SimpleCache: synchronously reply on reads from idle if data is in memory.

net/disk_cache/simple/simple_entry_impl.cc

 // Copyright (c) 2013 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/disk_cache/simple/simple_entry_impl.h"
 
 #include <algorithm>
 #include <cstring>
 #include <limits>
 #include <utility>
@@ skipping 369 matching lines @@
                                       offset < 0 || !buf_len)) {
     if (net_log_.IsCapturing()) {
       net_log_.AddEvent(net::NetLogEventType::SIMPLE_CACHE_ENTRY_READ_END,
                         CreateNetLogReadWriteCompleteCallback(0));
     }
 
     RecordReadResult(cache_type_, READ_RESULT_NONBLOCK_EMPTY_RETURN);
     return 0;
   }
 
-  // TODO(clamy): return immediatly when reading from stream 0.
-
   // TODO(felipeg): Optimization: Add support for truly parallel read

[pasko, 2017/04/07 16:13:39]

fyi: this one did not show much opportunity on the metrics
(SimpleCache.Http.WriteDependencyType, SimpleCache.Http.ReadIsParallelizable,
SimpleCache.Http.EntryOperationsPending) feel free to remove

   // operations.
   bool alone_in_queue =
       pending_operations_.size() == 0 && state_ == STATE_READY;
+  if (alone_in_queue && stream_index == 0) {
+    buf_len = std::min(buf_len, GetDataSize(stream_index) - offset);
+    return ReadStream0Data(buf, offset, buf_len);
+  }
+
   pending_operations_.push(SimpleEntryOperation::ReadOperation(
       this, stream_index, offset, buf_len, buf, callback, alone_in_queue));
   RunNextOperationIfNeeded();
   return net::ERR_IO_PENDING;
 }
 
 int SimpleEntryImpl::WriteData(int stream_index,
                                int offset,
                                net::IOBuffer* buf,
                                int buf_len,
@@ skipping 1156 matching lines @@
     }
     crc32s_end_offset_[stream_index] = offset + length;
   } else if (offset < crc32s_end_offset_[stream_index]) {
     // If a range for which the crc32 was already computed is rewritten, the
     // computation of the crc32 need to start from 0 again.
     crc32s_end_offset_[stream_index] = 0;
   }
 }
 
 }  // namespace disk_cache