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https://issues.apache.org/jira/browse/LOG4J2-254?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13658554#comment-13658554
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Edward Sargisson commented on LOG4J2-254:
-----------------------------------------
Hi all,
Thank you for looking at this defect and fixing it so quickly. I've had a look
and I'm not familiar enough with log4j2 code but it looks like
setOutputStream(( will refuse to set the os field if header is null.
AbstractLayout appears to be where header is stored and it states that header
may be null. The header == null probably just needs to be this.os = os;
> 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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