HttpCache::Transaction layer allowing parallel validation

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>
@@ skipping 138 matching lines @@
 
 HttpCache::Transaction::Transaction(RequestPriority priority, HttpCache* cache)
     : next_state_(STATE_NONE),
       request_(NULL),
       priority_(priority),
       cache_(cache->GetWeakPtr()),
       entry_(NULL),
       new_entry_(NULL),
       new_response_(NULL),
       mode_(NONE),
+      original_mode_(NONE),
       reading_(false),
       invalid_range_(false),
       truncated_(false),
       is_sparse_(false),
       range_requested_(false),
       handling_206_(false),
       cache_pending_(false),
       done_reading_(false),
       vary_mismatch_(false),
       couldnt_conditionalize_request_(false),
@@ skipping 20 matching lines @@
 }
 
 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 (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);
     }
   }
 }
 
 int HttpCache::Transaction::WriteMetadata(IOBuffer* buf, int buf_len,
                                           const CompletionCallback& callback) {
   DCHECK(buf);
   DCHECK_GT(buf_len, 0);
@@ skipping 339 matching lines @@
     ConnectionAttempts* out) const {
   ConnectionAttempts new_connection_attempts;
   if (network_trans_)
     network_trans_->GetConnectionAttempts(&new_connection_attempts);
 
   out->swap(new_connection_attempts);
   out->insert(out->begin(), old_connection_attempts_.begin(),
               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;
 }
 
 //-----------------------------------------------------------------------------
 
 // 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
+//   PartialHeadersReceived -> FinishHeaders*
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
 //
 // 2. Cached entry, no validation:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
-//   BeginCacheValidation() -> SetupEntryForRead()
+//   BeginCacheValidation() -> SetupEntryForRead() -> FinishHeaders*
 //
 //   Read():
 //   CacheReadData*
 //
 // 3. Cached entry, validation (304):
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   UpdateCachedResponse -> CacheWriteUpdatedResponse* ->
 //   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
-//   PartialHeadersReceived
+//   PartialHeadersReceived -> FinishHeaders*
 //
 //   Read():
 //   CacheReadData*
 //
 // 4. Cached entry, validation and replace (200):
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   OverwriteCachedResponse -> CacheWriteResponse* -> DoTruncateCachedData* ->
-//   TruncateCachedMetadata* -> PartialHeadersReceived
+//   TruncateCachedMetadata* -> PartialHeadersReceived -> FinishHeaders*
 //
 //   Read():
 //   NetworkRead* -> CacheWriteData*
 //
 // 5. Sparse entry, partially cached, byte range request:
 //   Start():
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheDispatchValidation -> BeginPartialCacheValidation() ->
 //   CacheQueryData* -> ValidateEntryHeadersAndContinue() ->
 //   StartPartialCacheValidation -> CompletePartialCacheValidation ->
 //   BeginCacheValidation() -> SendRequest* -> SuccessfulSendRequest ->
 //   UpdateCachedResponse -> CacheWriteUpdatedResponse* ->
 //   UpdateCachedResponseComplete -> OverwriteCachedResponse ->
-//   PartialHeadersReceived
+//   PartialHeadersReceived -> FinishHeaders*
 //
 //   Read() 1:
 //   NetworkRead* -> CacheWriteData*
 //
 //   Read() 2:
 //   NetworkRead* -> CacheWriteData* -> StartPartialCacheValidation ->
 //   CompletePartialCacheValidation -> CacheReadData* ->
 //
 //   Read() 3:
 //   CacheReadData* -> StartPartialCacheValidation ->
@@ skipping 20 matching lines @@
 //   Read():
 //   CacheReadData (returns 0)
 //
 // 9. HEAD. Cached entry, validation and replace (200):
 //   Pass through. The request dooms the old entry, as a HEAD won't be stored by
 //   itself.
 //   Start():
 //   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
 //   revalidation. Ignore missing ranges when deciding to revalidate. If the
 //   entry requires revalidation, ignore the whole request and go to full pass
 //   through (the result of the HEAD request will NOT update the entry).
 //
 //   Start(): Basically the same as example 7, as we never create a partial_
 //   object for this request.
 //
 // 11. Prefetch, not-cached entry:
 //   The same as example 1. The "unused_since_prefetch" bit is stored as true in
 //   UpdateCachedResponse.
 //
 // 12. Prefetch, cached entry:
 //   Like examples 2-4, only CacheToggleUnusedSincePrefetch* is inserted between
 //   CacheReadResponse* and CacheDispatchValidation if the unused_since_prefetch
 //   bit is unset.
 //
 // 13. Cached entry less than 5 minutes old, unused_since_prefetch is true:
 //   Skip validation, similar to example 2.
 //   GetBackend* -> InitEntry -> OpenEntry* -> AddToEntry* -> CacheReadResponse*
 //   -> CacheToggleUnusedSincePrefetch* -> CacheDispatchValidation ->
 //   BeginPartialCacheValidation() -> BeginCacheValidation() ->
-//   SetupEntryForRead()
+//   SetupEntryForRead() -> FinishHeaders*
 //
 //   Read():
 //   CacheReadData*
 //
 // 14. Cached entry more than 5 minutes old, unused_since_prefetch is true:
 //   Like examples 2-4, only CacheToggleUnusedSincePrefetch* is inserted between
 //   CacheReadResponse* and CacheDispatchValidation.
 int HttpCache::Transaction::DoLoop(int result) {
   DCHECK_NE(STATE_UNSET, next_state_);
   DCHECK_NE(STATE_NONE, next_state_);
   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);
 
