Re: [PATCH] config: retry acquiring config.lock for 100ms

[Junio C Hamano <gitster@pobox.com>]

Patrick Steinhardt <ps@pks.im> writes:

>> This bites in practice when running `git worktree add -b` concurrently
>> against the same repository. Each invocation makes several writes to
>> ".git/config" to set up branch tracking, and tooling that creates
>> worktrees in parallel sees intermittent failures. Worse, `git worktree
>> add` does not propagate the failed config write to its exit code: the
>> worktree is created and the command exits 0, but tracking
>> configuration is silently dropped.
>
> This very much sounds like a bug that is worth fixing independently.
>
>> The lock is held only for the duration of rewriting a small file, so
>> retrying for 100 ms papers over any realistic contention while still
>> failing fast if a stale lock has been left behind by a crashed
>> process. This mirrors what we already do for individual reference
>> locks (4ff0f01cb7 (refs: retry acquiring reference locks for 100ms,
>> 2017-08-21)).
>
> Famous last words :) Experience tells me that any timeout value that
> isn't excessive will eventually be hit in some production system. Which
> raises the question whether we want to make the timeout configurable,
> similar to "core.filesRefLockTimeout" and "core.packedRefsTimeout".
> ...
> Honestly though, I'm not really sure what to make with this. We could
> of course also add some validation that the configuration we want to set
> hasn't been modified meanwhile. But that would now lead to a situation
> where we have to update every single caller in our tree to make use of
> the new mechanism, which would be a bunch of work.
>
> And adding the timeout doesn't really change the status quo, either. We
> already have the case that we'll happily overwrite changes made by
> concurrent processes. The only thing that changes is that we make it
> more likely for concurrent changes to succeed.

We haven't heard any response to these points raised in the message
I am responding to.  Should I still keep the patch in my tree,
hoping that a responses may come some day?  I am tempted to discard
the topic as it has been quite a while since we last looked at it.

Thanks.