// Copyright (c) 2006-2010 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 "base/compiler_specific.h"

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

#include "base/histogram.h"
#include "base/ref_counted.h"
#include "base/string_util.h"
#include "base/time.h"
#include "net/base/io_buffer.h"
#include "net/base/load_flags.h"
#include "net/base/net_errors.h"
#include "net/base/net_log.h"
#include "net/base/ssl_cert_request_info.h"
#include "net/disk_cache/disk_cache.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"

using base::Time;

namespace net {

// disk cache entry data indices.
enum {
  kResponseInfoIndex,
  kResponseContentIndex,
  kMetadataIndex
};

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

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 HttpUtil::HeadersIterator& h,
                          const HeaderNameAndValue* search) {
  for (; search->name; ++search) {
    if (!LowerCaseEqualsASCII(h.name_begin(), h.name_end(), search->name))
      continue;

    if (!search->value)
      return true;

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

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

HttpCache::Transaction::Transaction(HttpCache* cache, bool enable_range_support)
    : next_state_(STATE_NONE),
      request_(NULL),
      cache_(cache->AsWeakPtr()),
      entry_(NULL),
      new_entry_(NULL),
      network_trans_(NULL),
      callback_(NULL),
      new_response_(NULL),
      mode_(NONE),
      target_state_(STATE_NONE),
      reading_(false),
      invalid_range_(false),
      enable_range_support_(enable_range_support),
      truncated_(false),
      server_responded_206_(false),
      cache_pending_(false),
      read_offset_(0),
      effective_load_flags_(0),
      final_upload_progress_(0),
      ALLOW_THIS_IN_INITIALIZER_LIST(
          io_callback_(this, &Transaction::OnIOComplete)),
      ALLOW_THIS_IN_INITIALIZER_LIST(
          cache_callback_(new CancelableCompletionCallback<Transaction>(
                this, &Transaction::OnIOComplete))),
      ALLOW_THIS_IN_INITIALIZER_LIST(
          write_headers_callback_(new CancelableCompletionCallback<Transaction>(
                this, &Transaction::OnIOComplete))) {
  COMPILE_ASSERT(HttpCache::Transaction::kNumValidationHeaders ==
                     ARRAYSIZE_UNSAFE(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_ = NULL;

  if (cache_) {
    if (entry_) {
      bool cancel_request = reading_ && enable_range_support_;
      if (cancel_request) {
        if (partial_.get()) {
          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);
    }
  }

  // If there is an outstanding callback, mark it as cancelled so running it
  // does nothing.
  cache_callback_->Cancel();
  write_headers_callback_->Cancel();

  // We could still have a cache read or write in progress, so we just null the
  // cache_ pointer to signal that we are dead.  See DoCacheReadCompleted.
  cache_.reset();
}

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

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

  if (!cache_)
    return ERR_UNEXPECTED;

  SetRequest(net_log, request);

  int rv;

  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;
      }
    }
  }

  // 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);
    rv = BeginNetworkRequest();
  } else {
    rv = AddToEntry();
  }

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

  return rv;
}

int HttpCache::Transaction::RestartIgnoringLastError(
    CompletionCallback* callback) {
  DCHECK(callback);

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

  if (!cache_)
    return ERR_UNEXPECTED;

  int rv = RestartNetworkRequest();

  if (rv == ERR_IO_PENDING)
    callback_ = callback;

  return rv;
}

int HttpCache::Transaction::RestartWithCertificate(
    X509Certificate* client_cert,
    CompletionCallback* callback) {
  DCHECK(callback);

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

  if (!cache_)
    return ERR_UNEXPECTED;

  int rv = RestartNetworkRequestWithCertificate(client_cert);

  if (rv == ERR_IO_PENDING)
    callback_ = callback;

  return rv;
}

int HttpCache::Transaction::RestartWithAuth(
    const std::wstring& username,
    const std::wstring& password,
    CompletionCallback* callback) {
  DCHECK(auth_response_.headers);
  DCHECK(callback);

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

  if (!cache_)
    return ERR_UNEXPECTED;

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

  int rv = RestartNetworkRequestWithAuth(username, password);

  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,
                                 CompletionCallback* callback) {
  DCHECK(buf);
  DCHECK_GT(buf_len, 0);
  DCHECK(callback);

