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

> 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.