     switch (state) {
       case STATE_GET_BACKEND:
         DCHECK_EQ(OK, rv);
         rv = DoGetBackend();
         break;
       case STATE_GET_BACKEND_COMPLETE:
         rv = DoGetBackendComplete(rv);
@@ skipping 116 matching lines @@
         DCHECK_EQ(OK, rv);
         rv = DoPartialHeadersReceived();
         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_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();
         break;
       case STATE_NETWORK_READ_COMPLETE:
         rv = DoNetworkReadComplete(rv);
         break;
       case STATE_CACHE_READ_DATA:
         DCHECK_EQ(OK, rv);
         rv = DoCacheReadData();
@@ skipping 16 matching lines @@
         break;
       default:
         NOTREACHED() << "bad state " << state;
         rv = ERR_FAILED;
         break;
     }
     DCHECK(next_state_ != STATE_UNSET) << "Previous state was " << state;
 
   } 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);
   }
 
   return rv;
 }
 
 int HttpCache::Transaction::DoGetBackend() {
   cache_pending_ = true;
   TransitionToState(STATE_GET_BACKEND_COMPLETE);
   net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_GET_BACKEND);
   return cache_->GetBackendForTransaction(this);
 }
 
 int HttpCache::Transaction::DoGetBackendComplete(int result) {
   DCHECK(result == OK || result == ERR_FAILED);
   net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_GET_BACKEND,
                                     result);
   cache_pending_ = false;
 
   if (!ShouldPassThrough()) {
     cache_key_ = cache_->GenerateCacheKey(request_);
 
     // Requested cache access mode.
     if (effective_load_flags_ & LOAD_ONLY_FROM_CACHE) {
       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;
     } else if (effective_load_flags_ & LOAD_BYPASS_CACHE) {
       mode_ = WRITE;
     } else {
       mode_ = READ_WRITE;
     }
 
     // Downgrade to UPDATE if the request has been externally conditionalized.
@@ skipping 18 matching lines @@
   // transaction behaves the same for GET and HEAD requests at this point: if it
   // was not modified, the entry is updated and a response is not returned from
   // the cache. If we receive 200, it doesn't matter if there was a validation
   // header or not.
   if (request_->method == "HEAD" && mode_ == WRITE)
     mode_ = NONE;
 
