HttpCache::Transaction layer allowing parallel validation

net/http/http_cache_transaction.cc

Index: net/http/http_cache_transaction.cc
diff --git a/net/http/http_cache_transaction.cc b/net/http/http_cache_transaction.cc
index 518944650747e6a3dc7f39c58b4b4d7cb1610758..48852bf7d33a18522b39afc147f1422f0aac566b 100644
--- a/net/http/http_cache_transaction.cc
+++ b/net/http/http_cache_transaction.cc
@@ -156,6 +156,7 @@ HttpCache::Transaction::Transaction(RequestPriority priority, HttpCache* cache)
       new_entry_(NULL),
       new_response_(NULL),
       mode_(NONE),
+      original_mode_(NONE),
       reading_(false),
       invalid_range_(false),
       truncated_(false),
@@ -196,16 +197,12 @@ HttpCache::Transaction::~Transaction() {
 
   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);
-        }
-      }
+      bool writing_incomplete = cache_->IsTransactionWritingIncomplete(
+          entry_, this, request_->method);
+      if (writing_incomplete && partial_)
+        entry_->disk_entry->CancelSparseIO();
 
-      cache_->DoneWithEntry(entry_, this, cancel_request);
+      cache_->DoneWithEntry(entry_, this, writing_incomplete);
     } else if (cache_pending_) {
       cache_->RemovePendingTransaction(this);
     }
@@ -565,6 +562,12 @@ void HttpCache::Transaction::GetConnectionAttempts(
               old_connection_attempts_.end());
 }
 
+void HttpCache::Transaction::SetValidatingCannotProceed() {
+  DCHECK(!reading_);
+  next_state_ = STATE_HEADERS_PHASE_CANNOT_PROCEED;
+  entry_ = nullptr;
+}
+
 size_t HttpCache::Transaction::EstimateMemoryUsage() const {
   // TODO(xunjieli): Consider improving the coverage. crbug.com/669108.
   return 0;
@@ -579,7 +582,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   GetBackend* -> InitEntry -> OpenEntry* -> CreateEntry* -> AddToEntry* ->
 //   SendRequest* -> SuccessfulSendRequest -> OverwriteCachedResponse ->
 //   CacheWriteResponse* -> TruncateCachedData* -> TruncateCachedMetadata* ->
-//   PartialHeadersReceived
+//   PartialHeadersReceived -> FinishHeaders*
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
@@ -588,7 +591,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
-//   BeginCacheValidation() -> SetupEntryForRead()
+//   BeginCacheValidation() -> SetupEntryForRead() -> FinishHeaders*
 //
 //   Read():
 //   CacheReadData*
@@ -600,7 +603,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   UpdateCachedResponse -> CacheWriteUpdatedResponse* ->
 //   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
-//   PartialHeadersReceived
+//   PartialHeadersReceived -> FinishHeaders*
 //
 //   Read():
 //   CacheReadData*
@@ -611,7 +614,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   OverwriteCachedResponse -> CacheWriteResponse* -> DoTruncateCachedData* ->
-//   TruncateCachedMetadata* -> PartialHeadersReceived
+//   TruncateCachedMetadata* -> PartialHeadersReceived -> FinishHeaders*
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
@@ -625,7 +628,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   UpdateCachedResponse -> CacheWriteUpdatedResponse* ->
 //   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
-//   PartialHeadersReceived
+//   PartialHeadersReceived -> FinishHeaders*
 //
 //   Read() 1:
 //   NetworkRead* -> CacheWriteData*
@@ -666,7 +669,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
-//   OverwriteCachedResponse
+//   OverwriteCachedResponse -> FinishHeaders*
 //
 // 10. HEAD. Sparse entry, partially cached:
 //   Serve the request from the cache, as long as it doesn't require
@@ -691,7 +694,7 @@ size_t HttpCache::Transaction::EstimateMemoryUsage() const {
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheToggleUnusedSincePrefetch* -> CacheDispatchValidation ->
 //   BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SetupEntryForRead()
+//   SetupEntryForRead() -> FinishHeaders*
 //
 //   Read():
 //   CacheReadData*
@@ -705,8 +708,9 @@ int HttpCache::Transaction::DoLoop(int result) {
   DCHECK(!in_do_loop_);
 
   int rv = result;
+  State state = next_state_;
   do {
-    State state = next_state_;
+    state = next_state_;
     next_state_ = STATE_UNSET;
     base::AutoReset<bool> scoped_in_do_loop(&in_do_loop_, true);
 
