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From "Alexander A. Strelets (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (ZOOKEEPER-2894) Memory and completions leak on zookeeper_close.
Date Fri, 08 Sep 2017 13:06:00 GMT

     [ https://issues.apache.org/jira/browse/ZOOKEEPER-2894?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Alexander A. Strelets updated ZOOKEEPER-2894:
---------------------------------------------
    Attachment: zk-will-free-zh-and-lose-completions.png

> 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
>              Labels: easyfix
>             Fix For: 3.4.10
>
>         Attachments: zk-will-free-zh-and-lose-completions.png
>
>
> 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 _zh.sent_requests_ queue+, they all
are removed from this queue and each of them is "completed" with personal fake response having
status ZCLOSING. Such fake responses are put into _zh.completions_to_process_ queue. It's
Ok
> 2. But then, _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).
> Alternatively, the same situation but in the case (a) is handled properly -- i.e., all
completion callback handlers are truly called with ZCLOSING and the memory is freed, both
for subcases (a.1) when there is a failure like connection-timeout, connection-closed, etc.,
or (a.2) there is not failure. The reason is that any callback handler (completion or watcher)
in the case (a) is called from the _process_completions()_ function which runs in the loop
until _zh.completions_to_process_ queue gets empty. So, this function guarantees this queue
to be completely processed even if new completions occur during reaction on previously queued
completions.
> *Consequently:*
> 1. At least there is definitely the +memory leak+ in the case (b) -- all the fake responses
put into _zh.completions_to_process_ queue are lost after _free(zh)_
> 2. And it looks like a great misbehavior not to call completions on sent requests in
the case (b) while they are called with ZCLOSING in the case (a) -- so, I think it's not "by
design" but a +completions leak+
> +To reproduce the case (b) do the following:+
> - open ZooKeeper session, connect to a server, receive and process connected-watch, etc.
> - then somewhere +from the main events loop+ 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 the following:+
> - the same as above, open ZooKeeper session, connect to a server, receive and process
connected-watch, etc.
> - the same as above, somewhere from the main events loop call for example _zoo_acreate()_
with valid arguments -- it shall return ZOK
> - but now don't call _zookeeper_close()_ immediately -- wait for completion callback
on the commenced request
> - when _zoo_acreate()_ completes, +from within its completion callback handler+, call
another _zoo_acreate()_ and immediately after it returned call _zookeeper_close()_
> - note that completion callback handler for the second _zoo_acreate()_ *will be called
with ZCLOSING, unlike the case (b) described above*
> 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)_.
> This is a proposed fix: https://github.com/apache/zookeeper/pull/363
> [upd]
> There are another tickets with about the same problem: ZOOKEEPER-1493, ZOOKEEPER-2073
(the "same" just according to their titles).
> However, as I can see, the corresponding patches were applied on the branch 3.4.10, but
the effect still persists -- so, this ticket does not duplicate the mentioned two.



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