Address benign data race in dcp/flow-control

WARNING: ThreadSanitizer: data race (pid=136910)
  Read of size 4 at 0x7d50000165c4 by main thread (mutexes: write M40586, write M2484):
    #0 void ConnHandler::addStat<unsigned int>(char const*, unsigned int const&, void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/tapconnection.h:291 (ep.so+0x00000004f3e3)
    couchbase#1 FlowControl::addStats(void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/flow-control.cc:133 (ep.so+0x00000005d789)
    couchbase#2 DcpConsumer::addStats(void (*)(char const*, unsigned short, char const*, unsigned int, void const*), void const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/consumer.cc:732 (ep.so+0x00000005acf8)
    couchbase#3 ConnStatBuilder::operator()(SingleThreadedRCPtr<ConnHandler>&) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/ep_engine.cc:3903 (ep.so+0x0000000d73b1)
    couchbase#4 EventuallyPersistentEngine::doDcpStats(void const*, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/ep_engine.cc:4160 (ep.so+0x0000000b9aca)
    couchbase#5 EventuallyPersistentEngine::getStats(void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/ep_engine.cc:4580 (ep.so+0x0000000bd624)
    #6 EvpGetStats(engine_interface*, void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/ep_engine.cc:216 (ep.so+0x0000000ad0ee)
    #7 mock_get_stats(engine_interface*, void const*, char const*, int, void (*)(char const*, unsigned short, char const*, unsigned int, void const*)) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/memcached/programs/engine_testapp/engine_testapp.cc:239 (engine_testapp+0x0000004c558d)
    #8 get_int_stat(engine_interface*, engine_interface_v1*, char const*, char const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/tests/ep_test_apis.cc:990 (ep_testsuite.so+0x0000000a11d4)
    #9 wait_for_stat_to_be(engine_interface*, engine_interface_v1*, char const*, int, char const*, long) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/tests/ep_test_apis.cc:1120 (ep_testsuite.so+0x0000000a70c3)
    #10 test_dcp_consumer_flow_control_aggressive(engine_interface*, engine_interface_v1*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/tests/ep_testsuite.cc:3876 (ep_testsuite.so+0x000000066e6d)
    #11 execute_test(test, char const*, char const*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/memcached/programs/engine_testapp/engine_testapp.cc:1090 (engine_testapp+0x0000004c4192)
    #12 __libc_start_main /build/buildd/eglibc-2.15/csu/libc-start.c:226 (libc.so.6+0x00000002176c)

  Previous write of size 4 at 0x7d50000165c4 by thread T17 (mutexes: write M2483):
    #0 FlowControl::setFlowControlBufSize(unsigned int) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/flow-control.cc:116 (ep.so+0x00000005d67c)
    couchbase#1 DcpConsumer::setFlowControlBufSize(unsigned int) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/consumer.cc:1000 (ep.so+0x00000005b092)
    couchbase#2 DcpFlowControlManagerAggressive::handleDisconnect(DcpConsumer*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/flow-control-manager.cc:211 (ep.so+0x00000005f0e4)
    couchbase#3 FlowControl::~FlowControl() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/flow-control.cc:43 (ep.so+0x00000005d051)
    couchbase#4 DcpConsumer::~DcpConsumer() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/consumer.cc:115 (ep.so+0x000000055824)
    couchbase#5 DcpConsumer::~DcpConsumer() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/dcp/consumer.cc:111 (ep.so+0x000000055e35)
    #6 DcpConnMap::manageConnections() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/atomic.h:271 (ep.so+0x000000046680)
    #7 ConnManager::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/connmap.cc:150 (ep.so+0x00000004c96e)
    #8 ExecutorThread::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/executorthread.cc:115 (ep.so+0x0000000eb94d)
    #9 launch_executor_thread(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000eb515)
    #10 platform_thread_wrap(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/platform/src/cb_pthreads.cc:53 (libplatform.so.0.1.0+0x0000000048bb)

Change-Id: Iae02308b71e7d416b35b6db3a84ddf26601d08a6
Reviewed-on: http://review.couchbase.org/56350
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>

Author: abhinavdangeti

src/dcp/flow-control.h

@@ -69,7 +69,7 @@ class FlowControl {
     bool pendingControl;
 
     /* Flow control buffer size */
-    uint32_t bufferSize;
+    Couchbase::RelaxedAtomic<uint32_t> bufferSize;
 
     /* Lock while updating buffersize and pendingControl */
     SpinLock bufferSizeLock;