@@ -843,6 +847,15 @@ int HttpCache::Transaction::DoLoop(int result) {
       case STATE_CACHE_READ_METADATA_COMPLETE:
         rv = DoCacheReadMetadataComplete(rv);
         break;
+      case STATE_HEADERS_PHASE_CANNOT_PROCEED:
+        rv = DoHeadersPhaseCannotProceed();
+        break;
+      case STATE_FINISH_HEADERS:
+        rv = DoFinishHeaders(rv);
+        break;
+      case STATE_FINISH_HEADERS_COMPLETE:
+        rv = DoFinishHeadersComplete(rv);
+        break;
       case STATE_NETWORK_READ:
         DCHECK_EQ(OK, rv);
         rv = DoNetworkRead();
@@ -879,8 +892,13 @@ int HttpCache::Transaction::DoLoop(int result) {
 
   } while (rv != ERR_IO_PENDING && next_state_ != STATE_NONE);
 
+  // Assert Start() state machine's allowed last state in successful cases when
+  // caching is happening.
+  DCHECK(reading_ || rv != OK || !entry_ ||
+         state == STATE_FINISH_HEADERS_COMPLETE);
+
   if (rv != ERR_IO_PENDING && !callback_.is_null()) {
-    read_buf_ = NULL;  // Release the buffer before invoking the callback.
+    read_buf_ = nullptr;  // Release the buffer before invoking the callback.
     base::ResetAndReturn(&callback_).Run(rv);
   }
 
@@ -907,7 +925,7 @@ int HttpCache::Transaction::DoGetBackendComplete(int result) {
     if (effective_load_flags_ & LOAD_ONLY_FROM_CACHE) {
       if (effective_load_flags_ & LOAD_BYPASS_CACHE) {
         // The client has asked for nonsense.
-        TransitionToState(STATE_NONE);
+        TransitionToState(STATE_FINISH_HEADERS);
         return ERR_CACHE_MISS;
       }
       mode_ = READ;
@@ -946,7 +964,7 @@ int HttpCache::Transaction::DoGetBackendComplete(int result) {
   // 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) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_MISS;
   }
 
@@ -963,6 +981,9 @@ int HttpCache::Transaction::DoGetBackendComplete(int result) {
   // This is only set if we have something to do with the response.
   range_requested_ = (partial_.get() != NULL);
 
+  // mode_ may change later, save the initial mode in case we need to restart
+  // this request.
+  original_mode_ = mode_;
   return OK;
 }
 
@@ -971,7 +992,7 @@ int HttpCache::Transaction::DoInitEntry() {
   DCHECK(!new_entry_);
 
   if (!cache_.get()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_UNEXPECTED;
   }
 
@@ -1008,7 +1029,7 @@ int HttpCache::Transaction::DoOpenEntryComplete(int result) {
   }
 
   if (result == ERR_CACHE_RACE) {
-    TransitionToState(STATE_INIT_ENTRY);
+    TransitionToState(STATE_HEADERS_PHASE_CANNOT_PROCEED);
     return OK;
   }
 
@@ -1034,7 +1055,7 @@ int HttpCache::Transaction::DoOpenEntryComplete(int result) {
 
   // The entry does not exist, and we are not permitted to create a new entry,
   // so we must fail.
-  TransitionToState(STATE_NONE);
+  TransitionToState(STATE_FINISH_HEADERS);
   return ERR_CACHE_MISS;
 }
 
