Fixes the cache lock issue.

net/http/http_cache_transaction.cc

 // 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"  // For OS_POSIX
 
 #if defined(OS_POSIX)
 #include <unistd.h>
 #endif
 
 #include <algorithm>
+#include <limits>
 #include <string>
+#include <utility>
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/compiler_specific.h"
 #include "base/format_macros.h"
 #include "base/location.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"  // For HexEncode.
 #include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"  // For LowerCaseEqualsASCII.
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/clock.h"
 #include "base/trace_event/trace_event.h"
 #include "base/values.h"
 #include "net/base/auth.h"
 #include "net/base/load_flags.h"
 #include "net/base/load_timing_info.h"
 #include "net/base/trace_constants.h"
 #include "net/base/upload_data_stream.h"
 #include "net/cert/cert_status_flags.h"
 #include "net/cert/x509_certificate.h"
 #include "net/disk_cache/disk_cache.h"
+#include "net/http/http_cache_shared_writers.h"
 #include "net/http/http_network_session.h"
 #include "net/http/http_request_info.h"
 #include "net/http/http_util.h"
 #include "net/log/net_log_event_type.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;
@@ skipping 123 matching lines @@
       truncated_(false),
       is_sparse_(false),
       range_requested_(false),
       handling_206_(false),
       cache_pending_(false),
       done_reading_(false),
       vary_mismatch_(false),
       couldnt_conditionalize_request_(false),
       bypass_lock_for_test_(false),
       fail_conditionalization_for_test_(false),
+      shared_(false),
+      initiate_shared_writing_(false),
+      shared_read_write_failure_result_(0),
       io_buf_len_(0),
       read_offset_(0),
       effective_load_flags_(0),
       write_len_(0),
       cache_entry_status_(CacheEntryStatus::ENTRY_UNDEFINED),
       validation_cause_(VALIDATION_CAUSE_UNDEFINED),
       total_received_bytes_(0),
       total_sent_bytes_(0),
       websocket_handshake_stream_base_create_helper_(NULL),
+      have_full_request_headers_(false),
       weak_factory_(this) {
   TRACE_EVENT0("io", "HttpCacheTransaction::Transaction");
   static_assert(HttpCache::Transaction::kNumValidationHeaders ==
                     arraysize(kValidationHeaders),
                 "invalid number of validation headers");
 
   io_callback_ = base::Bind(&Transaction::OnIOComplete,
                               weak_factory_.GetWeakPtr());
 }
 
 HttpCache::Transaction::~Transaction() {
   TRACE_EVENT0("io", "HttpCacheTransaction::~Transaction");
+
   // We may have to issue another IO, but we should never invoke the callback_
   // after this point.
   callback_.Reset();
 
+  if (shared_) {
+    // Remove transaction from shared writers and do any cleanup, if needed, for
+    // entry_->shared_writers and entry_.
+    RemoveTransactionFromSharedWriters();
+  }
+
   if (cache_) {
     if (entry_) {
       bool cancel_request = reading_ && response_.headers.get();
       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);
     }
   }
 }
 
+void HttpCache::Transaction::RemoveTransactionFromSharedWriters() {
+  switch (next_state_) {
+    case STATE_GET_BACKEND_COMPLETE:
+    case STATE_OPEN_ENTRY_COMPLETE:
+    case STATE_DOOM_ENTRY_COMPLETE:
+    case STATE_CREATE_ENTRY_COMPLETE:
+    case STATE_ADD_TO_ENTRY_COMPLETE:
+      // Pending transaction not yet added to an entry, so entry_ is null, let
+      // cache_ handle the deletion.
+      cache_->RemovePendingTransaction(this);
+      // Set cache_pending_ to false so that more cleanup is not attempted in
+      // the destructor.
+      cache_pending_ = false;
+      break;
+    case STATE_CACHE_READ_RESPONSE_COMPLETE:
+    case STATE_TOGGLE_UNUSED_SINCE_PREFETCH_COMPLETE:
+    case STATE_CACHE_QUERY_DATA_COMPLETE:
+    case STATE_COMPLETE_PARTIAL_CACHE_VALIDATION:
+    case STATE_SEND_REQUEST_COMPLETE:
+    case STATE_CACHE_WRITE_UPDATED_RESPONSE_COMPLETE:
+    case STATE_UPDATE_CACHED_RESPONSE_COMPLETE:
+    case STATE_CACHE_WRITE_RESPONSE_COMPLETE:
+    case STATE_TRUNCATE_CACHED_DATA_COMPLETE:
+    case STATE_TRUNCATE_CACHED_METADATA_COMPLETE:
+    case STATE_CACHE_READ_METADATA_COMPLETE:
+      entry_->shared_writers->RemoveValidatingTransaction(this);
+      break;
+    case STATE_SHARED_NETWORK_READ_COMPLETE:
+    case STATE_SHARED_CACHE_WRITE_DATA_COMPLETE:
+      // Current shared network transaction's consumer.
+      entry_->shared_writers->RemoveActiveTransaction(this);
+      break;
+    case STATE_SHARED_NETWORK_READ_WAIT_COMPLETE:
+      // Waiting on Read() operation to be completed by another
+      // transaction.
+      entry_->shared_writers->RemoveWaitingForReadTransaction(this);
+      break;
+    case STATE_CACHE_READ_DATA_COMPLETE:
+    case STATE_NONE:
+      entry_->shared_writers->RemoveIdleTransaction(this);
+      break;
+    default:
+      NOTREACHED() << "bad state";
+      break;
+  }
+
+  DCHECK(!shared_);
+
+  // Since any cleanup needed in entry_ and shared_writers is already done,
+  // set entry_ to null.
+  entry_ = nullptr;
+}
+
 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_ && !truncated_)
     return true;
 
