root/net/http/http_cache_transaction.cc

DEFINITIONS

1. NonErrorResponse
2. IsOfflineError
3. RecordOfflineStatus
4. RecordVaryHeaderHistogram
5. HeaderMatches
6. websocket_handshake_stream_base_create_helper_
7. WriteMetadata
8. AddTruncatedFlag
9. GetWriterLoadState
10. net_log
11. Start
12. RestartIgnoringLastError
13. RestartWithCertificate
14. RestartWithAuth
15. IsReadyToRestartForAuth
16. Read
17. StopCaching
18. GetFullRequestHeaders
19. GetTotalReceivedBytes
20. DoneReading
21. GetResponseInfo
22. GetLoadState
23. GetUploadProgress
24. SetQuicServerInfo
25. GetLoadTimingInfo
26. SetPriority
27. SetWebSocketHandshakeStreamCreateHelper
28. SetBeforeNetworkStartCallback
29. ResumeNetworkStart
30. DoCallback
31. HandleResult
32. DoLoop
33. DoGetBackend
34. DoGetBackendComplete
35. DoSendRequest
36. DoSendRequestComplete
37. DoSuccessfulSendRequest
38. DoNetworkRead
39. DoNetworkReadComplete
40. DoInitEntry
41. DoOpenEntry
42. DoOpenEntryComplete
43. DoCreateEntry
44. DoCreateEntryComplete
45. DoDoomEntry
46. DoDoomEntryComplete
47. DoAddToEntry
48. DoAddToEntryComplete
49. DoStartPartialCacheValidation
50. DoCompletePartialCacheValidation
51. DoUpdateCachedResponse
52. DoUpdateCachedResponseComplete
53. DoOverwriteCachedResponse
54. DoTruncateCachedData
55. DoTruncateCachedDataComplete
56. DoTruncateCachedMetadata
57. DoTruncateCachedMetadataComplete
58. DoPartialHeadersReceived
59. DoCacheReadResponse
60. DoCacheReadResponseComplete
61. DoCacheWriteResponse
62. DoCacheWriteTruncatedResponse
63. DoCacheWriteResponseComplete
64. DoCacheReadMetadata
65. DoCacheReadMetadataComplete
66. DoCacheQueryData
67. DoCacheQueryDataComplete
68. DoCacheReadData
69. DoCacheReadDataComplete
70. DoCacheWriteData
71. DoCacheWriteDataComplete
72. SetRequest
73. ShouldPassThrough
74. BeginCacheRead
75. BeginCacheValidation
76. BeginPartialCacheValidation
77. ValidateEntryHeadersAndContinue
78. BeginExternallyConditionalizedRequest
79. RestartNetworkRequest
80. RestartNetworkRequestWithCertificate
81. RestartNetworkRequestWithAuth
82. RequiresValidation
83. ConditionalizeRequest
84. ValidatePartialResponse
85. IgnoreRangeRequest
86. FailRangeRequest
87. SetupEntryForRead
88. ReadFromNetwork
89. ReadFromEntry
90. WriteToEntry
91. WriteResponseInfoToEntry
92. AppendResponseDataToEntry
93. DoneWritingToEntry
94. OnCacheReadError
95. DoomPartialEntry
96. DoPartialNetworkReadCompleted
97. DoPartialCacheReadCompleted
98. DoRestartPartialRequest
99. ResetNetworkTransaction
100. CanResume
101. UpdateTransactionPattern
102. RecordHistograms
103. OnIOComplete

// 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.

#include "net/http/http_cache_transaction.h"

#include "build/build_config.h"

#if defined(OS_POSIX)
#include <unistd.h>
#endif

#include <algorithm>
#include <string>

#include "base/bind.h"
#include "base/compiler_specific.h"
#include "base/memory/ref_counted.h"
#include "base/metrics/field_trial.h"
#include "base/metrics/histogram.h"
#include "base/metrics/sparse_histogram.h"
#include "base/rand_util.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/string_util.h"
#include "base/time/time.h"
#include "net/base/completion_callback.h"
#include "net/base/io_buffer.h"
#include "net/base/load_flags.h"
#include "net/base/load_timing_info.h"
#include "net/base/net_errors.h"
#include "net/base/net_log.h"
#include "net/base/upload_data_stream.h"
#include "net/cert/cert_status_flags.h"
#include "net/disk_cache/disk_cache.h"
#include "net/http/http_network_session.h"
#include "net/http/http_request_info.h"
#include "net/http/http_response_headers.h"
#include "net/http/http_transaction.h"
#include "net/http/http_util.h"
#include "net/http/partial_data.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "net/ssl/ssl_config_service.h"

using base::Time;
using base::TimeDelta;
using base::TimeTicks;

namespace {

// From http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-21#section-6
//      a "non-error response" is one with a 2xx (Successful) or 3xx
//      (Redirection) status code.
bool NonErrorResponse(int status_code) {
  int status_code_range = status_code / 100;
  return status_code_range == 2 || status_code_range == 3;
}

// Error codes that will be considered indicative of a page being offline/
// unreachable for LOAD_FROM_CACHE_IF_OFFLINE.
bool IsOfflineError(int error) {
  return (error == net::ERR_NAME_NOT_RESOLVED ||
          error == net::ERR_INTERNET_DISCONNECTED ||
          error == net::ERR_ADDRESS_UNREACHABLE ||
          error == net::ERR_CONNECTION_TIMED_OUT);
}

// Enum for UMA, indicating the status (with regard to offline mode) of
// a particular request.
enum RequestOfflineStatus {
  // A cache transaction hit in cache (data was present and not stale)
  // and returned it.
  OFFLINE_STATUS_FRESH_CACHE,

  // A network request was required for a cache entry, and it succeeded.
  OFFLINE_STATUS_NETWORK_SUCCEEDED,

  // A network request was required for a cache entry, and it failed with
  // a non-offline error.
  OFFLINE_STATUS_NETWORK_FAILED,

  // A network request was required for a cache entry, it failed with an
  // offline error, and we could serve stale data if
  // LOAD_FROM_CACHE_IF_OFFLINE was set.
  OFFLINE_STATUS_DATA_AVAILABLE_OFFLINE,

  // A network request was required for a cache entry, it failed with
  // an offline error, and there was no servable data in cache (even
  // stale data).
  OFFLINE_STATUS_DATA_UNAVAILABLE_OFFLINE,

  OFFLINE_STATUS_MAX_ENTRIES
};

void RecordOfflineStatus(int load_flags, RequestOfflineStatus status) {
  // Restrict to main frame to keep statistics close to
  // "would have shown them something useful if offline mode was enabled".
  if (load_flags & net::LOAD_MAIN_FRAME) {
    UMA_HISTOGRAM_ENUMERATION("HttpCache.OfflineStatus", status,
                              OFFLINE_STATUS_MAX_ENTRIES);
  }
}

// TODO(rvargas): Remove once we get the data.
void RecordVaryHeaderHistogram(const net::HttpResponseInfo* response) {
  enum VaryType {
    VARY_NOT_PRESENT,
    VARY_UA,
    VARY_OTHER,
    VARY_MAX
  };
  VaryType vary = VARY_NOT_PRESENT;
  if (response->vary_data.is_valid()) {
    vary = VARY_OTHER;
    if (response->headers->HasHeaderValue("vary", "user-agent"))
      vary = VARY_UA;
  }
  UMA_HISTOGRAM_ENUMERATION("HttpCache.Vary", vary, VARY_MAX);
}

}  // namespace

namespace net {

struct HeaderNameAndValue {
  const char* name;
  const char* value;
};

// If the request includes one of these request headers, then avoid caching
// to avoid getting confused.
static const HeaderNameAndValue kPassThroughHeaders[] = {
  { "if-unmodified-since", NULL },  // causes unexpected 412s
  { "if-match", NULL },             // causes unexpected 412s
  { "if-range", NULL },
  { NULL, NULL }
};

struct ValidationHeaderInfo {
  const char* request_header_name;
  const char* related_response_header_name;
};

static const ValidationHeaderInfo kValidationHeaders[] = {
  { "if-modified-since", "last-modified" },
  { "if-none-match", "etag" },
};

// If the request includes one of these request headers, then avoid reusing
// our cached copy if any.
static const HeaderNameAndValue kForceFetchHeaders[] = {
  { "cache-control", "no-cache" },
  { "pragma", "no-cache" },
  { NULL, NULL }
};

// If the request includes one of these request headers, then force our
// cached copy (if any) to be revalidated before reusing it.
static const HeaderNameAndValue kForceValidateHeaders[] = {
  { "cache-control", "max-age=0" },
  { NULL, NULL }
};

static bool HeaderMatches(const HttpRequestHeaders& headers,
                          const HeaderNameAndValue* search) {
  for (; search->name; ++search) {
    std::string header_value;
    if (!headers.GetHeader(search->name, &header_value))
      continue;

    if (!search->value)
      return true;

    HttpUtil::ValuesIterator v(header_value.begin(), header_value.end(), ',');
    while (v.GetNext()) {
      if (LowerCaseEqualsASCII(v.value_begin(), v.value_end(), search->value))
        return true;
    }
  }
  return false;
}

//-----------------------------------------------------------------------------

HttpCache::Transaction::Transaction(
    RequestPriority priority,
    HttpCache* cache)
    : next_state_(STATE_NONE),
      request_(NULL),
      priority_(priority),
      cache_(cache->AsWeakPtr()),
      entry_(NULL),
      new_entry_(NULL),
      new_response_(NULL),
      mode_(NONE),
      target_state_(STATE_NONE),
      reading_(false),
      invalid_range_(false),
      truncated_(false),
      is_sparse_(false),
      range_requested_(false),
      handling_206_(false),
      cache_pending_(false),
      done_reading_(false),
      vary_mismatch_(false),
      couldnt_conditionalize_request_(false),
      io_buf_len_(0),
      read_offset_(0),
      effective_load_flags_(0),
      write_len_(0),
      weak_factory_(this),
      io_callback_(base::Bind(&Transaction::OnIOComplete,
                              weak_factory_.GetWeakPtr())),
      transaction_pattern_(PATTERN_UNDEFINED),
      total_received_bytes_(0),
      websocket_handshake_stream_base_create_helper_(NULL) {
  COMPILE_ASSERT(HttpCache::Transaction::kNumValidationHeaders ==
                 arraysize(kValidationHeaders),
                 Invalid_number_of_validation_headers);
}

HttpCache::Transaction::~Transaction() {
  // We may have to issue another IO, but we should never invoke the callback_
  // after this point.
  callback_.Reset();

  if (cache_) {
    if (entry_) {
      bool cancel_request = reading_ && response_.headers;
      if (cancel_request) {
        if (partial_) {
          entry_->disk_entry->CancelSparseIO();
        } else {
          cancel_request &= (response_.headers->response_code() == 200);
        }
      }

      cache_->DoneWithEntry(entry_, this, cancel_request);
    } else if (cache_pending_) {
      cache_->RemovePendingTransaction(this);
    }
  }
}

int HttpCache::Transaction::WriteMetadata(IOBuffer* buf, int buf_len,
                                          const CompletionCallback& callback) {
  DCHECK(buf);
  DCHECK_GT(buf_len, 0);
  DCHECK(!callback.is_null());
  if (!cache_.get() || !entry_)
    return ERR_UNEXPECTED;