@@ -1053,8 +1074,9 @@ int HttpCache::Transaction::DoDoomEntryComplete(int result) {
   net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_DOOM_ENTRY,
                                     result);
   cache_pending_ = false;
-  TransitionToState(result == ERR_CACHE_RACE ? STATE_INIT_ENTRY
-                                             : STATE_CREATE_ENTRY);
+  TransitionToState(result == ERR_CACHE_RACE
+                        ? STATE_HEADERS_PHASE_CANNOT_PROCEED
+                        : STATE_CREATE_ENTRY);
   return OK;
 }
 
@@ -1081,7 +1103,7 @@ int HttpCache::Transaction::DoCreateEntryComplete(int result) {
       break;
 
     case ERR_CACHE_RACE:
-      TransitionToState(STATE_INIT_ENTRY);
+      TransitionToState(STATE_HEADERS_PHASE_CANNOT_PROCEED);
       break;
 
     default:
@@ -1162,13 +1184,13 @@ int HttpCache::Transaction::DoAddToEntryComplete(int result) {
   new_entry_ = NULL;
 
   if (result == ERR_CACHE_RACE) {
-    TransitionToState(STATE_INIT_ENTRY);
+    TransitionToState(STATE_HEADERS_PHASE_CANNOT_PROCEED);
     return OK;
   }
 
   if (result == ERR_CACHE_LOCK_TIMEOUT) {
     if (mode_ == READ) {
-      TransitionToState(STATE_NONE);
+      TransitionToState(STATE_FINISH_HEADERS);
       return ERR_CACHE_MISS;
     }
 
@@ -1182,12 +1204,15 @@ int HttpCache::Transaction::DoAddToEntryComplete(int result) {
     return OK;
   }
 
-  open_entry_last_used_ = entry_->disk_entry->GetLastUsed();
+  // Access timestamp for histograms only if entry is already written, to avoid
+  // data race since cache thread can also access this.
+  if (!cache_->IsWritingInProgress(entry_))
+    open_entry_last_used_ = entry_->disk_entry->GetLastUsed();
 
   // TODO(jkarlin): We should either handle the case or DCHECK.
   if (result != OK) {
     NOTREACHED();
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return result;
   }
 
@@ -1226,29 +1251,31 @@ int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
     return OnCacheReadError(result, true);
   }
 
-  int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
-  int64_t full_response_length = response_.headers->GetContentLength();
-
-  // Some resources may have slipped in as truncated when they're not.
-  if (full_response_length == current_size)
-    truncated_ = false;
-
-  // The state machine's handling of StopCaching unfortunately doesn't deal well
-  // with resources that are larger than 2GB when there is a truncated or sparse
-  // cache entry. While the state machine is reworked to resolve this, the
-  // following logic is put in place to defer such requests to the network. The
-  // cache should not be storing multi gigabyte resources. See
-  // http://crbug.com/89567.
-  if ((truncated_ || response_.headers->response_code() == 206) &&
-      !range_requested_ &&
-      full_response_length > std::numeric_limits<int32_t>::max()) {
-    // Does not release the cache entry. If another transaction wants to use
-    // this cache entry while this transaction is active, the second transaction
-    // will fall back to the network after the timeout.
-    DCHECK(!partial_);
-    mode_ = NONE;
-    TransitionToState(STATE_SEND_REQUEST);
-    return OK;
+  if (!cache_->IsWritingInProgress(entry_)) {

[Randy Smith (Not in Mondays), 2017/04/19 17:26:34]

This scares me.  Does this mean that we are opening up the possibility for a
rare race when we already have a writer and are dealing with a 2GB file &
StopCaching() is called?

I'd feel better just disabling the parallel validation for > 2GB files.  The
comment below suggests that this may be reasonable--the "do not release the
cache entry--if another transaction wants to use the cache entry" would suggest
that the writer is intending to hold on to the cache entry as long as it is
writing.  Can we extend that into your change and just (:-{) disable parallel
validation in this case?

[jkarlin, 2017/04/20 15:05:29]

Needs a comment so that we understand why we can't remove this condition in the
future without careful consideration.

