HttpCache::Transaction layer allowing parallel validation

net/http/http_cache.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.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 78 matching lines @@
                                         true,
                                         thread_,
                                         net_log,
                                         backend,
                                         callback);
 }
 
 //-----------------------------------------------------------------------------
 
 HttpCache::ActiveEntry::ActiveEntry(disk_cache::Entry* entry)
-    : disk_entry(entry),
-      writer(NULL),
-      will_process_pending_queue(false),
-      doomed(false) {
-}
+    : disk_entry(entry) {}
 
 HttpCache::ActiveEntry::~ActiveEntry() {
   if (disk_entry) {
     disk_entry->Close();
-    disk_entry = NULL;
+    disk_entry = nullptr;
   }
 }
 
 size_t HttpCache::ActiveEntry::EstimateMemoryUsage() const {
   // Skip |disk_entry| which is tracked in simple_backend_impl; Skip |readers|
-  // and |pending_queue| because the Transactions are owned by their respective
-  // URLRequestHttpJobs.
+  // and |add_to_entry_queue| because the Transactions are owned by their
+  // respective URLRequestHttpJobs.
   return 0;
 }
 
 bool HttpCache::ActiveEntry::HasNoTransactions() {
-  return !writer && readers.empty() && pending_queue.empty();
+  return !writer && readers.empty() && add_to_entry_queue.empty() &&
+         done_headers_queue.empty() && !headers_transaction;
+}
+
+bool HttpCache::ActiveEntry::HasNoActiveTransactions() {
+  return !writer && readers.empty() && !headers_transaction;
 }
 
 //-----------------------------------------------------------------------------
 
 // This structure keeps track of work items that are attempting to create or
 // open cache entries or the backend itself.
 struct HttpCache::PendingOp {
   PendingOp() : disk_entry(NULL) {}
   ~PendingOp() {}
 
@@ skipping 222 matching lines @@
     }
   }
 }
 
 HttpCache::~HttpCache() {
   // Transactions should see an invalid cache after this point; otherwise they
   // could see an inconsistent object (half destroyed).
   weak_factory_.InvalidateWeakPtrs();
 
   // If we have any active entries remaining, then we need to deactivate them.
-  // We may have some pending calls to OnProcessPendingQueue, but since those
-  // won't run (due to our destruction), we can simply ignore the corresponding
-  // will_process_pending_queue flag.
+  // We may have some pending tasks to process queued transactions ,but since
+  // those won't run (due to our destruction), we can simply ignore the
+  // corresponding flags.
   while (!active_entries_.empty()) {
     ActiveEntry* entry = active_entries_.begin()->second.get();
-    entry->will_process_pending_queue = false;
-    entry->pending_queue.clear();
+    entry->will_process_queued_transactions = false;
+    entry->add_to_entry_queue.clear();
     entry->readers.clear();
-    entry->writer = NULL;
+    entry->done_headers_queue.clear();
+    entry->headers_transaction = nullptr;
+    entry->writer = nullptr;
     DeactivateEntry(entry);
   }
 
   doomed_entries_.clear();
 
   // Before deleting pending_ops_, we have to make sure that the disk cache is
   // done with said operations, or it will attempt to use deleted data.
   disk_cache_.reset();
 
   for (auto pending_it = pending_ops_.begin(); pending_it != pending_ops_.end();
@@ skipping 239 matching lines @@
   // We keep track of doomed entries so that we can ensure that they are
   // cleaned up properly when the cache is destroyed.
   ActiveEntry* entry_ptr = entry.get();
   DCHECK_EQ(0u, doomed_entries_.count(entry_ptr));
   doomed_entries_[entry_ptr] = std::move(entry);
 
   entry_ptr->disk_entry->Doom();
   entry_ptr->doomed = true;
 
   DCHECK(entry_ptr->writer || !entry_ptr->readers.empty() ||
-         entry_ptr->will_process_pending_queue);
+         entry_ptr->headers_transaction ||
+         entry_ptr->will_process_queued_transactions);
   return OK;
 }
 
 int HttpCache::AsyncDoomEntry(const std::string& key, Transaction* trans) {
   std::unique_ptr<WorkItem> item =
       base::MakeUnique<WorkItem>(WI_DOOM_ENTRY, trans, nullptr);
   PendingOp* pending_op = GetPendingOp(key);
   if (pending_op->writer) {
     pending_op->pending_queue.push_back(std::move(item));
     return ERR_IO_PENDING;
@@ skipping 35 matching lines @@
   DCHECK(entry->doomed);
   DCHECK(entry->HasNoTransactions());
 
   auto it = doomed_entries_.find(entry);
   DCHECK(it != doomed_entries_.end());
   doomed_entries_.erase(it);
 }
 
