fix: unlock mailboxes after OOM

[ChristophWurst]

In contrast to try-catch and an error handler we can leverage a shutdown handler to still unlock mailboxes after running out of memory.

$ MEMPROF_PROFILE=dump_on_limit php -d memory_limit=59M ../../occ mail:account:sync 4783 -vvv
The current PHP memory limit is below the recommended value of 512MB.
[debug] account is up to date, skipping mailbox sync
[debug] Skipping mailbox sync for 8992
[debug] Skipping mailbox sync for 8993
[debug] Skipping mailbox sync for 8994
[debug] Skipping mailbox sync for 8995
[debug] Skipping mailbox sync for 8996
[debug] Skipping mailbox sync for 8997
[debug] Skipping mailbox sync for 8998
[debug] Skipping mailbox sync for 8999
[debug] Syncing 9000
[debug] Locking mailbox 9000 for new messages sync
[debug] Locking mailbox 9000 for changed messages sync
[debug] Locking mailbox 9000 for vanished messages sync
[debug] Running partial sync for 9000with criteria 42
[debug] partial sync 4783:Sent - get all known UIDs took 0s. 53/55MB memory used
[debug] partial sync 4783:Sent - get new messages via Horde took 1s. 54/55MB memory used
[debug] partial sync 4783:Sent - persist new messages took 0s. 55/55MB memory used
[debug] partial sync 4783:Sent - get changed messages via Horde took 0s. 55/55MB memory used
[debug] partial sync 4783:Sent - persist changed messages took 0s. 55/55MB memory used
[debug] partial sync 4783:Sent - get vanished messages via Horde took 0s. 55/55MB memory used
[debug] partial sync 4783:Sent - delete vanished messages took 0s. 55/55MB memory used
[debug] partial sync 4783:Sent took 1s
[debug] Unlocking mailbox 9000 from vanished messages sync
[debug] Unlocking mailbox 9000 from changed messages sync
[debug] Unlocking mailbox 9000 from new messages sync
[debug] Skipping mailbox sync for 9001
[debug] Skipping mailbox sync for 9002
[debug] Skipping mailbox sync for 9003
[debug] Skipping mailbox sync for 9004
[debug] Skipping mailbox sync for 9005
[debug] Skipping mailbox sync for 9006
[debug] Skipping mailbox sync for 9007
[debug] Skipping mailbox sync for 9008
[debug] Skipping mailbox sync for 9009
[debug] Skipping mailbox sync for 9010
[debug] Skipping mailbox sync for 9011
[debug] Skipping mailbox sync for 9012
[debug] Syncing 9013
[debug] Locking mailbox 9013 for new messages sync
[debug] Locking mailbox 9013 for changed messages sync
[debug] Locking mailbox 9013 for vanished messages sync
[debug] Running partial sync for 9013with criteria 42
PHP Fatal error:  Allowed memory size of 61865984 bytes exhausted (tried to allocate 20480 bytes) (memprof dumped to /tmp/memprof.callgrind.1856999238350022) in /nextcloud/3rdparty/doctrine/dbal/src/Driver/PDO/Result.php on line 119
Stack trace:
#0 /nextcloud/3rdparty/doctrine/dbal/src/Driver/PDO/Result.php(119): PDOStatement->fetchAll(2)
#1 /nextcloud/3rdparty/doctrine/dbal/src/Driver/PDO/Result.php(59): Doctrine\DBAL\Driver\PDO\Result->fetchAll(2)
#2 /nextcloud/3rdparty/doctrine/dbal/src/Result.php(103): Doctrine\DBAL\Driver\PDO\Result->fetchAllAssociative()
#3 /nextcloud/lib/private/DB/ResultAdapter.php(60): Doctrine\DBAL\Result->fetchAllAssociative()
#4 /nextcloud/apps/mail/lib/Db/MessageMapper.php(79): OC\DB\ResultAdapter->fetchAll()
#5 /nextcloud/apps/mail/lib/Db/MessageMapper.php(141): OCA\Mail\Db\MessageMapper->findUids(Object(OC\DB\QueryBuilder\QueryBuilder))
#6 /nextcloud/apps/mail/lib/Service/Sync/ImapToDbSynchronizer.php(415): OCA\Mail\Db\MessageMapper->findAllUids(Object(OCA\Mail\Db\Mailbox))
#7 /nextcloud/apps/mail/lib/Service/Sync/ImapToDbSynchronizer.php(277): OCA\Mail\Service\Sync\ImapToDbSynchronizer->runPartialSync(Object(OCA\Mail\IMAP\HordeImapClient), Object(OCA\Mail\Account), Object(OCA\Mail\Db\Mailbox), Object(OCA\Mail\Support\ConsoleLoggerDecorator), true, 42, NULL)
#8 /nextcloud/apps/mail/lib/Service/Sync/ImapToDbSynchronizer.php(134): OCA\Mail\Service\Sync\ImapToDbSynchronizer->sync(Object(OCA\Mail\Account), Object(OCA\Mail\IMAP\HordeImapClient), Object(OCA\Mail\Db\Mailbox), Object(OCA\Mail\Support\ConsoleLoggerDecorator), 42, NULL, false, true)
#9 /nextcloud/apps/mail/lib/Command/SyncAccount.php(88): OCA\Mail\Service\Sync\ImapToDbSynchronizer->syncAccount(Object(OCA\Mail\Account), Object(OCA\Mail\Support\ConsoleLoggerDecorator), false)
#10 /nextcloud/apps/mail/lib/Command/SyncAccount.php(72): OCA\Mail\Command\SyncAccount->sync(Object(OCA\Mail\Account), false, Object(Symfony\Component\Console\Output\ConsoleOutput))
#11 /nextcloud/3rdparty/symfony/console/Command/Command.php(326): OCA\Mail\Command\SyncAccount->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#12 /nextcloud/3rdparty/symfony/console/Application.php(1078): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#13 /nextcloud/3rdparty/symfony/console/Application.php(324): Symfony\Component\Console\Application->doRunCommand(Object(OCA\Mail\Command\SyncAccount), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#14 /nextcloud/3rdparty/symfony/console/Application.php(175): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#15 /nextcloud/lib/private/Console/Application.php(187): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#16 /nextcloud/console.php(92): OC\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput))
#17 /nextcloud/occ(33): require_once('/nextcloud...')
#18 {main}
[debug] Unlocking mailbox 9013 from vanished messages sync
[debug] Unlocking mailbox 9013 from changed messages sync
[debug] Unlocking mailbox 9013 from new messages sync

On main I would have to wait 5 minutes or unlock the mailbox manually. Here I can try again right away.

This doesn't fix the root cause of the OOM, but it makes it a bit more pleasant to retry and continue using the app.

[ChristophWurst]

/backport to stable5.7

[kesselb]

Suggested change

	$oomBuffer = str_repeat(' ', 1024 * 1024);
	$oomBuffer = random_bytes(1024);

[kesselb]

Passing $oomBuffer as reference seems to break psalm.
It seems to work also without the oomBuffer 🤷

[ChristophWurst]

Yes it probably works because we already have loaded the classes necessary for unlocking