[shivanisha, 2017/04/21 23:06:00]

Talked with asanka@ yesterday about this and here is what I understand, happens:
If a cache entry has a prefix of a >2G response already cached, then we start
getting the rest of the response in 2G chunks via range requests and if
StopCaching is invoked while any of these chunks are being read, then the state
machine returns after the current chunk and does not send a range request for
the next chunk. For this reason if the cached content-length header of the
prefix imply a size > 2G then we pre-emptively send the request to the network.
In this case if I also include dooming the entry here , then no further
transaction will map to this entry and the race would not happen. Added
doomentry here.

Also added a comment here as per feedback.

+    int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
+    int64_t full_response_length = response_.headers->GetContentLength();
+
+    // Some resources may have slipped in as truncated when they're not.
+    if (full_response_length == current_size)
+      truncated_ = false;
+
+    // The state machine's handling of StopCaching unfortunately doesn't deal
+    // well with resources that are larger than 2GB when there is a truncated or
+    // sparse cache entry. While the state machine is reworked to resolve this,
+    // the following logic is put in place to defer such requests to the
+    // network. The cache should not be storing multi gigabyte resources. See
+    // http://crbug.com/89567.
+    if ((truncated_ || response_.headers->response_code() == 206) &&
+        !range_requested_ &&
+        full_response_length > std::numeric_limits<int32_t>::max()) {
+      // Does not release the cache entry. If another transaction wants to use
+      // this cache entry while this transaction is active, the second
+      // transaction will fall back to the network after the timeout.
+      DCHECK(!partial_);
+      mode_ = NONE;
+      TransitionToState(STATE_SEND_REQUEST);
+      return OK;
+    }
   }
 
   if (response_.unused_since_prefetch !=
@@ -1330,7 +1357,7 @@ int HttpCache::Transaction::DoCacheQueryData() {
 int HttpCache::Transaction::DoCacheQueryDataComplete(int result) {
   DCHECK_EQ(OK, result);
   if (!cache_.get()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_UNEXPECTED;
   }
 
@@ -1340,7 +1367,7 @@ int HttpCache::Transaction::DoCacheQueryDataComplete(int result) {
 // We may end up here multiple times for a given request.
 int HttpCache::Transaction::DoStartPartialCacheValidation() {
   if (mode_ == NONE) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return OK;
   }
 
@@ -1357,12 +1384,12 @@ int HttpCache::Transaction::DoCompletePartialCacheValidation(int result) {
       cache_->DoneReadingFromEntry(entry_, this);
       entry_ = NULL;
     }
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return result;
   }
 
   if (result < 0) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return result;
   }
 
@@ -1388,7 +1415,7 @@ int HttpCache::Transaction::DoSendRequest() {
   int rv =
       cache_->network_layer_->CreateTransaction(priority_, &network_trans_);
   if (rv != OK) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return rv;
   }
   network_trans_->SetBeforeNetworkStartCallback(before_network_start_callback_);
@@ -1410,7 +1437,7 @@ int HttpCache::Transaction::DoSendRequest() {
 int HttpCache::Transaction::DoSendRequestComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoSendRequestComplete");
   if (!cache_.get()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_UNEXPECTED;
   }
 
@@ -1441,7 +1468,7 @@ int HttpCache::Transaction::DoSendRequestComplete(int result) {
     DoneWritingToEntry(true);
   }
 
-  TransitionToState(STATE_NONE);
+  TransitionToState(STATE_FINISH_HEADERS);
   return result;
 }
 
@@ -1455,7 +1482,7 @@ int HttpCache::Transaction::DoSuccessfulSendRequest() {
       new_response->headers->response_code() == 407) {
     SetAuthResponse(*new_response);
     if (!reading_) {
-      TransitionToState(STATE_NONE);
+      TransitionToState(STATE_FINISH_HEADERS);
       return OK;
     }
 
@@ -1480,7 +1507,7 @@ int HttpCache::Transaction::DoSuccessfulSendRequest() {
     mode_ = NONE;
     partial_.reset();
     ResetNetworkTransaction();
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_AUTH_FAILURE_AFTER_READ;
   }
 
