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 77 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 81 matching lines @@
   disk_cache::Backend** backend_;
 };
 
 //-----------------------------------------------------------------------------
 
 // This class encapsulates a transaction whose only purpose is to write metadata
 // to a given entry.
 class HttpCache::MetadataWriter {
  public:
   explicit MetadataWriter(HttpCache::Transaction* trans)
-      : transaction_(trans),
-        verified_(false),
-        buf_len_(0) {
-  }
+      : verified_(false), buf_len_(0), transaction_(trans) {}
 
   ~MetadataWriter() {}
 
   // Implements the bulk of HttpCache::WriteMetadata.
   void Write(const GURL& url,
              base::Time expected_response_time,
              IOBuffer* buf,
              int buf_len);
 
  private:
   void VerifyResponse(int result);
   void SelfDestroy();
   void OnIOComplete(int result);
 
-  std::unique_ptr<HttpCache::Transaction> transaction_;
   bool verified_;
   scoped_refptr<IOBuffer> buf_;
   int buf_len_;
   base::Time expected_response_time_;
   HttpRequestInfo request_info_;
+
+  // |transaction_| to come after |request_info_| so that |request_info_| is not
+  // destroyed earlier.
+  std::unique_ptr<HttpCache::Transaction> transaction_;
   DISALLOW_COPY_AND_ASSIGN(MetadataWriter);
 };
 
 void HttpCache::MetadataWriter::Write(const GURL& url,
                                       base::Time expected_response_time,
                                       IOBuffer* buf,
                                       int buf_len) {
   DCHECK_GT(buf_len, 0);
   DCHECK(buf);
   DCHECK(buf->data());
@@ skipping 79 matching lines @@
   session->SetServerPushDelegate(
       base::MakeUnique<HttpCacheLookupManager>(this));
 }
 
 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 240 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_partial) {
+  // 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;
+
+  // TODO(crbug.com/715913, crbug.com/715974, crbug.com/715920,
+  // crbug.com/715911): Convert the CHECKs in this function to DCHECKs once
+  // canary is clear of any crashes.
+  CHECK_EQ(entry->headers_transaction, transaction);
+
+  entry->headers_transaction = nullptr;
+
+  // If transaction is responsible for writing the response body, then do not go
+  // through done_headers_queue for performance benefit. (Also, in case of
+  // writer transaction, the consumer sometimes depend on synchronous behaviour
+  // e.g. while computing raw headers size. (crbug.com/711766))
+  if (transaction->mode() & Transaction::WRITE) {
+    // Partial requests may have write mode even when there is a writer present
+    // since they may be reader for a particular range and writer for another
+    // range.
+    CHECK(is_partial || (!entry->writer && entry->done_headers_queue.empty()));
+
+    if (!entry->writer) {
+      entry->writer = transaction;
+      ProcessQueuedTransactions(entry);
+      return OK;
     }
-  } 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,
-                              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);
-
+  }
+
+  // If this is not the first transaction in done_headers_queue, it should be a
+  // read-mode transaction except if it is a partial request.
+  CHECK(is_partial || (entry->done_headers_queue.empty() ||
+                       !(transaction->mode() & Transaction::WRITE)));
+
+  entry->done_headers_queue.push_back(transaction);
+  ProcessQueuedTransactions(entry);
+  return ERR_IO_PENDING;
+}
+
+void HttpCache::DoneWithEntry(ActiveEntry* entry,
+                              Transaction* transaction,
+                              bool process_cancel,
+                              bool is_partial) {
+  bool cancel = false;
+  if (process_cancel) {
+    cancel = IsTransactionExclusivelyWriting(entry, transaction, is_partial);
+    if (cancel && is_partial)
+      entry->disk_entry->CancelSparseIO();

[jkarlin, 2017/05/30 14:42:38]

What if we didn't call CancelSparseIO? Would anything bad happen? From the
description it doesn't seem like it. Then we could remove the logically complex
IsTransactionExclusivelyWriting function.

[shivanisha, 2017/05/31 17:32:13]

IsTransactionExclusivelyWriting's return value is not just used for deciding to
invoke CancelSparseIO but also for other cases further down.