   // If must use cache, then we must fail.  This can happen for back/forward
   // navigations to a page generated via a form post.
   if (!(mode_ & READ) && effective_load_flags_ & LOAD_ONLY_FROM_CACHE) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_MISS;
   }
 
   if (mode_ == NONE) {
     if (partial_) {
       partial_->RestoreHeaders(&custom_request_->extra_headers);
       partial_.reset();
     }
     TransitionToState(STATE_SEND_REQUEST);
   } else {
     TransitionToState(STATE_INIT_ENTRY);
   }
 
   // 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;
 }
 
 int HttpCache::Transaction::DoInitEntry() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoInitEntry");
   DCHECK(!new_entry_);
 
   if (!cache_.get()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_UNEXPECTED;
   }
 
   if (mode_ == WRITE) {
     TransitionToState(STATE_DOOM_ENTRY);
     return OK;
   }
 
   TransitionToState(STATE_OPEN_ENTRY);
   return OK;
@@ skipping 16 matching lines @@
   // transaction attached.
   net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_OPEN_ENTRY,
                                     result);
   cache_pending_ = false;
   if (result == OK) {
     TransitionToState(STATE_ADD_TO_ENTRY);
     return OK;
   }
 
   if (result == ERR_CACHE_RACE) {
-    TransitionToState(STATE_INIT_ENTRY);
+    TransitionToState(STATE_HEADERS_PHASE_CANNOT_PROCEED);
     return OK;
   }
 
   if (request_->method == "PUT" || request_->method == "DELETE" ||
       (request_->method == "HEAD" && mode_ == READ_WRITE)) {
     DCHECK(mode_ == READ_WRITE || mode_ == WRITE || request_->method == "HEAD");
     mode_ = NONE;
     TransitionToState(STATE_SEND_REQUEST);
     return OK;
   }
 
   if (mode_ == READ_WRITE) {
     mode_ = WRITE;
     TransitionToState(STATE_CREATE_ENTRY);
     return OK;
   }
   if (mode_ == UPDATE) {
     // There is no cache entry to update; proceed without caching.
     mode_ = NONE;
     TransitionToState(STATE_SEND_REQUEST);
     return OK;
   }
 
   // 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;
 }
 
 int HttpCache::Transaction::DoDoomEntry() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoDoomEntry");
   TransitionToState(STATE_DOOM_ENTRY_COMPLETE);
   cache_pending_ = true;
   if (first_cache_access_since_.is_null())
     first_cache_access_since_ = TimeTicks::Now();
   net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_DOOM_ENTRY);
   return cache_->DoomEntry(cache_key_, this);
 }
 
 int HttpCache::Transaction::DoDoomEntryComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoDoomEntryComplete");
   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;
 }
 
 int HttpCache::Transaction::DoCreateEntry() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCreateEntry");
   DCHECK(!new_entry_);
   TransitionToState(STATE_CREATE_ENTRY_COMPLETE);
   cache_pending_ = true;
   net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_CREATE_ENTRY);
   return cache_->CreateEntry(cache_key_, &new_entry_, this);
 }
 
 int HttpCache::Transaction::DoCreateEntryComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCreateEntryComplete");
   // It is important that we go to STATE_ADD_TO_ENTRY whenever the result is
   // OK, otherwise the cache will end up with an active entry without any
   // transaction attached.
   net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_CREATE_ENTRY,
                                     result);
   cache_pending_ = false;
   switch (result) {
     case OK:
       TransitionToState(STATE_ADD_TO_ENTRY);
       break;
 
     case ERR_CACHE_RACE:
-      TransitionToState(STATE_INIT_ENTRY);
+      TransitionToState(STATE_HEADERS_PHASE_CANNOT_PROCEED);
       break;
 
     default:
       // 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_)
@@ skipping 60 matching lines @@
   DCHECK(new_entry_);
   cache_pending_ = false;
 
   if (result == OK)
     entry_ = new_entry_;
 
   // If there is a failure, the cache should have taken care of new_entry_.
   new_entry_ = NULL;
 
   if (result == ERR_CACHE_RACE) {
-    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;
     }
 
