Fix data race on mock_connstruct::references

Add identified by ThreadSanitizer. mock_connstruct::references is
accessed concurrently from two threads; update to use std::atomic.

Details:

WARNING: ThreadSanitizer: data race (pid=32054)
  Write of size 4 at 0x7d280000eff0 by main thread:
    #0 mock_cookie_reserve() /memcached/programs/engine_testapp/mock_server.cc:129:5 (engine_testapp+0x0000004c77c3)
    #1 EventuallyPersistentEngine::reserveCookie() /ep-engine/src/ep_engine.cc:2016:28 (ep.so+0x0000000a2f05)
    #2 EventuallyPersistentEngine::createTapQueue() /ep-engine/src/ep_engine.cc:2574 (ep.so+0x0000000a2f05)
    #3 EvpGetTapIterator() /ep-engine/src/ep_engine.cc:1463:17 (ep.so+0x00000009f943)
    #4 mock_get_tap_iterator() /memcached/programs/engine_testapp/engine_testapp.cc:467:20 (engine_testapp+0x0000004c59ce)
    #5 test_tap_ack_stream() /ep-engine/tests/ep_testsuite.cc:7341:20 (ep_testsuite.so+0x00000003edf0)
    #6 execute_test() /memcached/programs/engine_testapp/engine_testapp.cc:1090:19 (engine_testapp+0x0000004c4142)
    memcached#7 main /memcached/programs/engine_testapp/engine_testapp.cc:1439 (engine_testapp+0x0000004c4142)

  Previous write of size 4 at 0x7d280000eff0 by thread T10 (mutexes: write M2181):
    #0 mock_cookie_release() /memcached/programs/engine_testapp/mock_server.cc:136:5 (engine_testapp+0x0000004c7801)
    #1 EventuallyPersistentEngine::releaseCookie() /ep-engine/src/ep_engine.cc:2025:28 (ep.so+0x0000000a1142)
    #2 ConnHandler::releaseReference() /ep-engine/src/tapconnection.cc:311:9 (ep.so+0x0000000fc5d2)
    #3 TapConnMap::manageConnections() /ep-engine/src/connmap.cc:532:10 (ep.so+0x00000003e972)
    #4 ConnManager::run() /ep-engine/src/connmap.cc:150:9 (ep.so+0x00000004a60e)
    #5 ExecutorThread::run() /ep-engine/src/executorthread.cc:115:26 (ep.so+0x0000000d8a9e)
    #6 launch_executor_thread() /ep-engine/src/executorthread.cc:33:9 (ep.so+0x0000000d8615)
    memcached#7 platform_thread_wrap /platform/src/cb_pthreads.c:23:5 (libplatform.so.0.1.0+0x000000003c31)

Change-Id: Ib1861cf9b66e31b5f50e6b114119da72d54cde10
Reviewed-on: http://review.couchbase.org/55866
Reviewed-by: abhinav dangeti <[email protected]>
Tested-by: buildbot <[email protected]>

Author: daverigby

programs/engine_testapp/mock_server.cc

@@ -133,8 +133,8 @@ static ENGINE_ERROR_CODE mock_cookie_reserve(const void *cookie) {
 static ENGINE_ERROR_CODE mock_cookie_release(const void *cookie) {
     struct mock_connstruct *c = (struct mock_connstruct *)cookie;
 
-    c->references--;
-    if (c->references == 0) {
+    const int new_rc = --c->references;
+    if (new_rc == 0) {
         free(c);
     }
     return ENGINE_SUCCESS;

programs/engine_testapp/mock_server.h

@@ -4,6 +4,7 @@
 #include <memcached/engine.h>
 #include <platform/platform.h>
 
+#include <atomic>
 #include <string>
 
 struct mock_connstruct {
@@ -23,7 +24,7 @@ struct mock_connstruct {
     bool handle_datatype_support;
     cb_mutex_t mutex;
     cb_cond_t cond;
-    int references;
+    std::atomic<int> references;
 };
 
 struct mock_callbacks {