DBConnectionPool::flush should fully flush connection pool

Thread 2 (Thread 0x7f5c64838700 (LWP 5100)):

#0  __lll_lock_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:132

#1  0x00007f5c6ae34065 in _L_lock_858 () from /lib/x86_64-linux-gnu/libpthread.so.0

#2  0x00007f5c6ae33eba in __pthread_mutex_lock (mutex=0x7f5c6b1724a0) at pthread_mutex_lock.c:61

#3  0x0000000000dabe18 in boost::timed_mutex::lock (this=0x7f5c6b1724a0) at src/third_party/boost/boost/thread/pthread/mutex.hpp:154

#4  0x0000000000dae256 in boost::unique_lock<boost::timed_mutex>::lock (this=0x7f5c648369a0) at src/third_party/boost/boost/thread/locks.hpp:412

#5  0x0000000000dad031 in boost::unique_lock<boost::timed_mutex>::unique_lock (this=0x7f5c648369a0, m_=...)

    at src/third_party/boost/boost/thread/locks.hpp:290

#6  0x0000000000dac173 in mongo::mutex::scoped_lock::scoped_lock (this=0x7f5c648369a0, m=...) at src/mongo/util/concurrency/mutex.h:115

#7  0x0000000000da9912 in mongo::DBConnectionPool::_get (this=0x20bb1e0 <mongo::pool>, ident=..., socketTimeout=5)

    at src/mongo/client/connpool.cpp:205

#8  0x0000000000da9bc4 in mongo::DBConnectionPool::get (this=0x20bb1e0 <mongo::pool>, url=..., socketTimeout=5)

    at src/mongo/client/connpool.cpp:234

#9  0x0000000000e03dea in mongo::ScopedDbConnection::ScopedDbConnection (this=0x7f5c64836b70, host=..., socketTimeout=5)

    at src/mongo/client/connpool.h:308

#10 0x0000000000dfebba in mongo::ReplicaSetMonitor::Refresher::_refreshUntilMatches (this=0x7f5c64836c40, criteria=0x7f5c64836cd0)

---Type <return> to continue, or q <return> to quit---

    at src/mongo/client/replica_set_monitor.cpp:760

#11 0x0000000000e03567 in mongo::ReplicaSetMonitor::Refresher::refreshUntilMatches (this=0x7f5c64836c40, criteria=...)

    at src/mongo/client/replica_set_monitor.h:239

#12 0x0000000000dfb70f in mongo::ReplicaSetMonitor::getHostOrRefresh (this=0x7f5c68c47120, criteria=...)

    at src/mongo/client/replica_set_monitor.cpp:288

#13 0x0000000000dfb8ae in mongo::ReplicaSetMonitor::getMasterOrUassert (this=0x7f5c68c47120) at src/mongo/client/replica_set_monitor.cpp:302

#14 0x0000000000dcea2f in mongo::DBClientReplicaSet::checkMaster (this=0x7f5c69542c20) at src/mongo/client/dbclient_rs.cpp:291

#15 0x0000000000dd1b25 in mongo::DBClientReplicaSet::findOne (this=0x7f5c69542c20, ns=..., query=..., fieldsToReturn=0x0, queryOptions=0)

    at src/mongo/client/dbclient_rs.cpp:630

#16 0x0000000000db5da5 in mongo::DBClientWithCommands::runCommand (this=0x7f5c69542c20, dbname=..., cmd=..., info=..., options=0)

    at src/mongo/client/dbclient.cpp:448

#17 0x0000000000db86b4 in mongo::DBClientWithCommands::isMaster (this=0x7f5c69542c20, isMaster=@0x7f5c6483724e: true, info=0x7f5c648371e0)

    at src/mongo/client/dbclient.cpp:776

#18 0x0000000000da9276 in mongo::PoolForHost::flush (this=0x7f5c6b1902c0) at src/mongo/client/connpool.cpp:127

#19 0x0000000000daa4ba in mongo::DBConnectionPool::flush (this=0x20bb1e0 <mongo::pool>) at src/mongo/client/connpool.cpp:302

#20 0x0000000000e90dea in mongo::PoolFlushCmd::run (this=0x20bdce0 <mongo::poolFlushCmd>, txn=0x0, result=...) at src/mongo/db/commands.cpp:356

#21 0x0000000001185197 in mongo::Command::execCommandClientBasic (txn=0x0, c=0x20bdce0 <mongo::poolFlushCmd>, client=..., queryOptions=0, 

    ns=0x7f5c695aa434 "admin.$cmd", cmdObj=..., result=..., fromRepl=false) at src/mongo/s/s_only.cpp:148

#22 0x000000000110b36c in mongo::Command::runAgainstRegistered (ns=0x7f5c695aa434 "admin.$cmd", jsobj=..., anObjBuilder=..., queryOptions=0)

    at src/mongo/s/commands_public.cpp:2696

#23 0x000000000119c49b in mongo::Strategy::clientCommandOp (this=0x7f5c6b2386e0, r=...) at src/mongo/s/strategy.cpp:296

#24 0x0000000001183f39 in mongo::Request::process (this=0x7f5c64837b50, attempt=0) at src/mongo/s/request.cpp:122

#25 0x0000000000d633af in mongo::ShardedMessageHandler::process (this=0x7fff0c363a60, m=..., p=0x7f5c69546fa0, le=0x7f5c68b2c1a0)

    at src/mongo/s/server.cpp:148

#26 0x000000000124f601 in mongo::PortMessageServer::handleIncomingMsg (arg=0x7f5c68b01220) at src/mongo/util/net/message_server_port.cpp:234

#27 0x00007f5c6ae31e9a in start_thread (arg=0x7f5c64838700) at pthread_create.c:308

#28 0x00007f5c69f3dccd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112

#29 0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7f5c6b24a740 (LWP 4703)):

#0  0x00007f5c69f37033 in select () at ../sysdeps/unix/syscall-template.S:82

#1  0x00000000012479f3 in mongo::Listener::initAndListen (this=0x7f5c695983e8) at src/mongo/util/net/listen.cpp:256

#2  0x000000000124f0ea in mongo::PortMessageServer::run (this=0x7f5c695983e0) at src/mongo/util/net/message_server_port.cpp:158

#3  0x0000000000d5e6bd in mongo::start (opts=...) at src/mongo/s/server.cpp:200

#4  0x0000000000d5edea in runMongosServer (doUpgrade=false) at src/mongo/s/server.cpp:297

#5  0x0000000000d5f21a in _main () at src/mongo/s/server.cpp:364

#6  0x0000000000d5f62b in mongoSMain (argc=10, argv=0x7fff0c364148, envp=0x7fff0c3641a0) at src/mongo/s/server.cpp:434

#7  0x0000000000d5f928 in main (argc=10, argv=0x7fff0c364148, envp=0x7fff0c3641a0) at src/mongo/s/server.cpp:468