Repository: borgbackup/borg

Issue:
Non fatal bug in removing lock after archive creation
Hi,

I run borg 1.2.6 like this:

`borg create --verbose --filter AME --list --stats --show-rc --compression auto,lzma,6 --noflags --noacls --noxattrs --exclude-caches "::${BORG_BACKUP_NAME_PREFIX}$(date '+%Y-%m-%d_%H-%M-%S')" "${BORG_BACKUP_SOURCE}" --exclude-from "${BORG_BACKUP_EXCLUDE_LIST}"`
(with BORG_REPO and BORG_PASSPHRASE set in environment)

and after most backups I get this non fatal exception and rc 2:

```
------------------------------------------------------------------------------
Repository: [snip]
Archive name: [snip]-2023-11-14_21-21-07
Archive fingerprint: [snip]
Time (start): Tue, 2023-11-14 21:21:08
Time (end):   Tue, 2023-11-14 21:22:01
Duration: 53.10 seconds
Number of files: 552009
Utilization of max. archive size: 0%
------------------------------------------------------------------------------
                       Original size      Compressed size    Deduplicated size
This archive:               31.26 GB             19.19 GB            451.76 kB
All archives:               29.34 TB             26.86 TB            772.90 GB

                       Unique chunks         Total chunks
Chunk index:                 1196589             38684220
------------------------------------------------------------------------------
Exception ignored in: <function Repository.__del__ at 0x7f9f2378a020>
Traceback (most recent call last):
  File "/usr/lib64/python3.11/site-packages/borg/repository.py", line 189, in __del__
    self.close()
  File "/usr/lib64/python3.11/site-packages/borg/repository.py", line 478, in close
    self.lock.release()
  File "/usr/lib64/python3.11/site-packages/borg/locking.py", line 417, in release
    self._roster.modify(EXCLUSIVE, REMOVE)
  File "/usr/lib64/python3.11/site-packages/borg/locking.py", line 316, in modify
    elements.remove(self.id)
KeyError: ('[snip]@141812767217169', 565630, 0)
Local Exception
OSError: [Errno 95] Operation not supported

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib64/python3.11/site-packages/borg/archiver.py", line 5324, in main
    exit_code = archiver.run(args)
                ^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/site-packages/borg/archiver.py", line 5255, in run
    return set_ec(func(args))
                  ^^^^^^^^^^
  File "/usr/lib64/python3.11/site-packages/borg/archiver.py", line 168, in wrapper
    with repository:
  File "/usr/lib64/python3.11/site-packages/borg/repository.py", line 214, in __exit__
    self.close()
  File "/usr/lib64/python3.11/site-packages/borg/repository.py", line 478, in close
    self.lock.release()
  File "/usr/lib64/python3.11/site-packages/borg/locking.py", line 417, in release
    self._roster.modify(EXCLUSIVE, REMOVE)
  File "/usr/lib64/python3.11/site-packages/borg/locking.py", line 320, in modify
    self.save(roster)
  File "/usr/lib64/python3.11/site-packages/borg/locking.py", line 291, in save
    with open(self.path, "w") as f:
OSError: [Errno 95] Operation not supported

Platform: Linux [snip] 6.5.10-200.fc38.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Nov  2 19:59:55 UTC 2023 x86_64
Linux: Unknown Linux  
Borg: 1.2.6  Python: CPython 3.11.6 msgpack: 1.0.4 fuse: llfuse 1.4.2 [pyfuse3,llfuse]
PID: 565630  CWD: /home/[snip]
sys.argv: ['/usr/bin/borg', 'create', '--verbose', '--filter', 'AME', '--list', '--stats', '--show-rc', '--compression', 'auto,lzma,6', '--noflags', '--noacls', '--noxattrs', '--exclude-caches', '::[snip]-2023-11-14_21-21-07', '/home/[snip]', '--exclude-from', '/home/[snip]']
SSH_ORIGINAL_COMMAND: None

terminating with error status, rc 2
```

after some local messing around with the borg python files I found that the lock here gets "removed" twice:
https://github.com/borgbackup/borg/blob/2cefe8f53db4a9e0cdd1eeff1b06ebdab0f18ae8/src/borg/locking.py#L316

When looking at the trace back the last superfluous unlock comes from error handler in 

https://github.com/borgbackup/borg/blob/2cefe8f53db4a9e0cdd1eeff1b06ebdab0f18ae8/src/borg/archiver.py#L5361

As workaround in locking.py#L316 just replaced the 
`elements.remove(self.id)`
with
`elements.discard(self.id)`

I would have expected to now see the issue that triggered the exception handling in archiver.py, but I get no (visible) exceptions anymore.


Discussion:
Isn't the main issue here that your underlying OS answered `Operation not supported` to `open()`?
Indeed it looks like this, but actually its not.
The file is perfectly present and writable by Python with same user and when instrumenting the code, the Key Error during unlocking happens first and only during its Error handling the second exceptions comes. Maybe because the unlock still has the same file open??

Also evidence to the open not being the root problem is that locking and unlocking works multiple times during the archive creation, only fails in final steps.
And finally, as stated above: when changing the code so the key error is impossible to happen in this place, then no OS error is thrown. 
See also: #7154 (and also quite some other issues, this seems to be a frequent issue at teardown).
Yeah, guess it is better to use `set.discard` there, thanks for this workaround!

Just out of curiosity: what filesystem does your borg use to access the repository?


Resolving patch:
diff --git a/src/borg/locking.py b/src/borg/locking.py
--- a/src/borg/locking.py
+++ b/src/borg/locking.py
@@ -7,7 +7,7 @@
 from .helpers import Error, ErrorWithTraceback
 from .logger import create_logger
 
-ADD, REMOVE = 'add', 'remove'
+ADD, REMOVE, REMOVE2 = 'add', 'remove', 'remove2'
 SHARED, EXCLUSIVE = 'shared', 'exclusive'
 
 logger = create_logger(__name__)
@@ -285,6 +285,11 @@ def modify(self, key, op):
         if op == ADD:
             elements.add(self.id)
         elif op == REMOVE:
+            # note: we ignore it if the element is already not present anymore.
+            # this has been frequently seen in teardowns involving Repository.__del__ and Repository.__exit__.
+            elements.discard(self.id)
+        elif op == REMOVE2:
+            # needed for callers that do not want to ignore.
             elements.remove(self.id)
         else:
             raise ValueError('Unknown LockRoster op %r' % op)
@@ -299,7 +304,7 @@ def migrate_lock(self, key, old_id, new_id):
         killing, self.kill_stale_locks = self.kill_stale_locks, False
         try:
             try:
-                self.modify(key, REMOVE)
+                self.modify(key, REMOVE2)
             except KeyError:
                 # entry was not there, so no need to add a new one, but still update our id
                 self.id = new_id
diff --git a/src/borg/testsuite/locking.py b/src/borg/testsuite/locking.py
--- a/src/borg/testsuite/locking.py
+++ b/src/borg/testsuite/locking.py
@@ -162,7 +162,7 @@ def test_kill_stale(self, lockpath, free_pid):
             assert roster.get(SHARED) == {our_id}
         assert roster.get(EXCLUSIVE) == set()
         assert roster.get(SHARED) == set()
-        with pytest.raises(KeyError):
+        with pytest.raises(NotLocked):
             dead_lock.release()
 
         with Lock(lockpath, id=cant_know_if_dead_id, exclusive=True):


Test patch: