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

net/disk_cache/simple/simple_entry_impl.cc

Index: net/disk_cache/simple/simple_entry_impl.cc
diff --git a/net/disk_cache/simple/simple_entry_impl.cc b/net/disk_cache/simple/simple_entry_impl.cc
index 2b354cd0f68043a93b4f70956991a7261a2e422b..ea30b674fb851fc2f3af637315bf825f0be13076 100644
--- a/net/disk_cache/simple/simple_entry_impl.cc
+++ b/net/disk_cache/simple/simple_entry_impl.cc
@@ -387,12 +387,15 @@ int SimpleEntryImpl::ReadData(int stream_index,
     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();