 HttpCache::ActiveEntry* HttpCache::FindActiveEntry(const std::string& key) {
   auto it = active_entries_.find(key);
-  return it != active_entries_.end() ? it->second.get() : NULL;
+  return it != active_entries_.end() ? it->second.get() : nullptr;
 }
 
 HttpCache::ActiveEntry* HttpCache::ActivateEntry(
     disk_cache::Entry* disk_entry) {
   DCHECK(!FindActiveEntry(disk_entry->GetKey()));
   ActiveEntry* entry = new ActiveEntry(disk_entry);
   active_entries_[disk_entry->GetKey()] = base::WrapUnique(entry);
   return entry;
 }
 
 void HttpCache::DeactivateEntry(ActiveEntry* entry) {
-  DCHECK(!entry->will_process_pending_queue);
+  DCHECK(!entry->will_process_queued_transactions);
   DCHECK(!entry->doomed);
   DCHECK(entry->disk_entry);
   DCHECK(entry->HasNoTransactions());
 
   std::string key = entry->disk_entry->GetKey();
   if (key.empty())
     return SlowDeactivateEntry(entry);
 
   auto it = active_entries_.find(key);
   DCHECK(it != active_entries_.end());
@@ skipping 109 matching lines @@
 }
 
 void HttpCache::DestroyEntry(ActiveEntry* entry) {
   if (entry->doomed) {
     FinalizeDoomedEntry(entry);
   } else {
     DeactivateEntry(entry);
   }
 }
 