-  if (!CanResume(true))
+  if (!cache_->CanResumeEntry(true, request_->method, &response_, entry_))
     return false;
 
   // We may have received the whole resource already.
   if (done_reading_)
     return true;
 
   truncated_ = true;
   next_state_ = STATE_CACHE_WRITE_TRUNCATED_RESPONSE;
   DoLoop(OK);
   return true;
 }
 
 LoadState HttpCache::Transaction::GetWriterLoadState() const {
-  if (network_trans_.get())
-    return network_trans_->GetLoadState();
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    return network_transaction->GetLoadState();
   if (entry_ || !request_)
     return LOAD_STATE_IDLE;
   return LOAD_STATE_WAITING_FOR_CACHE;
 }
 
 const NetLogWithSource& HttpCache::Transaction::net_log() const {
   return net_log_;
 }
 
 int HttpCache::Transaction::Start(const HttpRequestInfo* request,
@@ skipping 89 matching lines @@
 }
 
 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) {
+  if (next_state_ == STATE_SHARED_READ_WRITE_FAILED) {
+    if (net_log_.IsCapturing())
+      net_log_.EndEventWithNetErrorCode(
+          NetLogEventType::HTTP_CACHE_SHARED_READ_WRITE_FAILED,
+          shared_read_write_failure_result_);
+
+    return shared_read_write_failure_result_;
+  }
+
   DCHECK_EQ(next_state_, STATE_NONE);
   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) {
     UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_OTHER);
     DCHECK(mode_ & WRITE);
     DoneWritingToEntry(mode_ == READ_WRITE);
     mode_ = NONE;
   }
 
   reading_ = true;
   read_buf_ = buf;
   io_buf_len_ = buf_len;
-  if (network_trans_) {
-    DCHECK(mode_ == WRITE || mode_ == NONE ||
-           (mode_ == READ_WRITE && partial_));
-    next_state_ = STATE_NETWORK_READ;
-  } else {
-    DCHECK(mode_ == READ || (mode_ == READ_WRITE && partial_));
-    next_state_ = STATE_CACHE_READ_DATA;
+
+  if (shared_) {
+    int disk_entry_size =
+        entry_->disk_entry->GetDataSize(kResponseContentIndex);
+    if (read_offset_ == disk_entry_size) {
+      next_state_ = STATE_SHARED_NETWORK_READ;
+    } else {
+      DCHECK_LT(read_offset_, disk_entry_size);
+      next_state_ = STATE_CACHE_READ_DATA;
+    }
+  } else {  // not shared.
+    if (network_trans_) {
+      DCHECK(mode_ == WRITE || mode_ == NONE ||
+             (mode_ == READ_WRITE && partial_));
+      next_state_ = STATE_NETWORK_READ;
+    } else {  // read-only
+      DCHECK(mode_ == READ || (mode_ == READ_WRITE && partial_));
+      next_state_ = STATE_CACHE_READ_DATA;
+    }
   }
 
   int rv = DoLoop(OK);
 
   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_) {
+  if (shared_) {
+    // This might or might not stop caching based on whether other consumers
+    // exist for this resource or not. If it does, shared_ will be set to false.
+    entry_->shared_writers->StopCaching(this);
+  }
+  if (!shared_ && 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);
-
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction) {
+    return network_transaction->GetFullRequestHeaders(headers);
+  } else if (have_full_request_headers_) {
+    *headers = full_request_headers_;
+    return true;
+  }
   // TODO(juliatuttle): Read headers from cache.
   return false;
 }
 
 int64_t HttpCache::Transaction::GetTotalReceivedBytes() const {
   int64_t total_received_bytes = total_received_bytes_;
-  if (network_trans_)
-    total_received_bytes += network_trans_->GetTotalReceivedBytes();
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    total_received_bytes += network_transaction->GetTotalReceivedBytes();
   return total_received_bytes;
 }
 
 int64_t HttpCache::Transaction::GetTotalSentBytes() const {
   int64_t total_sent_bytes = total_sent_bytes_;
-  if (network_trans_)
-    total_sent_bytes += network_trans_->GetTotalSentBytes();
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    total_sent_bytes += network_transaction->GetTotalSentBytes();
   return total_sent_bytes;
 }
 
 void HttpCache::Transaction::DoneReading() {
   if (cache_.get() && entry_) {
     DCHECK_NE(mode_, UPDATE);
+    bool perform_entry_cleanup = true;
+    if (shared_) {
+      entry_->shared_writers->DoneReading(this);
+
+      // shared_ should have been set to false.
+      DCHECK(!shared_);
+
+      DoneWritingToEntry(true, false);
+      return;
+    }
+
     if (mode_ & WRITE) {
-      DoneWritingToEntry(true);
+      DoneWritingToEntry(true, perform_entry_cleanup);
     } 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;
     }
   }
 }
 
