Zombie FileHandler locks can exhaust all available log file locks. (original) (raw)

Daniel Fuchs daniel.fuchs at oracle.com
Mon Jun 23 14:42:33 UTC 2014

Hi,

I wonder whether the following patch would solve the issue. In the case where the lock file already exists, we check whether it's writable, and whether its directory is writable, and if so, we open it for 'write+append', otherwise, we continue with the next lock name.

I we have manage to open the lock file, then we proceed with trying to lock it, as before. If locking succeeds - we use that file. If locking throws exception - we only fall through if we actually created the file, otherwise we proceed with the next lock name.

Would that make sense? It passes the CheckLockLocationTest that was added for 6244047. Note: I also added a deleteOnExit() hook for good measure.

-- daniel

--- a/src/share/classes/java/util/logging/FileHandler.java +++ b/src/share/classes/java/util/logging/FileHandler.java @@ -25,6 +25,7 @@

package java.util.logging;

+import static java.nio.file.StandardOpenOption.APPEND; import static java.nio.file.StandardOpenOption.CREATE_NEW; import static java.nio.file.StandardOpenOption.WRITE;

@@ -35,6 +36,8 @@ import java.io.OutputStream; import java.nio.channels.FileChannel; import java.nio.file.FileAlreadyExistsException; +import java.nio.file.Files; +import java.nio.file.Path; import java.nio.file.Paths; import java.security.AccessController; import java.security.PrivilegedAction; @@ -434,24 +437,44 @@ continue; }

FileChannel.open(Paths.get(lockFileName),

Files.isWritable(lockFilePath.getParent())) {

supported // on the target directory. We have to proceed without

On 6/23/14 3:18 PM, Daniel Fuchs wrote:

Hi Jason,

On 6/23/14 2:51 PM, Jason Mehrens wrote: Daniel,

My understanding is that changing CREATENEW to use CREATE would make it work like does in JDK7. Closing the lock files when the FileHandler is unreferenced I is probably the fix for JDK-6774110: lock file is not deleted when child logger is used. I'll see if I can dig more info about the reason for the change. Thanks for pointing at JDK-6774110 - I'll have a look at that as well. -- daniel

If we could prove that system FileLock is mandatory we could use the JDK7 behavior otherwise if they are advisory then use the JDK8 behavior. If we knew the boot time of the JVM host O/S we could delete all lock files older than the boot time + some constant. Jason

---------------------------------------- Date: Mon, 23 Jun 2014 11:48:18 +0200 From: daniel.fuchs at oracle.com To: jasonmehrens at hotmail.com; core-libs-dev at openjdk.java.net Subject: Re: Zombie FileHandler locks can exhaust all available log file locks. Hi Jason, Looking at the diff for 6244047 - I see that, as you pointed out, the unwanted behavior described comes from the fact that the new code is using CREATENEW - which prevents the 'zombie lock files' from being reused. I am not an expert in file locks - but I wonder whether we could revert to using CREATE instead: wouldn't tryLock() later tell us if the file is used by another entity? Another possibility would be to combine CREATENEW and DELETEONCLOSE, which according to StandardOpenOptions will attempt to delete the file on JVM termination if close() hasn't been called. This probably wouldn't help in case on VM crash, but it would help in the case demonstrated by your test below. I have however some reluctance because I see that we call FileChannel.close() in the case were the lock can't be obtained, and I'm not sure what that would do... Also StandardOpenOptions has some warning about using DELETEONCLOSE for files that can be opened concurrently by other entities - so I'm not sure it would be appropriate. The last possibility I see would be to change the lock HashMap to take instances of a subclass of WeakReference as values (instead of String), and add some code that attempts to close & remove the lock file when the FileHandler is no longer referenced. Again - this will probably not help in the case of crash, and also adds the question on when the weak reference queue should be polled to ensure that the no longer referenced FileChannel are closed in a timely fashion. All in all - I feel our best options would be to revert to using CREATE, or use CREATENEW + DELETEONCLOSE, or not do anything and live with the issue. Hopefully some nio experts will chime in ;-) On another track - we could also make MAX configurable - but that would just be shifting the issue around - wouldn't it? best regards, -- daniel On 6/20/14 10:54 PM, Jason Mehrens wrote: Daniel, Jim,

In JDK8 the FileHandler file locking was changed to use FileChannel.open with CREATENEW. If the file exists (locked or not) the FileHandler will rotate due to safety concerns about writing to the same log file. The FileHandler has an upper bound of 100 as the number of file locks that can be attempted to be acquired. Given the right pattern, enough time, and enough failures it seems at it is possible for a program to end up in a state where the FileHandler is surrounded by zombie lock files refuses log or perform a clean up action. (A.K.A Farmer Rick Grimes). This means that the lck files have to manually be deleted or the FileHandler will just fail with an IOException every time it is created. A simple test to emulate crashing is to run (JDK7 vs. JDK8) the following code twice without deleting the log and lck files: public static void main(String[] args) throws Exception { System.out.println(System.getProperty("java.runtime.version")); ReferenceQueue q = new ReferenceQueue<>(); for (int i=0; i<100; i++) {_ _WeakReference h = new WeakReference<>( new FileHandler("./test%u.log", 10000, 2, true), q); while (q.poll() != h) { System.runFinalization(); System.gc(); System.runFinalization(); Thread.yield(); } } } I understand that if you can't trust that the file locking always works then there isn't much that can be done. Leaving the number of locks as unbounded isn't really an option either. Seems like there should be a way to identify zombie lock files (ATOMICMOVE) and delete them. Any thoughts on this? The source discussion on this can be found at http://stackoverflow.com/questions/24321098/is-java-util-logging-filehandler-in-java-8-broken

Regards, Jason

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