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https://issues.apache.org/jira/browse/ZOOKEEPER-4311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17355698#comment-17355698
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Enrico Olivelli commented on ZOOKEEPER-4311:
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very interesting!
I will review the patch
> Fsync errors are ignored in AtomicFileWritingIdiom
> --------------------------------------------------
>
> Key: ZOOKEEPER-4311
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-4311
> Project: ZooKeeper
> Issue Type: Bug
> Components: leaderElection, server
> Reporter: Dmitrii Kovalkov
> Priority: Major
> Labels: pull-request-available
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Class AtomicFileOutputStream has a non-trivial logic in its 'close' method.
> ([code|https://github.com/apache/zookeeper/blob/5c102298f8a160ea996be7b6d6f95189d4ff2f41/zookeeper-server/src/main/java/org/apache/zookeeper/common/AtomicFileOutputStream.java#L76-L106]).
> It ensures that data is persistently stored on the disk via 'flush' and
> 'fsync' to .tmp file, then tries to rename the file. In case of any errors,
> .tmp file is deleted and exception is thrown.
> AtomicFileWritingIdiom, which is based on AtomicFileOutputStream, only calls
> 'flush' explicitly. 'close' method is called via IOUtils.closeStream
> ([code|https://github.com/apache/zookeeper/blob/5c102298f8a160ea996be7b6d6f95189d4ff2f41/zookeeper-server/src/main/java/org/apache/zookeeper/common/AtomicFileWritingIdiom.java#L87]).
> But docs says that IOUtils.closeStream ignores IOException, which can happen
> during fsync.
> ([docs|https://hadoop.apache.org/docs/r2.7.3/api/org/apache/hadoop/io/IOUtils.html#closeStream(java.io.Closeable)]).
> As a result, in case of fsync errors, .tmp file is deleted, main file is not
> updated, but zookeeper ignores an exception and assumes that everything is ok.
> AtomicFileWritingIdiom is used in leader election to store 'currentEpoch' and
> 'acceptedEpoch' files. This bug theoreticly can lead to electing two leaders
> in one epoch in case of disk failures.
>
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