@@ skipping 18 matching lines @@
     return cache_->GetLoadStateForPendingTransaction(this);
 
   return LOAD_STATE_IDLE;
 }
 
 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);
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    return network_transaction->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;
 }
 
 bool HttpCache::Transaction::GetRemoteEndpoint(IPEndPoint* endpoint) const {
-  if (network_trans_)
-    return network_trans_->GetRemoteEndpoint(endpoint);
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    return network_transaction->GetRemoteEndpoint(endpoint);
 
   if (!old_remote_endpoint_.address().empty()) {
     *endpoint = old_remote_endpoint_;
     return true;
   }
 
   return false;
 }
 
 void HttpCache::Transaction::PopulateNetErrorDetails(
     NetErrorDetails* details) const {
-  if (network_trans_)
-    return network_trans_->PopulateNetErrorDetails(details);
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    network_transaction->PopulateNetErrorDetails(details);
   return;
 }
 
 void HttpCache::Transaction::SetPriority(RequestPriority priority) {
   priority_ = priority;
   if (network_trans_)
     network_trans_->SetPriority(priority_);
+  else if (shared_ && entry_ && entry_->shared_writers)
+    entry_->shared_writers->PriorityChanged();
 }
 
 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(
@@ skipping 10 matching lines @@
 
 int HttpCache::Transaction::ResumeNetworkStart() {
   if (network_trans_)
     return network_trans_->ResumeNetworkStart();
   return ERR_UNEXPECTED;
 }
 
 void HttpCache::Transaction::GetConnectionAttempts(
     ConnectionAttempts* out) const {
   ConnectionAttempts new_connection_attempts;
-  if (network_trans_)
-    network_trans_->GetConnectionAttempts(&new_connection_attempts);
+  HttpTransaction* network_transaction = GetCurrentNetworkTransaction();
+  if (network_transaction)
+    network_transaction->GetConnectionAttempts(&new_connection_attempts);
 
   out->swap(new_connection_attempts);
   out->insert(out->begin(), old_connection_attempts_.begin(),
               old_connection_attempts_.end());
 }
 
 //-----------------------------------------------------------------------------
 
 // A few common patterns: (Foo* means Foo -> FooComplete)
 //
 // 1. Not-cached entry:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> CreateEntry* -> AddToEntry* ->
 //   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse ->
 //   CacheWriteResponse* -> TruncateCachedData* -> TruncateCachedMetadata* ->
 //   PartialHeadersReceived
 //
-//   Read():
+//   Read(): (For transactions that are not eligible for shared writing)
 //   NetworkRead* -> CacheWriteData*
 //
+//   Read(): (For transactions that are eligible for shared writing)
+//   SharedNetworkRead* -> SharedCacheWriteData*
+//
+//   Read(): (For a transaction that is shared and another read is already in
+//   progress)
+//   SharedNetworkRead -> SharedNetworkReadWaitComplete
+//
 // 2. Cached entry, no validation:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SetupEntryForRead()
 //
-//   Read():
+//   Read(): (When response is already written to the cache.)
 //   CacheReadData*
 //
+//   Read(): (When response is currently being written to the cache by shared
+//   writing.)
+//   SharedNetworkRead* -> SharedCacheWriteData*
+//
 // 3. Cached entry, validation (304):
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   UpdateCachedResponse -> CacheWriteUpdatedResponse* ->
 //   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
 //   PartialHeadersReceived
 //
-//   Read():
+//   Read(): () (When response is already written to the cache.)
 //   CacheReadData*
 //
+//   Read(): (When response is currently being written to the cache by shared
+//   writing.)
+//   SharedNetworkRead* -> SharedCacheWriteData*
+//
 // 4. Cached entry, validation and replace (200):
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   OverwriteCachedResponse -> CacheWriteResponse* -> DoTruncateCachedData* ->
 //   TruncateCachedMetadata* -> PartialHeadersReceived
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
@@ skipping 241 matching lines @@
       case STATE_CACHE_WRITE_DATA_COMPLETE:
         rv = DoCacheWriteDataComplete(rv);
         break;
       case STATE_CACHE_WRITE_TRUNCATED_RESPONSE:
         DCHECK_EQ(OK, rv);
         rv = DoCacheWriteTruncatedResponse();
         break;
       case STATE_CACHE_WRITE_TRUNCATED_RESPONSE_COMPLETE:
         rv = DoCacheWriteTruncatedResponseComplete(rv);
         break;
+      case STATE_SHARED_NETWORK_READ:
+        DCHECK_EQ(OK, rv);
+        rv = DoSharedNetworkRead();
+        break;
+      case STATE_SHARED_NETWORK_READ_COMPLETE:
+        rv = DoSharedNetworkReadComplete(rv);
+        break;
+      case STATE_SHARED_NETWORK_READ_WAIT_COMPLETE:
+        rv = DoSharedNetworkReadWaitComplete(rv);
+        break;
+      case STATE_SHARED_CACHE_WRITE_DATA:
+        rv = DoSharedCacheWriteData(rv);
+        break;
+      case STATE_SHARED_CACHE_WRITE_DATA_COMPLETE:
+        rv = DoSharedCacheWriteDataComplete(rv);
+        break;
+      case STATE_SHARED_READ_WRITE_FAILED:
+        rv = DoSharedReadWriteFailed();
+        break;
       default:
         NOTREACHED() << "bad state";
         rv = ERR_FAILED;
         break;
     }
   } while (rv != ERR_IO_PENDING && next_state_ != STATE_NONE);
 
   if (rv != ERR_IO_PENDING && !callback_.is_null()) {
     read_buf_ = NULL;  // Release the buffer before invoking the callback.
     base::ResetAndReturn(&callback_).Run(rv);
@@ skipping 617 matching lines @@
 