  DCHECK(!callback_);

  if (!cache_)
    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 && mode_ != NONE) {
    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_);
    callback_ = callback;
  }
  return rv;
}

void HttpCache::Transaction::StopCaching() {
}

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

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

uint64 HttpCache::Transaction::GetUploadProgress() const {
  if (network_trans_.get())
    return network_trans_->GetUploadProgress();
  return final_upload_progress_;
}

int HttpCache::Transaction::WriteMetadata(IOBuffer* buf, int buf_len,
                                          CompletionCallback* callback) {
  DCHECK(buf);
  DCHECK_GT(buf_len, 0);
  DCHECK(callback);
  if (!cache_ || !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);

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

  // Double check that there is something worth keeping.
  if (!entry_->disk_entry->GetDataSize(kResponseContentIndex))
    return false;

  if (response_.headers->GetContentLength() <= 0 ||
      response_.headers->HasHeaderValue("Accept-Ranges", "none") ||
      !response_.headers->HasStrongValidators())
    return false;

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

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

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

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

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

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_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_PARTIAL_CACHE_VALIDATION:
        DCHECK_EQ(OK, rv);
        rv = DoPartialCacheValidation();
        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_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::DoSendRequest() {
  DCHECK(mode_ & WRITE || mode_ == NONE);
  DCHECK(!network_trans_.get());

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

  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_)
    return ERR_UNEXPECTED;

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

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

  if (!ValidatePartialResponse(new_response->headers, &server_responded_206_) &&
      !auth_response_.headers) {
    // 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.
    response_ = HttpResponseInfo();
    network_trans_.reset();
    next_state_ = STATE_SEND_REQUEST;
    return OK;
  }
  if (server_responded_206_ && mode_ == READ_WRITE && !truncated_ &&
      response_.headers->response_code() == 200) {
    // We have stored the full entry, but it changed and the server is
    // sending a range. We have to delete the old entry.
    DoneWritingToEntry(false);
  }

  HistogramHeaders(new_response->headers);

  new_response_ = new_response;
  // Are we expecting a response to a conditional query?
  if (mode_ == READ_WRITE || mode_ == UPDATE) {
    if (new_response->headers->response_code() == 304 ||
        server_responded_206_) {
      next_state_ = STATE_UPDATE_CACHED_RESPONSE;
      return OK;
    }
    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_, io_buf_len_, &io_callback_);
}

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

  if (!cache_)
    return ERR_UNEXPECTED;

  next_state_ = STATE_CACHE_WRITE_DATA;
  return result;
}

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

  if (!cache_)
    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);
  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_.EndEvent(NetLog::TYPE_HTTP_CACHE_OPEN_ENTRY);
  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 (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)
    DLOG(INFO) << "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_.EndEvent(NetLog::TYPE_HTTP_CACHE_CREATE_ENTRY);
  cache_pending_ = false;
  next_state_ = STATE_ADD_TO_ENTRY;

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

  if (result != OK) {
    // 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;
  net_log_.BeginEvent(NetLog::TYPE_HTTP_CACHE_DOOM_ENTRY);
  return cache_->DoomEntry(cache_key_, this);
}

int HttpCache::Transaction::DoDoomEntryComplete(int result) {
  net_log_.EndEvent(NetLog::TYPE_HTTP_CACHE_DOOM_ENTRY);
  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_WAITING);
  return cache_->AddTransactionToEntry(new_entry_, this);
}

int HttpCache::Transaction::DoAddToEntryComplete(int result) {
  net_log_.EndEvent(NetLog::TYPE_HTTP_CACHE_WAITING);
  DCHECK(new_entry_);
  cache_pending_ = false;

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

  if (result != OK) {
    // If there is a failure, the cache should have taken care of new_entry_.
    NOTREACHED();
    new_entry_ = NULL;
    return result;
  }

  entry_ = new_entry_;
  new_entry_ = NULL;

  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;
}

int HttpCache::Transaction::DoPartialCacheValidation() {
  if (mode_ == NONE)
    return OK;

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

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

  if (rv < 0) {
    DCHECK(rv != ERR_IO_PENDING);
    return rv;
  }

  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);
  response_.response_time = new_response_->response_time;
  response_.request_time = new_response_->request_time;

  if (response_.headers->HasHeaderValue("cache-control", "no-store")) {
    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(!server_responded_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_ && !server_responded_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();
    network_trans_.reset();
  }
  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 (server_responded_206_ && partial_.get())
    partial_->FixContentLength(new_response_->headers);

  response_ = *new_response_;
  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;