  // We don't need to track this operation for anything.
  // It could be possible to check if there is something already written and
  // avoid writing again (it should be the same, right?), but let's allow the
  // caller to "update" the contents with something new.
  return entry_->disk_entry->WriteData(kMetadataIndex, 0, buf, buf_len,
                                       callback, true);
}

bool HttpCache::Transaction::AddTruncatedFlag() {
  DCHECK(mode_ & WRITE || mode_ == NONE);

  // Don't set the flag for sparse entries.
  if (partial_.get() && !truncated_)
    return true;

  if (!CanResume(true))
    return false;

  // We may have received the whole resource already.
  if (done_reading_)
    return true;

  truncated_ = true;
  target_state_ = STATE_NONE;
  next_state_ = STATE_CACHE_WRITE_TRUNCATED_RESPONSE;
  DoLoop(OK);
  return true;
}

LoadState HttpCache::Transaction::GetWriterLoadState() const {
  if (network_trans_.get())
    return network_trans_->GetLoadState();
  if (entry_ || !request_)
    return LOAD_STATE_IDLE;
  return LOAD_STATE_WAITING_FOR_CACHE;
}

const BoundNetLog& HttpCache::Transaction::net_log() const {
  return net_log_;
}

int HttpCache::Transaction::Start(const HttpRequestInfo* request,
                                  const CompletionCallback& callback,
                                  const BoundNetLog& net_log) {
  DCHECK(request);
  DCHECK(!callback.is_null());

  // Ensure that we only have one asynchronous call at a time.
  DCHECK(callback_.is_null());
  DCHECK(!reading_);
  DCHECK(!network_trans_.get());
  DCHECK(!entry_);

  if (!cache_.get())
    return ERR_UNEXPECTED;

  SetRequest(net_log, request);

  // We have to wait until the backend is initialized so we start the SM.
  next_state_ = STATE_GET_BACKEND;
  int rv = DoLoop(OK);

  // Setting this here allows us to check for the existence of a callback_ to
  // determine if we are still inside Start.
  if (rv == ERR_IO_PENDING)
    callback_ = callback;

  return rv;
}

int HttpCache::Transaction::RestartIgnoringLastError(
    const CompletionCallback& callback) {
  DCHECK(!callback.is_null());

  // Ensure that we only have one asynchronous call at a time.
  DCHECK(callback_.is_null());

  if (!cache_.get())
    return ERR_UNEXPECTED;

  int rv = RestartNetworkRequest();

  if (rv == ERR_IO_PENDING)
    callback_ = callback;

  return rv;
}

int HttpCache::Transaction::RestartWithCertificate(
    X509Certificate* client_cert,
    const CompletionCallback& callback) {
  DCHECK(!callback.is_null());

  // Ensure that we only have one asynchronous call at a time.
  DCHECK(callback_.is_null());

  if (!cache_.get())
    return ERR_UNEXPECTED;

  int rv = RestartNetworkRequestWithCertificate(client_cert);

  if (rv == ERR_IO_PENDING)
    callback_ = callback;

  return rv;
}

int HttpCache::Transaction::RestartWithAuth(
    const AuthCredentials& credentials,
    const CompletionCallback& callback) {
  DCHECK(auth_response_.headers.get());
  DCHECK(!callback.is_null());

  // Ensure that we only have one asynchronous call at a time.
  DCHECK(callback_.is_null());

  if (!cache_.get())
    return ERR_UNEXPECTED;

  // Clear the intermediate response since we are going to start over.
  auth_response_ = HttpResponseInfo();

  int rv = RestartNetworkRequestWithAuth(credentials);

  if (rv == ERR_IO_PENDING)
    callback_ = callback;

  return rv;
}

bool HttpCache::Transaction::IsReadyToRestartForAuth() {
  if (!network_trans_.get())
    return false;
  return network_trans_->IsReadyToRestartForAuth();
}

int HttpCache::Transaction::Read(IOBuffer* buf, int buf_len,
                                 const CompletionCallback& callback) {
  DCHECK(buf);
  DCHECK_GT(buf_len, 0);
  DCHECK(!callback.is_null());

  DCHECK(callback_.is_null());

  if (!cache_.get())
    return ERR_UNEXPECTED;

  // If we have an intermediate auth response at this point, then it means the
  // user wishes to read the network response (the error page).  If there is a
  // previous response in the cache then we should leave it intact.
  if (auth_response_.headers.get() && mode_ != NONE) {
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    DCHECK(mode_ & WRITE);
    DoneWritingToEntry(mode_ == READ_WRITE);
    mode_ = NONE;
  }

  reading_ = true;
  int rv;

  switch (mode_) {
    case READ_WRITE:
      DCHECK(partial_.get());
      if (!network_trans_.get()) {
        // We are just reading from the cache, but we may be writing later.
        rv = ReadFromEntry(buf, buf_len);
        break;
      }
    case NONE:
    case WRITE:
      DCHECK(network_trans_.get());
      rv = ReadFromNetwork(buf, buf_len);
      break;
    case READ:
      rv = ReadFromEntry(buf, buf_len);
      break;
    default:
      NOTREACHED();
      rv = ERR_FAILED;
  }

  if (rv == ERR_IO_PENDING) {
    DCHECK(callback_.is_null());
    callback_ = callback;
  }
  return rv;
}

void HttpCache::Transaction::StopCaching() {
  // We really don't know where we are now. Hopefully there is no operation in
  // progress, but nothing really prevents this method to be called after we
  // returned ERR_IO_PENDING. We cannot attempt to truncate the entry at this
  // point because we need the state machine for that (and even if we are really
  // free, that would be an asynchronous operation). In other words, keep the
  // entry how it is (it will be marked as truncated at destruction), and let
  // the next piece of code that executes know that we are now reading directly
  // from the net.
  // TODO(mmenke):  This doesn't release the lock on the cache entry, so a
  //                future request for the resource will be blocked on this one.
  //                Fix this.
  if (cache_.get() && entry_ && (mode_ & WRITE) && network_trans_.get() &&
      !is_sparse_ && !range_requested_) {
    mode_ = NONE;
  }
}

bool HttpCache::Transaction::GetFullRequestHeaders(
    HttpRequestHeaders* headers) const {
  if (network_trans_)
    return network_trans_->GetFullRequestHeaders(headers);

  // TODO(ttuttle): Read headers from cache.
  return false;
}

int64 HttpCache::Transaction::GetTotalReceivedBytes() const {
  int64 total_received_bytes = total_received_bytes_;
  if (network_trans_)
    total_received_bytes += network_trans_->GetTotalReceivedBytes();
  return total_received_bytes;
}

void HttpCache::Transaction::DoneReading() {
  if (cache_.get() && entry_) {
    DCHECK_NE(mode_, UPDATE);
    if (mode_ & WRITE) {
      DoneWritingToEntry(true);
    } else if (mode_ & READ) {
      // It is necessary to check mode_ & READ because it is possible
      // for mode_ to be NONE and entry_ non-NULL with a write entry
      // if StopCaching was called.
      cache_->DoneReadingFromEntry(entry_, this);
      entry_ = NULL;
    }
  }
}

const HttpResponseInfo* HttpCache::Transaction::GetResponseInfo() const {
  // Null headers means we encountered an error or haven't a response yet
  if (auth_response_.headers.get())
    return &auth_response_;
  return (response_.headers.get() || response_.ssl_info.cert.get() ||
          response_.cert_request_info.get())
             ? &response_
             : NULL;
}

LoadState HttpCache::Transaction::GetLoadState() const {
  LoadState state = GetWriterLoadState();
  if (state != LOAD_STATE_WAITING_FOR_CACHE)
    return state;

  if (cache_.get())
    return cache_->GetLoadStateForPendingTransaction(this);

  return LOAD_STATE_IDLE;
}

UploadProgress HttpCache::Transaction::GetUploadProgress() const {
  if (network_trans_.get())
    return network_trans_->GetUploadProgress();
  return final_upload_progress_;
}

void HttpCache::Transaction::SetQuicServerInfo(
    QuicServerInfo* quic_server_info) {}

bool HttpCache::Transaction::GetLoadTimingInfo(
    LoadTimingInfo* load_timing_info) const {
  if (network_trans_)
    return network_trans_->GetLoadTimingInfo(load_timing_info);

  if (old_network_trans_load_timing_) {
    *load_timing_info = *old_network_trans_load_timing_;
    return true;
  }

  if (first_cache_access_since_.is_null())
    return false;

  // If the cache entry was opened, return that time.
  load_timing_info->send_start = first_cache_access_since_;
  // This time doesn't make much sense when reading from the cache, so just use
  // the same time as send_start.
  load_timing_info->send_end = first_cache_access_since_;
  return true;
}

void HttpCache::Transaction::SetPriority(RequestPriority priority) {
  priority_ = priority;
  if (network_trans_)
    network_trans_->SetPriority(priority_);
}

void HttpCache::Transaction::SetWebSocketHandshakeStreamCreateHelper(
    WebSocketHandshakeStreamBase::CreateHelper* create_helper) {
  websocket_handshake_stream_base_create_helper_ = create_helper;
  if (network_trans_)
    network_trans_->SetWebSocketHandshakeStreamCreateHelper(create_helper);
}

void HttpCache::Transaction::SetBeforeNetworkStartCallback(
    const BeforeNetworkStartCallback& callback) {
  DCHECK(!network_trans_);
  before_network_start_callback_ = callback;
}

int HttpCache::Transaction::ResumeNetworkStart() {
  if (network_trans_)
    return network_trans_->ResumeNetworkStart();
  return ERR_UNEXPECTED;
}

//-----------------------------------------------------------------------------

void HttpCache::Transaction::DoCallback(int rv) {
  DCHECK(rv != ERR_IO_PENDING);
  DCHECK(!callback_.is_null());

  read_buf_ = NULL;  // Release the buffer before invoking the callback.