+  }
+
+  // Transaction is waiting in the done_headers_queue.
+  auto it = std::find(entry->done_headers_queue.begin(),
+                      entry->done_headers_queue.end(), transaction);
+  if (it != entry->done_headers_queue.end()) {
+    entry->done_headers_queue.erase(it);
+    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);
+    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);
+
+  if (transaction == entry->writer)
+    entry->writer = nullptr;
+  else
+    entry->headers_transaction = nullptr;
+
+  // If writer fails, restart the headers_transaction by setting its state.
+  // Since the headers_transactions is awaiting an asynchronous operation
+  // completion, when it's IO callback is invoked, it will 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(transaction);
+  DCHECK(it != entry->readers.end());
+  entry->readers.erase(it);
+
+  ProcessQueuedTransactions(entry);
+}
+
+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 {
-    DoneReadingFromEntry(entry, trans);
-  }
-}
-
-void HttpCache::DoneWritingToEntry(ActiveEntry* entry, bool success) {
-  DCHECK(entry->readers.empty());
-
-  entry->writer = NULL;
-
-  if (success) {
-    ProcessPendingQueue(entry);
+    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::ProcessAddToEntryQueue(ActiveEntry* entry) {
+  DCHECK(!entry->add_to_entry_queue.empty());
+
+  // Note the entry may be new or may already have a response body written to
+  // it. In both cases, a transaction needs to wait since only one transaction
+  // can be in the headers phase at a time.
+  if (entry->headers_transaction) {
+    return;
+  }
+  Transaction* transaction = entry->add_to_entry_queue.front();
+  entry->add_to_entry_queue.erase(entry->add_to_entry_queue.begin());
+  entry->headers_transaction = transaction;
+
+  transaction->io_callback().Run(OK);
+}
+
+void HttpCache::ProcessDoneHeadersQueue(ActiveEntry* entry) {
+  DCHECK(!entry->writer);
+  DCHECK(!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 {
-    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) {
-  DCHECK(!entry->writer);
-
-  auto it = entry->readers.find(trans);
-  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);
+    // If a transaction is in front of this queue with only read mode set and
+    // there is no writer, it implies response body is already written, convert
+    // to a reader.
+    auto return_val = entry->readers.insert(transaction);
+    DCHECK_EQ(return_val.second, true);
+  }
+
+  // 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::CanTransactionWriteResponseHeaders(ActiveEntry* entry,
+                                                   Transaction* transaction,
+                                                   bool is_match) const {
+  if (transaction != entry->headers_transaction)
+    return false;
+
+  if (!(transaction->mode() & Transaction::WRITE))
+    return false;
+
+  // If its not a match then check if it is the transaction responsible for
+  // writing the response body.
+  if (!is_match) {
+    return !entry->writer && entry->done_headers_queue.empty() &&
+           entry->readers.empty();
+  }
+
+  return true;
+}
+
+bool HttpCache::IsTransactionExclusivelyWriting(ActiveEntry* entry,
+                                                Transaction* transaction,
+                                                bool is_partial) const {
+  if (transaction == entry->writer)
+    return true;
+
+  if (transaction->method() == "HEAD" || transaction->method() == "DELETE")
+    return false;
+
+  // Check if transaction is 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 (!is_partial) {
+    // If a transaction is completing headers or done with headers phase with
+    // write mode then it should be the future writer. Just checking the front
+    // of done_headers_queue since the rest should anyways be READ mode
+    // transactions as they would be a result of validation match (for non-range
+    // requests).
+    return transaction == entry->headers_transaction ||
+           transaction == entry->done_headers_queue.front();
+  }
+
+  // Range requests specific cases.
+
+  // For range requests, it is possible to have WRITE mode set even if another
+  // transaction is the writer.
+  if (entry->writer)
+    return false;
+
+  if (transaction == entry->headers_transaction ||
+      transaction == entry->done_headers_queue.front()) {
+    return true;
+  }
+
+  // If transaction is not in front of done_headers_queue, see if its the first
+  // writer transaction in the queue.
+  for (auto* done_headers : entry->done_headers_queue) {
+    if (transaction == done_headers)
+      return true;
+    if (done_headers->mode() & Transaction::WRITE)
+      return false;
+  }
+
+  return false;
+}
+
+bool HttpCache::IsWritingInProgress(ActiveEntry* entry) const {
+  return entry->writer != nullptr;
 }
 
 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;
+void HttpCache::OnProcessQueuedTransactions(ActiveEntry* entry) {
+  entry->will_process_queued_transactions = false;
 
-  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);
+  // Note that this function should only invoke one transaction's IO callback
+  // since its possible for IO callbacks' consumers to destroy the cache/entry.
 
   // If no one is interested in this entry, then we can deactivate it.
   if (entry->HasNoTransactions()) {
     DestroyEntry(entry);
     return;
   }
 
-  if (entry->pending_queue.empty())
+  if (entry->done_headers_queue.empty() && entry->add_to_entry_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.
+  // To maintain FIFO order of transactions, done_headers_queue should be
+  // checked for processing before add_to_entry_queue.
 
-  entry->pending_queue.erase(entry->pending_queue.begin());
+  // 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;
+  }
 
-  int rv = AddTransactionToEntry(entry, next);
-  if (rv != ERR_IO_PENDING) {
-    next->io_callback().Run(rv);
-  }
+  if (!entry->add_to_entry_queue.empty())
+    ProcessAddToEntryQueue(entry);
 }
 
 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);
@@ skipping 126 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