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From "Francois Ferrand (Jira)" <j...@apache.org>
Subject [jira] [Created] (SSHD-966) Deadlock on disconnection at the end of key-exchange
Date Mon, 10 Feb 2020 14:11:00 GMT
Francois Ferrand created SSHD-966:
-------------------------------------

             Summary: Deadlock on disconnection at the end of key-exchange
                 Key: SSHD-966
                 URL: https://issues.apache.org/jira/browse/SSHD-966
             Project: MINA SSHD
          Issue Type: Bug
            Reporter: Francois Ferrand


We are using git-repo to download projects from Gerrit server, using SSH.
Gerrit is in version 2.16.16. which uses SSHD 2.0.0 and Mina 2.0.17 with NIO2 backend.

One particularity of this setup is that git-repo creates a single control master channel,
and then downloads *lots* of Git repositories (500 repositories, some of them relatively large),
with some degree of parallelism. This takes a long time, lots of data, and the multiplexed
connections are handled by gerrit in multiple threads.

In some cases, we experience a deadlock when an error happens at the end of the key exchange,
while sending pending packets:

{noformat}
Warning, the following threads are deadlocked : SSH git-upload-pack /project1 (myuser), sshd-SshServer[df5f657]-nio2-thread-3

"SSH git-upload-pack /project1 (myuser)" prio=1 BLOCKED
	org.apache.sshd.common.session.helpers.AbstractSession.writePacket(AbstractSession.java:1107)
	org.apache.sshd.common.channel.AbstractChannel.writePacket(AbstractChannel.java:798)
	org.apache.sshd.common.channel.ChannelOutputStream.flush(ChannelOutputStream.java:227)
	org.apache.sshd.common.channel.ChannelOutputStream.write(ChannelOutputStream.java:127)
	org.eclipse.jgit.transport.UploadPack$ResponseBufferedOutputStream.write(UploadPack.java:2183)
	org.eclipse.jgit.transport.SideBandOutputStream.writeBuffer(SideBandOutputStream.java:174)
	org.eclipse.jgit.transport.SideBandOutputStream.write(SideBandOutputStream.java:153)
	org.eclipse.jgit.internal.storage.pack.PackOutputStream.write(PackOutputStream.java:132)
	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs2(PackFile.java:614)
	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs(PackFile.java:433)
	org.eclipse.jgit.internal.storage.file.WindowCursor.copyObjectAsIs(WindowCursor.java:221)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjectImpl(PackWriter.java:1644)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObject(PackWriter.java:1621)
	org.eclipse.jgit.internal.storage.pack.PackOutputStream.writeObject(PackOutputStream.java:171)
	org.eclipse.jgit.internal.storage.file.WindowCursor.writeObjects(WindowCursor.java:229)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1609)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1597)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writePack(PackWriter.java:1154)
	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:2133)
	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:1947)
	org.eclipse.jgit.transport.UploadPack.service(UploadPack.java:971)
	org.eclipse.jgit.transport.UploadPack.upload(UploadPack.java:776)
	com.google.gerrit.sshd.commands.Upload.runImpl(Upload.java:77)
	com.google.gerrit.sshd.AbstractGitCommand.service(AbstractGitCommand.java:98)
	com.google.gerrit.sshd.AbstractGitCommand.access$000(AbstractGitCommand.java:31)
	com.google.gerrit.sshd.AbstractGitCommand$1.run(AbstractGitCommand.java:63)
	com.google.gerrit.sshd.BaseCommand$TaskThunk.run(BaseCommand.java:467)
	com.google.gerrit.server.logging.LoggingContextAwareRunnable.run(LoggingContextAwareRunnable.java:83)
	java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	java.util.concurrent.FutureTask.run(FutureTask.java:266)
	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	com.google.gerrit.server.git.WorkQueue$Task.run(WorkQueue.java:646)
	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	java.lang.Thread.run(Thread.java:748)