   RecordNoStoreHeaderHistogram(request_->load_flags, new_response);
 
   if (new_response_->headers->response_code() == 416 &&
       (request_->method == "GET" || request_->method == "POST")) {
     // If there is an active entry it may be destroyed with this transaction.
     SetResponse(*new_response_);
     return OK;
   }
 
+  // If the transaction is shared and it is a 200 or an error response, then
+  // doom the entry and let this transaction continue without writing to the
+  // cache if shared writers contain more transactions. If not, continue
+  // writing to the cache and also transfer the network transaction to shared
+  // writers.
+  if (shared_ && new_response->headers->response_code() != 304) {
+    network_trans_ = entry_->shared_writers->OnValidationNoMatch(
+        cache_key_, this, std::move(network_trans_), priority_);
+    if (!shared_) {
+      DCHECK(network_trans_);
+      if (cache_entry_status_ == CacheEntryStatus::ENTRY_UNDEFINED) {
+        UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_DOOM_SHARED_WRITING);
+      }
+      DoneWritingToEntry(false, false);
+      return OK;
+    } else {
+      DCHECK(!network_trans_);
+    }
+  }
+
   // Are we expecting a response to a conditional query?
   if (mode_ == READ_WRITE || mode_ == UPDATE) {
     if (new_response->headers->response_code() == 304 || handling_206_) {
       UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_VALIDATED);
       next_state_ = STATE_UPDATE_CACHED_RESPONSE;
       return OK;
     }
     UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_UPDATED);
     mode_ = WRITE;
   }
@@ skipping 102 matching lines @@
   SetResponse(*new_response_);
 
   if (request_->method == "HEAD") {
     // This response is replacing the cached one.
     DoneWritingToEntry(false);
     mode_ = NONE;
     new_response_ = NULL;
     return OK;
   }
 
-  if (handling_206_ && !CanResume(false)) {
+  if (handling_206_ &&
+      !cache_->CanResumeEntry(false, request_->method, &response_, entry_)) {
     // There is no point in storing this resource because it will never be used.
     // This may change if we support LOAD_ONLY_FROM_CACHE with sparse entries.
     DoneWritingToEntry(false);
     if (partial_)
       partial_->FixResponseHeaders(response_.headers.get(), true);
     next_state_ = STATE_PARTIAL_HEADERS_RECEIVED;
     return OK;
   }
 
   next_state_ = STATE_CACHE_WRITE_RESPONSE;
   return OK;
 }
 
 int HttpCache::Transaction::DoCacheWriteResponse() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteResponse");
   next_state_ = STATE_CACHE_WRITE_RESPONSE_COMPLETE;
   return WriteResponseInfoToEntry(truncated_);
 }
 
 int HttpCache::Transaction::DoCacheWriteResponseComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteResponseComplete");
   next_state_ = STATE_TRUNCATE_CACHED_DATA;
   return OnWriteResponseInfoToEntryComplete(result);
 }
 
+void HttpCache::Transaction::SetShared() {
+  shared_ = true;
+}
+
+void HttpCache::Transaction::ResetShared(bool continue_network_reading,
+                                         bool continue_cache_reading) {
+  if (!continue_network_reading) {
+    SaveSharedNetworkTransactionInfo();
+  }
+  if (continue_cache_reading) {
+    mode_ = READ;
+  }
+  shared_ = false;
+}
+
 int HttpCache::Transaction::DoTruncateCachedData() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoTruncateCachedData");
   next_state_ = STATE_TRUNCATE_CACHED_DATA_COMPLETE;
   if (!entry_)
     return OK;
   if (net_log_.IsCapturing())
     net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_WRITE_DATA);
   // Truncate the stream.
   return WriteToEntry(kResponseContentIndex, 0, NULL, 0, io_callback_);
 }
@@ skipping 28 matching lines @@
     if (net_log_.IsCapturing()) {
       net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_WRITE_INFO,
                                         result);
     }
   }
 
   next_state_ = STATE_PARTIAL_HEADERS_RECEIVED;
   return OK;
 }
 
+void HttpCache::Transaction::ProcessForSharedWriting() {
+  // Should not be already reading.
+  if (reading_)
+    return;
+
+  // If not already part of SharedWriters, then check if one should be
+  // created.
+  if (!shared_ && !IsEligibleForSharedWriting())
+    return;
+
+  if (shared_) {
+    // Non 304 case is already handled in DoSuccessfulSendRequest.
+    if (response_.headers->response_code() != 304) {
+      return;
+    }
+    UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_JOIN_SHARED_WRITING);
+    entry_->shared_writers->OnValidationMatch(this, priority_);
+    network_trans_.reset();
+    return;
+  }
+
+  // Do not create a SharedWriters if its a Redirect response or if its a
+  // no-store response.
+  if (response_.headers->response_code() != 200)
+    return;
+
+  if (!entry_)
+    return;
+
+  DCHECK(cache_entry_status_ == CacheEntryStatus::ENTRY_NOT_IN_CACHE ||
+         cache_entry_status_ == CacheEntryStatus::ENTRY_UPDATED ||
+         cache_entry_status_ == CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE ||
+         cache_entry_status_ == CacheEntryStatus::ENTRY_OTHER);
+  // Measure how many requests out of the total of "not cached" and "updated"
+  // are initiating shared writing.
+  initiate_shared_writing_ = true;
+  DCHECK(entry_ && network_trans_);
+
+  // An instance of SharedWriters will be created when the first writer has
+  // written the new response headers in the cache. Transfer network
+  // transaction’s ownership to SharedWriters so it can be used by any of the
+  // transactions for subsequent reading from the network.
+  SharedWriters::Create(this, std::move(network_trans_), cache_, priority_);
+}
+
 int HttpCache::Transaction::DoPartialHeadersReceived() {
   new_response_ = NULL;
   if (entry_ && !partial_ && entry_->disk_entry->GetDataSize(kMetadataIndex))
     next_state_ = STATE_CACHE_READ_METADATA;
 
