LockRoster.modify: no KeyError if element was already gone, fixes #7937

The intention of LockRoster.modify(key, REMOVE) is to remove self.id.

Using set.discard will just ignore it if self.id is not present there anymore.

Previously, using set.remove triggered a KeyError that has been frequently
seen in tracebacks of teardowns involving Repository.__del__ and Repository.__exit__.

I added a REMOVE2 op to serve one caller that needs to get the KeyError if
self.id was not present.

Thanks to @herrmanntom for the workaround!

src/borg/locking.py

@@ -8,7 +8,7 @@ from . import platform
 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__)
@@ -326,6 +326,11 @@ class LockRoster:
         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)
@@ -340,7 +345,7 @@ class LockRoster:
         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

src/borg/testsuite/locking.py

@@ -280,7 +280,7 @@ class TestLock:
             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):