I saw a hang once when my C++ application called the zookeeper_close() method of the multi-threaded Zookeeper client library. The stack trace of the hung thread was the following:
Thread 8 (Thread 5644):
#0 0x00007f5d7bb5bbe4 in __lll_lock_wait () from /lib/libpthread.so.0
#1 0x00007f5d7bb59ad0 in pthread_cond_broadcast@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#2 0x00007f5d793628f6 in unlock_completion_list (l=0x32b4d68) at .../zookeeper/src/c/src/mt_adaptor.c:66
#3 0x00007f5d79354d4b in free_completions (zh=0x32b4c80, callCompletion=1, reason=-116) at .../zookeeper/src/c/src/zookeeper.c:1069
#4 0x00007f5d79355008 in cleanup_bufs (zh=0x32b4c80, callCompletion=1, rc=-116) at .../thirdparty/zookeeper/src/c/src/zookeeper.c:1125
#5 0x00007f5d79353200 in destroy (zh=0x32b4c80) at .../thirdparty/zookeeper/src/c/src/zookeeper.c:366
#6 0x00007f5d79358e0e in zookeeper_close (zh=0x32b4c80) at .../zookeeper/src/c/src/zookeeper.c:2326
#7 0x00007f5d79356d18 in api_epilog (zh=0x32b4c80, rc=0) at .../zookeeper/src/c/src/zookeeper.c:1661
#8 0x00007f5d79362f2f in adaptor_finish (zh=0x32b4c80) at .../zookeeper/src/c/src/mt_adaptor.c:205
#9 0x00007f5d79358c8c in zookeeper_close (zh=0x32b4c80) at .../zookeeper/src/c/src/zookeeper.c:2297
The omitted part of the stack trace is entirely within my application, and contains no other calls to/from the Zookeeper client. In particular, I am not calling zookeeper_close() from within a completion handler or any of the library's threads.
I haven't been able to reproduce this, and when I encountered this I wasn't capturing logging from the client library, so unfortunately I don't have any more information at this time. But I will update this JIRA if I see it again.