[RocksDB] Avoid RocksDBKVStore::delVBucket() to delete a live DB instance

(gdb) bt

#0  rocksdb::DBImpl::GetAggregatedIntProperty (this=0x7f03141df400, property=..., aggregated_value=0x7f0465ded2d8) at db/db_impl.cc:1773

#1  0x00007f04760a20e6 in rocksdb::MemoryUtil::GetApproximateMemoryUsageByType (dbs=std::vector of length 127, capacity 128 = {...}, cache_set=std::unordered_set with 2 elements = {...}, 

    usage_by_type=usage_by_type@entry=0x7f0465ded3e0) at utilities/memory/memory_util.cc:23

#2  0x00007f047676aca2 in RocksDBKVStore::getStat (this=0x7f025cc2a400, name=0x7f047678e3c2 "io_total_write_bytes", value=@0x7f0465ded490: 139657227933936)

    at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/rocksdb-kvstore/rocksdb-kvstore.cc:620

#3  0x00007f04766ed9de in KVBucket::getKVStoreStat (this=<optimized out>, name=0x7f047678e3c2 "io_total_write_bytes", value=@0x7f0465ded500: 837943367603616, option=<optimized out>)

    at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/kv_bucket.cc:2178

#4  0x00007f04766a7bc9 in EventuallyPersistentEngine::doEngineStats (this=0x7f045476a000, cookie=0x7f0454356b78, add_stat=0x471b30 <append_stats(char const*, uint16_t, char const*, uint32_t, void const*)>)

    at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/ep_engine.cc:2630

#5  0x00007f04766af400 in EventuallyPersistentEngine::getStats (this=this@entry=0x7f045476a000, cookie=cookie@entry=0x7f0454356b78, stat_key=stat_key@entry=0x0, nkey=nkey@entry=0, 

    add_stat=add_stat@entry=0x471b30 <append_stats(char const*, uint16_t, char const*, uint32_t, void const*)>) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/ep_engine.cc:3586

#6  0x00007f04766b0288 in EvpGetStats (handle=0x7f045476a000, cookie=0x7f0454356b78, stat_key=0x0, nkey=0, add_stat=0x471b30 <append_stats(char const*, uint16_t, char const*, uint32_t, void const*)>)

    at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/ep_engine.cc:316

#7  0x0000000000476f8a in get_stats (k=..., this=0x7f0454736050) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/protocol/mcbp/stats_context.cc:924

#8  StatsCommandContext::step (this=0x7f0454736050) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/protocol/mcbp/stats_context.cc:850

#9  0x0000000000479a84 in SteppableCommandContext::drive (this=this@entry=0x7f0454736050) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/protocol/mcbp/steppable_command_context.cc:33

#10 0x0000000000455655 in stat_executor (cookie=...) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/mcbp_executors.cc:233

#11 0x0000000000457511 in operator() (__args#0=..., this=0x18bf600 <handlers+512>) at /usr/include/c++/4.8.2/functional:2471

#12 execute_request_packet (request=..., cookie=...) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/mcbp_executors.cc:844

#13 mcbp_execute_packet (cookie=...) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/mcbp_executors.cc:925

#14 0x000000000047c214 in conn_execute (connection=...) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/statemachine_mcbp.cc:372

#15 0x000000000047d53e in McbpStateMachine::execute (this=this@entry=0x7f0454356998) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/statemachine_mcbp.cc:170

#16 0x000000000044526c in McbpConnection::runStateMachinery (this=0x7f0454356880) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/connection_mcbp.cc:783

#17 0x00000000004452ab in McbpConnection::runEventLoop (this=0x7f0454356880, which=<optimized out>) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/connection_mcbp.cc:952

#18 0x0000000000447ffe in run_event_loop (c=0x7f0454356880, which=<optimized out>) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/connections.cc:159

#19 0x0000000000418dbc in event_handler (fd=23743, which=<optimized out>, arg=0x7f0454356880) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/memcached.cc:1040

#20 0x00007f047c23542c in event_process_active_single_queue.isra.26 () from /opt/couchbase/bin/../lib/libevent_core.so.2.1.8

#21 0x00007f047c23887f in event_base_loop () from /opt/couchbase/bin/../lib/libevent_core.so.2.1.8

#22 0x0000000000432804 in worker_libevent (arg=0x7f046e38ec80) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/daemon/thread.cc:232

#23 0x00007f047d11802e in run (this=0x7f046e29ece0) at /home/couchbase/jenkins/workspace/couchbase-server-unix/platform/src/cb_pthreads.cc:59

#24 platform_thread_wrap (arg=0x7f046e29ece0) at /home/couchbase/jenkins/workspace/couchbase-server-unix/platform/src/cb_pthreads.cc:72

