MB-16500 [BP]: Address possible data race in ConnHandler: lastWalkTime

WARNING: ThreadSanitizer: data race (pid=26185)

  Write of size 4 at 0x7d4c0000a154 by main thread:
    #0 ConnHandler::setLastWalkTime() /home/abhinav/couchbase/ep-engine/src/tapconnection.h:356 (ep.so+0x000000065641)
    couchbase#1 EvpDcpStep(engine_interface*, void const*, dcp_message_producers*) /home/abhinav/couchbase/ep-engine/src/ep_engine.cc:1478 (ep.so+0x0000000b4d5b)
    couchbase#2 mock_dcp_step(engine_interface*, void const*, dcp_message_producers*) /home/abhinav/couchbase/memcached/programs/engine_testapp/engine_testapp.cc:476 (engine_testapp+0x0000000baf95)
    couchbase#3 dcp_stream(engine_interface*, engine_interface_v1*, char const*, void const*, unsigned short, unsigned int, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, int, int, int, int, bool, bool, unsigned char, bool, unsigned long*, bool) /home/abhinav/couchbase/ep-engine/tests/ep_testsuite.cc:4164 (ep_testsuite.so+0x0000000990df)
    couchbase#4 test_dcp_producer_stream_req_dgm(engine_interface*, engine_interface_v1*) /home/abhinav/couchbase/ep-engine/tests/ep_testsuite.cc:4564 (ep_testsuite.so+0x000000077634)
    couchbase#5 execute_test(test, char const*, char const*) /home/abhinav/couchbase/memcached/programs/engine_testapp/engine_testapp.cc:1090 (engine_testapp+0x0000000b946c)
    #6 __libc_start_main /build/buildd/eglibc-2.19/csu/libc-start.c:287 (libc.so.6+0x000000021ec4)

  Previous read of size 4 at 0x7d4c0000a154 by thread T10 (mutexes: write M1367):
    #0 ConnHandler::getLastWalkTime() /home/abhinav/couchbase/ep-engine/src/tapconnection.h:360 (ep.so+0x000000049cbe)
    couchbase#1 ConnManager::run() /home/abhinav/couchbase/ep-engine/src/connmap.cc:150 (ep.so+0x00000005031e)
    couchbase#2 ExecutorThread::run() /home/abhinav/couchbase/ep-engine/src/executorthread.cc:112 (ep.so+0x0000000f8746)
    couchbase#3 launch_executor_thread(void*) /home/abhinav/couchbase/ep-engine/src/executorthread.cc:33 (ep.so+0x0000000f82e5)
    couchbase#4 platform_thread_wrap /home/abhinav/couchbase/platform/src/cb_pthreads.c:23 (libplatform.so.0.1.0+0x000000003d31)

Change-Id: I2c5024afde6cb749aad901572bfd68734f6d7d5d
Reviewed-on: http://review.couchbase.org/55780
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-on: http://review.couchbase.org/56062

Author: abhinavdangeti

src/tapconnection.h

@@ -353,11 +353,11 @@ class ConnHandler : public RCValue {
     }
 
     void setLastWalkTime() {
-        lastWalkTime = ep_current_time();
+        lastWalkTime.store(ep_current_time());
     }
 
     rel_time_t getLastWalkTime() {
-        return lastWalkTime;
+        return lastWalkTime.load();
     }
 
     void setConnected(bool s) {
@@ -417,7 +417,7 @@ class ConnHandler : public RCValue {
     rel_time_t created;
 
     //! The last time this connection's step function was called
-    rel_time_t lastWalkTime;
+    AtomicValue<rel_time_t> lastWalkTime;
 
     //! Should we disconnect as soon as possible?
     bool disconnect;