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https://issues.apache.org/jira/browse/LOG4J2-254?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13657772#comment-13657772
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Gary Gregory commented on LOG4J2-254:
-------------------------------------
Looking at OutputStreamManager, it looks like the footer and header should also
be volatile. Alternatively, and preferably, they should be *final*. In order to
make them final, the OutputStreamManager should be passed in the layout, which
trickles down into the factory code. The current logic of not overwritting the
foot and layout feels like a workaround for the fact that footer and headers
are not declared final.
> 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
>
> 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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