  // Since Run may result in Read being called, clear callback_ up front.
  CompletionCallback c = callback_;
  callback_.Reset();
  c.Run(rv);
}

int HttpCache::Transaction::HandleResult(int rv) {
  DCHECK(rv != ERR_IO_PENDING);
  if (!callback_.is_null())
    DoCallback(rv);

  return rv;
}

// A few common patterns: (Foo* means Foo -> FooComplete)
//
// Not-cached entry:
//   Start():
//   GetBackend* -> InitEntry -> OpenEntry* -> CreateEntry* -> AddToEntry* ->
//   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse ->
//   CacheWriteResponse* -> TruncateCachedData* -> TruncateCachedMetadata* ->
//   PartialHeadersReceived
//
//   Read():
//   NetworkRead* -> CacheWriteData*
//
// Cached entry, no validation:
//   Start():
//   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
//   -> BeginPartialCacheValidation() -> BeginCacheValidation()
//
//   Read():
//   CacheReadData*
//
// Cached entry, validation (304):
//   Start():
//   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
//   SendRequest* -> SuccessfulSendRequest -> UpdateCachedResponse ->
//   CacheWriteResponse* -> UpdateCachedResponseComplete ->
//   OverwriteCachedResponse -> PartialHeadersReceived
//
//   Read():
//   CacheReadData*
//
// Cached entry, validation and replace (200):
//   Start():
//   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
//   -> BeginPartialCacheValidation() -> BeginCacheValidation() ->
//   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse ->
//   CacheWriteResponse* -> DoTruncateCachedData* -> TruncateCachedMetadata* ->
//   PartialHeadersReceived
//
//   Read():
//   NetworkRead* -> CacheWriteData*
//
// Sparse entry, partially cached, byte range request:
//   Start():
//   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
//   -> BeginPartialCacheValidation() -> CacheQueryData* ->
//   ValidateEntryHeadersAndContinue() -> StartPartialCacheValidation ->
//   CompletePartialCacheValidation -> BeginCacheValidation() -> SendRequest* ->
//   SuccessfulSendRequest -> UpdateCachedResponse -> CacheWriteResponse* ->
//   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
//   PartialHeadersReceived
//
//   Read() 1:
//   NetworkRead* -> CacheWriteData*
//
//   Read() 2:
//   NetworkRead* -> CacheWriteData* -> StartPartialCacheValidation ->
//   CompletePartialCacheValidation -> CacheReadData* ->
//
//   Read() 3:
//   CacheReadData* -> StartPartialCacheValidation ->
//   CompletePartialCacheValidation -> BeginCacheValidation() -> SendRequest* ->
//   SuccessfulSendRequest -> UpdateCachedResponse* -> OverwriteCachedResponse
//   -> PartialHeadersReceived -> NetworkRead* -> CacheWriteData*
//
int HttpCache::Transaction::DoLoop(int result) {
  DCHECK(next_state_ != STATE_NONE);

  int rv = result;
  do {
    State state = next_state_;
    next_state_ = STATE_NONE;
    switch (state) {
      case STATE_GET_BACKEND:
        DCHECK_EQ(OK, rv);
        rv = DoGetBackend();
        break;
      case STATE_GET_BACKEND_COMPLETE:
        rv = DoGetBackendComplete(rv);
        break;
      case STATE_SEND_REQUEST:
        DCHECK_EQ(OK, rv);
        rv = DoSendRequest();
        break;
      case STATE_SEND_REQUEST_COMPLETE:
        rv = DoSendRequestComplete(rv);
        break;
      case STATE_SUCCESSFUL_SEND_REQUEST:
        DCHECK_EQ(OK, rv);
        rv = DoSuccessfulSendRequest();
        break;
      case STATE_NETWORK_READ:
        DCHECK_EQ(OK, rv);
        rv = DoNetworkRead();
        break;
      case STATE_NETWORK_READ_COMPLETE:
        rv = DoNetworkReadComplete(rv);
        break;
      case STATE_INIT_ENTRY:
        DCHECK_EQ(OK, rv);
        rv = DoInitEntry();
        break;
      case STATE_OPEN_ENTRY:
        DCHECK_EQ(OK, rv);
        rv = DoOpenEntry();
        break;
      case STATE_OPEN_ENTRY_COMPLETE:
        rv = DoOpenEntryComplete(rv);
        break;
      case STATE_CREATE_ENTRY:
        DCHECK_EQ(OK, rv);
        rv = DoCreateEntry();
        break;
      case STATE_CREATE_ENTRY_COMPLETE:
        rv = DoCreateEntryComplete(rv);
        break;
      case STATE_DOOM_ENTRY:
        DCHECK_EQ(OK, rv);
        rv = DoDoomEntry();
        break;
      case STATE_DOOM_ENTRY_COMPLETE:
        rv = DoDoomEntryComplete(rv);
        break;
      case STATE_ADD_TO_ENTRY:
        DCHECK_EQ(OK, rv);
        rv = DoAddToEntry();
        break;
      case STATE_ADD_TO_ENTRY_COMPLETE:
        rv = DoAddToEntryComplete(rv);
        break;
      case STATE_START_PARTIAL_CACHE_VALIDATION:
        DCHECK_EQ(OK, rv);
        rv = DoStartPartialCacheValidation();
        break;
      case STATE_COMPLETE_PARTIAL_CACHE_VALIDATION:
        rv = DoCompletePartialCacheValidation(rv);
        break;
      case STATE_UPDATE_CACHED_RESPONSE:
        DCHECK_EQ(OK, rv);
        rv = DoUpdateCachedResponse();
        break;
      case STATE_UPDATE_CACHED_RESPONSE_COMPLETE:
        rv = DoUpdateCachedResponseComplete(rv);
        break;
      case STATE_OVERWRITE_CACHED_RESPONSE:
        DCHECK_EQ(OK, rv);
        rv = DoOverwriteCachedResponse();
        break;
      case STATE_TRUNCATE_CACHED_DATA:
        DCHECK_EQ(OK, rv);
        rv = DoTruncateCachedData();
        break;
      case STATE_TRUNCATE_CACHED_DATA_COMPLETE:
        rv = DoTruncateCachedDataComplete(rv);
        break;
      case STATE_TRUNCATE_CACHED_METADATA:
        DCHECK_EQ(OK, rv);
        rv = DoTruncateCachedMetadata();
        break;
      case STATE_TRUNCATE_CACHED_METADATA_COMPLETE:
        rv = DoTruncateCachedMetadataComplete(rv);
        break;
      case STATE_PARTIAL_HEADERS_RECEIVED:
        DCHECK_EQ(OK, rv);
        rv = DoPartialHeadersReceived();
        break;
      case STATE_CACHE_READ_RESPONSE:
        DCHECK_EQ(OK, rv);
        rv = DoCacheReadResponse();
        break;
      case STATE_CACHE_READ_RESPONSE_COMPLETE:
        rv = DoCacheReadResponseComplete(rv);
        break;
      case STATE_CACHE_WRITE_RESPONSE:
        DCHECK_EQ(OK, rv);
        rv = DoCacheWriteResponse();
        break;
      case STATE_CACHE_WRITE_TRUNCATED_RESPONSE:
        DCHECK_EQ(OK, rv);
        rv = DoCacheWriteTruncatedResponse();
        break;
      case STATE_CACHE_WRITE_RESPONSE_COMPLETE:
        rv = DoCacheWriteResponseComplete(rv);
        break;
      case STATE_CACHE_READ_METADATA:
        DCHECK_EQ(OK, rv);
        rv = DoCacheReadMetadata();
        break;
      case STATE_CACHE_READ_METADATA_COMPLETE:
        rv = DoCacheReadMetadataComplete(rv);
        break;
      case STATE_CACHE_QUERY_DATA:
        DCHECK_EQ(OK, rv);
        rv = DoCacheQueryData();
        break;
      case STATE_CACHE_QUERY_DATA_COMPLETE:
        rv = DoCacheQueryDataComplete(rv);
        break;
      case STATE_CACHE_READ_DATA:
        DCHECK_EQ(OK, rv);
        rv = DoCacheReadData();
        break;
      case STATE_CACHE_READ_DATA_COMPLETE:
        rv = DoCacheReadDataComplete(rv);
        break;
      case STATE_CACHE_WRITE_DATA:
        rv = DoCacheWriteData(rv);
        break;
      case STATE_CACHE_WRITE_DATA_COMPLETE:
        rv = DoCacheWriteDataComplete(rv);
        break;
      default:
        NOTREACHED() << "bad state";
        rv = ERR_FAILED;
        break;
    }
  } while (rv != ERR_IO_PENDING && next_state_ != STATE_NONE);

  if (rv != ERR_IO_PENDING)
    HandleResult(rv);

  return rv;
}

int HttpCache::Transaction::DoGetBackend() {
  cache_pending_ = true;
  next_state_ = STATE_GET_BACKEND_COMPLETE;
  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_GET_BACKEND);
  return cache_->GetBackendForTransaction(this);
}

int HttpCache::Transaction::DoGetBackendComplete(int result) {
  DCHECK(result == OK || result == ERR_FAILED);
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_GET_BACKEND,
                                    result);
  cache_pending_ = false;

  if (!ShouldPassThrough()) {
    cache_key_ = cache_->GenerateCacheKey(request_);

    // Requested cache access mode.
    if (effective_load_flags_ & LOAD_ONLY_FROM_CACHE) {
      mode_ = READ;
    } else if (effective_load_flags_ & LOAD_BYPASS_CACHE) {
      mode_ = WRITE;
    } else {
      mode_ = READ_WRITE;
    }

    // Downgrade to UPDATE if the request has been externally conditionalized.
    if (external_validation_.initialized) {
      if (mode_ & WRITE) {
        // Strip off the READ_DATA bit (and maybe add back a READ_META bit
        // in case READ was off).
        mode_ = UPDATE;
      } else {
        mode_ = NONE;
      }
    }
  }

  // Use PUT and DELETE only to invalidate existing stored entries.
  if ((request_->method == "PUT" || request_->method == "DELETE") &&
      mode_ != READ_WRITE && mode_ != WRITE) {
    mode_ = NONE;
  }

  // If must use cache, then we must fail.  This can happen for back/forward
  // navigations to a page generated via a form post.
  if (!(mode_ & READ) && effective_load_flags_ & LOAD_ONLY_FROM_CACHE)
    return ERR_CACHE_MISS;

  if (mode_ == NONE) {
    if (partial_.get()) {
      partial_->RestoreHeaders(&custom_request_->extra_headers);
      partial_.reset();
    }
    next_state_ = STATE_SEND_REQUEST;
  } else {
    next_state_ = STATE_INIT_ENTRY;
  }

  // This is only set if we have something to do with the response.
  range_requested_ = (partial_.get() != NULL);

  return OK;
}

int HttpCache::Transaction::DoSendRequest() {
  DCHECK(mode_ & WRITE || mode_ == NONE);
  DCHECK(!network_trans_.get());

  send_request_since_ = TimeTicks::Now();

  // Create a network transaction.
  int rv = cache_->network_layer_->CreateTransaction(priority_,
                                                     &network_trans_);
  if (rv != OK)
    return rv;
  network_trans_->SetBeforeNetworkStartCallback(before_network_start_callback_);

  // Old load timing information, if any, is now obsolete.
  old_network_trans_load_timing_.reset();

  if (websocket_handshake_stream_base_create_helper_)
    network_trans_->SetWebSocketHandshakeStreamCreateHelper(
        websocket_handshake_stream_base_create_helper_);

  next_state_ = STATE_SEND_REQUEST_COMPLETE;
  rv = network_trans_->Start(request_, io_callback_, net_log_);
  return rv;
}

int HttpCache::Transaction::DoSendRequestComplete(int result) {
  if (!cache_.get())
    return ERR_UNEXPECTED;