  // Truncate the stream.
  int rv = WriteToEntry(kResponseContentIndex, 0, NULL, 0, NULL);
  DCHECK(rv != ERR_IO_PENDING);
  rv = WriteToEntry(kMetadataIndex, 0, NULL, 0, NULL);
  DCHECK(rv != ERR_IO_PENDING);
  return OK;
}

int HttpCache::Transaction::DoTruncateCachedDataComplete(int result) {
  // If this response is a redirect, then we can stop writing now.  (We don't
  // need to cache the response body of a redirect.)
  if (response_.headers->IsRedirect(NULL))
    DoneWritingToEntry(true);
  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);
  }
  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);
  cache_callback_->AddRef();  // Balanced in DoCacheReadResponseComplete.
  return entry_->disk_entry->ReadData(kResponseInfoIndex, 0, read_buf_,
                                      io_buf_len_, cache_callback_);
}

int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
  cache_callback_->Release();  // Balance the AddRef from DoCacheReadResponse.
  net_log_.EndEvent(NetLog::TYPE_HTTP_CACHE_READ_INFO);
  if (result != io_buf_len_ ||
      !HttpCache::ParseResponseInfo(read_buf_->data(), io_buf_len_,
                                    &response_, &truncated_)) {
    DLOG(ERROR) << "ReadData failed: " << result;
    return ERR_CACHE_READ_FAILURE;
  }

  // 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:
      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() {
  return WriteResponseInfoToEntry(false);
}

int HttpCache::Transaction::DoCacheWriteTruncatedResponse() {
  return WriteResponseInfoToEntry(true);
}

int HttpCache::Transaction::DoCacheWriteResponseComplete(int result) {
  next_state_ = target_state_;
  target_state_ = STATE_NONE;
  if (!entry_)
    return OK;

  // Balance the AddRef from WriteResponseInfoToEntry.
  write_headers_callback_->Release();
  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);
  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);
  cache_callback_->AddRef();  // Balanced in DoCacheReadMetadataComplete.
  return entry_->disk_entry->ReadData(kMetadataIndex, 0, response_.metadata,
                                      response_.metadata->size(),
                                      cache_callback_);
}

int HttpCache::Transaction::DoCacheReadMetadataComplete(int result) {
  cache_callback_->Release();  // Balance the AddRef from DoCacheReadMetadata.
  net_log_.EndEvent(NetLog::TYPE_HTTP_CACHE_READ_INFO);
  if (result != response_.metadata->size()) {
    DLOG(ERROR) << "ReadData failed: " << result;
    return ERR_CACHE_READ_FAILURE;
  }

  return OK;
}

int HttpCache::Transaction::DoCacheQueryData() {
  next_state_ = STATE_CACHE_QUERY_DATA_COMPLETE;

  // Balanced in ValidateEntryHeadersAndContinue.
  cache_callback_->AddRef();
  return entry_->disk_entry->ReadyForSparseIO(cache_callback_);
}

int HttpCache::Transaction::DoCacheQueryDataComplete(int result) {
  DCHECK_EQ(OK, result);
  // Balance the AddRef from BeginPartialCacheValidation.
  cache_callback_->Release();
  if (!cache_)
    return ERR_UNEXPECTED;

  return ValidateEntryHeadersAndContinue(true);
}

int HttpCache::Transaction::DoCacheReadData() {
  DCHECK(entry_);
  next_state_ = STATE_CACHE_READ_DATA_COMPLETE;
  cache_callback_->AddRef();  // Balanced in DoCacheReadDataComplete.
  if (partial_.get()) {
    return partial_->CacheRead(entry_->disk_entry, read_buf_, io_buf_len_,
                               cache_callback_);
  }

  return entry_->disk_entry->ReadData(kResponseContentIndex, read_offset_,
                                      read_buf_, io_buf_len_, cache_callback_);
}

int HttpCache::Transaction::DoCacheReadDataComplete(int result) {
  cache_callback_->Release();  // Balance the AddRef from DoCacheReadData.

  if (!cache_)
    return ERR_UNEXPECTED;

  if (partial_.get())
    return DoPartialCacheReadCompleted(result);