     // The cache is busy, bypass it for this transaction.
     mode_ = NONE;
     TransitionToState(STATE_SEND_REQUEST);
     if (partial_) {
       partial_->RestoreHeaders(&custom_request_->extra_headers);
       partial_.reset();
     }
     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;
   }
 
   if (mode_ == WRITE) {
     if (partial_)
       partial_->RestoreHeaders(&custom_request_->extra_headers);
     TransitionToState(STATE_SEND_REQUEST);
   } else {
     // We have to read the headers from the cached entry.
     DCHECK(mode_ & READ_META);
@@ skipping 18 matching lines @@
 int HttpCache::Transaction::DoCacheReadResponseComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheReadResponseComplete");
   net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_READ_INFO,
                                     result);
   if (result != io_buf_len_ ||
       !HttpCache::ParseResponseInfo(read_buf_->data(), io_buf_len_, &response_,
                                     &truncated_)) {
     return OnCacheReadError(result, true);
   }
 
-  int current_size = entry_->disk_entry->GetDataSize(kResponseContentIndex);
-  int64_t full_response_length = response_.headers->GetContentLength();
+  if (!cache_->IsWritingInProgress(entry_)) {
+    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;
+    // 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;
+    // 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 !=
       !!(request_->load_flags & LOAD_PREFETCH)) {
     // Either this is the first use of an entry since it was prefetched XOR
     // this is a prefetch. The value of response.unused_since_prefetch is
     // valid for this transaction but the bit needs to be flipped in storage.
     TransitionToState(STATE_TOGGLE_UNUSED_SINCE_PREFETCH);
     return OK;
   }
@@ skipping 61 matching lines @@
 }
 
 int HttpCache::Transaction::DoCacheQueryData() {
   TransitionToState(STATE_CACHE_QUERY_DATA_COMPLETE);
   return entry_->disk_entry->ReadyForSparseIO(io_callback_);
 }
 
 int HttpCache::Transaction::DoCacheQueryDataComplete(int result) {
   DCHECK_EQ(OK, result);
   if (!cache_.get()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_UNEXPECTED;
   }
 
   return ValidateEntryHeadersAndContinue();
 }
 
 // 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;
   }
 
   TransitionToState(STATE_COMPLETE_PARTIAL_CACHE_VALIDATION);
   return partial_->ShouldValidateCache(entry_->disk_entry, io_callback_);
 }
 
 int HttpCache::Transaction::DoCompletePartialCacheValidation(int result) {
   if (!result) {
     // This is the end of the request.
     if (mode_ & WRITE) {
       DoneWritingToEntry(true);
     } else {
       cache_->DoneReadingFromEntry(entry_, this);
       entry_ = NULL;
     }
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return result;
   }
 
   if (result < 0) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return result;
   }
 
   partial_->PrepareCacheValidation(entry_->disk_entry,
                                    &custom_request_->extra_headers);
 
   if (reading_ && partial_->IsCurrentRangeCached()) {
     TransitionToState(STATE_CACHE_READ_DATA);
     return OK;
   }
 
   return BeginCacheValidation();
 }
 
 int HttpCache::Transaction::DoSendRequest() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoSendRequest");
   DCHECK(mode_ & WRITE || mode_ == NONE);
   DCHECK(!network_trans_.get());
 
   send_request_since_ = TimeTicks::Now();
 
   // Create a network transaction.
   int rv =
       cache_->network_layer_->CreateTransaction(priority_, &network_trans_);
   if (rv != OK) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return rv;
   }
   network_trans_->SetBeforeNetworkStartCallback(before_network_start_callback_);
   network_trans_->SetBeforeHeadersSentCallback(before_headers_sent_callback_);
 
   // Old load timing information, if any, is now obsolete.
   old_network_trans_load_timing_.reset();
   old_remote_endpoint_ = IPEndPoint();
 
   if (websocket_handshake_stream_base_create_helper_)
     network_trans_->SetWebSocketHandshakeStreamCreateHelper(
         websocket_handshake_stream_base_create_helper_);
 
   TransitionToState(STATE_SEND_REQUEST_COMPLETE);
   rv = network_trans_->Start(request_, io_callback_, net_log_);
   return rv;
 }
 
 int HttpCache::Transaction::DoSendRequestComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoSendRequestComplete");
   if (!cache_.get()) {
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_UNEXPECTED;
   }
 