  // If requested, and we have a readable cache entry, and we have
  // an error indicating that we're offline as opposed to in contact
  // with a bad server, read from cache anyway.
  if (IsOfflineError(result)) {
    if (mode_ == READ_WRITE && entry_ && !partial_) {
      RecordOfflineStatus(effective_load_flags_,
                          OFFLINE_STATUS_DATA_AVAILABLE_OFFLINE);
      if (effective_load_flags_ & LOAD_FROM_CACHE_IF_OFFLINE) {
        UpdateTransactionPattern(PATTERN_NOT_COVERED);
        response_.server_data_unavailable = true;
        return SetupEntryForRead();
      }
    } else {
      RecordOfflineStatus(effective_load_flags_,
                          OFFLINE_STATUS_DATA_UNAVAILABLE_OFFLINE);
    }
  } else {
    RecordOfflineStatus(effective_load_flags_,
                        (result == OK ? OFFLINE_STATUS_NETWORK_SUCCEEDED :
                                        OFFLINE_STATUS_NETWORK_FAILED));
  }

  // If we tried to conditionalize the request and failed, we know
  // we won't be reading from the cache after this point.
  if (couldnt_conditionalize_request_)
    mode_ = WRITE;

  if (result == OK) {
    next_state_ = STATE_SUCCESSFUL_SEND_REQUEST;
    return OK;
  }

  // Do not record requests that have network errors or restarts.
  UpdateTransactionPattern(PATTERN_NOT_COVERED);
  if (IsCertificateError(result)) {
    const HttpResponseInfo* response = network_trans_->GetResponseInfo();
    // If we get a certificate error, then there is a certificate in ssl_info,
    // so GetResponseInfo() should never return NULL here.
    DCHECK(response);
    response_.ssl_info = response->ssl_info;
  } else if (result == ERR_SSL_CLIENT_AUTH_CERT_NEEDED) {
    const HttpResponseInfo* response = network_trans_->GetResponseInfo();
    DCHECK(response);
    response_.cert_request_info = response->cert_request_info;
  } else if (response_.was_cached) {
    DoneWritingToEntry(true);
  }
  return result;
}

// We received the response headers and there is no error.
int HttpCache::Transaction::DoSuccessfulSendRequest() {
  DCHECK(!new_response_);
  const HttpResponseInfo* new_response = network_trans_->GetResponseInfo();
  bool authentication_failure = false;

  if (new_response->headers->response_code() == 401 ||
      new_response->headers->response_code() == 407) {
    auth_response_ = *new_response;
    if (!reading_)
      return OK;

    // We initiated a second request the caller doesn't know about. We should be
    // able to authenticate this request because we should have authenticated
    // this URL moments ago.
    if (IsReadyToRestartForAuth()) {
      DCHECK(!response_.auth_challenge.get());
      next_state_ = STATE_SEND_REQUEST_COMPLETE;
      // In theory we should check to see if there are new cookies, but there
      // is no way to do that from here.
      return network_trans_->RestartWithAuth(AuthCredentials(), io_callback_);
    }

    // We have to perform cleanup at this point so that at least the next
    // request can succeed.
    authentication_failure = true;
    if (entry_)
      DoomPartialEntry(false);
    mode_ = NONE;
    partial_.reset();
  }

  new_response_ = new_response;
  if (authentication_failure ||
      (!ValidatePartialResponse() && !auth_response_.headers.get())) {
    // Something went wrong with this request and we have to restart it.
    // If we have an authentication response, we are exposed to weird things
    // hapenning if the user cancels the authentication before we receive
    // the new response.
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    response_ = HttpResponseInfo();
    ResetNetworkTransaction();
    new_response_ = NULL;
    next_state_ = STATE_SEND_REQUEST;
    return OK;
  }

  if (handling_206_ && mode_ == READ_WRITE && !truncated_ && !is_sparse_) {
    // We have stored the full entry, but it changed and the server is
    // sending a range. We have to delete the old entry.
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    DoneWritingToEntry(false);
  }

  if (mode_ == WRITE &&
      transaction_pattern_ != PATTERN_ENTRY_CANT_CONDITIONALIZE) {
    UpdateTransactionPattern(PATTERN_ENTRY_NOT_CACHED);
  }

  if (mode_ == WRITE &&
      (request_->method == "PUT" || request_->method == "DELETE")) {
    if (NonErrorResponse(new_response->headers->response_code())) {
      int ret = cache_->DoomEntry(cache_key_, NULL);
      DCHECK_EQ(OK, ret);
    }
    cache_->DoneWritingToEntry(entry_, true);
    entry_ = NULL;
    mode_ = NONE;
  }

  if (request_->method == "POST" &&
      NonErrorResponse(new_response->headers->response_code())) {
    cache_->DoomMainEntryForUrl(request_->url);
  }

  RecordVaryHeaderHistogram(new_response);

  if (new_response_->headers->response_code() == 416 &&
      (request_->method == "GET" || request_->method == "POST")) {
    // If there is an ective entry it may be destroyed with this transaction.
    response_ = *new_response_;
    return OK;
  }

  // Are we expecting a response to a conditional query?
  if (mode_ == READ_WRITE || mode_ == UPDATE) {
    if (new_response->headers->response_code() == 304 || handling_206_) {
      UpdateTransactionPattern(PATTERN_ENTRY_VALIDATED);
      next_state_ = STATE_UPDATE_CACHED_RESPONSE;
      return OK;
    }
    UpdateTransactionPattern(PATTERN_ENTRY_UPDATED);
    mode_ = WRITE;
  }

  next_state_ = STATE_OVERWRITE_CACHED_RESPONSE;
  return OK;
}

int HttpCache::Transaction::DoNetworkRead() {
  next_state_ = STATE_NETWORK_READ_COMPLETE;
  return network_trans_->Read(read_buf_.get(), io_buf_len_, io_callback_);
}

int HttpCache::Transaction::DoNetworkReadComplete(int result) {
  DCHECK(mode_ & WRITE || mode_ == NONE);

  if (!cache_.get())
    return ERR_UNEXPECTED;

  // If there is an error or we aren't saving the data, we are done; just wait
  // until the destructor runs to see if we can keep the data.
  if (mode_ == NONE || result < 0)
    return result;

  next_state_ = STATE_CACHE_WRITE_DATA;
  return result;
}

int HttpCache::Transaction::DoInitEntry() {
  DCHECK(!new_entry_);

  if (!cache_.get())
    return ERR_UNEXPECTED;

  if (mode_ == WRITE) {
    next_state_ = STATE_DOOM_ENTRY;
    return OK;
  }

  next_state_ = STATE_OPEN_ENTRY;
  return OK;
}

int HttpCache::Transaction::DoOpenEntry() {
  DCHECK(!new_entry_);
  next_state_ = STATE_OPEN_ENTRY_COMPLETE;
  cache_pending_ = true;
  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_OPEN_ENTRY);
  first_cache_access_since_ = TimeTicks::Now();
  return cache_->OpenEntry(cache_key_, &new_entry_, this);
}

int HttpCache::Transaction::DoOpenEntryComplete(int result) {
  // It is important that we go to STATE_ADD_TO_ENTRY whenever the result is
  // OK, otherwise the cache will end up with an active entry without any
  // transaction attached.
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_OPEN_ENTRY, result);
  cache_pending_ = false;
  if (result == OK) {
    next_state_ = STATE_ADD_TO_ENTRY;
    return OK;
  }

  if (result == ERR_CACHE_RACE) {
    next_state_ = STATE_INIT_ENTRY;
    return OK;
  }

  if (request_->method == "PUT" || request_->method == "DELETE") {
    DCHECK(mode_ == READ_WRITE || mode_ == WRITE);
    mode_ = NONE;
    next_state_ = STATE_SEND_REQUEST;
    return OK;
  }

  if (mode_ == READ_WRITE) {
    mode_ = WRITE;
    next_state_ = STATE_CREATE_ENTRY;
    return OK;
  }
  if (mode_ == UPDATE) {
    // There is no cache entry to update; proceed without caching.
    mode_ = NONE;
    next_state_ = STATE_SEND_REQUEST;
    return OK;
  }
  if (cache_->mode() == PLAYBACK)
    DVLOG(1) << "Playback Cache Miss: " << request_->url;

  // The entry does not exist, and we are not permitted to create a new entry,
  // so we must fail.
  return ERR_CACHE_MISS;
}

int HttpCache::Transaction::DoCreateEntry() {
  DCHECK(!new_entry_);
  next_state_ = STATE_CREATE_ENTRY_COMPLETE;
  cache_pending_ = true;
  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_CREATE_ENTRY);
  return cache_->CreateEntry(cache_key_, &new_entry_, this);
}

int HttpCache::Transaction::DoCreateEntryComplete(int result) {
  // It is important that we go to STATE_ADD_TO_ENTRY whenever the result is
  // OK, otherwise the cache will end up with an active entry without any
  // transaction attached.
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_CREATE_ENTRY,
                                    result);
  cache_pending_ = false;
  next_state_ = STATE_ADD_TO_ENTRY;

  if (result == ERR_CACHE_RACE) {
    next_state_ = STATE_INIT_ENTRY;
    return OK;
  }

  if (result == OK) {
    UMA_HISTOGRAM_BOOLEAN("HttpCache.OpenToCreateRace", false);
  } else {
    UMA_HISTOGRAM_BOOLEAN("HttpCache.OpenToCreateRace", true);
    // We have a race here: Maybe we failed to open the entry and decided to
    // create one, but by the time we called create, another transaction already
    // created the entry. If we want to eliminate this issue, we need an atomic
    // OpenOrCreate() method exposed by the disk cache.
    DLOG(WARNING) << "Unable to create cache entry";
    mode_ = NONE;
    if (partial_.get())
      partial_->RestoreHeaders(&custom_request_->extra_headers);
    next_state_ = STATE_SEND_REQUEST;
  }
  return OK;
}

int HttpCache::Transaction::DoDoomEntry() {
  next_state_ = STATE_DOOM_ENTRY_COMPLETE;
  cache_pending_ = true;
  if (first_cache_access_since_.is_null())
    first_cache_access_since_ = TimeTicks::Now();
  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_DOOM_ENTRY);
  return cache_->DoomEntry(cache_key_, this);
}

int HttpCache::Transaction::DoDoomEntryComplete(int result) {
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_DOOM_ENTRY, result);
  next_state_ = STATE_CREATE_ENTRY;
  cache_pending_ = false;
  if (result == ERR_CACHE_RACE)
    next_state_ = STATE_INIT_ENTRY;
  return OK;
}

int HttpCache::Transaction::DoAddToEntry() {
  DCHECK(new_entry_);
  cache_pending_ = true;
  next_state_ = STATE_ADD_TO_ENTRY_COMPLETE;
  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_ADD_TO_ENTRY);
  DCHECK(entry_lock_waiting_since_.is_null());
  entry_lock_waiting_since_ = TimeTicks::Now();
  return cache_->AddTransactionToEntry(new_entry_, this);
}

int HttpCache::Transaction::DoAddToEntryComplete(int result) {
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_ADD_TO_ENTRY,
                                    result);
  const TimeDelta entry_lock_wait =
      TimeTicks::Now() - entry_lock_waiting_since_;
  UMA_HISTOGRAM_TIMES("HttpCache.EntryLockWait", entry_lock_wait);

  entry_lock_waiting_since_ = TimeTicks();
  DCHECK(new_entry_);
  cache_pending_ = false;

  if (result == OK)
    entry_ = new_entry_;

