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

[ThomasWaldmann]

Problem

When Lock.acquire() for an exclusive lock succeeded in creating its lock object but then timed out waiting for existing non-exclusive locks to go away, it exited the retry loop via break and raised LockTimeout without deleting the exclusive lock it had just created.

That zombie exclusive lock then blocked every other client:

• clients on other hosts were locked out until the lock expired as stale (up to 30 minutes by default),
• clients on the same host recovered earlier only because process_alive() detects the dead owner process.

Easy to hit in practice: e.g. borg compact (exclusive) timing out against a long-running borg create (shared) leaves the repo unusable for other clients for the staleness period.

Fix

Delete our lock before giving up on the timeout path, same as the other back-off paths in acquire() already do.

Testing

Added test_exclusive_lock_timeout_leaves_no_lock: a shared lock is held, an exclusive acquire times out, and the store must contain only the shared lock afterwards. The test fails against the unfixed code (the zombie exclusive lock is found) and passes with the fix.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.12%. Comparing base (cbc94ae) to head (88f470e).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9871   +/-   ##
=======================================
  Coverage   85.12%   85.12%           
=======================================
  Files          93       93           
  Lines       15458    15459    +1     
  Branches     2337     2337           
=======================================
+ Hits        13158    13159    +1     
  Misses       1599     1599           
  Partials      701      701           

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