+  ProcessForSharedWriting();
+
   if (!partial_)
     return OK;
 
   if (reading_) {
     if (network_trans_.get()) {
       next_state_ = STATE_NETWORK_READ;
     } else {
       next_state_ = STATE_CACHE_READ_DATA;
     }
   } else if (mode_ != NONE) {
@@ skipping 30 matching lines @@
 }
 
 int HttpCache::Transaction::DoNetworkRead() {
   TRACE_EVENT0("io", "HttpCacheTransaction::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) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoNetworkReadComplete");
+  if (net_log_.IsCapturing())
+    net_log_.EndEventWithNetErrorCode(
+        NetLogEventType::HTTP_CACHE_NETWORK_READ_COMPLETE, 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::DoSharedNetworkRead() {
+  TRACE_EVENT0("io", "HttpCacheTransaction::DoSharedNetworkRead");
+  if (net_log_.IsCapturing())
+    net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_SHARED_NETWORK_READ);
+
+  next_state_ = STATE_SHARED_NETWORK_READ_COMPLETE;
+  bool shared_read_in_progress = false;
+  int result = entry_->shared_writers->Read(
+      read_buf_, io_buf_len_, io_callback_, this, &shared_read_in_progress);
+  if (shared_read_in_progress) {
+    next_state_ = STATE_SHARED_NETWORK_READ_WAIT_COMPLETE;
+  }
+  return result;
+}
+
+int HttpCache::Transaction::DoSharedNetworkReadComplete(int result) {
+  TRACE_EVENT0("io", "HttpCacheTransaction::DoSharedNetworkReadComplete");
+  if (net_log_.IsCapturing())
+    net_log_.EndEventWithNetErrorCode(
+        NetLogEventType::HTTP_CACHE_SHARED_NETWORK_READ_COMPLETE, result);
+
+  if (result <= 0) {
+    // Set entry as null so that the destructor does not invoke DoneWithEntry
+    // again as entry_ is already cleaned up by SharedWriters.
+    entry_ = nullptr;
+    return result;
+  }
+
+  read_offset_ += result;
+
+  next_state_ = STATE_SHARED_CACHE_WRITE_DATA;
+  return result;
+}
+
+int HttpCache::Transaction::DoSharedCacheWriteData(int result) {
+  TRACE_EVENT0("io", "HttpCacheTransaction::DoSharedCacheWriteData");
+  if (net_log_.IsCapturing())
+    net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_SHARED_CACHE_WRITE_DATA);
+
+  next_state_ = STATE_SHARED_CACHE_WRITE_DATA_COMPLETE;
+  write_len_ = result;
+  return entry_->shared_writers->CacheWrite(read_buf_, write_len_, io_callback_,
+                                            this);
+}
+
+int HttpCache::Transaction::DoSharedCacheWriteDataComplete(int result) {
+  TRACE_EVENT0("io", "HttpCacheTransaction::DoSharedCacheWriteDataComplete");
+  if (net_log_.IsCapturing())
+    net_log_.EndEventWithNetErrorCode(
+        NetLogEventType::HTTP_CACHE_SHARED_CACHE_WRITE_DATA_COMPLETE, result);
+
+  OnCacheWriteDataComplete(true, &result);
+
+  if (result == 0) {
+    DoneWritingToEntry(true, false);
+  }
+  return result;
+}
+
+void HttpCache::Transaction::OnCacheWriteDataComplete(bool was_shared,
+                                                      int* result) {
+  if (*result != write_len_) {
+    DoneWritingToEntry(false, !was_shared);
+    // We want to ignore errors writing to disk and just keep reading from
+    // the network.
+    *result = write_len_;
+  } else if (!done_reading_ && entry_ && (!partial_ || truncated_)) {
+    done_reading_ = cache_->IsResponseCompleted(entry_, &response_);
+  }
+}
+
+int HttpCache::Transaction::DoSharedNetworkReadWaitComplete(int result) {
+  TRACE_EVENT0("io", "HttpCacheTransaction::DoSharedNetworkReadWaitComplete");
+  if (net_log_.IsCapturing())
+    net_log_.EndEventWithNetErrorCode(
+        NetLogEventType::HTTP_CACHE_SHARED_NETWORK_READ_WAIT_COMPLETE, result);
+
+  // If its a network read failure or cache write failure, we just return the
+  // result. If its a cache write success, read_buf_ would have been filled
+  // with the read data by SharedWriters.
+  if (result == 0) {                  // response is complete.
+    DoneWritingToEntry(true, false);  // changes the mode_ to NONE.
+  } else if (result > 0) {
+    read_offset_ += result;
+  } else {
+    // Set entry as null so that the destructor does not invoke DoneWithEntry
+    // again as entry_ is already cleaned up by SharedWriters.
+    entry_ = nullptr;
+  }
+
+  return result;
+}
+
 int HttpCache::Transaction::DoCacheReadData() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheReadData");
   if (request_->method == "HEAD")
     return 0;
 