@@ -1518,7 +1545,7 @@ int HttpCache::Transaction::DoSuccessfulSendRequest() {
       int ret = cache_->DoomEntry(cache_key_, NULL);
       DCHECK_EQ(OK, ret);
     }
-    cache_->DoneWritingToEntry(entry_, true);
+    cache_->DoneWritingToEntry(entry_, true, this);
     entry_ = NULL;
     mode_ = NONE;
   }
@@ -1536,7 +1563,7 @@ int HttpCache::Transaction::DoSuccessfulSendRequest() {
       (request_->method == "GET" || request_->method == "POST")) {
     // If there is an active entry it may be destroyed with this transaction.
     SetResponse(*new_response_);
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return OK;
   }
 
@@ -1624,7 +1651,6 @@ int HttpCache::Transaction::DoUpdateCachedResponseComplete(int result) {
   } else if (entry_ && !handling_206_) {
     DCHECK_EQ(READ_WRITE, mode_);
     if (!partial_ || partial_->IsLastRange()) {
-      cache_->ConvertWriterToReader(entry_);
       mode_ = READ;
     }
     // We no longer need the network transaction, so destroy it.
@@ -1662,7 +1688,7 @@ int HttpCache::Transaction::DoOverwriteCachedResponse() {
     DoneWritingToEntry(false);
     mode_ = NONE;
     new_response_ = NULL;
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return OK;
   }
 
@@ -1683,6 +1709,19 @@ int HttpCache::Transaction::DoOverwriteCachedResponse() {
 int HttpCache::Transaction::DoCacheWriteResponse() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteResponse");
   TransitionToState(STATE_CACHE_WRITE_RESPONSE_COMPLETE);
+
+  // Invalidate any current entry with a successful response if this transaction
+  // cannot write to this entry. This transaction then continues to read from
+  // the network without writing to the backend.
+  if (entry_ && response_.headers &&
+      !cache_->CanTransactionWriteResponseHeaders(
+          entry_, this, response_.headers->response_code())) {
+    cache_->DoneWritingToEntry(entry_, false, this);
+    entry_ = nullptr;
+    mode_ = NONE;
+    return OK;
+  }
+
   return WriteResponseInfoToEntry(truncated_);
 }
 
@@ -1744,10 +1783,12 @@ int HttpCache::Transaction::DoPartialHeadersReceived() {
   new_response_ = NULL;
 
   if (!partial_) {
-    if (entry_ && entry_->disk_entry->GetDataSize(kMetadataIndex))
+    if (entry_ && entry_->disk_entry->GetDataSize(kMetadataIndex)) {
       TransitionToState(STATE_CACHE_READ_METADATA);
-    else
-      TransitionToState(STATE_NONE);
+    } else {
+      DCHECK(!reading_);
+      TransitionToState(STATE_FINISH_HEADERS);
+    }
     return OK;
   }
 
@@ -1761,13 +1802,63 @@ int HttpCache::Transaction::DoPartialHeadersReceived() {
     // We are about to return the headers for a byte-range request to the user,
     // so let's fix them.
     partial_->FixResponseHeaders(response_.headers.get(), true);
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
   } else {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
   }
   return OK;
 }
 
