrypt_len);

    //DescribeBuffers(LS_VERBOSE, "Encrypt In ", out_buf.desc());
    SECURITY_STATUS res = EncryptMessage(&impl_->ctx, 0, out_buf.desc(), 0);
    //DescribeBuffers(LS_VERBOSE, "Encrypt Out ", out_buf.desc());

    if (FAILED(res)) {
      Error("EncryptMessage", res, false);
      return SOCKET_ERROR;
    }

    // We assume that the header and data segments do not change length,
    // or else encrypting the concatenated packet in-place is wrong.
    ASSERT(out_buf[0].cbBuffer == impl_->sizes.cbHeader);
    ASSERT(out_buf[1].cbBuffer == static_cast<unsigned long>(encrypt_len));

    // However, the length of the trailer may change due to padding.
    ASSERT(out_buf[2].cbBuffer <= impl_->sizes.cbTrailer);

    packet_len = out_buf[0].cbBuffer
               + out_buf[1].cbBuffer
               + out_buf[2].cbBuffer;

    written += encrypt_len;
    outbuf.insert(outbuf.end(), &message[0], &message[packet_len-1]+1);
  }

  if (int err = Flush()) {
    state_ = SSL_ERROR;
    SetError(err);
    return SOCKET_ERROR;
  }

  return static_cast<int>(written);
}

int
SChannelAdapter::Recv(void* pv, size_t cb) {
  switch (state_) {
  case SSL_NONE:
    return AsyncSocketAdapter::Recv(pv, cb);

  case SSL_WAIT:
  case SSL_CONNECTING:
    SetError(EWOULDBLOCK);
    return SOCKET_ERROR;

  case SSL_CONNECTED:
    break;

  case SSL_ERROR:
  default:
    return SOCKET_ERROR;
  }

  SChannelBuffer& readable = impl_->readable;
  if (readable.empty()) {
    SetError(EWOULDBLOCK);
    return SOCKET_ERROR;
  }
  size_t read = _min(cb, readable.size());
  memcpy(pv, &readable[0], read);
  if (size_t remaining = readable.size() - read) {
    memmove(&readable[0], &readable[read], remaining);
    readable.resize(remaining);
  } else {
    readable.clear();
  }

  PostEvent();
  return static_cast<int>(read);
}

int
SChannelAdapter::Close() {
  if (!impl_->readable.empty()) {
    LOG(WARNING) << "SChannelAdapter::Close with readable data";
    // Note: this isn't strictly an error, but we're using it temporarily to
    // track bugs.
    //ASSERT(false);
  }
  if (state_ == SSL_CONNECTED) {
    DWORD token = SCHANNEL_SHUTDOWN;
    CSecBufferBundle<1> sb_in;
    sb_in[0].BufferType = SECBUFFER_TOKEN;
    sb_in[0].cbBuffer = sizeof(token);
    sb_in[0].pvBuffer = &token;
    ApplyControlToken(&impl_->ctx, sb_in.desc());
    // TODO: In theory, to do a nice shutdown, we need to begin shutdown
    // negotiation with more calls to InitializeSecurityContext.  Since the
    // socket api doesn't support nice shutdown at this point, we don't bother.
  }
  Cleanup();
  impl_ = new SSLImpl;
  state_ = restartable_ ? SSL_WAIT : SSL_NONE;
  signal_close_ = false;
  message_pending_ = false;
  return AsyncSocketAdapter::Close();
}

Socket::ConnState
SChannelAdapter::GetState() const {
  if (signal_close_)
    return CS_CONNECTED;
  ConnState state = socket_->GetState();
  if ((state == CS_CONNECTED)
      && ((state_ == SSL_WAIT) || (state_ == SSL_CONNECTING)))
    state = CS_CONNECTING;
  return state;
}

void
SChannelAdapter::OnConnectEvent(AsyncSocket* socket) {
  LOG(LS_VERBOSE) << "SChannelAdapter::OnConnectEvent";
  if (state_ != SSL_WAIT) {
    ASSERT(state_ == SSL_NONE);
    AsyncSocketAdapter::OnConnectEvent(socket);
    return;
  }

  state_ = SSL_CONNECTING;
  if (int err = BeginSSL()) {
    Error("BeginSSL", err);
  }
}

void
SChannelAdapter::OnReadEvent(AsyncSocket* socket) {
  if (state_ == SSL_NONE) {
    AsyncSocketAdapter::OnReadEvent(socket);
    return;
  }

  if (int err = Read()) {
    Error("Read", err);
    return;
  }

  if (impl_->inbuf.empty())
    return;

  if (state_ == SSL_CONNECTED) {
    if (int err = DecryptData()) {
      Error("DecryptData", err);
    } else if (!impl_->readable.empty()) {
      AsyncSocketAdapter::OnReadEvent(this);
    }
  } else if (state_ == SSL_CONNECTING) {
    if (int err = ContinueSSL()) {
      Error("ContinueSSL", err);
    }
  }
}

void
SChannelAdapter::OnWriteEvent(AsyncSocket* socket) {
  if (state_ == SSL_NONE) {
    AsyncSocketAdapter::OnWriteEvent(socket);
    return;
  }

  if (int err = Flush()) {
    Error("Flush", err);
    return;
  }

  // See if we have more data to write
  if (!impl_->outbuf.empty())
    return;

  // Buffer is empty, submit notification
  if (state_ == SSL_CONNECTED) {
    AsyncSocketAdapter::OnWriteEvent(socket);
  }
}

void
SChannelAdapter::OnCloseEvent(AsyncSocket* socket, int err) {
  if ((state_ == SSL_NONE) || impl_->readable.empty()) {
    AsyncSocketAdapter::OnCloseEvent(socket, err);
    return;
  }

  // If readable is non-empty, then we have a pending Message
  // that will allow us to signal close (eventually).
  signal_close_ = true;
}

void
SChannelAdapter::OnMessage(Message* pmsg) {
  if (!message_pending_)
    return;  // This occurs when socket is closed

  message_pending_ = false;
  if (!impl_->readable.empty()) {
    AsyncSocketAdapter::OnReadEvent(this);
  } else if (signal_close_) {
    signal_close_ = false;
    AsyncSocketAdapter::OnCloseEvent(this, 0); // TODO: cache this error?
  }
}

} // namespace rtc