   DCHECK(entry_);
   next_state_ = STATE_CACHE_READ_DATA_COMPLETE;
 
   if (net_log_.IsCapturing())
     net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_READ_DATA);
@@ skipping 41 matching lines @@
   next_state_ = STATE_CACHE_WRITE_DATA_COMPLETE;
   write_len_ = num_bytes;
   if (entry_) {
     if (net_log_.IsCapturing())
       net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_WRITE_DATA);
   }
 
   if (!entry_ || !num_bytes)
     return num_bytes;
 
+  if (partial_) {
+    return partial_->CacheWrite(entry_->disk_entry, read_buf_.get(), num_bytes,
+                                io_callback_);
+  }
+
   int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
-  return WriteToEntry(kResponseContentIndex, current_size, read_buf_.get(),
-                      num_bytes, io_callback_);
+  return entry_->disk_entry->WriteData(kResponseContentIndex, current_size,
+                                       read_buf_.get(), num_bytes, io_callback_,
+                                       true);
 }
 
 int HttpCache::Transaction::DoCacheWriteDataComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteDataComplete");
   if (entry_) {
     if (net_log_.IsCapturing()) {
       net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_WRITE_DATA,
                                         result);
     }
   }
   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_ && (!partial_ || truncated_)) {
-    int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
-    int64_t body_size = response_.headers->GetContentLength();
-    if (body_size >= 0 && body_size <= current_size)
-      done_reading_ = true;
-  }
+  OnCacheWriteDataComplete(false, &result);
 
   if (partial_) {
     // This may be the last request.
     if (result != 0 || truncated_ ||
         !(partial_->IsLastRange() || mode_ == WRITE)) {
       return DoPartialNetworkReadCompleted(result);
     }
   }
 
+  // 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 (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_ ||
-        response_.headers->GetContentLength() <= 0) {
+        response_.headers->GetContentLength() <= 0)
       DoneWritingToEntry(true);
-    }
   }
 
   return result;
 }
 
 int HttpCache::Transaction::DoCacheWriteTruncatedResponse() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteTruncatedResponse");
   next_state_ = STATE_CACHE_WRITE_TRUNCATED_RESPONSE_COMPLETE;
   return WriteResponseInfoToEntry(true);
 }
 
 int HttpCache::Transaction::DoCacheWriteTruncatedResponseComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteTruncatedResponse");
 
   return OnWriteResponseInfoToEntryComplete(result);
 }
 
+int HttpCache::Transaction::DoSharedReadWriteFailed() {
+  if (net_log_.IsCapturing())
+    net_log_.EndEventWithNetErrorCode(
+        NetLogEventType::HTTP_CACHE_SHARED_READ_WRITE_FAILED,
+        shared_read_write_failure_result_);
+
+  return shared_read_write_failure_result_;
+}
+
 //-----------------------------------------------------------------------------
 
 void HttpCache::Transaction::SetRequest(const NetLogWithSource& net_log,
                                         const HttpRequestInfo* request) {
   net_log_ = net_log;
   request_ = request;
   effective_load_flags_ = request_->load_flags;
 
   if (cache_->mode() == DISABLE)
     effective_load_flags_ |= LOAD_DISABLE_CACHE;
@@ skipping 173 matching lines @@
 
   if (partial_ && (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) {
-    UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_USED);
-    return SetupEntryForRead();
+    if (shared_) {
+      UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_JOIN_SHARED_WRITING);
+      entry_->shared_writers->OnValidationMatch(this, priority_);
+    } else {
+      UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_USED);
+      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;
       UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE);
@@ skipping 498 matching lines @@
       IsCertStatusError(response_.ssl_info.cert_status)) {
     DoneWritingToEntry(false);
     if (net_log_.IsCapturing())
       net_log_.EndEvent(NetLogEventType::HTTP_CACHE_WRITE_INFO);
     return OK;
   }
 
   if (truncated)
     DCHECK_EQ(200, response_.headers->response_code());
 
-  // When writing headers, we normally only write the non-transient headers.
-  bool skip_transient_headers = true;
-  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);
+  return cache_->WriteResponseInfo(entry_, &response_, io_callback_, truncated,
+                                   &io_buf_len_);
 }
 
 int HttpCache::Transaction::OnWriteResponseInfoToEntryComplete(int result) {
   if (!entry_)
     return OK;
   if (net_log_.IsCapturing()) {
     net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_WRITE_INFO,
                                       result);
   }
 
   if (result != io_buf_len_) {
     DLOG(ERROR) << "failed to write response info to cache";
     DoneWritingToEntry(false);
   }
   return OK;
 }
 
-void HttpCache::Transaction::DoneWritingToEntry(bool success) {
+void HttpCache::Transaction::DoneWritingToEntry(bool success,
+                                                bool perform_entry_cleanup) {
   if (!entry_)
     return;
 
   RecordHistograms();
 
-  cache_->DoneWritingToEntry(entry_, success);
-  entry_ = NULL;
+  if (perform_entry_cleanup) {
+    cache_->DoneWritingToEntry(entry_, success);
+  }
+  entry_ = nullptr;
   mode_ = NONE;  // switch to 'pass through' mode
 }
 
