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https://issues.apache.org/jira/browse/ZOOKEEPER-1400?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13215121#comment-13215121
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Marshall McMullen commented on ZOOKEEPER-1400:
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Please see attached path that provides my new proposed functionality.
> Allow logging via callback instead of raw FILE pointer
> ------------------------------------------------------
>
> Key: ZOOKEEPER-1400
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1400
> Project: ZooKeeper
> Issue Type: Improvement
> Components: c client
> Affects Versions: 3.4.2
> Environment: Linux
> Reporter: Marshall McMullen
> Fix For: 3.4.2
>
> Attachments: case-2739.patch
>
>
> The existing logging framework inside the C client uses a raw FILE*. Using a
> FILE* is very limiting and potentially dangerous. A safer alternative is to
> just provide a callback that the C client will call for each message. In our
> environment, we saw some really nasty issues with multiple threads all
> connecting to zookeeper via the C Client related to the use of a raw FILE*.
> Specifically, if the FILE * is closed and that file descriptor is reused by
> the kernel before the C client is notified then the C client will use it's
> static global logStream pointer for subsequent logging messages. That FILE*
> is now a loose cannon! In our environment, we saw zookeeper log messages
> ending up in other sockets and even in our core data path. Clearly this is
> dangerous. In our particular case, we'd omitted a call to
> zoo_set_log_stream(NULL) to notify C client that the FILE* has been closed.
> However, even with that bug fixed, there's still a race condition where log
> messages in flight may be sent before the C client is notified of the FILE
> closure, and the same problem can happen.
> Other issues we've seen involved multiple threads, wherein one would close
> the FILE*, and that's a global change that affects all threads connected
> within that process. That's a pretty nasty limitation as well.
> My proposed change is to allow setting a callback for log messages. A
> callback is used in preference to a raw FILE*. If no callback is set, then it
> will fallback to the existing FILE*. If that's not set, then it falls back to
> stderr as it always has.
> While refactoring this code, I removed the need for the double parens in all
> the LOG macros as that wasn't necessary and didn't fit with my new approach.
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