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From "Alexander A. Strelets (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-2894) Memory and completions leak on zookeeper_close.
Date Fri, 08 Sep 2017 11:27:00 GMT
Alexander A. Strelets created ZOOKEEPER-2894:
------------------------------------------------

             Summary: Memory and completions leak on zookeeper_close.
                 Key: ZOOKEEPER-2894
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2894
             Project: ZooKeeper
          Issue Type: Bug
          Components: c client
    Affects Versions: 3.4.10
         Environment: Linux ubuntu 4.4.0-87-generic
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

https://github.com/apache/zookeeper.git
branch-3.4
            Reporter: Alexander A. Strelets
            Priority: Critical
             Fix For: 3.4.10


ZooKeeper C Client *+single thread+* build

First of all, ZooKeeper C Client design allows calling _zookeeper_close()_ in two ways:
a) from a ZooKeeper callback handler (completion or watcher) which in turn is called through
_zookeeper_process()_
b) and from other places -- i.e., when the call-stack does not pass through any of zookeeper
mechanics prior to enter into mentioned _zookeeper_close()_

The issue described here below is +specific only to the case (b)+. So, it's Ok with the case
(a).

When _zookeeper_close()_ is called in the (b) way, the following happens:
1. If there are requests waiting for responses in the _zh.sent_requests_ queue, they all are
removed from this queue and each of them is "completed" with the fake response having status
ZCLOSING. Such fake responses are put into the _zh.completions_to_process_ queue. It's Ok
2. But then, the _zh.completions_to_process_ queue is left unhandled. +Neither completion
callbacks are called, nor dynamic memory allocated for fake responses is freed.+
3. Different structures within _zh_ are dismissed and finally _zh_ is freed

This is illustrated on the screenshot attached to this ticket: you may see that the next instruction
to execute will be _free(zh)_ while _zh.completions_to_process_ queue is not empty (see the
"Variables" tab to the right).

*Consequently:*
1. At least there is definitely the +memory leak+ in the case (b)
2. And it looks like a great misbehavior not to call completions on sent requests in the case
(b) as soon as they are called with ZCLOSING in the case (a) -- so, the +completions leak+

To reproduce the case (b) do the following:
- open ZooKeeper session, connect to a server, receive and process connected-watch, etc.
- then call for example _zoo_acreate()_ with valid arguments -- it shall return ZOK
- then, immediately after it returned, call _zookeeper_close()_
- note that completion callback handler for _zoo_acreate()_ will not be called

To reproduce the case (a) do about the same, but call _zookeeper_close()_ from a watcher or
another completion callback handler.

To fix this I propose calling to _process_completions()_ from _destroy(zhandle_t *zh)_ as
it is done in _handle_error(zhandle_t *zh,int rc)_.



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