Data race observed with vbucket's pendingBGFetches

11:56:19 WARNING: ThreadSanitizer: data race (pid=47003)

11:56:19   Read of size 8 at 0x7d6400050df8 by main thread (mutexes: write M45364, write M44294):

11:56:19     #0 std::_Hashtable<std::string, std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > >, std::allocator<std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > > >, std::__detail::_Select1st, std::equal_to<std::string>, std::hash<std::string>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true> >::size() const /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/hashtable.h:500 (ep.so+0x00000008d98e)

11:56:19     #1 std::unordered_map<std::string, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> >, std::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > > > >::size() const /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/unordered_map.h:264 (ep.so+0x000000085be0)

11:56:19     #2 VBucket::numPendingBGFetchItems() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/vbucket.h:333 (ep.so+0x000000101789)

11:56:19     #3 EventuallyPersistentStore::bgFetch(std::string const&, unsigned short, unsigned long, void const*, bool) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep.cc:1651 (ep.so+0x0000000d71f7)

11:56:19     #4 EventuallyPersistentStore::getInternal(std::string const&, unsigned short, void const*, bool, bool, vbucket_state_t, bool) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep.cc:1708 (ep.so+0x0000000e3e21)

11:56:19     #5 EventuallyPersistentStore::get(std::string const&, unsigned short, void const*, bool, bool, bool) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep.h:242 (ep.so+0x00000019eee9)

11:56:19     #6 EventuallyPersistentEngine::get(void const*, void**, void const*, int, unsigned short, bool) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep_engine.h:259 (ep.so+0x00000016fe75)

11:56:19     #7 EvpGet(engine_interface*, void const*, void**, void const*, int, unsigned short) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/ep_engine.cc:202 (ep.so+0x000000136911)

11:56:19     #8 mock_get /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/memcached/programs/engine_testapp/engine_testapp.c (engine_testapp+0x0000004c6e84)

11:56:19     #9 get_item_info(engine_interface*, engine_interface_v1*, item_info*, char const*, unsigned short) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/ep_test_apis.cc:356 (ep_testsuite.so+0x0000000e2f2d)

11:56:19     #10 check_key_value(engine_interface*, engine_interface_v1*, char const*, char const*, unsigned long, unsigned short) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/ep_testsuite.cc:155 (ep_testsuite.so+0x0000000b2590)

11:56:19     #11 test_duplicate_items_disk(engine_interface*, engine_interface_v1*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/tests/ep_testsuite.cc:7839 (ep_testsuite.so+0x00000005a4b6)

11:56:19     #12 execute_test /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/memcached/programs/engine_testapp/engine_testapp.c (engine_testapp+0x0000004c4e9f)

11:56:19     #13 main crtstuff.c (engine_testapp+0x0000004c2e01)

11:56:19 

11:56:19   Previous write of size 8 at 0x7d6400050df8 by thread T1 (mutexes: write M44318):

11:56:19     #0 std::_Hashtable<std::string, std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > >, std::allocator<std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > > >, std::__detail::_Select1st, std::equal_to<std::string>, std::hash<std::string>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<true, false, true> >::clear() /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/hashtable.h:1943 (ep.so+0x000000087ba8)

11:56:19     #1 std::unordered_map<std::string, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> >, std::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > > > >::clear() /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/unordered_map.h:528 (ep.so+0x000000086f40)

11:56:19     #2 VBucket::getBGFetchItems(std::unordered_map<std::string, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> >, std::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, std::list<VBucketBGFetchItem*, std::allocator<VBucketBGFetchItem*> > > > >&) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/vbucket.cc:294 (ep.so+0x0000002b4026)

11:56:19     #3 BgFetcher::run(GlobalTask*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/bgfetcher.cc:155 (ep.so+0x00000008449f)

11:56:19     #4 BgFetcherTask::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/tasks.cc:89 (ep.so+0x00000025205e)

11:56:19     #5 ExecutorThread::run() /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/executorthread.cc:109 (ep.so+0x0000001e38f1)

11:56:19     #6 launch_executor_thread(void*) /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/ep-engine/src/executorthread.cc:34 (ep.so+0x0000001e2f1a)

11:56:19     #7 platform_thread_wrap /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-3.0.x/platform/src/cb_pthreads.c (libplatform.so.0.1.0+0x00000000377c)