"sshd-SshServer[df5f657]-nio2-thread-3" daemon prio=5 BLOCKED
	org.apache.sshd.common.channel.ChannelOutputStream.close(ChannelOutputStream.java:249)
	org.apache.sshd.common.util.io.IoUtils.closeQuietly(IoUtils.java:151)
	org.apache.sshd.server.channel.ChannelSession.doCloseImmediately(ChannelSession.java:205)
	org.apache.sshd.common.util.closeable.AbstractCloseable.close(AbstractCloseable.java:83)
	org.apache.sshd.common.channel.AbstractChannel.close(AbstractChannel.java:576)
	org.apache.sshd.common.util.closeable.ParallelCloseable.doClose(ParallelCloseable.java:65)
	org.apache.sshd.common.util.closeable.SimpleCloseable.close(SimpleCloseable.java:63)
	org.apache.sshd.common.util.closeable.AbstractInnerCloseable.doCloseImmediately(AbstractInnerCloseable.java:46)
	org.apache.sshd.common.util.closeable.AbstractCloseable.close(AbstractCloseable.java:83)
	org.apache.sshd.common.util.closeable.ParallelCloseable.doClose(ParallelCloseable.java:65)
	org.apache.sshd.common.util.closeable.SimpleCloseable.close(SimpleCloseable.java:63)
	org.apache.sshd.common.util.closeable.SequentialCloseable$1.operationComplete(SequentialCloseable.java:56)
	org.apache.sshd.common.util.closeable.SequentialCloseable$1.operationComplete(SequentialCloseable.java:45)
	org.apache.sshd.common.util.closeable.SequentialCloseable.doClose(SequentialCloseable.java:69)
	org.apache.sshd.common.util.closeable.SimpleCloseable.close(SimpleCloseable.java:63)
	org.apache.sshd.common.util.closeable.AbstractInnerCloseable.doCloseImmediately(AbstractInnerCloseable.java:46)
	org.apache.sshd.common.util.closeable.AbstractCloseable.close(AbstractCloseable.java:83)
	org.apache.sshd.common.session.helpers.AbstractSession.exceptionCaught(AbstractSession.java:973)
	org.apache.sshd.common.session.helpers.AbstractSessionIoHandler.exceptionCaught(AbstractSessionIoHandler.java:53)
	org.apache.sshd.common.io.nio2.Nio2Session.exceptionCaught(Nio2Session.java:186)
	org.apache.sshd.common.io.nio2.Nio2Session.handleWriteCycleFailure(Nio2Session.java:460)
	org.apache.sshd.common.io.nio2.Nio2Session$2.onFailed(Nio2Session.java:415)
	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.lambda$failed$1(Nio2CompletionHandler.java:46)
	org.apache.sshd.common.io.nio2.Nio2CompletionHandler$$Lambda$627/1874891543.run(Unknown Source)
	java.security.AccessController.doPrivileged(Native Method)
	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.failed(Nio2CompletionHandler.java:45)
	sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:128)
	sun.nio.ch.Invoker.invokeDirect(Invoker.java:157)
	sun.nio.ch.UnixAsynchronousSocketChannelImpl.implWrite(UnixAsynchronousSocketChannelImpl.java:736)
	sun.nio.ch.AsynchronousSocketChannelImpl.write(AsynchronousSocketChannelImpl.java:382)
	sun.nio.ch.AsynchronousSocketChannelImpl.write(AsynchronousSocketChannelImpl.java:399)
	org.apache.sshd.common.io.nio2.Nio2Session.doWriteCycle(Nio2Session.java:401)
	org.apache.sshd.common.io.nio2.Nio2Session.startWriting(Nio2Session.java:386)
	org.apache.sshd.common.io.nio2.Nio2Session.writePacket(Nio2Session.java:169)
	org.apache.sshd.common.session.helpers.AbstractSession.doWritePacket(AbstractSession.java:1181)
	org.apache.sshd.common.session.helpers.AbstractSession.sendPendingPackets(AbstractSession.java:910)
	org.apache.sshd.common.session.helpers.AbstractSession.handleNewKeys(AbstractSession.java:875)
	org.apache.sshd.common.session.helpers.AbstractSession.doHandleMessage(AbstractSession.java:606)
	org.apache.sshd.common.session.helpers.AbstractSession.handleMessage(AbstractSession.java:555)
	org.apache.sshd.common.session.helpers.AbstractSession.decode(AbstractSession.java:1527)
	org.apache.sshd.common.session.helpers.AbstractSession.messageReceived(AbstractSession.java:516)
	org.apache.sshd.common.session.helpers.AbstractSessionIoHandler.messageReceived(AbstractSessionIoHandler.java:63)
	org.apache.sshd.common.io.nio2.Nio2Session.handleReadCycleCompletion(Nio2Session.java:339)
	org.apache.sshd.common.io.nio2.Nio2Session$1.onCompleted(Nio2Session.java:318)
	org.apache.sshd.common.io.nio2.Nio2Session$1.onCompleted(Nio2Session.java:315)
	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.lambda$completed$0(Nio2CompletionHandler.java:38)
	org.apache.sshd.common.io.nio2.Nio2CompletionHandler$$Lambda$349/1690110070.run(Unknown Source)
	java.security.AccessController.doPrivileged(Native Method)
	org.apache.sshd.common.io.nio2.Nio2CompletionHandler.completed(Nio2CompletionHandler.java:37)
	sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:126)
	sun.nio.ch.Invoker$2.run(Invoker.java:218)
	sun.nio.ch.AsynchronousChannelGroupImpl$1.run(AsynchronousChannelGroupImpl.java:112)
	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	java.lang.Thread.run(Thread.java:748)