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

int HttpCache::Transaction::DoCacheWriteData(int num_bytes) {
  next_state_ = STATE_CACHE_WRITE_DATA_COMPLETE;
  cache_callback_->AddRef();  // Balanced in DoCacheWriteDataComplete.

  return AppendResponseDataToEntry(read_buf_, num_bytes, cache_callback_);
}

int HttpCache::Transaction::DoCacheWriteDataComplete(int result) {
  // Balance the AddRef from DoCacheWriteData.
  cache_callback_->Release();
  if (!cache_)
    return ERR_UNEXPECTED;

  if (result < 0)
    return result;

  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.
    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 },
  };

  std::string new_extra_headers;
  bool range_found = false;
  bool external_validation_error = false;

  // scan request headers to see if we have any that would impact our load flags
  HttpUtil::HeadersIterator it(request_->extra_headers.begin(),
                               request_->extra_headers.end(),
                               "\r\n");
  while (it.GetNext()) {
    if (!LowerCaseEqualsASCII(it.name(), "range")) {
      new_extra_headers.append(it.name_begin(), it.values_end());
      new_extra_headers.append("\r\n");
    } else {
      if (enable_range_support_) {
        range_found = true;
      } else {
        effective_load_flags_ |= LOAD_DISABLE_CACHE;
        continue;
      }
    }
    for (size_t i = 0; i < ARRAYSIZE_UNSAFE(kSpecialHeaders); ++i) {
      if (HeaderMatches(it, 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_UNSAFE(kValidationHeaders); ++i) {
      const ValidationHeaderInfo& info = kValidationHeaders[i];
      if (LowerCaseEqualsASCII(it.name_begin(), it.name_end(),
                               info.request_header_name)) {
        if (!external_validation_.values[i].empty() || it.values().empty())
          external_validation_error = true;
        external_validation_.values[i] = it.values();
        external_validation_.initialized = true;
        break;
      }
    }
  }

  // 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)) {
    partial_.reset(new PartialData);
    if (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_));
      request_ = custom_request_.get();
      custom_request_->extra_headers = new_extra_headers;
      partial_->SetHeaders(new_extra_headers);
    } else {
      // The range is invalid or we cannot handle it properly.
      LOG(INFO) << "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 && request_->upload_data->identifier())
    return false;

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

int HttpCache::Transaction::AddToEntry() {
  next_state_ = STATE_INIT_ENTRY;
  cache_pending_ = false;
  return DoLoop(OK);
}

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);

  if ((effective_load_flags_ & LOAD_PREFERRING_CACHE ||
       !RequiresValidation()) && !partial_.get()) {
    cache_->ConvertWriterToReader(entry_);
    mode_ = READ;
    if (entry_ && entry_->disk_entry->GetDataSize(kMetadataIndex))
      next_state_ = STATE_CACHE_READ_METADATA;
  } 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.  We'll switch to
    // either READ or WRITE mode once we hear back from the server.
    if (!ConditionalizeRequest())
      mode_ = WRITE;
    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();

  if (!enable_range_support_)
    return BeginCacheValidation();

  bool byte_range_requested = partial_.get() != NULL;
  if (byte_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(false);
}

int HttpCache::Transaction::ValidateEntryHeadersAndContinue(
    bool byte_range_requested) {
  DCHECK(mode_ == READ_WRITE);

  if (!partial_->UpdateFromStoredHeaders(response_.headers, entry_->disk_entry,
                                         truncated_)) {
    // 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(!byte_range_requested);
    mode_ = WRITE;
    truncated_ = false;
    next_state_ = STATE_INIT_ENTRY;
    return OK;
  }

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

  next_state_ = STATE_PARTIAL_CACHE_VALIDATION;
  return OK;
}

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

  for (size_t i = 0;  i < ARRAYSIZE_UNSAFE(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.
      DoneWritingToEntry(true);
    }
  }

  next_state_ = STATE_SEND_REQUEST;
  return OK;
}

int HttpCache::Transaction::BeginNetworkRequest() {
  next_state_ = STATE_SEND_REQUEST;
  return DoLoop(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 std::wstring& username,
    const std::wstring& password) {
  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(username, password, &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 (effective_load_flags_ & LOAD_VALIDATE_CACHE)
    return true;

  if (response_.headers->response_code() == 206 && !enable_range_support_)
    return true;

