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https://issues.apache.org/jira/browse/LOG4J2-254?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13658333#comment-13658333
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Ralph Goers commented on LOG4J2-254:
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Remko, in practical terms what you are describing won't happen.
RollingFileManager needs to insure that the file is being rolled by only a
single thread or everything will be a mess. Notice that the setOutputStream is
only ever called from methods that are within synchronized constructs. So the
change in (1) is not needed.
Removing the synchronization on the write method as you suggest in (2) may
break things. Look at the comments on the method. We aren't synchronizing
because the value of os may change but because the OutputStream itself might
not be thread safe.
The third recommendation, while technically correct, has the problem that if
you have multiple threads calling close something is very broken. Again, only
one thread should ever attempt to close the stream. However, I don't see any
harm in making this change.
> Race condition when setting new filename in RollingFileAppender related code
> ----------------------------------------------------------------------------
>
> Key: LOG4J2-254
> URL: https://issues.apache.org/jira/browse/LOG4J2-254
> Project: Log4j 2
> Issue Type: Bug
> Components: Appenders
> Affects Versions: 2.0-beta7
> Reporter: Edward Sargisson
> Fix For: 2.0-beta7
>
>
> I've come across what very much looks like a race condition in log4j1. In
> reviewing the log4j 2 code I believe the same condition exists.
> OutputStreamManager.setOutputStream and OutputStreamManager.write need to
> have a happens-before edge inserted. You could either make
> OutputStreamManager.os volatile (best) or make setOutputStream synchronized.
> When the RollingFileAppender decides to roll the file it creates a new
> OutputStream and calls setOutputStream with it. If there is no happens-before
> edge then that write to OutputStreamManager.os may not be visible to all
> threads.
> Background:
> I've been attempting to find a way to have applications write logs to Flume
> without being halted if Flume is down or its channels fill up. My approach
> was to use the RollingFileAppender from apache-log4j-extras and configure it
> to roll every minute. Then I setup a Flume spooling directory source to read
> those files and forward them on.
> I've been having problems with Flume complaining that the rolled log file has
> changed. The spooling directory source checks this so that people do not
> attempt to use it on logs that are currently being written to.
> I caught an instance of this this afternoon.
> File: castellan-reader.20130514T2058.log.COMPLETED
> 2013-05-14 20:57:05,330 INFO ...
> File: castellan-reader.20130514T2058.log
> 2013-05-14 21:23:05,709 DEBUG ...
> Why would an event from 2123 be written into a file from 2058?
> My analysis of log4j 1 code is:
> My understanding of log4j shows that the RollingFileAppenders end up calling
> this:
> FileAppender:
> public synchronized void setFile(String fileName, boolean append, boolean
> bufferedIO, int bufferSize)
> Which shortly calls:
> this.qw = new QuietWriter(writer, errorHandler);
> However, the code to actually write to the writer is this:
> protected
> void subAppend(LoggingEvent event) {
> this.qw.write(this.layout.format(event));
> Unless I'm mistaken there's no happens-before edge between setting the qw and
> calling subappend. The code path to get to subAppend appears not to go
> through any method synchronized on FileAppender's monitor. this.qw is not
> volatile.
> Note that I haven't tested log4j 2 for this probable defect - I am raising
> this work item based on my reading of the code.
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