locking: do not leave a lock behind when exclusive acquire times out

src/borg/storelocking.py

@@ -189,6 +189,9 @@ def acquire(self):
                                 return self
                             time.sleep(self.other_locks_go_away_delay)
                         logger.debug("LOCK-ACQUIRE: timeout while waiting for non-exclusive locks to go away.")
+                        # we won't get the exclusive lock, so do not leave our lock behind:
+                        # it would needlessly block other clients until it expired as stale.
+                        self._delete_lock(key, ignore_not_found=True, update_last_refresh=True)
                         break  # timeout
                     else:
                         logger.debug("LOCK-ACQUIRE: someone else also created an exclusive lock, deleting ours.")

src/borg/testsuite/storelocking_test.py

@@ -45,6 +45,17 @@ def test_exclusive_lock(self, lockstore):
             with pytest.raises(LockTimeout):
                 Lock(lockstore, exclusive=True, id=ID2).acquire()
 
+    def test_exclusive_lock_timeout_leaves_no_lock(self, lockstore):
+        # When acquiring an exclusive lock times out because a non-exclusive lock does not go away,
+        # the not-acquired exclusive lock must not stay behind in the store: it would block all
+        # other clients (even on other hosts) until it expired as stale.
+        with Lock(lockstore, exclusive=False, id=ID1) as shared_lock:
+            with pytest.raises(LockTimeout):
+                Lock(lockstore, exclusive=True, id=ID2).acquire()
+            locks = shared_lock._get_locks()
+            assert len(locks) == 1  # only the non-exclusive lock of ID1 is left
+            assert not any(lock["exclusive"] for lock in locks.values())
+
     def test_double_nonexclusive_lock_succeeds(self, lockstore):
         with Lock(lockstore, exclusive=False, id=ID1):
             with Lock(lockstore, exclusive=False, id=ID2):