  // If there is a failure, the cache should have taken care of new_entry_.
  new_entry_ = NULL;

  if (result == ERR_CACHE_RACE) {
    next_state_ = STATE_INIT_ENTRY;
    return OK;
  }

  if (result != OK) {
    NOTREACHED();
    return result;
  }

  if (mode_ == WRITE) {
    if (partial_.get())
      partial_->RestoreHeaders(&custom_request_->extra_headers);
    next_state_ = STATE_SEND_REQUEST;
  } else {
    // We have to read the headers from the cached entry.
    DCHECK(mode_ & READ_META);
    next_state_ = STATE_CACHE_READ_RESPONSE;
  }
  return OK;
}

// We may end up here multiple times for a given request.
int HttpCache::Transaction::DoStartPartialCacheValidation() {
  if (mode_ == NONE)
    return OK;

  next_state_ = STATE_COMPLETE_PARTIAL_CACHE_VALIDATION;
  return partial_->ShouldValidateCache(entry_->disk_entry, io_callback_);
}

int HttpCache::Transaction::DoCompletePartialCacheValidation(int result) {
  if (!result) {
    // This is the end of the request.
    if (mode_ & WRITE) {
      DoneWritingToEntry(true);
    } else {
      cache_->DoneReadingFromEntry(entry_, this);
      entry_ = NULL;
    }
    return result;
  }

  if (result < 0)
    return result;

  partial_->PrepareCacheValidation(entry_->disk_entry,
                                   &custom_request_->extra_headers);

  if (reading_ && partial_->IsCurrentRangeCached()) {
    next_state_ = STATE_CACHE_READ_DATA;
    return OK;
  }

  return BeginCacheValidation();
}

// We received 304 or 206 and we want to update the cached response headers.
int HttpCache::Transaction::DoUpdateCachedResponse() {
  next_state_ = STATE_UPDATE_CACHED_RESPONSE_COMPLETE;
  int rv = OK;
  // Update cached response based on headers in new_response.
  // TODO(wtc): should we update cached certificate (response_.ssl_info), too?
  response_.headers->Update(*new_response_->headers.get());
  response_.response_time = new_response_->response_time;
  response_.request_time = new_response_->request_time;
  response_.network_accessed = new_response_->network_accessed;

  if (response_.headers->HasHeaderValue("cache-control", "no-store")) {
    if (!entry_->doomed) {
      int ret = cache_->DoomEntry(cache_key_, NULL);
      DCHECK_EQ(OK, ret);
    }
  } else {
    // If we are already reading, we already updated the headers for this
    // request; doing it again will change Content-Length.
    if (!reading_) {
      target_state_ = STATE_UPDATE_CACHED_RESPONSE_COMPLETE;
      next_state_ = STATE_CACHE_WRITE_RESPONSE;
      rv = OK;
    }
  }
  return rv;
}

int HttpCache::Transaction::DoUpdateCachedResponseComplete(int result) {
  if (mode_ == UPDATE) {
    DCHECK(!handling_206_);
    // We got a "not modified" response and already updated the corresponding
    // cache entry above.
    //
    // By closing the cached entry now, we make sure that the 304 rather than
    // the cached 200 response, is what will be returned to the user.
    DoneWritingToEntry(true);
  } else if (entry_ && !handling_206_) {
    DCHECK_EQ(READ_WRITE, mode_);
    if (!partial_.get() || partial_->IsLastRange()) {
      cache_->ConvertWriterToReader(entry_);
      mode_ = READ;
    }
    // We no longer need the network transaction, so destroy it.
    final_upload_progress_ = network_trans_->GetUploadProgress();
    ResetNetworkTransaction();
  } else if (entry_ && handling_206_ && truncated_ &&
             partial_->initial_validation()) {
    // We just finished the validation of a truncated entry, and the server
    // is willing to resume the operation. Now we go back and start serving
    // the first part to the user.
    ResetNetworkTransaction();
    new_response_ = NULL;
    next_state_ = STATE_START_PARTIAL_CACHE_VALIDATION;
    partial_->SetRangeToStartDownload();
    return OK;
  }
  next_state_ = STATE_OVERWRITE_CACHED_RESPONSE;
  return OK;
}

int HttpCache::Transaction::DoOverwriteCachedResponse() {
  if (mode_ & READ) {
    next_state_ = STATE_PARTIAL_HEADERS_RECEIVED;
    return OK;
  }

  // We change the value of Content-Length for partial content.
  if (handling_206_ && partial_.get())
    partial_->FixContentLength(new_response_->headers.get());

  response_ = *new_response_;

  if (handling_206_ && !CanResume(false)) {
    // There is no point in storing this resource because it will never be used.
    DoneWritingToEntry(false);
    if (partial_.get())
      partial_->FixResponseHeaders(response_.headers.get(), true);
    next_state_ = STATE_PARTIAL_HEADERS_RECEIVED;
    return OK;
  }

  target_state_ = STATE_TRUNCATE_CACHED_DATA;
  next_state_ = truncated_ ? STATE_CACHE_WRITE_TRUNCATED_RESPONSE :
                             STATE_CACHE_WRITE_RESPONSE;
  return OK;
}

int HttpCache::Transaction::DoTruncateCachedData() {
  next_state_ = STATE_TRUNCATE_CACHED_DATA_COMPLETE;
  if (!entry_)
    return OK;
  if (net_log_.IsLogging())
    net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_WRITE_DATA);
  // Truncate the stream.
  return WriteToEntry(kResponseContentIndex, 0, NULL, 0, io_callback_);
}

int HttpCache::Transaction::DoTruncateCachedDataComplete(int result) {
  if (entry_) {
    if (net_log_.IsLogging()) {
      net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_WRITE_DATA,
                                        result);
    }
  }

  next_state_ = STATE_TRUNCATE_CACHED_METADATA;
  return OK;
}

int HttpCache::Transaction::DoTruncateCachedMetadata() {
  next_state_ = STATE_TRUNCATE_CACHED_METADATA_COMPLETE;
  if (!entry_)
    return OK;

  if (net_log_.IsLogging())
    net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_WRITE_INFO);
  return WriteToEntry(kMetadataIndex, 0, NULL, 0, io_callback_);
}

int HttpCache::Transaction::DoTruncateCachedMetadataComplete(int result) {
  if (entry_) {
    if (net_log_.IsLogging()) {
      net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_WRITE_INFO,
                                        result);
    }
  }

  next_state_ = STATE_PARTIAL_HEADERS_RECEIVED;
  return OK;
}

int HttpCache::Transaction::DoPartialHeadersReceived() {
  new_response_ = NULL;
  if (entry_ && !partial_.get() &&
      entry_->disk_entry->GetDataSize(kMetadataIndex))
    next_state_ = STATE_CACHE_READ_METADATA;

  if (!partial_.get())
    return OK;

  if (reading_) {
    if (network_trans_.get()) {
      next_state_ = STATE_NETWORK_READ;
    } else {
      next_state_ = STATE_CACHE_READ_DATA;
    }
  } else if (mode_ != NONE) {
    // We are about to return the headers for a byte-range request to the user,
    // so let's fix them.
    partial_->FixResponseHeaders(response_.headers.get(), true);
  }
  return OK;
}

int HttpCache::Transaction::DoCacheReadResponse() {
  DCHECK(entry_);
  next_state_ = STATE_CACHE_READ_RESPONSE_COMPLETE;

  io_buf_len_ = entry_->disk_entry->GetDataSize(kResponseInfoIndex);
  read_buf_ = new IOBuffer(io_buf_len_);

  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_READ_INFO);
  return entry_->disk_entry->ReadData(kResponseInfoIndex, 0, read_buf_.get(),
                                      io_buf_len_, io_callback_);
}

int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_READ_INFO, result);
  if (result != io_buf_len_ ||
      !HttpCache::ParseResponseInfo(read_buf_->data(), io_buf_len_,
                                    &response_, &truncated_)) {
    return OnCacheReadError(result, true);
  }

  // Some resources may have slipped in as truncated when they're not.
  int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
  if (response_.headers->GetContentLength() == current_size)
    truncated_ = false;

  // We now have access to the cache entry.
  //
  //  o if we are a reader for the transaction, then we can start reading the
  //    cache entry.
  //
  //  o if we can read or write, then we should check if the cache entry needs
  //    to be validated and then issue a network request if needed or just read
  //    from the cache if the cache entry is already valid.
  //
  //  o if we are set to UPDATE, then we are handling an externally
  //    conditionalized request (if-modified-since / if-none-match). We check
  //    if the request headers define a validation request.
  //
  switch (mode_) {
    case READ:
      UpdateTransactionPattern(PATTERN_ENTRY_USED);
      result = BeginCacheRead();
      break;
    case READ_WRITE:
      result = BeginPartialCacheValidation();
      break;
    case UPDATE:
      result = BeginExternallyConditionalizedRequest();
      break;
    case WRITE:
    default:
      NOTREACHED();
      result = ERR_FAILED;
  }
  return result;
}

int HttpCache::Transaction::DoCacheWriteResponse() {
  if (entry_) {
    if (net_log_.IsLogging())
      net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_WRITE_INFO);
  }
  return WriteResponseInfoToEntry(false);
}

int HttpCache::Transaction::DoCacheWriteTruncatedResponse() {
  if (entry_) {
    if (net_log_.IsLogging())
      net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_WRITE_INFO);
  }
  return WriteResponseInfoToEntry(true);
}

int HttpCache::Transaction::DoCacheWriteResponseComplete(int result) {
  next_state_ = target_state_;
  target_state_ = STATE_NONE;
  if (!entry_)
    return OK;
  if (net_log_.IsLogging()) {
    net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_WRITE_INFO,
                                      result);
  }

  // Balance the AddRef from WriteResponseInfoToEntry.
  if (result != io_buf_len_) {
    DLOG(ERROR) << "failed to write response info to cache";
    DoneWritingToEntry(false);
  }
  return OK;
}

int HttpCache::Transaction::DoCacheReadMetadata() {
  DCHECK(entry_);
  DCHECK(!response_.metadata.get());
  next_state_ = STATE_CACHE_READ_METADATA_COMPLETE;

  response_.metadata =
      new IOBufferWithSize(entry_->disk_entry->GetDataSize(kMetadataIndex));