   // If we tried to conditionalize the request and failed, we know
   // we won't be reading from the cache after this point.
   if (couldnt_conditionalize_request_)
     mode_ = WRITE;
 
   if (result == OK) {
     TransitionToState(STATE_SUCCESSFUL_SEND_REQUEST);
@@ skipping 10 matching lines @@
     // so GetResponseInfo() should never return NULL here.
     DCHECK(response);
     response_.ssl_info = response->ssl_info;
   } else if (result == ERR_SSL_CLIENT_AUTH_CERT_NEEDED) {
     DCHECK(response);
     response_.cert_request_info = response->cert_request_info;
   } else if (response_.was_cached) {
     DoneWritingToEntry(true);
   }
 
-  TransitionToState(STATE_NONE);
+  TransitionToState(STATE_FINISH_HEADERS);
   return result;
 }
 
 // We received the response headers and there is no error.
 int HttpCache::Transaction::DoSuccessfulSendRequest() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoSuccessfulSendRequest");
   DCHECK(!new_response_);
   const HttpResponseInfo* new_response = network_trans_->GetResponseInfo();
 
   if (new_response->headers->response_code() == 401 ||
       new_response->headers->response_code() == 407) {
     SetAuthResponse(*new_response);
     if (!reading_) {
-      TransitionToState(STATE_NONE);
+      TransitionToState(STATE_FINISH_HEADERS);
       return OK;
     }
 
     // We initiated a second request the caller doesn't know about. We should be
     // able to authenticate this request because we should have authenticated
     // this URL moments ago.
     if (IsReadyToRestartForAuth()) {
       DCHECK(!response_.auth_challenge.get());
       TransitionToState(STATE_SEND_REQUEST_COMPLETE);
       // In theory we should check to see if there are new cookies, but there
       // is no way to do that from here.
       return network_trans_->RestartWithAuth(AuthCredentials(), io_callback_);
     }
 
     // We have to perform cleanup at this point so that at least the next
     // request can succeed.  We do not retry at this point, because data
     // has been read and we have no way to gather credentials.  We would
     // fail again, and potentially loop.  This can happen if the credentials
     // expire while chrome is suspended.
     if (entry_)
       DoomPartialEntry(false);
     mode_ = NONE;
     partial_.reset();
     ResetNetworkTransaction();
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return ERR_CACHE_AUTH_FAILURE_AFTER_READ;
   }
 
   new_response_ = new_response;
   if (!ValidatePartialResponse() && !auth_response_.headers.get()) {
     // Something went wrong with this request and we have to restart it.
     // If we have an authentication response, we are exposed to weird things
     // hapenning if the user cancels the authentication before we receive
     // the new response.
     net_log_.AddEvent(NetLogEventType::HTTP_CACHE_RE_SEND_PARTIAL_REQUEST);
@@ skipping 17 matching lines @@
     UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_NOT_IN_CACHE);
   }
 
   // Invalidate any cached GET with a successful PUT or DELETE.
   if (mode_ == WRITE &&
       (request_->method == "PUT" || request_->method == "DELETE")) {
     if (NonErrorResponse(new_response->headers->response_code())) {
       int ret = cache_->DoomEntry(cache_key_, NULL);
       DCHECK_EQ(OK, ret);
     }
-    cache_->DoneWritingToEntry(entry_, true);
+    cache_->DoneWritingToEntry(entry_, true, this);
     entry_ = NULL;
     mode_ = NONE;
   }
 
   // Invalidate any cached GET with a successful POST.
   if (!(effective_load_flags_ & LOAD_DISABLE_CACHE) &&
       request_->method == "POST" &&
       NonErrorResponse(new_response->headers->response_code())) {
     cache_->DoomMainEntryForUrl(request_->url);
   }
 
   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_);
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return OK;
   }
 