+void HttpCache::Transaction::ContinueWithoutSharedWriting(
+    std::unique_ptr<HttpTransaction> network_transaction,
+    bool needs_entry) {
+  shared_ = false;
+  if (!needs_entry)
+    entry_ = nullptr;
+  mode_ = NONE;
+  network_trans_ = std::move(network_transaction);
+}
+
 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);
+    if (shared_) {
+      entry_->shared_writers->RemoveValidatingTransaction(this);
+    } else {
+      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::OnAddToEntryTimeout(base::TimeTicks start_time) {
   if (entry_lock_waiting_since_ != start_time)
     return;
 
   DCHECK_EQ(next_state_, STATE_ADD_TO_ENTRY_COMPLETE);
 
   if (!cache_)
     return;
 
   cache_->RemovePendingTransaction(this);
   OnIOComplete(ERR_CACHE_LOCK_TIMEOUT);
 }
 
 void HttpCache::Transaction::DoomPartialEntry(bool delete_object) {
+  // Partial requests not supported with shared writing as of now.
+  DCHECK(!shared_);
   DVLOG(2) << "DoomPartialEntry";
   int rv = cache_->DoomEntry(cache_key_, NULL);
   DCHECK_EQ(OK, rv);
   cache_->DoneWithEntry(entry_, this, false);
   entry_ = NULL;
   is_sparse_ = false;
   truncated_ = false;
   if (delete_object)
     partial_.reset(NULL);
 }
@@ skipping 40 matching lines @@
   if (!delete_object) {
     // The simplest way to re-initialize partial_ is to create a new object.
     partial_.reset(new PartialData());
     if (partial_->Init(request_->extra_headers))
       partial_->SetHeaders(custom_request_->extra_headers);
     else
       partial_.reset();
   }
 }
 
-void HttpCache::Transaction::ResetNetworkTransaction() {
+void HttpCache::Transaction::SaveNetworkTransactionInfo(
+    const HttpTransaction* transaction) {
   DCHECK(!old_network_trans_load_timing_);
-  DCHECK(network_trans_);
+  DCHECK(transaction);
   LoadTimingInfo load_timing;
-  if (network_trans_->GetLoadTimingInfo(&load_timing))
+  if (transaction->GetLoadTimingInfo(&load_timing))
     old_network_trans_load_timing_.reset(new LoadTimingInfo(load_timing));
-  total_received_bytes_ += network_trans_->GetTotalReceivedBytes();
-  total_sent_bytes_ += network_trans_->GetTotalSentBytes();
+  total_received_bytes_ += transaction->GetTotalReceivedBytes();
+  total_sent_bytes_ += transaction->GetTotalSentBytes();
   ConnectionAttempts attempts;
-  network_trans_->GetConnectionAttempts(&attempts);
+  transaction->GetConnectionAttempts(&attempts);
   for (const auto& attempt : attempts)
     old_connection_attempts_.push_back(attempt);
   old_remote_endpoint_ = IPEndPoint();
-  network_trans_->GetRemoteEndpoint(&old_remote_endpoint_);
+  transaction->GetRemoteEndpoint(&old_remote_endpoint_);
+}
+
+void HttpCache::Transaction::SaveSharedNetworkTransactionInfo() {
+  // If network_trans_ is still present, this transaction has not started using
+  // the "shared" network transaction.
+  if (network_trans_) {
+    SaveNetworkTransactionInfo(network_trans_.get());
+    return;
+  }
+
+  DCHECK(!old_network_trans_load_timing_);
+
+  // If the transaction is being deleted while still in the waiting queue,
+  // entry_ is not yet set, do nothing.
+  if (!entry_)
+    return;
+  DCHECK(entry_->shared_writers);
+  HttpTransaction* network_transaction =
+      entry_->shared_writers->network_transaction();
+  SaveNetworkTransactionInfo(network_transaction);
+  have_full_request_headers_ =
+      network_transaction->GetFullRequestHeaders(&full_request_headers_);
+}
+
+void HttpCache::Transaction::ResetNetworkTransaction() {
+  SaveNetworkTransactionInfo(network_trans_.get());
   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;
+HttpTransaction* HttpCache::Transaction::GetCurrentNetworkTransaction() const {
+  if (network_trans_)
+    return network_trans_.get();
+  if (shared_ && entry_ && entry_->shared_writers)
+    return entry_->shared_writers->network_transaction();
+  return nullptr;
 }
 
 void HttpCache::Transaction::SetResponse(const HttpResponseInfo& response) {
   response_ = response;
   SyncCacheEntryStatusToResponse();
 }
 
 void HttpCache::Transaction::SetAuthResponse(
     const HttpResponseInfo& auth_response) {
   auth_response_ = auth_response;
@@ skipping 105 matching lines @@
     }
   }
 
   CACHE_STATUS_HISTOGRAMS("");
 
   if (cache_entry_status_ == CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE) {
     UMA_HISTOGRAM_ENUMERATION("HttpCache.CantConditionalizeCause",
                               validation_cause_, VALIDATION_CAUSE_MAX);
   }
 
+  // This may also exclude shared writing counts for these cases.
   if (cache_entry_status_ == CacheEntryStatus::ENTRY_OTHER)
     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);
 
