AfsExceptionExclusiveAttemptConflict: File is already used by a different exclusive user on restart

[hg-ms]

Caused by: one.microstream.afs.exceptions.AfsExceptionExclusiveAttemptConflict: File is already used by a different exclusive user: one.microstream.afs.types.AFile$Default@6327b54d("storage/channel_3/transactions_3.sft"). Exclusive user: (java.lang.Thread@8674c87) Thread[MicroStream-StorageChannel-3,5,]. Attempting user: (java.lang.Thread@6a0b3fb) Thread[MicroStream-StorageChannel-3,5,main].
	at one.microstream.afs.types.AccessManager$Default$2.handleExclusiveAttemptConflict(AccessManager.java:776)
	at one.microstream.afs.types.AccessManager$Default.internalUseWriting(AccessManager.java:664)
	at one.microstream.afs.types.AccessManager$Default.useWriting(AccessManager.java:497)
	at one.microstream.afs.types.AFile.useWriting(AFile.java:108)
	at one.microstream.afs.types.AFS.applyWriting(AFS.java:272)
	at one.microstream.afs.types.AFS.applyWriting(AFS.java:262)
	at one.microstream.afs.types.AFile.ensureExists(AFile.java:157)
	at one.microstream.storage.types.StorageFileManager$Default.createTransactionsFile(StorageFileManager.java:1117)
	at one.microstream.storage.types.StorageFileManager$Default.readTransactionsFile(StorageFileManager.java:727)
	at one.microstream.storage.types.StorageFileManager$Default.readStorage(StorageFileManager.java:712)
	at one.microstream.storage.types.StorageChannel$Default.readStorage(StorageChannel.java:745)
	at one.microstream.storage.types.StorageChannelTaskInitialize$Default.internalProcessBy(StorageChannelTaskInitialize.java:195)
	at one.microstream.storage.types.StorageChannelTaskInitialize$Default.internalProcessBy(StorageChannelTaskInitialize.java:1)
	at one.microstream.storage.types.StorageChannelTask$Abstract.processBy(StorageChannelTask.java:222)
	at one.microstream.storage.types.StorageChannel$Default.work(StorageChannel.java:422)
	at one.microstream.storage.types.StorageChannel$Default.run(StorageChannel.java:509)
	at java.lang.Thread.run(Thread.java:748)

After fixing #279 that exception gets reproducible thrown after restarting a storage several times if the continuous Backup is enabled.

The Exception is thrown because the AccessManager instance does not deregister the affected file on shutdown. At the next restart the error is caused.

[hg-ms]

Here is why the transaction log file is not released during the shutdown:

The shutdown in the StorageSystem class has several steps:

1. The continuous backup is stopped
2. The channels are stopped
3. When all channels are stopped successfully the StorageOperationController gets disabled.

The StorageOperationController gets stopped in StorageChannelTaskShutdown.Default.succeed(StorageChannel, Void)
The continuous backup has a dependency to the StorageOperationController therefore it gets stopped first.
When the Backup gets stopped the StorageBackupHandler will process all currently enqueued items before it will stop. But after that it is still possible that the still active StorageChannels enqueue new backup tasks. Those tasks are created during the housekeeping phase of the storage channels.
With the backup-handler already disabled those tasks won’t be processed any more an the “release” of the related files in the AccessManager will not happen.