  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_READ_INFO);
  return entry_->disk_entry->ReadData(kMetadataIndex, 0,
                                      response_.metadata.get(),
                                      response_.metadata->size(),
                                      io_callback_);
}

int HttpCache::Transaction::DoCacheReadMetadataComplete(int result) {
  net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_READ_INFO, result);
  if (result != response_.metadata->size())
    return OnCacheReadError(result, false);
  return OK;
}

int HttpCache::Transaction::DoCacheQueryData() {
  next_state_ = STATE_CACHE_QUERY_DATA_COMPLETE;
  return entry_->disk_entry->ReadyForSparseIO(io_callback_);
}

int HttpCache::Transaction::DoCacheQueryDataComplete(int result) {
  if (result == ERR_NOT_IMPLEMENTED) {
    // Restart the request overwriting the cache entry.
    // TODO(pasko): remove this workaround as soon as the SimpleBackendImpl
    // supports Sparse IO.
    return DoRestartPartialRequest();
  }
  DCHECK_EQ(OK, result);
  if (!cache_.get())
    return ERR_UNEXPECTED;

  return ValidateEntryHeadersAndContinue();
}

int HttpCache::Transaction::DoCacheReadData() {
  DCHECK(entry_);
  next_state_ = STATE_CACHE_READ_DATA_COMPLETE;

  if (net_log_.IsLogging())
    net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_READ_DATA);
  if (partial_.get()) {
    return partial_->CacheRead(entry_->disk_entry, read_buf_.get(), io_buf_len_,
                               io_callback_);
  }

  return entry_->disk_entry->ReadData(kResponseContentIndex, read_offset_,
                                      read_buf_.get(), io_buf_len_,
                                      io_callback_);
}

int HttpCache::Transaction::DoCacheReadDataComplete(int result) {
  if (net_log_.IsLogging()) {
    net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_READ_DATA,
                                      result);
  }

  if (!cache_.get())
    return ERR_UNEXPECTED;

  if (partial_.get()) {
    // Partial requests are confusing to report in histograms because they may
    // have multiple underlying requests.
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    return DoPartialCacheReadCompleted(result);
  }

  if (result > 0) {
    read_offset_ += result;
  } else if (result == 0) {  // End of file.
    RecordHistograms();
    cache_->DoneReadingFromEntry(entry_, this);
    entry_ = NULL;
  } else {
    return OnCacheReadError(result, false);
  }
  return result;
}

int HttpCache::Transaction::DoCacheWriteData(int num_bytes) {
  next_state_ = STATE_CACHE_WRITE_DATA_COMPLETE;
  write_len_ = num_bytes;
  if (entry_) {
    if (net_log_.IsLogging())
      net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_WRITE_DATA);
  }

  return AppendResponseDataToEntry(read_buf_.get(), num_bytes, io_callback_);
}

int HttpCache::Transaction::DoCacheWriteDataComplete(int result) {
  if (entry_) {
    if (net_log_.IsLogging()) {
      net_log_.EndEventWithNetErrorCode(NetLog::TYPE_HTTP_CACHE_WRITE_DATA,
                                        result);
    }
  }
  // Balance the AddRef from DoCacheWriteData.
  if (!cache_.get())
    return ERR_UNEXPECTED;

  if (result != write_len_) {
    DLOG(ERROR) << "failed to write response data to cache";
    DoneWritingToEntry(false);

    // We want to ignore errors writing to disk and just keep reading from
    // the network.
    result = write_len_;
  } else if (!done_reading_ && entry_) {
    int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
    int64 body_size = response_.headers->GetContentLength();
    if (body_size >= 0 && body_size <= current_size)
      done_reading_ = true;
  }

  if (partial_.get()) {
    // This may be the last request.
    if (!(result == 0 && !truncated_ &&
          (partial_->IsLastRange() || mode_ == WRITE)))
      return DoPartialNetworkReadCompleted(result);
  }

  if (result == 0) {
    // End of file. This may be the result of a connection problem so see if we
    // have to keep the entry around to be flagged as truncated later on.
    if (done_reading_ || !entry_ || partial_.get() ||
        response_.headers->GetContentLength() <= 0)
      DoneWritingToEntry(true);
  }

  return result;
}

//-----------------------------------------------------------------------------

void HttpCache::Transaction::SetRequest(const BoundNetLog& net_log,
                                        const HttpRequestInfo* request) {
  net_log_ = net_log;
  request_ = request;
  effective_load_flags_ = request_->load_flags;

  switch (cache_->mode()) {
    case NORMAL:
      break;
    case RECORD:
      // When in record mode, we want to NEVER load from the cache.
      // The reason for this is beacuse we save the Set-Cookie headers
      // (intentionally).  If we read from the cache, we replay them
      // prematurely.
      effective_load_flags_ |= LOAD_BYPASS_CACHE;
      break;
    case PLAYBACK:
      // When in playback mode, we want to load exclusively from the cache.
      effective_load_flags_ |= LOAD_ONLY_FROM_CACHE;
      break;
    case DISABLE:
      effective_load_flags_ |= LOAD_DISABLE_CACHE;
      break;
  }

  // Some headers imply load flags.  The order here is significant.
  //
  //   LOAD_DISABLE_CACHE   : no cache read or write
  //   LOAD_BYPASS_CACHE    : no cache read
  //   LOAD_VALIDATE_CACHE  : no cache read unless validation
  //
  // The former modes trump latter modes, so if we find a matching header we
  // can stop iterating kSpecialHeaders.
  //
  static const struct {
    const HeaderNameAndValue* search;
    int load_flag;
  } kSpecialHeaders[] = {
    { kPassThroughHeaders, LOAD_DISABLE_CACHE },
    { kForceFetchHeaders, LOAD_BYPASS_CACHE },
    { kForceValidateHeaders, LOAD_VALIDATE_CACHE },
  };

  bool range_found = false;
  bool external_validation_error = false;

  if (request_->extra_headers.HasHeader(HttpRequestHeaders::kRange))
    range_found = true;

  for (size_t i = 0; i < ARRAYSIZE_UNSAFE(kSpecialHeaders); ++i) {
    if (HeaderMatches(request_->extra_headers, kSpecialHeaders[i].search)) {
      effective_load_flags_ |= kSpecialHeaders[i].load_flag;
      break;
    }
  }

  // Check for conditionalization headers which may correspond with a
  // cache validation request.
  for (size_t i = 0; i < arraysize(kValidationHeaders); ++i) {
    const ValidationHeaderInfo& info = kValidationHeaders[i];
    std::string validation_value;
    if (request_->extra_headers.GetHeader(
            info.request_header_name, &validation_value)) {
      if (!external_validation_.values[i].empty() ||
          validation_value.empty()) {
        external_validation_error = true;
      }
      external_validation_.values[i] = validation_value;
      external_validation_.initialized = true;
    }
  }

  // We don't support ranges and validation headers.
  if (range_found && external_validation_.initialized) {
    LOG(WARNING) << "Byte ranges AND validation headers found.";
    effective_load_flags_ |= LOAD_DISABLE_CACHE;
  }

  // If there is more than one validation header, we can't treat this request as
  // a cache validation, since we don't know for sure which header the server
  // will give us a response for (and they could be contradictory).
  if (external_validation_error) {
    LOG(WARNING) << "Multiple or malformed validation headers found.";
    effective_load_flags_ |= LOAD_DISABLE_CACHE;
  }

  if (range_found && !(effective_load_flags_ & LOAD_DISABLE_CACHE)) {
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    partial_.reset(new PartialData);
    if (request_->method == "GET" && partial_->Init(request_->extra_headers)) {
      // We will be modifying the actual range requested to the server, so
      // let's remove the header here.
      custom_request_.reset(new HttpRequestInfo(*request_));
      custom_request_->extra_headers.RemoveHeader(HttpRequestHeaders::kRange);
      request_ = custom_request_.get();
      partial_->SetHeaders(custom_request_->extra_headers);
    } else {
      // The range is invalid or we cannot handle it properly.
      VLOG(1) << "Invalid byte range found.";
      effective_load_flags_ |= LOAD_DISABLE_CACHE;
      partial_.reset(NULL);
    }
  }
}

bool HttpCache::Transaction::ShouldPassThrough() {
  // We may have a null disk_cache if there is an error we cannot recover from,
  // like not enough disk space, or sharing violations.
  if (!cache_->disk_cache_.get())
    return true;

  // When using the record/playback modes, we always use the cache
  // and we never pass through.
  if (cache_->mode() == RECORD || cache_->mode() == PLAYBACK)
    return false;

  if (effective_load_flags_ & LOAD_DISABLE_CACHE)
    return true;

  if (request_->method == "GET")
    return false;

  if (request_->method == "POST" && request_->upload_data_stream &&
      request_->upload_data_stream->identifier()) {
    return false;
  }

  if (request_->method == "PUT" && request_->upload_data_stream)
    return false;

  if (request_->method == "DELETE")
    return false;

  // TODO(darin): add support for caching HEAD responses
  return true;
}

int HttpCache::Transaction::BeginCacheRead() {
  // We don't support any combination of LOAD_ONLY_FROM_CACHE and byte ranges.
  if (response_.headers->response_code() == 206 || partial_.get()) {
    NOTREACHED();
    return ERR_CACHE_MISS;
  }

  // We don't have the whole resource.
  if (truncated_)
    return ERR_CACHE_MISS;

  if (entry_->disk_entry->GetDataSize(kMetadataIndex))
    next_state_ = STATE_CACHE_READ_METADATA;

  return OK;
}

int HttpCache::Transaction::BeginCacheValidation() {
  DCHECK(mode_ == READ_WRITE);

  bool skip_validation = !RequiresValidation();

  if (truncated_) {
    // Truncated entries can cause partial gets, so we shouldn't record this
    // load in histograms.
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    skip_validation = !partial_->initial_validation();
  }

  if (partial_.get() && (is_sparse_ || truncated_) &&
      (!partial_->IsCurrentRangeCached() || invalid_range_)) {
    // Force revalidation for sparse or truncated entries. Note that we don't
    // want to ignore the regular validation logic just because a byte range was
    // part of the request.
    skip_validation = false;
  }

  if (skip_validation) {
    UpdateTransactionPattern(PATTERN_ENTRY_USED);
    RecordOfflineStatus(effective_load_flags_, OFFLINE_STATUS_FRESH_CACHE);
    return SetupEntryForRead();
  } else {
    // Make the network request conditional, to see if we may reuse our cached
    // response.  If we cannot do so, then we just resort to a normal fetch.
    // Our mode remains READ_WRITE for a conditional request.  Even if the
    // conditionalization fails, we don't switch to WRITE mode until we
    // know we won't be falling back to using the cache entry in the
    // LOAD_FROM_CACHE_IF_OFFLINE case.
    if (!ConditionalizeRequest()) {
      couldnt_conditionalize_request_ = true;
      UpdateTransactionPattern(PATTERN_ENTRY_CANT_CONDITIONALIZE);
      if (partial_.get())
        return DoRestartPartialRequest();

      DCHECK_NE(206, response_.headers->response_code());
    }
    next_state_ = STATE_SEND_REQUEST;
  }
  return OK;
}

int HttpCache::Transaction::BeginPartialCacheValidation() {
  DCHECK(mode_ == READ_WRITE);

  if (response_.headers->response_code() != 206 && !partial_.get() &&
      !truncated_) {
    return BeginCacheValidation();
  }