-int HttpCache::AddTransactionToEntry(ActiveEntry* entry, Transaction* trans) {
+int HttpCache::AddTransactionToEntry(ActiveEntry* entry,
+                                     Transaction* transaction) {
   DCHECK(entry);
   DCHECK(entry->disk_entry);
-
-  // We implement a basic reader/writer lock for the disk cache entry.  If
-  // there is already a writer, then everyone has to wait for the writer to
-  // finish before they can access the cache entry.  There can be multiple
-  // readers.
-  //
-  // NOTE: If the transaction can only write, then the entry should not be in
-  // use (since any existing entry should have already been doomed).
-
-  if (entry->writer || entry->will_process_pending_queue) {
-    entry->pending_queue.push_back(trans);
-    return ERR_IO_PENDING;
-  }
-
-  if (trans->mode() & Transaction::WRITE) {
-    // transaction needs exclusive access to the entry
-    if (entry->readers.empty()) {
-      entry->writer = trans;
-    } else {
-      entry->pending_queue.push_back(trans);
-      return ERR_IO_PENDING;
-    }
+  // Always add a new transaction to the queue to maintain FIFO order.
+  entry->add_to_entry_queue.push_back(transaction);
+  ProcessQueuedTransactions(entry);
+  return ERR_IO_PENDING;
+}
+
+int HttpCache::DoneWithResponseHeaders(ActiveEntry* entry,
+                                       Transaction* transaction,
+                                       bool is_match) {
+  // If |transaction| is the current writer, do nothing. This can happen for
+  // range requests since they can go back to headers phase after starting to
+  // write.
+  if (entry->writer == transaction)
+    return OK;
+
+  // Proceed only if |transaction| is a headers_transaction and not already a
+  // reader.
+  if (entry->headers_transaction != transaction)
+    return OK;
+
+  entry->headers_transaction = nullptr;
+
+  if (!is_match) {
+    // Doom or destroy this entry based on whether it has active consumers.
+    ProcessEntryFailure(entry);
+    return OK;
+  }
+
+  entry->done_headers_queue.push_back(transaction);
+  ProcessQueuedTransactions(entry);
+  return ERR_IO_PENDING;
+}
+
+void HttpCache::RemoveAllQueuedTransactions(ActiveEntry* entry,
+                                            TransactionList* list) {
+  // Process done_headers_queue before add_to_entry_queue to maintain FIFO
+  // order.
+  for (auto* transaction : entry->done_headers_queue)
+    list->push_back(transaction);
+  entry->done_headers_queue.clear();
+
+  for (auto* transaction : entry->add_to_entry_queue)
+    list->push_back(transaction);
+  entry->add_to_entry_queue.clear();
+}
+
+void HttpCache::ProcessEntryFailure(ActiveEntry* entry) {
+  // Failure case is either writer failing to completely write the response to
+  // the cache or validating transaction received a non-304 response.
+  TransactionList list;
+  if (entry->HasNoActiveTransactions() &&
+      !entry->will_process_queued_transactions) {
+    entry->disk_entry->Doom();
+    RemoveAllQueuedTransactions(entry, &list);
+    DestroyEntry(entry);
   } else {
-    // transaction needs read access to the entry
-    entry->readers.insert(trans);
-  }
-
-  // We do this before calling EntryAvailable to force any further calls to
-  // AddTransactionToEntry to add their transaction to the pending queue, which
-  // ensures FIFO ordering.
-  if (!entry->writer && !entry->pending_queue.empty())
-    ProcessPendingQueue(entry);
-
-  return OK;
-}
-
-void HttpCache::DoneWithEntry(ActiveEntry* entry, Transaction* trans,
+    DoomActiveEntry(entry->disk_entry->GetKey());
+    RemoveAllQueuedTransactions(entry, &list);
+  }
+  // ERR_CACHE_RACE causes the transaction to restart the whole process.
+  for (auto* transaction : list)
+    transaction->io_callback().Run(net::ERR_CACHE_RACE);
+}
+
+void HttpCache::ProcessQueuedTransactions(ActiveEntry* entry) {
+  // Multiple readers may finish with an entry at once, so we want to batch up
+  // calls to OnProcessQueuedTransactions. This flag also tells us that we
+  // should not delete the entry before OnProcessQueuedTransactions runs.
+  if (entry->will_process_queued_transactions)
+    return;
+
+  entry->will_process_queued_transactions = true;
+
+  // Post a task instead of invoking the io callback of another transaction here
+  // to avoid re-entrancy.
+  base::ThreadTaskRunnerHandle::Get()->PostTask(
+      FROM_HERE,
+      base::Bind(&HttpCache::OnProcessQueuedTransactions, GetWeakPtr(), entry));
+}
+
+void HttpCache::OnProcessQueuedTransactions(ActiveEntry* entry) {
+  entry->will_process_queued_transactions = false;
+
+  // If no one is interested in this entry, then we can deactivate it.
+  if (entry->HasNoTransactions()) {
+    DestroyEntry(entry);
+    return;
+  }
+
+  if (entry->done_headers_queue.empty() && entry->add_to_entry_queue.empty())
+    return;
+
+  // To maintain FIFO order of transactions, done_headers_queue should be
+  // processed first, and then add_to_entry_queue.
+  // If another transaction is writing the response, let validated transactions
+  // wait till the response is complete. If the response is not yet started, the
+  // done_headers_queue transaction should start writing it.
+  if (!entry->writer && !entry->done_headers_queue.empty()) {
+    ProcessDoneHeadersQueue(entry);
+    return;
+  }
+
+  if (!entry->add_to_entry_queue.empty())
+    ProcessAddToEntryQueue(entry);
+}
+
+void HttpCache::ProcessAddToEntryQueue(ActiveEntry* entry) {
+  if (entry->headers_transaction) {
+    // Note the entry may be new or may already have a response body written to
+    // it. In both cases, a transaction with write mode set, needs to wait
+    // because only one transaction is allowed to be validating at a time.
+    // If transaction is read-only and entry is not yet complete, then it
+    // needs to wait. If entry is complete, it needs to wait because its
+    // possible that the headers_transaction dooms the entry if its not a match.
+    return;
+  }
+
+  Transaction* transaction = entry->add_to_entry_queue.front();
+
+  // If a writer is present, read-only transactions cannot proceed.
+  if (entry->writer && !(transaction->mode() & Transaction::WRITE))
+    return;
+
+  entry->add_to_entry_queue.erase(entry->add_to_entry_queue.begin());
+
+  // This state may be reached either during the start of an entry or when the
+  // response is written or being written.
+  if (transaction->mode() & Transaction::WRITE) {
+    entry->headers_transaction = transaction;
+  } else {
+    // A read-only transaction can start reading the response.
+    // If a read transaction is here, then it has to be an already created
+    // entry as the other case is already handled in the Transaction state
+    // machine.
+    entry->readers.insert(transaction);
+    transaction->ConvertToReadMode();