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

  // Since Vary header computation is fairly expensive, we save it for last.
  if (response_.vary_data.is_valid() &&
          !response_.vary_data.MatchesRequest(*request_, *response_.headers))
    return true;

  return false;
}

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

  if (!enable_range_support_ && response_.headers->response_code() != 200) {
    // This only makes sense for cached 200 responses.
    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;

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

  std::string etag_value;
  response_.headers->EnumerateHeader(NULL, "etag", &etag_value);

  std::string last_modified_value;
  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());

  if (!etag_value.empty()) {
    if (partial_.get() && !partial_->IsCurrentRangeCached()) {
      // 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.append("If-Range: ");
    } else {
      custom_request_->extra_headers.append("If-None-Match: ");
    }
    custom_request_->extra_headers.append(etag_value);
    custom_request_->extra_headers.append("\r\n");
    // 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 (partial_.get() && !partial_->IsCurrentRangeCached()) {
      custom_request_->extra_headers.append("If-Range: ");
    } else {
      custom_request_->extra_headers.append("If-Modified-Since: ");
    }
    custom_request_->extra_headers.append(last_modified_value);
    custom_request_->extra_headers.append("\r\n");
  }

  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, bool* partial_content) {
  int response_code = headers->response_code();
  bool partial_response = enable_range_support_ ? response_code == 206 : false;
  *partial_content = false;

  if (!entry_)
    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
    if (partial_response || response_code == 200 || response_code == 304) {
      DoomPartialEntry(true);
      mode_ = NONE;
    } else {
      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)) {
      *partial_content = true;
      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.
    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?).
  if (mode_ & WRITE) {
    DoneWritingToEntry(mode_ != WRITE);
  } else if (mode_ & READ && entry_) {
    cache_->DoneReadingFromEntry(entry_, this);
  }

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

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,
                                         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);
  }

  if (rv != ERR_IO_PENDING && rv != data_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.
    rv = data_len;
  }
  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);
    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();

  // Balanced in DoCacheWriteResponseComplete.  We may be running from the
  // destructor of this object so cache_callback_ may be currently in use.
  write_headers_callback_->AddRef();
  io_buf_len_ = data->pickle()->size();
  return entry_->disk_entry->WriteData(kResponseInfoIndex, 0, data, io_buf_len_,
                                       write_headers_callback_, true);
}

int HttpCache::Transaction::AppendResponseDataToEntry(
    IOBuffer* data, int data_len, 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;

  if (cache_->mode() == RECORD)
    DLOG(INFO) << "Recorded: " << request_->method << request_->url
               << " status: " << response_.headers->response_code();

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

void HttpCache::Transaction::DoomPartialEntry(bool delete_object) {
  int rv = cache_->DoomEntry(cache_key_, NULL);
  DCHECK_EQ(OK, rv);
  cache_->DoneWithEntry(entry_, this, false);
  entry_ = NULL;
  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.
    network_trans_.reset();
    next_state_ = STATE_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_PARTIAL_CACHE_VALIDATION;
  }
  return result;
}

// For a 200 response we'll add a histogram with one bit set per header:
//   0x01 Content-Length
//   0x02 Date
//   0x04 Last-Modified
//   0x08 Etag
//   0x10 Accept-Ranges: bytes
//   0x20 Accept-Ranges: none
//
// TODO(rvargas): remove after having some results.
void HttpCache::Transaction::HistogramHeaders(
    const HttpResponseHeaders* headers) {
  if (headers->response_code() != 200)
    return;

  int64 content_length = headers->GetContentLength();
  int value = 0;
  if (content_length > 0)
    value = 1;

  Time date;
  if (headers->GetDateValue(&date))
    value += 2;
  if (headers->GetLastModifiedValue(&date))
    value += 4;

  std::string etag;
  headers->EnumerateHeader(NULL, "etag", &etag);
  if (!etag.empty())
    value += 8;

  std::string accept_ranges("Accept-Ranges");
  if (headers->HasHeaderValue(accept_ranges, "bytes"))
    value += 0x10;
  if (headers->HasHeaderValue(accept_ranges, "none"))
    value += 0x20;

  // |value| goes from 0 to 63. Actually, the max value should be 47 (0x2f)
  // but we'll see.
  UMA_HISTOGRAM_ENUMERATION("HttpCache.ResponseHeaders", value, 65);
}

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

}  // namespace net