  // Partial requests should not be recorded in histograms.
  UpdateTransactionPattern(PATTERN_NOT_COVERED);
  if (range_requested_) {
    next_state_ = STATE_CACHE_QUERY_DATA;
    return OK;
  }
  // The request is not for a range, but we have stored just ranges.
  partial_.reset(new PartialData());
  partial_->SetHeaders(request_->extra_headers);
  if (!custom_request_.get()) {
    custom_request_.reset(new HttpRequestInfo(*request_));
    request_ = custom_request_.get();
  }

  return ValidateEntryHeadersAndContinue();
}

// This should only be called once per request.
int HttpCache::Transaction::ValidateEntryHeadersAndContinue() {
  DCHECK(mode_ == READ_WRITE);

  if (!partial_->UpdateFromStoredHeaders(
          response_.headers.get(), entry_->disk_entry, truncated_)) {
    return DoRestartPartialRequest();
  }

  if (response_.headers->response_code() == 206)
    is_sparse_ = true;

  if (!partial_->IsRequestedRangeOK()) {
    // The stored data is fine, but the request may be invalid.
    invalid_range_ = true;
  }

  next_state_ = STATE_START_PARTIAL_CACHE_VALIDATION;
  return OK;
}

int HttpCache::Transaction::BeginExternallyConditionalizedRequest() {
  DCHECK_EQ(UPDATE, mode_);
  DCHECK(external_validation_.initialized);

  for (size_t i = 0;  i < arraysize(kValidationHeaders); i++) {
    if (external_validation_.values[i].empty())
      continue;
    // Retrieve either the cached response's "etag" or "last-modified" header.
    std::string validator;
    response_.headers->EnumerateHeader(
        NULL,
        kValidationHeaders[i].related_response_header_name,
        &validator);

    if (response_.headers->response_code() != 200 || truncated_ ||
        validator.empty() || validator != external_validation_.values[i]) {
      // The externally conditionalized request is not a validation request
      // for our existing cache entry. Proceed with caching disabled.
      UpdateTransactionPattern(PATTERN_NOT_COVERED);
      DoneWritingToEntry(true);
    }
  }

  next_state_ = STATE_SEND_REQUEST;
  return OK;
}

int HttpCache::Transaction::RestartNetworkRequest() {
  DCHECK(mode_ & WRITE || mode_ == NONE);
  DCHECK(network_trans_.get());
  DCHECK_EQ(STATE_NONE, next_state_);

  next_state_ = STATE_SEND_REQUEST_COMPLETE;
  int rv = network_trans_->RestartIgnoringLastError(io_callback_);
  if (rv != ERR_IO_PENDING)
    return DoLoop(rv);
  return rv;
}

int HttpCache::Transaction::RestartNetworkRequestWithCertificate(
    X509Certificate* client_cert) {
  DCHECK(mode_ & WRITE || mode_ == NONE);
  DCHECK(network_trans_.get());
  DCHECK_EQ(STATE_NONE, next_state_);

  next_state_ = STATE_SEND_REQUEST_COMPLETE;
  int rv = network_trans_->RestartWithCertificate(client_cert, io_callback_);
  if (rv != ERR_IO_PENDING)
    return DoLoop(rv);
  return rv;
}

int HttpCache::Transaction::RestartNetworkRequestWithAuth(
    const AuthCredentials& credentials) {
  DCHECK(mode_ & WRITE || mode_ == NONE);
  DCHECK(network_trans_.get());
  DCHECK_EQ(STATE_NONE, next_state_);

  next_state_ = STATE_SEND_REQUEST_COMPLETE;
  int rv = network_trans_->RestartWithAuth(credentials, io_callback_);
  if (rv != ERR_IO_PENDING)
    return DoLoop(rv);
  return rv;
}

bool HttpCache::Transaction::RequiresValidation() {
  // TODO(darin): need to do more work here:
  //  - make sure we have a matching request method
  //  - watch out for cached responses that depend on authentication

  // In playback mode, nothing requires validation.
  if (cache_->mode() == net::HttpCache::PLAYBACK)
    return false;

  if (response_.vary_data.is_valid() &&
      !response_.vary_data.MatchesRequest(*request_,
                                          *response_.headers.get())) {
    vary_mismatch_ = true;
    return true;
  }

  if (effective_load_flags_ & LOAD_PREFERRING_CACHE)
    return false;

  if (effective_load_flags_ & LOAD_VALIDATE_CACHE)
    return true;

  if (request_->method == "PUT" || request_->method == "DELETE")
    return true;

  if (response_.headers->RequiresValidation(
          response_.request_time, response_.response_time, Time::Now())) {
    return true;
  }

  return false;
}

bool HttpCache::Transaction::ConditionalizeRequest() {
  DCHECK(response_.headers.get());

  if (request_->method == "PUT" || request_->method == "DELETE")
    return false;

  // This only makes sense for cached 200 or 206 responses.
  if (response_.headers->response_code() != 200 &&
      response_.headers->response_code() != 206) {
    return false;
  }

  // We should have handled this case before.
  DCHECK(response_.headers->response_code() != 206 ||
         response_.headers->HasStrongValidators());

  // Just use the first available ETag and/or Last-Modified header value.
  // TODO(darin): Or should we use the last?

  std::string etag_value;
  if (response_.headers->GetHttpVersion() >= HttpVersion(1, 1))
    response_.headers->EnumerateHeader(NULL, "etag", &etag_value);

  std::string last_modified_value;
  if (!vary_mismatch_) {
    response_.headers->EnumerateHeader(NULL, "last-modified",
                                       &last_modified_value);
  }

  if (etag_value.empty() && last_modified_value.empty())
    return false;

  if (!partial_.get()) {
    // Need to customize the request, so this forces us to allocate :(
    custom_request_.reset(new HttpRequestInfo(*request_));
    request_ = custom_request_.get();
  }
  DCHECK(custom_request_.get());

  bool use_if_range = partial_.get() && !partial_->IsCurrentRangeCached() &&
                      !invalid_range_;

  if (!etag_value.empty()) {
    if (use_if_range) {
      // We don't want to switch to WRITE mode if we don't have this block of a
      // byte-range request because we may have other parts cached.
      custom_request_->extra_headers.SetHeader(
          HttpRequestHeaders::kIfRange, etag_value);
    } else {
      custom_request_->extra_headers.SetHeader(
          HttpRequestHeaders::kIfNoneMatch, etag_value);
    }
    // For byte-range requests, make sure that we use only one way to validate
    // the request.
    if (partial_.get() && !partial_->IsCurrentRangeCached())
      return true;
  }

  if (!last_modified_value.empty()) {
    if (use_if_range) {
      custom_request_->extra_headers.SetHeader(
          HttpRequestHeaders::kIfRange, last_modified_value);
    } else {
      custom_request_->extra_headers.SetHeader(
          HttpRequestHeaders::kIfModifiedSince, last_modified_value);
    }
  }

  return true;
}

// We just received some headers from the server. We may have asked for a range,
// in which case partial_ has an object. This could be the first network request
// we make to fulfill the original request, or we may be already reading (from
// the net and / or the cache). If we are not expecting a certain response, we
// just bypass the cache for this request (but again, maybe we are reading), and
// delete partial_ (so we are not able to "fix" the headers that we return to
// the user). This results in either a weird response for the caller (we don't
// expect it after all), or maybe a range that was not exactly what it was asked
// for.
//
// If the server is simply telling us that the resource has changed, we delete
// the cached entry and restart the request as the caller intended (by returning
// false from this method). However, we may not be able to do that at any point,
// for instance if we already returned the headers to the user.
//
// WARNING: Whenever this code returns false, it has to make sure that the next
// time it is called it will return true so that we don't keep retrying the
// request.
bool HttpCache::Transaction::ValidatePartialResponse() {
  const HttpResponseHeaders* headers = new_response_->headers.get();
  int response_code = headers->response_code();
  bool partial_response = (response_code == 206);
  handling_206_ = false;

  if (!entry_ || request_->method != "GET")
    return true;

  if (invalid_range_) {
    // We gave up trying to match this request with the stored data. If the
    // server is ok with the request, delete the entry, otherwise just ignore
    // this request
    DCHECK(!reading_);
    if (partial_response || response_code == 200) {
      DoomPartialEntry(true);
      mode_ = NONE;
    } else {
      if (response_code == 304)
        FailRangeRequest();
      IgnoreRangeRequest();
    }
    return true;
  }

  if (!partial_.get()) {
    // We are not expecting 206 but we may have one.
    if (partial_response)
      IgnoreRangeRequest();

    return true;
  }

  // TODO(rvargas): Do we need to consider other results here?.
  bool failure = response_code == 200 || response_code == 416;

  if (partial_->IsCurrentRangeCached()) {
    // We asked for "If-None-Match: " so a 206 means a new object.
    if (partial_response)
      failure = true;

    if (response_code == 304 && partial_->ResponseHeadersOK(headers))
      return true;
  } else {
    // We asked for "If-Range: " so a 206 means just another range.
    if (partial_response && partial_->ResponseHeadersOK(headers)) {
      handling_206_ = true;
      return true;
    }

    if (!reading_ && !is_sparse_ && !partial_response) {
      // See if we can ignore the fact that we issued a byte range request.
      // If the server sends 200, just store it. If it sends an error, redirect
      // or something else, we may store the response as long as we didn't have
      // anything already stored.
      if (response_code == 200 ||
          (!truncated_ && response_code != 304 && response_code != 416)) {
        // The server is sending something else, and we can save it.
        DCHECK((truncated_ && !partial_->IsLastRange()) || range_requested_);
        partial_.reset();
        truncated_ = false;
        return true;
      }
    }

    // 304 is not expected here, but we'll spare the entry (unless it was
    // truncated).
    if (truncated_)
      failure = true;
  }

  if (failure) {
    // We cannot truncate this entry, it has to be deleted.
    UpdateTransactionPattern(PATTERN_NOT_COVERED);
    DoomPartialEntry(false);
    mode_ = NONE;
    if (!reading_ && !partial_->IsLastRange()) {
      // We'll attempt to issue another network request, this time without us
      // messing up the headers.
      partial_->RestoreHeaders(&custom_request_->extra_headers);
      partial_.reset();
      truncated_ = false;
      return false;
    }
    LOG(WARNING) << "Failed to revalidate partial entry";
    partial_.reset();
    return true;
  }