[Randy Smith (Not in Mondays), 2017/03/31 19:26:12]

What transition in the transaction mode is actually happening here? 
Specifically, looking at the definition of HC::T::Mode, I see three bits: 0 is
READ_META, 1 is READ_DATA, and 2 is WIRTE.  Here and below we're only converting
if WRITE isn't set, and READ == READ_META | READ_DATA.  So it seems as if the
implication of ConvertToReadMode() is wrong; either we aren't changing the mode
at all, or we're upgrading it (we had zero or one of READ_META & READ_DATA, and
we've moving to having both).  Is that correct?  Is there a better name for this
function if it is?

[shivanisha, 2017/04/03 20:16:48]

Thanks for pointing this out. We are actually converting the mode_ to READ and
resetting network transaction in all the places where validation was a match
before we come here. So we do not need this function at all. This logic was a
remnant from the initial patches where there was a conditional
ConvertWriterToReader.

+
+    // More readers can join too.
+    ProcessQueuedTransactions(entry);
+  }
+
+  transaction->io_callback().Run(OK);
+}
+
+void HttpCache::ProcessDoneHeadersQueue(ActiveEntry* entry) {
+  DCHECK(!entry->writer && !entry->done_headers_queue.empty());
+  Transaction* transaction = entry->done_headers_queue.front();
+  // If this transaction is responsible for writing the response body.
+  if (transaction->mode() & Transaction::WRITE) {
+    entry->writer = transaction;
+  } else {
+    // Response body is already written, convert to a reader.
+    auto return_val = entry->readers.insert(transaction);
+    DCHECK_EQ(return_val.second, true);
+    transaction->ConvertToReadMode();
+  }
+  // Post another task to give a chance to more transactions to either join
+  // readers or another transaction to start parallel validation.
+  ProcessQueuedTransactions(entry);
+  entry->done_headers_queue.erase(entry->done_headers_queue.begin());
+  transaction->io_callback().Run(OK);
+}
+
+bool HttpCache::IsTransactionCurrentOrFutureWriter(
+    ActiveEntry* entry,
+    Transaction* transaction,
+    const std::string& method) const {
+  if (method == "HEAD" || method == "DELETE")
+    return false;
+
+  if (transaction == entry->writer)
+    return true;
+
+  // Check if transaction was about to start writing to the cache.
+
+  // Transaction's mode may have been set to NONE if StopCaching was invoked.
+  if (!(transaction->mode() & Transaction::WRITE ||
+        transaction->mode() == Transaction::NONE))
+    return false;
+
+  if (transaction == entry->headers_transaction)
+    return !entry->writer;
+
+  if (transaction == entry->done_headers_queue.front())

[Randy Smith (Not in Mondays), 2017/03/31 19:26:12]

Why is the check against the front of the done_headers queue rather than for any
place in the queue?

[shivanisha, 2017/04/03 20:16:48]

The invariant is that the first transaction that created the entry will go from
headers_transaction -> done_headers_queue -> writer and will write the response.
Any later transactions added to done_headers_queue should actually be a result
of validation match and thus have mode as READ. Added a comment to that effect.

[Randy Smith (Not in Mondays), 2017/04/04 21:21:16]

So does that mean that if the writer completes and we have only readers, this
function might return true for a late arriving reader?  

[shivanisha, 2017/04/05 15:00:37]

The check on mode takes care of a reader always returning false. Also, the
latest patch updates the transaction == entry->headers_transaction condition to
return !entry->writer && entry->done_headers_queue.empty();