#25 0x00007f047cadbdc5 in start_thread () from /lib64/libpthread.so.0

#26 0x00007f047ac7876d in clone () from /lib64/libc.so.6

(gdb) t 130

[Switching to thread 130 (Thread 0x7f02535f4700 (LWP 14207))]

#0  0x00007f047cae21bd in __lll_lock_wait () from /lib64/libpthread.so.0

(gdb) bt

#0  0x00007f047cae21bd in __lll_lock_wait () from /lib64/libpthread.so.0

#1  0x00007f047caddd02 in _L_lock_791 () from /lib64/libpthread.so.0

#2  0x00007f047caddc08 in pthread_mutex_lock () from /lib64/libpthread.so.0

#3  0x00007f0475fdbda9 in rocksdb::port::Mutex::Lock (this=this@entry=0x7f03141df830) at port/port_posix.cc:60

#4  0x00007f0475fb7255 in rocksdb::InstrumentedMutex::LockInternal (this=this@entry=0x7f03141df830) at monitoring/instrumented_mutex.cc:38

#5  0x00007f0475fb7346 in rocksdb::InstrumentedMutex::Lock (this=0x7f03141df830) at monitoring/instrumented_mutex.cc:26

#6  0x00007f0475e9d051 in rocksdb::ColumnFamilyData::~ColumnFamilyData (this=0x7f025c4c6400, __in_chrg=<optimized out>) at db/column_family.cc:453

#7  0x00007f0475e9df28 in rocksdb::ColumnFamilySet::~ColumnFamilySet (this=0x7f025cd76da0, __in_chrg=<optimized out>) at db/column_family.cc:1002

#8  0x00007f0475f6914b in operator() (this=<optimized out>, __ptr=0x7f025cd76da0) at /usr/include/c++/4.8.2/bits/unique_ptr.h:67

#9  reset (__p=0x7f025cd76da0, this=0x7f025c5bf940) at /usr/include/c++/4.8.2/bits/unique_ptr.h:262

#10 rocksdb::VersionSet::~VersionSet (this=0x7f025c5bf940, __in_chrg=<optimized out>) at db/version_set.cc:2340

#11 0x00007f0475edd8f3 in operator() (this=<optimized out>, __ptr=0x7f025c5bf940) at /usr/include/c++/4.8.2/bits/unique_ptr.h:67

#12 reset (__p=0x7f025c5bf940, this=0x7f03141df418) at /usr/include/c++/4.8.2/bits/unique_ptr.h:262

#13 rocksdb::DBImpl::~DBImpl (this=0x7f03141df400, __in_chrg=<optimized out>) at db/db_impl.cc:349

#14 0x00007f0475eddf01 in rocksdb::DBImpl::~DBImpl (this=0x7f03141df400, __in_chrg=<optimized out>) at db/db_impl.cc:357

#15 0x00007f04767674b0 in operator() (this=<optimized out>, __ptr=<optimized out>) at /usr/include/c++/4.8.2/bits/unique_ptr.h:67

#16 ~unique_ptr (this=0x7f025c538540, __in_chrg=<optimized out>) at /usr/include/c++/4.8.2/bits/unique_ptr.h:184

#17 ~KVRocksDB (this=0x7f025c538540, __in_chrg=<optimized out>) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/rocksdb-kvstore/rocksdb-kvstore.cc:164

#18 operator() (this=<optimized out>, __ptr=0x7f025c538540) at /usr/include/c++/4.8.2/bits/unique_ptr.h:67

#19 reset (__p=0x7f025c538540, this=<optimized out>) at /usr/include/c++/4.8.2/bits/unique_ptr.h:262

#20 RocksDBKVStore::delVBucket (this=0x7f025cc2a400, vbid=<optimized out>, vb_version=<optimized out>) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/rocksdb-kvstore/rocksdb-kvstore.cc:534

#21 0x00007f047672f79a in VBucketMemoryAndDiskDeletionTask::run (this=0x7f02660d25b8) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/vbucketdeletiontask.cc:87

#22 0x00007f04766cddc1 in ExecutorThread::run (this=0x7f0259592c00) at /home/couchbase/jenkins/workspace/couchbase-server-unix/kv_engine/engines/ep/src/executorthread.cc:122

#23 0x00007f047d11802e in run (this=0x7f02595ec120) at /home/couchbase/jenkins/workspace/couchbase-server-unix/platform/src/cb_pthreads.cc:59

#24 platform_thread_wrap (arg=0x7f02595ec120) at /home/couchbase/jenkins/workspace/couchbase-server-unix/platform/src/cb_pthreads.cc:72

#25 0x00007f047cadbdc5 in start_thread () from /lib64/libpthread.so.0

#26 0x00007f047ac7876d in clone () from /lib64/libc.so.6