"SSH git-upload-pack /project2 (myuser)" prio=1 BLOCKED
	org.apache.sshd.common.session.helpers.AbstractSession.writePacket(AbstractSession.java:1107)
	org.apache.sshd.common.channel.AbstractChannel.writePacket(AbstractChannel.java:798)
	org.apache.sshd.common.channel.ChannelOutputStream.flush(ChannelOutputStream.java:227)
	org.apache.sshd.common.channel.ChannelOutputStream.write(ChannelOutputStream.java:127)
	org.eclipse.jgit.transport.UploadPack$ResponseBufferedOutputStream.write(UploadPack.java:2183)
	org.eclipse.jgit.transport.SideBandOutputStream.writeBuffer(SideBandOutputStream.java:174)
	org.eclipse.jgit.transport.SideBandOutputStream.write(SideBandOutputStream.java:153)
	org.eclipse.jgit.internal.storage.pack.PackOutputStream.write(PackOutputStream.java:132)
	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs2(PackFile.java:614)
	org.eclipse.jgit.internal.storage.file.PackFile.copyAsIs(PackFile.java:433)
	org.eclipse.jgit.internal.storage.file.WindowCursor.copyObjectAsIs(WindowCursor.java:221)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjectImpl(PackWriter.java:1644)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObject(PackWriter.java:1621)
	org.eclipse.jgit.internal.storage.pack.PackOutputStream.writeObject(PackOutputStream.java:171)
	org.eclipse.jgit.internal.storage.file.WindowCursor.writeObjects(WindowCursor.java:229)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1609)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writeObjects(PackWriter.java:1597)
	org.eclipse.jgit.internal.storage.pack.PackWriter.writePack(PackWriter.java:1154)
	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:2133)
	org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:1947)
	org.eclipse.jgit.transport.UploadPack.service(UploadPack.java:971)
	org.eclipse.jgit.transport.UploadPack.upload(UploadPack.java:776)
	com.google.gerrit.sshd.commands.Upload.runImpl(Upload.java:77)
	com.google.gerrit.sshd.AbstractGitCommand.service(AbstractGitCommand.java:98)
	com.google.gerrit.sshd.AbstractGitCommand.access$000(AbstractGitCommand.java:31)
	com.google.gerrit.sshd.AbstractGitCommand$1.run(AbstractGitCommand.java:63)
	com.google.gerrit.sshd.BaseCommand$TaskThunk.run(BaseCommand.java:467)
	com.google.gerrit.server.logging.LoggingContextAwareRunnable.run(LoggingContextAwareRunnable.java:83)
	java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	java.util.concurrent.FutureTask.run(FutureTask.java:266)
	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	com.google.gerrit.server.git.WorkQueue$Task.run(WorkQueue.java:646)
	java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	java.lang.Thread.run(Thread.java:748)
{noformat}

In AbstractSession.handleNewKeys(), there is a lock on {{pendingEvents}}, while trying to
send these packets; when sending the packets fails, an exception is generated, which causes
the channel to get closed by calling ChannelOutputStream.close(), which is a synchronized
method.

At the same time, some other threads are trying to write data to the session: it calls ChannelOutputStream.write(),
which is also a synchronized method, which calls AbstractSession.writePacket() which attempts
to lock {{pendingPackets}} to queue the packets.

*NOTE*: in our setup, we can reproduce this quite easily by increasing the parallelism in
git-repo, reducing the values of waitTimeout (WAIT_FOR_SPACE_TIMEOUT), rekeyBytesLimit (REKEY_TIME_LIMIT)
and rekeyTimeLimit (REKEY_TIME_LIMIT) configuration options.



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