+    return !entry->writer;
+
+  return false;
+}
+
+void HttpCache::DoneWithEntry(ActiveEntry* entry,
+                              Transaction* transaction,
                               bool cancel) {
-  // If we already posted a task to move on to the next transaction and this was
-  // the writer, there is nothing to cancel.
-  if (entry->will_process_pending_queue && entry->readers.empty())
-    return;
-
-  if (entry->writer) {
-    DCHECK(trans == entry->writer);
-
+  // Transaction is waiting in the done_headers_queue.
+  auto i = std::find(entry->done_headers_queue.begin(),
+                     entry->done_headers_queue.end(), transaction);
+  if (i != entry->done_headers_queue.end()) {
+    entry->done_headers_queue.erase(i);
+    if (cancel)
+      ProcessEntryFailure(entry);
+    return;
+  }
+
+  // Transaction is removed in the headers phase.
+  if (transaction == entry->headers_transaction) {
+    // If the response is not written (cancel is true), consider it a failure.
+    DoneWritingToEntry(entry, !cancel, transaction);
+    return;
+  }
+
+  // Transaction is removed in the writing phase.
+  if (transaction == entry->writer) {
     // Assume there was a failure.
     bool success = false;
     if (cancel) {
       DCHECK(entry->disk_entry);
       // This is a successful operation in the sense that we want to keep the
       // entry.
-      success = trans->AddTruncatedFlag();
+      success = transaction->AddTruncatedFlag();
       // The previous operation may have deleted the entry.
-      if (!trans->entry())
+      if (!transaction->entry())
         return;
     }
-    DoneWritingToEntry(entry, success);
-  } else {
-    DoneReadingFromEntry(entry, trans);
-  }
-}
-
-void HttpCache::DoneWritingToEntry(ActiveEntry* entry, bool success) {
-  DCHECK(entry->readers.empty());
-
-  entry->writer = NULL;
-
-  if (success) {
-    ProcessPendingQueue(entry);
-  } else {
-    DCHECK(!entry->will_process_pending_queue);
-
-    // We failed to create this entry.
-    TransactionList pending_queue;
-    pending_queue.swap(entry->pending_queue);
-
-    entry->disk_entry->Doom();
-    DestroyEntry(entry);
-
-    // We need to do something about these pending entries, which now need to
-    // be added to a new entry.
-    while (!pending_queue.empty()) {
-      // ERR_CACHE_RACE causes the transaction to restart the whole process.
-      pending_queue.front()->io_callback().Run(ERR_CACHE_RACE);
-      pending_queue.pop_front();
-    }
-  }
-}
-
-void HttpCache::DoneReadingFromEntry(ActiveEntry* entry, Transaction* trans) {
+    DoneWritingToEntry(entry, success, transaction);
+    return;
+  }
+
+  // Transaction is reading from the entry.
+  DoneReadingFromEntry(entry, transaction);
+}
+
+void HttpCache::DoneWritingToEntry(ActiveEntry* entry,
+                                   bool success,
+                                   Transaction* transaction) {
+  DCHECK(transaction == entry->writer ||
+         transaction == entry->headers_transaction);
+
+  // Transaction is done writing to entry in the response body phase.

[jkarlin, 2017/04/03 14:32:48]

Does this comment only apply to the transaction == entry->writer case? If so,
please put the comment inside that. Right now it also looks like it applies to
the else.

[shivanisha, 2017/04/03 20:16:48]

Removed the comment since it is clear from the condition.

+  if (transaction == entry->writer)
+    entry->writer = nullptr;
+  else
+    entry->headers_transaction = nullptr;
+
+  // If writer fails, all transactions should be restarted.
+  if (!success && entry->headers_transaction) {
+    entry->headers_transaction->SetValidatingCannotProceed();
+    entry->headers_transaction = nullptr;
+    DCHECK(entry->HasNoActiveTransactions());
+  }
+  if (!success)
+    ProcessEntryFailure(entry);
+  else
+    ProcessQueuedTransactions(entry);
+}
+
+void HttpCache::DoneReadingFromEntry(ActiveEntry* entry,
+                                     Transaction* transaction) {
   DCHECK(!entry->writer);
-
-  auto it = entry->readers.find(trans);
+  auto it = entry->readers.find(transaction);
   DCHECK(it != entry->readers.end());
-
   entry->readers.erase(it);
 
-  ProcessPendingQueue(entry);
-}
-
-void HttpCache::ConvertWriterToReader(ActiveEntry* entry) {
-  DCHECK(entry->writer);
-  DCHECK(entry->writer->mode() == Transaction::READ_WRITE);
-  DCHECK(entry->readers.empty());
-
-  Transaction* trans = entry->writer;
-
-  entry->writer = NULL;
-  entry->readers.insert(trans);
-
-  ProcessPendingQueue(entry);
+  ProcessQueuedTransactions(entry);
 }
 
