[Bug 49693] Intermittently Log4j fails to bind with log file

[bu...@apache.org]

https://issues.apache.org/bugzilla/show_bug.cgi?id=49693

--- Comment #2 from Fabrice <fa...@eileo.com> ---
Hi,

I came accross this issue while researching a similar behavior on our systems.

When rolling at midnight (using companions, we create a folder based on date)
we intermittently lose the logs.

Bascially the appender gets broken in a way a no new log file is created.

Looking at the code there is a race condition in FileAppender in the following
block:

String parentName = new File(fileName).getParent();
      if (parentName != null) {
        File parentDir = new File(parentName);
        if ((!parentDir.exists()) && (parentDir.mkdirs()))
          ostream = new FileOutputStream(fileName, append);
        else
          throw ex;
      }

This is run when a new folder structure must be created by the appender. If two
different appenders try to create the parent directory at the same time, then
both will get "false" when testing for parent dir existence, but only one will
get "true" upon directory creation. The second appender will then fail and
rethrow the original exception, and log4j presumably closes the appender after
that, which prevents any further attempt to write to the appender.

Only solution so far is to reload log4j configuration.

My assumption is that this code tries to protect against errors while trying to
create the directories. To keep the protection a double check pattern should be
added to re-test for parent dir existence and only fail if it does not exist
during the second test. Another option would be to synchronize on a static
monitor object at the FileAppender level so that two different appenders could
not enter that section at the same time.

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