Re: [PATCH] lockfile: add PID file for debugging stale locks

> From: Paulo Casaretto <pcasaretto@gmail.com>
> 
> When a lock file is held, it can be helpful to know which process owns
> it, especially when debugging stale locks left behind by crashed
> processes. Add an optional feature that creates a companion .lock.pid
> file alongside each lock file, containing the PID of the lock holder.
> 
> The .lock.pid file is created when a lock is acquired (if enabled), and
> automatically cleaned up when the lock is released (via commit or
> rollback). The file is registered as a tempfile so it gets cleaned up
> by signal and atexit handlers if the process terminates abnormally.
> 
> When a lock conflict occurs, the code checks if the PID from the .pid
> file is still running using kill(pid, 0). This allows providing
> context-aware error messages. With PID info enabled:
> 
>   Lock is held by process 12345. Wait for it to finish, or remove
>   the lock file to continue.
> 
> Or for a stale lock:
> 
>   Lock was held by process 12345, which is no longer running.
>   Remove the stale lock file to continue.
> 
> Without PID info (default):
> 
>   Another git process seems to be running in this repository.
>   Wait for it to finish, or remove the lock file to continue.
> 
> The feature is opt-in via GIT_LOCK_PID_INFO=1 environment variable.