 LoadState HttpCache::GetLoadStateForPendingTransaction(
       const Transaction* trans) {
   auto i = active_entries_.find(trans->key());
   if (i == active_entries_.end()) {
     // If this is really a pending transaction, and it is not part of
     // active_entries_, we should be creating the backend or the entry.
     return LOAD_STATE_WAITING_FOR_CACHE;
   }
@@ skipping 29 matching lines @@
 
   for (auto k = doomed_entries_.begin(); k != doomed_entries_.end() && !found;
        ++k) {
     found = RemovePendingTransactionFromEntry(k->first, trans);
   }
 
   DCHECK(found) << "Pending transaction not found";
 }
 
 bool HttpCache::RemovePendingTransactionFromEntry(ActiveEntry* entry,
-                                                  Transaction* trans) {
-  TransactionList& pending_queue = entry->pending_queue;
+                                                  Transaction* transaction) {
+  TransactionList& add_to_entry_queue = entry->add_to_entry_queue;
 
-  auto j = find(pending_queue.begin(), pending_queue.end(), trans);
-  if (j == pending_queue.end())
+  auto j =
+      find(add_to_entry_queue.begin(), add_to_entry_queue.end(), transaction);
+  if (j == add_to_entry_queue.end())
     return false;
 
-  pending_queue.erase(j);
+  add_to_entry_queue.erase(j);
   return true;
 }
 
 bool HttpCache::RemovePendingTransactionFromPendingOp(PendingOp* pending_op,
                                                       Transaction* trans) {
   if (pending_op->writer->Matches(trans)) {
     pending_op->writer->ClearTransaction();
     pending_op->writer->ClearEntry();
     return true;
   }
   WorkItemList& pending_queue = pending_op->pending_queue;
 
   for (auto it = pending_queue.begin(); it != pending_queue.end(); ++it) {
     if ((*it)->Matches(trans)) {
       pending_queue.erase(it);
       return true;
     }
   }
   return false;
 }
 
-void HttpCache::ProcessPendingQueue(ActiveEntry* entry) {
-  // Multiple readers may finish with an entry at once, so we want to batch up
-  // calls to OnProcessPendingQueue.  This flag also tells us that we should
-  // not delete the entry before OnProcessPendingQueue runs.
-  if (entry->will_process_pending_queue)
-    return;
-  entry->will_process_pending_queue = true;
-
-  base::ThreadTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE,
-      base::Bind(&HttpCache::OnProcessPendingQueue, GetWeakPtr(), entry));
-}
-
-void HttpCache::OnProcessPendingQueue(ActiveEntry* entry) {
-  entry->will_process_pending_queue = false;
-  DCHECK(!entry->writer);
-
-  // If no one is interested in this entry, then we can deactivate it.
-  if (entry->HasNoTransactions()) {
-    DestroyEntry(entry);
-    return;
-  }
-
-  if (entry->pending_queue.empty())
-    return;
-
-  // Promote next transaction from the pending queue.
-  Transaction* next = entry->pending_queue.front();
-  if ((next->mode() & Transaction::WRITE) && !entry->readers.empty())
-    return;  // Have to wait.
-
-  entry->pending_queue.erase(entry->pending_queue.begin());
-
-  int rv = AddTransactionToEntry(entry, next);
-  if (rv != ERR_IO_PENDING) {
-    next->io_callback().Run(rv);
-  }
-}
-
 void HttpCache::OnIOComplete(int result, PendingOp* pending_op) {
   WorkItemOperation op = pending_op->writer->operation();
 
   // Completing the creation of the backend is simpler than the other cases.
   if (op == WI_CREATE_BACKEND)
     return OnBackendCreated(result, pending_op);
 
   std::unique_ptr<WorkItem> item = std::move(pending_op->writer);
   bool fail_requests = false;
 
@@ skipping 124 matching lines @@
     building_backend_ = false;
     DeletePendingOp(pending_op);
   }
 
   // The cache may be gone when we return from the callback.
   if (!item->DoCallback(result, disk_cache_.get()))
     item->NotifyTransaction(result, NULL);
 }
 
 }  // namespace net