+  if (cache_entry_status_ == CacheEntryStatus::ENTRY_JOIN_SHARED_WRITING) {
+    UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.JoinSharedWriting", total_time);
+  } else if (cache_entry_status_ ==
+             CacheEntryStatus::ENTRY_DOOM_SHARED_WRITING) {
+    UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.DoomSharedWriting", total_time);
+  } else if (initiate_shared_writing_) {
+    UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.InitiateSharedWriting",
+                        total_time);
+  }
+
   bool did_send_request = !send_request_since_.is_null();
   DCHECK(
       (did_send_request &&
        (cache_entry_status_ == CacheEntryStatus::ENTRY_NOT_IN_CACHE ||
         cache_entry_status_ == CacheEntryStatus::ENTRY_VALIDATED ||
         cache_entry_status_ == CacheEntryStatus::ENTRY_UPDATED ||
-        cache_entry_status_ == CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE)) ||
+        cache_entry_status_ == CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE ||
+        cache_entry_status_ == CacheEntryStatus::ENTRY_JOIN_SHARED_WRITING ||
+        cache_entry_status_ == CacheEntryStatus::ENTRY_DOOM_SHARED_WRITING)) ||
       (!did_send_request &&
-       cache_entry_status_ == CacheEntryStatus::ENTRY_USED));
+       (cache_entry_status_ == CacheEntryStatus::ENTRY_USED ||
+        cache_entry_status_ == CacheEntryStatus::ENTRY_JOIN_SHARED_WRITING)));
 
   if (!did_send_request) {
-    DCHECK(cache_entry_status_ == CacheEntryStatus::ENTRY_USED);
-    UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.Used", total_time);
+    if (cache_entry_status_ == CacheEntryStatus::ENTRY_USED) {
+      UMA_HISTOGRAM_TIMES("HttpCache.AccessToDone.Used", total_time);
+    }
     return;
   }
 
   TimeDelta before_send_time = send_request_since_ - first_cache_access_since_;
   int64_t before_send_percent = (total_time.ToInternalValue() == 0)
                                     ? 0
                                     : before_send_time * 100 / total_time;
   DCHECK_GE(before_send_percent, 0);
   DCHECK_LE(before_send_percent, 100);
   base::HistogramBase::Sample before_send_sample =
       static_cast<base::HistogramBase::Sample>(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_sample);
 
   // TODO(gavinp): Remove or minimize these histograms, particularly the ones
   // below this comment after we have received initial data.
   switch (cache_entry_status_) {
     case CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE: {
       UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.CantConditionalize",
                           before_send_time);
       UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.CantConditionalize",
                                before_send_sample);
+      if (initiate_shared_writing_) {
+        UMA_HISTOGRAM_TIMES(
+            "HttpCache.BeforeSend.CantConditionalize.InitiateSharedWriting",
+            before_send_time);
+        UMA_HISTOGRAM_PERCENTAGE(
+            "HttpCache.PercentBeforeSend.CantConditionalize."
+            "InitiateSharedWriting",
+            before_send_sample);
+      }
+
       break;
     }
     case CacheEntryStatus::ENTRY_NOT_IN_CACHE: {
       UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.NotCached", before_send_time);
       UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.NotCached",
                                before_send_sample);
+      if (initiate_shared_writing_) {
+        UMA_HISTOGRAM_TIMES(
+            "HttpCache.BeforeSend.NotCached.InitiateSharedWriting",
+            before_send_time);
+        UMA_HISTOGRAM_PERCENTAGE(
+            "HttpCache.PercentBeforeSend.NotCached.InitiateSharedWriting",
+            before_send_sample);
+      }
+
       break;
     }
     case CacheEntryStatus::ENTRY_VALIDATED: {
       UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.Validated", before_send_time);
       UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.Validated",
                                before_send_sample);
       break;
     }
     case CacheEntryStatus::ENTRY_UPDATED: {
       UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.Updated", before_send_time);
       UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.Updated",
                                before_send_sample);
+      if (initiate_shared_writing_) {
+        UMA_HISTOGRAM_TIMES(
+            "HttpCache.BeforeSend.Updated.InitiateSharedWriting",
+            before_send_time);
+        UMA_HISTOGRAM_PERCENTAGE(
+            "HttpCache.PercentBeforeSend.Updated.InitiateSharedWriting",
+            before_send_sample);
+      }
+      break;
+    }
+    case CacheEntryStatus::ENTRY_JOIN_SHARED_WRITING: {
+      UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.JoinSharedWriting",
+                          before_send_time);
+      UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.JoinSharedWriting",
+                               before_send_sample);
+      break;
+    }
+    case CacheEntryStatus::ENTRY_DOOM_SHARED_WRITING: {
+      UMA_HISTOGRAM_TIMES("HttpCache.BeforeSend.DoomSharedWriting",
+                          before_send_time);
+      UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.DoomSharedWriting",
+                               before_send_sample);
       break;
     }
     default:
       NOTREACHED();
   }
 }
 
+bool HttpCache::Transaction::IsEligibleForSharedWriting() const {
+  if (!(mode_ & WRITE))
+    return false;
+
+  DCHECK(request_);
+  if (request_->method != "GET")
+    return false;
+
+  if (partial_ || range_requested_)
+    return false;
+
+  if (truncated_)
+    return false;
+
+  return true;
+}
+
+void HttpCache::Transaction::SetSharedWritingFailState(int result) {
+  next_state_ = STATE_SHARED_READ_WRITE_FAILED;
+  shared_read_write_failure_result_ = result;
+  entry_ = nullptr;
+}
+
 void HttpCache::Transaction::OnIOComplete(int result) {
   DoLoop(result);
 }
 
 }  // namespace net