From f50b6cd93438638fccf7d7b7ceb6129f074bddc5 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Thu, 23 Oct 2014 01:34:07 -0700
Subject: [PATCH] Bug 1087834 - Reduce heap churn involving
 |input_overflow_buf_|. r=billm.

--HG--
extra : rebase_source : f842309c5d75479fefc266d405c0b8aceb7c247f
---
 .../src/chrome/common/ipc_channel_posix.cc    | 24 ++++++++++++-------
 .../src/chrome/common/ipc_channel_posix.h     |  2 ++
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/ipc/chromium/src/chrome/common/ipc_channel_posix.cc b/ipc/chromium/src/chrome/common/ipc_channel_posix.cc
index 697cec5b4608..66d87ccc22c4 100644
--- a/ipc/chromium/src/chrome/common/ipc_channel_posix.cc
+++ b/ipc/chromium/src/chrome/common/ipc_channel_posix.cc
@@ -383,14 +383,20 @@ bool Channel::ChannelImpl::EnqueueHelloMessage() {
   return true;
 }
 
-static void
-ClearAndShrink(std::string& s, size_t capacity)
+void Channel::ChannelImpl::ClearAndShrinkInputOverflowBuf()
 {
-  // This swap trick is the closest thing C++ has to a guaranteed way to
-  // shrink the capacity of a string.
-  std::string tmp;
-  tmp.reserve(capacity);
-  s.swap(tmp);
+  // If input_overflow_buf_ has grown, shrink it back to its normal size.
+  static size_t previousCapacityAfterClearing = 0;
+  if (input_overflow_buf_.capacity() > previousCapacityAfterClearing) {
+    // This swap trick is the closest thing C++ has to a guaranteed way
+    // to shrink the capacity of a string.
+    std::string tmp;
+    tmp.reserve(Channel::kReadBufferSize);
+    input_overflow_buf_.swap(tmp);
+    previousCapacityAfterClearing = input_overflow_buf_.capacity();
+  } else {
+    input_overflow_buf_.clear();
+  }
 }
 
 bool Channel::ChannelImpl::Connect() {
@@ -519,7 +525,7 @@ bool Channel::ChannelImpl::ProcessIncomingMessages() {
     } else {
       if (input_overflow_buf_.size() >
          static_cast<size_t>(kMaximumMessageSize - bytes_read)) {
-        ClearAndShrink(input_overflow_buf_, Channel::kReadBufferSize);
+        ClearAndShrinkInputOverflowBuf();
         CHROMIUM_LOG(ERROR) << "IPC message is too big";
         return false;
       }
@@ -628,7 +634,7 @@ bool Channel::ChannelImpl::ProcessIncomingMessages() {
       }
     }
     if (end == p) {
-      ClearAndShrink(input_overflow_buf_, Channel::kReadBufferSize);
+      ClearAndShrinkInputOverflowBuf();
     } else if (!overflowp) {
       // p is from input_buf_
       input_overflow_buf_.assign(p, end - p);
diff --git a/ipc/chromium/src/chrome/common/ipc_channel_posix.h b/ipc/chromium/src/chrome/common/ipc_channel_posix.h
index c97e2e6ae47b..25ce3907087b 100644
--- a/ipc/chromium/src/chrome/common/ipc_channel_posix.h
+++ b/ipc/chromium/src/chrome/common/ipc_channel_posix.h
@@ -59,6 +59,8 @@ class Channel::ChannelImpl : public MessageLoopForIO::Watcher {
   bool ProcessIncomingMessages();
   bool ProcessOutgoingMessages();
 
+  void ClearAndShrinkInputOverflowBuf();
+
   // MessageLoopForIO::Watcher implementation.
   virtual void OnFileCanReadWithoutBlocking(int fd);
   virtual void OnFileCanWriteWithoutBlocking(int fd);