  IgnoreRangeRequest();
  return true;
}

void HttpCache::Transaction::IgnoreRangeRequest() {
  // We have a problem. We may or may not be reading already (in which case we
  // returned the headers), but we'll just pretend that this request is not
  // using the cache and see what happens. Most likely this is the first
  // response from the server (it's not changing its mind midway, right?).
  UpdateTransactionPattern(PATTERN_NOT_COVERED);
  if (mode_ & WRITE)
    DoneWritingToEntry(mode_ != WRITE);
  else if (mode_ & READ && entry_)
    cache_->DoneReadingFromEntry(entry_, this);

  partial_.reset(NULL);
  entry_ = NULL;
  mode_ = NONE;
}

void HttpCache::Transaction::FailRangeRequest() {
  response_ = *new_response_;
  partial_->FixResponseHeaders(response_.headers.get(), false);
}

int HttpCache::Transaction::SetupEntryForRead() {
  if (network_trans_)
    ResetNetworkTransaction();
  if (partial_.get()) {
    if (truncated_ || is_sparse_ || !invalid_range_) {
      // We are going to return the saved response headers to the caller, so
      // we may need to adjust them first.
      next_state_ = STATE_PARTIAL_HEADERS_RECEIVED;
      return OK;
    } else {
      partial_.reset();
    }
  }
  cache_->ConvertWriterToReader(entry_);
  mode_ = READ;

  if (entry_->disk_entry->GetDataSize(kMetadataIndex))
    next_state_ = STATE_CACHE_READ_METADATA;
  return OK;
}


int HttpCache::Transaction::ReadFromNetwork(IOBuffer* data, int data_len) {
  read_buf_ = data;
  io_buf_len_ = data_len;
  next_state_ = STATE_NETWORK_READ;
  return DoLoop(OK);
}

int HttpCache::Transaction::ReadFromEntry(IOBuffer* data, int data_len) {
  read_buf_ = data;
  io_buf_len_ = data_len;
  next_state_ = STATE_CACHE_READ_DATA;
  return DoLoop(OK);
}

int HttpCache::Transaction::WriteToEntry(int index, int offset,
                                         IOBuffer* data, int data_len,
                                         const CompletionCallback& callback) {
  if (!entry_)
    return data_len;

  int rv = 0;
  if (!partial_.get() || !data_len) {
    rv = entry_->disk_entry->WriteData(index, offset, data, data_len, callback,
                                       true);
  } else {
    rv = partial_->CacheWrite(entry_->disk_entry, data, data_len, callback);
  }
  return rv;
}

int HttpCache::Transaction::WriteResponseInfoToEntry(bool truncated) {
  next_state_ = STATE_CACHE_WRITE_RESPONSE_COMPLETE;
  if (!entry_)
    return OK;

  // Do not cache no-store content (unless we are record mode).  Do not cache
  // content with cert errors either.  This is to prevent not reporting net
  // errors when loading a resource from the cache.  When we load a page over
  // HTTPS with a cert error we show an SSL blocking page.  If the user clicks
  // proceed we reload the resource ignoring the errors.  The loaded resource
  // is then cached.  If that resource is subsequently loaded from the cache,
  // no net error is reported (even though the cert status contains the actual
  // errors) and no SSL blocking page is shown.  An alternative would be to
  // reverse-map the cert status to a net error and replay the net error.
  if ((cache_->mode() != RECORD &&
       response_.headers->HasHeaderValue("cache-control", "no-store")) ||
      net::IsCertStatusError(response_.ssl_info.cert_status)) {
    DoneWritingToEntry(false);
    if (net_log_.IsLogging())
      net_log_.EndEvent(NetLog::TYPE_HTTP_CACHE_WRITE_INFO);
    return OK;
  }

  // When writing headers, we normally only write the non-transient
  // headers; when in record mode, record everything.
  bool skip_transient_headers = (cache_->mode() != RECORD);

  if (truncated)
    DCHECK_EQ(200, response_.headers->response_code());

  scoped_refptr<PickledIOBuffer> data(new PickledIOBuffer());
  response_.Persist(data->pickle(), skip_transient_headers, truncated);
  data->Done();

  io_buf_len_ = data->pickle()->size();
  return entry_->disk_entry->WriteData(kResponseInfoIndex, 0, data.get(),
                                       io_buf_len_, io_callback_, true);
}

int HttpCache::Transaction::AppendResponseDataToEntry(
    IOBuffer* data, int data_len, const CompletionCallback& callback) {
  if (!entry_ || !data_len)
    return data_len;

  int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
  return WriteToEntry(kResponseContentIndex, current_size, data, data_len,
                      callback);
}

void HttpCache::Transaction::DoneWritingToEntry(bool success) {
  if (!entry_)
    return;

  RecordHistograms();

  cache_->DoneWritingToEntry(entry_, success);
  entry_ = NULL;
  mode_ = NONE;  // switch to 'pass through' mode
}

int HttpCache::Transaction::OnCacheReadError(int result, bool restart) {
  DLOG(ERROR) << "ReadData failed: " << result;
  const int result_for_histogram = std::max(0, -result);
  if (restart) {
    UMA_HISTOGRAM_SPARSE_SLOWLY("HttpCache.ReadErrorRestartable",
                                result_for_histogram);
  } else {
    UMA_HISTOGRAM_SPARSE_SLOWLY("HttpCache.ReadErrorNonRestartable",
                                result_for_histogram);
  }

  // Avoid using this entry in the future.
  if (cache_.get())
    cache_->DoomActiveEntry(cache_key_);

  if (restart) {
    DCHECK(!reading_);
    DCHECK(!network_trans_.get());
    cache_->DoneWithEntry(entry_, this, false);
    entry_ = NULL;
    is_sparse_ = false;
    partial_.reset();
    next_state_ = STATE_GET_BACKEND;
    return OK;
  }

  return ERR_CACHE_READ_FAILURE;
}

void HttpCache::Transaction::DoomPartialEntry(bool delete_object) {
  DVLOG(2) << "DoomPartialEntry";
  int rv = cache_->DoomEntry(cache_key_, NULL);
  DCHECK_EQ(OK, rv);
  cache_->DoneWithEntry(entry_, this, false);
  entry_ = NULL;
  is_sparse_ = false;
  if (delete_object)
    partial_.reset(NULL);
}

int HttpCache::Transaction::DoPartialNetworkReadCompleted(int result) {
  partial_->OnNetworkReadCompleted(result);

  if (result == 0) {
    // We need to move on to the next range.
    ResetNetworkTransaction();
    next_state_ = STATE_START_PARTIAL_CACHE_VALIDATION;
  }
  return result;
}

int HttpCache::Transaction::DoPartialCacheReadCompleted(int result) {
  partial_->OnCacheReadCompleted(result);

  if (result == 0 && mode_ == READ_WRITE) {
    // We need to move on to the next range.
    next_state_ = STATE_START_PARTIAL_CACHE_VALIDATION;
  } else if (result < 0) {
    return OnCacheReadError(result, false);
  }
  return result;
}

int HttpCache::Transaction::DoRestartPartialRequest() {
  // The stored data cannot be used. Get rid of it and restart this request.
  // We need to also reset the |truncated_| flag as a new entry is created.
  DoomPartialEntry(!range_requested_);
  mode_ = WRITE;
  truncated_ = false;
  next_state_ = STATE_INIT_ENTRY;
  return OK;
}

void HttpCache::Transaction::ResetNetworkTransaction() {
  DCHECK(!old_network_trans_load_timing_);
  DCHECK(network_trans_);
  LoadTimingInfo load_timing;
  if (network_trans_->GetLoadTimingInfo(&load_timing))
    old_network_trans_load_timing_.reset(new LoadTimingInfo(load_timing));
  total_received_bytes_ += network_trans_->GetTotalReceivedBytes();
  network_trans_.reset();
}

// Histogram data from the end of 2010 show the following distribution of
// response headers:
//
//   Content-Length............... 87%
//   Date......................... 98%
//   Last-Modified................ 49%
//   Etag......................... 19%
//   Accept-Ranges: bytes......... 25%
//   Accept-Ranges: none.......... 0.4%
//   Strong Validator............. 50%
//   Strong Validator + ranges.... 24%
//   Strong Validator + CL........ 49%
//
bool HttpCache::Transaction::CanResume(bool has_data) {
  // Double check that there is something worth keeping.
  if (has_data && !entry_->disk_entry->GetDataSize(kResponseContentIndex))
    return false;

  if (request_->method != "GET")
    return false;

  // Note that if this is a 206, content-length was already fixed after calling
  // PartialData::ResponseHeadersOK().
  if (response_.headers->GetContentLength() <= 0 ||
      response_.headers->HasHeaderValue("Accept-Ranges", "none") ||
      !response_.headers->HasStrongValidators()) {
    return false;
  }

  return true;
}

void HttpCache::Transaction::UpdateTransactionPattern(
    TransactionPattern new_transaction_pattern) {
  if (transaction_pattern_ == PATTERN_NOT_COVERED)
    return;
  DCHECK(transaction_pattern_ == PATTERN_UNDEFINED ||
         new_transaction_pattern == PATTERN_NOT_COVERED);
  transaction_pattern_ = new_transaction_pattern;
}

void HttpCache::Transaction::RecordHistograms() {
  DCHECK_NE(PATTERN_UNDEFINED, transaction_pattern_);
  if (!cache_.get() || !cache_->GetCurrentBackend() ||
      cache_->GetCurrentBackend()->GetCacheType() != DISK_CACHE ||
      cache_->mode() != NORMAL || request_->method != "GET") {
    return;
  }
  UMA_HISTOGRAM_ENUMERATION(
      "HttpCache.Pattern", transaction_pattern_, PATTERN_MAX);
  if (transaction_pattern_ == PATTERN_NOT_COVERED)
    return;
  DCHECK(!range_requested_);
  DCHECK(!first_cache_access_since_.is_null());

  TimeDelta total_time = base::TimeTicks::Now() - first_cache_access_since_;

  UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone", total_time);

  bool did_send_request = !send_request_since_.is_null();
  DCHECK(
      (did_send_request &&
       (transaction_pattern_ == PATTERN_ENTRY_NOT_CACHED ||
        transaction_pattern_ == PATTERN_ENTRY_VALIDATED ||
        transaction_pattern_ == PATTERN_ENTRY_UPDATED ||
        transaction_pattern_ == PATTERN_ENTRY_CANT_CONDITIONALIZE)) ||
      (!did_send_request && transaction_pattern_ == PATTERN_ENTRY_USED));

  if (!did_send_request) {
    DCHECK(transaction_pattern_ == PATTERN_ENTRY_USED);
    UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.Used", total_time);
    return;
  }

  TimeDelta before_send_time = send_request_since_ - first_cache_access_since_;
  int before_send_percent =
      total_time.ToInternalValue() == 0 ? 0
                                        : before_send_time * 100 / total_time;
  DCHECK_LE(0, before_send_percent);
  DCHECK_GE(100, before_send_percent);

  UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.SentRequest", total_time);
  UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend", before_send_time);
  UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend", before_send_percent);

  // TODO(gavinp): Remove or minimize these histograms, particularly the ones
  // below this comment after we have received initial data.
  switch (transaction_pattern_) {
    case PATTERN_ENTRY_CANT_CONDITIONALIZE: {
      UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.CantConditionalize",
                          before_send_time);
      UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.CantConditionalize",
                               before_send_percent);
      break;
    }
    case PATTERN_ENTRY_NOT_CACHED: {
      UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.NotCached", before_send_time);
      UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.NotCached",
                               before_send_percent);
      break;
    }
    case PATTERN_ENTRY_VALIDATED: {
      UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.Validated", before_send_time);
      UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.Validated",
                               before_send_percent);
      break;
    }
    case PATTERN_ENTRY_UPDATED: {
      UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.Updated", before_send_time);
      UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.Updated",
                               before_send_percent);
      break;
    }
    default:
      NOTREACHED();
  }
}

void HttpCache::Transaction::OnIOComplete(int result) {
  DoLoop(result);
}

}  // namespace net