MB-19259: Fix data race on DcpConsumer::backoffs

This member is read by stats processing concurrently with DcpConsumer
updating it. Change to RelaxedAtomic<>

As reported by ThreadSanitizer:

WARNING: ThreadSanitizer: data race (pid=24629)
  Read of size 4 at 0x7d5000016b9c by main thread (mutexes: write M27034, write M2479):
    #0 void ConnHandler::addStat<unsigned int>(), void const*) ep-engine/src/tapconnection.h:291:18 (ep.so+0x00000004e993)
    #1 DcpConsumer::addStats(), void const*) ep-engine/src/dcp/consumer.cc:731:5 (ep.so+0x00000005a2c3)
    ...

  Previous write of size 4 at 0x7d5000016b9c by thread T10:
    #0 DcpConsumer::processBufferedItems() ep-engine/src/dcp/consumer.cc:755:17 (ep.so+0x0000000539c3)
    #1 Processer::run() ep-engine/src/dcp/consumer.cc:57:13 (ep.so+0x00000005376b)
    #2 ExecutorThread::run() ep-engine/src/executorthread.cc:115:26 (ep.so+0x0000000e944e)
    ...

Change-Id: Iabddcc06213fbb80815d4b464c459adb922a0eff
Reviewed-on: http://review.couchbase.org/62977
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>
Tested-by: buildbot <build@couchbase.com>

diff --git a/src/dcp-consumer.h b/src/dcp-consumer.h
index 9c54334..700d405 100644 (file)
--- a/src/dcp-consumer.h
+++ b/src/dcp-consumer.h
@@ -22,6 +22,9 @@
 
 #include "tapconnection.h"
 #include "dcp-stream.h"
+
+#include <relaxed_atomic.h>
+
 typedef RCPtr<PassiveStream> passive_stream_t;
 
 class DcpResponse;
@@ -126,7 +129,7 @@ private:
     opaque_map opaqueMap_;
 
     rel_time_t lastNoopTime;
-    uint32_t backoffs;
+    Couchbase::RelaxedAtomic<uint32_t> backoffs;
     uint32_t noopInterval;
     bool enableNoop;
     bool sendNoopInterval;