   // Are we expecting a response to a conditional query?
   if (mode_ == READ_WRITE || mode_ == UPDATE) {
     if (new_response->headers->response_code() == 304 || handling_206_) {
       UpdateCacheEntryStatus(CacheEntryStatus::ENTRY_VALIDATED);
       TransitionToState(STATE_UPDATE_CACHED_RESPONSE);
       return OK;
     }
@@ skipping 67 matching lines @@
     DCHECK(!handling_206_);
     // We got a "not modified" response and already updated the corresponding
     // cache entry above.
     //
     // By closing the cached entry now, we make sure that the 304 rather than
     // the cached 200 response, is what will be returned to the user.
     DoneWritingToEntry(true);
   } else if (entry_ && !handling_206_) {
     DCHECK_EQ(READ_WRITE, mode_);
     if (!partial_ || partial_->IsLastRange()) {
-      cache_->ConvertWriterToReader(entry_);
       mode_ = READ;
     }
     // We no longer need the network transaction, so destroy it.
     ResetNetworkTransaction();
   } else if (entry_ && handling_206_ && truncated_ &&
              partial_->initial_validation()) {
     // We just finished the validation of a truncated entry, and the server
     // is willing to resume the operation. Now we go back and start serving
     // the first part to the user.
     ResetNetworkTransaction();
@@ skipping 17 matching lines @@
   if (handling_206_ && partial_)
     partial_->FixContentLength(new_response_->headers.get());
 
   SetResponse(*new_response_);
 
   if (request_->method == "HEAD") {
     // This response is replacing the cached one.
     DoneWritingToEntry(false);
     mode_ = NONE;
     new_response_ = NULL;
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
     return OK;
   }
 
   if (handling_206_ && !CanResume(false)) {
     // 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);
     TransitionToState(STATE_PARTIAL_HEADERS_RECEIVED);
     return OK;
   }
 
   TransitionToState(STATE_CACHE_WRITE_RESPONSE);
   return OK;
 }
 
 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_);
 }
 
 int HttpCache::Transaction::DoCacheWriteResponseComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheWriteResponseComplete");
   TransitionToState(STATE_TRUNCATE_CACHED_DATA);
   return OnWriteResponseInfoToEntryComplete(result);
 }
 
 int HttpCache::Transaction::DoTruncateCachedData() {
@@ skipping 41 matching lines @@
   }
 
   TransitionToState(STATE_PARTIAL_HEADERS_RECEIVED);
   return OK;
 }
 
 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;
   }
 
   if (reading_) {
     if (network_trans_.get()) {
       TransitionToState(STATE_NETWORK_READ);
     } else {
       TransitionToState(STATE_CACHE_READ_DATA);
     }
   } else if (mode_ != NONE) {
     // We are about to return the headers for a byte-range request to the user,
     // so let's fix them.
     partial_->FixResponseHeaders(response_.headers.get(), true);
-    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_);
   DCHECK(!response_.metadata.get());
   TransitionToState(STATE_CACHE_READ_METADATA_COMPLETE);
 
   response_.metadata =
       new IOBufferWithSize(entry_->disk_entry->GetDataSize(kMetadataIndex));
 
   net_log_.BeginEvent(NetLogEventType::HTTP_CACHE_READ_INFO);
   return entry_->disk_entry->ReadData(kMetadataIndex, 0,
                                       response_.metadata.get(),
                                       response_.metadata->size(),
                                       io_callback_);
 }
 
 int HttpCache::Transaction::DoCacheReadMetadataComplete(int result) {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoCacheReadMetadataComplete");
   net_log_.EndEventWithNetErrorCode(NetLogEventType::HTTP_CACHE_READ_INFO,
                                     result);
   if (result != response_.metadata->size())
     return OnCacheReadError(result, false);
-  TransitionToState(STATE_NONE);
+
+  TransitionToState(STATE_FINISH_HEADERS);
   return OK;
 }
 
 int HttpCache::Transaction::DoNetworkRead() {
   TRACE_EVENT0("io", "HttpCacheTransaction::DoNetworkRead");
   TransitionToState(STATE_NETWORK_READ_COMPLETE);
   return network_trans_->Read(read_buf_.get(), io_buf_len_, io_callback_);
 }
 
