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From "Richard Shapiro (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COMPRESS-357) BZip2CompressorOutputStream can affect output stream incorrectly
Date Mon, 13 Jun 2016 01:28:21 GMT

    [ https://issues.apache.org/jira/browse/COMPRESS-357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15326728#comment-15326728
] 

Richard Shapiro commented on COMPRESS-357:
------------------------------------------

I just want to point out that the real problem with this is the presence of a finalize() method.
Any code that requires that finalize() be run is almost certainly incorrect, as you can never
rely on it ever being called. Since the class is thread-unsafe, there is in practice no additional
performance cost to synchronizing both flush() and finish(), since if the lock isn't essentially
free, you aren't using the class correctly either (i.e., you'd better be calling write() and
flush() inside some other synchronized block, or else guarantee they are on the same thread).


The other thing to remember is that if it's incorrect, it doesn't matter how fast something
is, and bzip2 is already so slow that I doubt you will ever see the synchronization cost even
if you do multi-thread it.

By far the most correct solution is to get rid of finish().   

> BZip2CompressorOutputStream can affect output stream incorrectly
> ----------------------------------------------------------------
>
>                 Key: COMPRESS-357
>                 URL: https://issues.apache.org/jira/browse/COMPRESS-357
>             Project: Commons Compress
>          Issue Type: Bug
>          Components: Compressors
>    Affects Versions: 1.9, 1.11
>         Environment: multithreaded
>            Reporter: Richard Shapiro
>              Labels: easyfix
>             Fix For: 1.12
>
>   Original Estimate: 1h
>  Remaining Estimate: 1h
>
> BZip2CompressorOutputStream has an unsynchronized finished() method, and an unsynchronized
finalize method. Finish checks to see if the output stream is null, and if it is not it calls
various methods, some of which write to the output stream. 
> Now, consider something like this sequence.
> BZip2OutputStream s = ...
> ...
> s.close();
> s = null;
> After the s = null, the stream is garbage. At some point the garbage collector call finalize(),
which calls finish(). But, since the GC may be on a different thread, there is no guarantee
that the assignment this.out = null in finish() has actually been made visible to the GC thread,
which results in bad data in the output stream.
> This is not a theoretical problem; In a part of a large project I'm working on, this
happens about 2% of the time. 
> The fixes are simple
> 1) synchronize finish() or
>   
> 2) don't call finish from finalize().
> A workaround is to derive a class and override the finalize() method. 



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