LOG4J2-3452: Fix race condition in FileUtils.mkdir()

[stefanvodita]

A race condition could appear when 2 threads were trying to create the
same directory. Both would check that the directory did not exist, then
one would create the directory and the other would throw an exception.

This was reproduced with a unit test included this commit and fixed by
using Files.createDirectories() instead of File.mkdirs(), so that the
existence check and directory creation are done as an atomic operation.

Note: I didn't manage to run the tests with Maven, but I ran the tests in
FileUtilsTest from IntelliJ. I'll come back to this when I have more time.

[ppkarwasz]

@stefanvodita,
Thank you for your contribution.

[merkisoft]

if the directory exists, it still tries to create it - which breaks when using a policy file - you should have left "if exists just return"

[stefanvodita]

Just to check that I understand the problem - is there an exception you are encountering, which we could have caught when calling Files.createDirectories(dir.toPath());? If you have a test that reproduces the issue, that would be very helpful.

[merkisoft]

the problem is that althought the logs dir exists, you call to create it and java security manager kicks in and it requires these two lines (i only had the 2nd initially, because in my view log4j should not attempt to write the dir):

permission java.io.FilePermission "${user.dir}/logs", "read,write";    //  <<< this was missing and grants mkdir on the parent
permission java.io.FilePermission "${user.dir}/logs/-", "read,write";

config - using log4j 1-2 bridge:
log4j.appender.file=org.apache.log4j.DailyRollingFileAppender
log4j.appender.file.File=logs/kurs.log

anyhow: the java security manager / policy files are not often used and are apparently on the way out, no need to fix - hopefully the one who has the same issue finds this comment :-)

[ppkarwasz]

@merkisoft,

The main point of @stefanvodita's PR is that File.mkdirs returns false if the directory exits or a file system error occurs, we can not differentiate the two cases. Files.createDirectories on the other hand throws an exception only in the latter case.

I don't see anything wrong in adding a Files.exists call in front of Files.createDirectories. You can create a PR for it.

Remark: with Files.exists you still need to add a "read" permission on the log dir.