 int HttpCache::Transaction::DoNetworkReadComplete(int result) {
@@ skipping 276 matching lines @@
     return false;
 
   return true;
 }
 
 int HttpCache::Transaction::BeginCacheRead() {
   // We don't support any combination of LOAD_ONLY_FROM_CACHE and byte ranges.
   // 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;
   }
 
   if (request_->method == "HEAD")
     FixHeadersForHead();
 
   if (entry_->disk_entry->GetDataSize(kMetadataIndex))
     TransitionToState(STATE_CACHE_READ_METADATA);
   else
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
 
   return OK;
 }
 
 int HttpCache::Transaction::BeginCacheValidation() {
   DCHECK_EQ(mode_, READ_WRITE);
 
   bool skip_validation = !RequiresValidation();
 
   if (request_->method == "HEAD" &&
@@ skipping 439 matching lines @@
   if (partial_) {
     if (truncated_ || is_sparse_ || !invalid_range_) {
       // We are going to return the saved response headers to the caller, so
       // we may need to adjust them first.
       TransitionToState(STATE_PARTIAL_HEADERS_RECEIVED);
       return OK;
     } else {
       partial_.reset();
     }
   }
-  cache_->ConvertWriterToReader(entry_);
+
   mode_ = READ;
 
   if (request_->method == "HEAD")
     FixHeadersForHead();
 
   if (entry_->disk_entry->GetDataSize(kMetadataIndex))
     TransitionToState(STATE_CACHE_READ_METADATA);
   else
-    TransitionToState(STATE_NONE);
+    TransitionToState(STATE_FINISH_HEADERS);
   return OK;
 }
 
 int HttpCache::Transaction::WriteToEntry(int index, int offset,
                                          IOBuffer* data, int data_len,
                                          const CompletionCallback& callback) {
   if (!entry_)
     return data_len;
 
   int rv = 0;
@@ skipping 58 matching lines @@
   }
   return OK;
 }
 
 void HttpCache::Transaction::DoneWritingToEntry(bool success) {
   if (!entry_)
     return;
 
   RecordHistograms();
 
-  cache_->DoneWritingToEntry(entry_, success);
+  cache_->DoneWritingToEntry(entry_, success, this);
   entry_ = NULL;
   mode_ = NONE;  // switch to 'pass through' mode
 }
 
 int HttpCache::Transaction::OnCacheReadError(int result, bool restart) {
   DLOG(ERROR) << "ReadData failed: " << result;
   const int result_for_histogram = std::max(0, -result);
   if (restart) {
     UMA_HISTOGRAM_SPARSE_SLOWLY("HttpCache.ReadErrorRestartable",
                                 result_for_histogram);
@@ skipping 190 matching lines @@
   bool validation_request =
       cache_entry_status_ == CacheEntryStatus::ENTRY_VALIDATED ||
       cache_entry_status_ == CacheEntryStatus::ENTRY_UPDATED;
 
   bool stale_request =
       validation_cause_ == VALIDATION_CAUSE_STALE &&
       (validation_request ||
        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.

[jkarlin, 2017/04/20 15:05:31]

Can you add a crbug link to the comment?

[shivanisha, 2017/04/21 23:06:01]

done

+
     // 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 =
         (time_since_use * 1000) / stale_entry_freshness_;
 
     if (validation_request) {
       int64_t age_in_freshness_periods =
           (stale_entry_age_ * 100) / stale_entry_freshness_;
       if (cache_entry_status_ == CacheEntryStatus::ENTRY_VALIDATED) {
         UMA_HISTOGRAM_COUNTS("HttpCache.StaleEntry.Validated.Age",
@@ skipping 133 matching lines @@
 }
 
 void HttpCache::Transaction::TransitionToState(State state) {
   // Ensure that the state is only set once per Do* state.
   DCHECK(in_do_loop_);
   DCHECK_EQ(STATE_UNSET, next_state_) << "Next state is " << state;
   next_state_ = state;
 }
 
 }  // namespace net