[crac] RFR: Linux file system watcher support

[joeylee]

On Tue, 16 May 2023 08:30:41 GMT, Radim Vansa <duke at openjdk.org> wrote:

>> inotify monitors changes on filesystem, this support automatic restore for LinuxFileWatcher.
>> 
>> FileWatcherAfterRestoreTest verifies watcher service works after restore.
>> FileWatcherTest verifies automatic closing inotiify fd
>> 
>> The watcher keys are still managed by user, so exception will be thrown if no watcher keys are leaked, as in FileWatcherWithOpenKeysTest
>
> I was hoping that a fastdebug build with assertions on would fail somewhere earlier but it is not the case. I would suspect that since this NULL dereference is happening when the thread already exits there's an earlier event that goes wrong.

@rvansa  Yes, it seems a obj monitor in in use list is wrong. This is my [gdb debug history](https://gist.github.com/joeyleeeeeee97/96213d4dfd655894d86f7e36531f03f2)

I noticed 

(gdb) p ObjectSynchronizer::_in_use_list->_head->_next_om->_next_om->_object->_obj
$9 = (oop *) 0x36383631706d6574

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PR Comment: https://git.openjdk.org/crac/pull/72#issuecomment-1549238512