+int HttpCache::Transaction::DoHeadersPhaseCannotProceed() {
+  // If its the Start state machine and it cannot proceed due to a cache
+  // failure, restart this transaction.
+  DCHECK(!reading_);
+  TransitionToState(STATE_INIT_ENTRY);
+  cache_entry_status_ = CacheEntryStatus::ENTRY_UNDEFINED;
+  entry_ = nullptr;
+  mode_ = original_mode_;
+  if (network_trans_)
+    network_trans_.reset();
+  return OK;
+}
+
+int HttpCache::Transaction::DoFinishHeaders(int result) {
+  if (!entry_ || result != OK) {
+    TransitionToState(STATE_NONE);
+    return result;
+  }
+
+  TransitionToState(STATE_FINISH_HEADERS_COMPLETE);
+
+  // If it was an auth failure or 416, this transaction should continue to be
+  // headers_transaction till consumer takes an action, so no need to do
+  // anything now.
+  if (auth_response_.headers.get() ||
+      (new_response_ && new_response_->headers &&
+       new_response_->headers->response_code() == 416))
+    return OK;
+
+  // If there is no response body to be written or read, it does not need to
+  // wait.
+  if (request_->method == "HEAD")
+    return OK;
+
+  // If the transaction needs to wait because another transaction is still
+  // writing the response body, it will return ERR_IO_PENDING now and the
+  // io_callback_ will be invoked when the wait is done.
+  return cache_->DoneWithResponseHeaders(entry_, this);
+}
+
+int HttpCache::Transaction::DoFinishHeadersComplete(int rv) {
+  if (rv == ERR_CACHE_RACE) {
+    TransitionToState(STATE_HEADERS_PHASE_CANNOT_PROCEED);
+    return OK;
+  }
+
+  TransitionToState(STATE_NONE);
+  return rv;
+}
+
 int HttpCache::Transaction::DoCacheReadMetadata() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheReadMetadata");
   DCHECK(entry_);
@@ -1790,7 +1881,8 @@ int HttpCache::Transaction::DoCacheReadMetadataComplete(int result) {
                                     result);
   if (result != response_.metadata->size())
     return OnCacheReadError(result, false);
-  TransitionToState(STATE_NONE);
+
+  TransitionToState(STATE_FINISH_HEADERS);
   return OK;
 }
 
@@ -2087,18 +2179,18 @@ int HttpCache::Transaction::BeginCacheRead() {
   // TODO(jkarlin): Either handle this case or DCHECK.
   if (response_.headers->response_code() == 206 || partial_) {
     NOTREACHED();
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_MISS;
   }
 
   // We don't have the whole resource.
   if (truncated_) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_MISS;
   }
 
   if (RequiresValidation()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_MISS;
   }
 
@@ -2108,7 +2200,7 @@ int HttpCache::Transaction::BeginCacheRead() {
   if (entry_->disk_entry->GetDataSize(kMetadataIndex))
     TransitionToState(STATE_CACHE_READ_METADATA);
   else
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
 
   return OK;
 }
@@ -2568,7 +2660,7 @@ int HttpCache::Transaction::SetupEntryForRead() {
       partial_.reset();
     }
   }
-  cache_->ConvertWriterToReader(entry_);
+
   mode_ = READ;
 
   if (request_->method == "HEAD")
@@ -2577,7 +2669,7 @@ int HttpCache::Transaction::SetupEntryForRead() {
   if (entry_->disk_entry->GetDataSize(kMetadataIndex))
     TransitionToState(STATE_CACHE_READ_METADATA);
   else
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
   return OK;
 }
 
@@ -2656,7 +2748,7 @@ void HttpCache::Transaction::DoneWritingToEntry(bool success) {
 
   RecordHistograms();
 
-  cache_->DoneWritingToEntry(entry_, success);
+  cache_->DoneWritingToEntry(entry_, success, this);
   entry_ = NULL;
   mode_ = NONE;  // switch to 'pass through' mode
 }
@@ -2867,10 +2959,14 @@ void HttpCache::Transaction::RecordHistograms() {
        cache_entry_status_ == CacheEntryStatus::ENTRY_CANT_CONDITIONALIZE);
   int64_t freshness_periods_since_last_used = 0;
 
-  if (stale_request) {
+  if (stale_request && !open_entry_last_used_.is_null()) {
+    // Note that we are not able to capture those transactions' histograms which
+    // when added to entry the response was being written by another transaction
+    // because getting the last used timestamp might lead to a data race in that
+    // case.
+
     // For stale entries, record how many freshness periods have elapsed since
     // the entry was last used.
-    DCHECK(!open_entry_last_used_.is_null());
     DCHECK(!stale_entry_freshness_.is_zero());
     base::TimeDelta time_since_use = base::Time::Now() - open_entry_last_used_;
     freshness_periods_since_last_used =