Re: [PATCH] refs: fix interleaving hook calls with reference-transaction hook

[Patrick Steinhardt <ps@pks.im>]

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On Fri, Aug 07, 2020 at 03:58:37AM -0400, Jeff King wrote:
> On Fri, Aug 07, 2020 at 09:05:58AM +0200, Patrick Steinhardt wrote:
> 
> > In order to not repeatedly search for the reference-transaction hook in
> > case it's getting called multiple times, we use a caching mechanism to
> > only call `find_hook()` once. What was missed though is that the return
> > value of `find_hook()` actually comes from a static strbuf, which means
> > it will get overwritten when calling `find_hook()` again. As a result,
> > we may call the wrong hook with parameters of the reference-transaction
> > hook.
> > 
> > This scenario was spotted in the wild when executing a git-push(1) with
> > multiple references, where there are interleaving calls to both the
> > update and the reference-transaction hook. While initial calls to the
> > reference-transaction hook work as expected, it will stop working after
> > the next invocation of the update hook. The result is that we now start
> > calling the update hook with parameters and stdin of the
> > reference-transaction hook.
> 
> That makes sense. I think of push as a single transaction, but that's
> only if the caller sends the "atomic" capability. Otherwise get one per
> ref.
> 
> > diff --git a/refs.c b/refs.c
> > index 2dd851fe81..17e515b288 100644
> > --- a/refs.c
> > +++ b/refs.c
> > @@ -2044,7 +2044,7 @@ static int run_transaction_hook(struct ref_transaction *transaction,
> >  	if (hook == &hook_not_found)
> >  		return ret;
> >  	if (!hook)
> > -		hook = find_hook("reference-transaction");
> > +		hook = xstrdup_or_null(find_hook("reference-transaction"));
> >  	if (!hook) {
> >  		hook = &hook_not_found;
> >  		return ret;
> 
> The fix here looks obviously correct, though I have to wonder if the
> caching is even worth it. It's saving us an access() call, but we're
> about to exec a whole sub-process.
> 
> It's perhaps more justifiable when there isn't a hook (we're still just
> paying that one access(), but it's proportionally bigger). I kind of
> doubt it's measurable, though, since a ref write requires a bunch of
> syscalls anyway.

Yeah, this really was done to not have to pay a performance penalty if
updating thousands of refs if no reference-transaction hook exists. E.g.
if doing a non-atomic push of n reference, we'd have n calls to
access(3P). See [1] for reference.

I've just did another quick benchmark without the cache, and it still
consistently shows a non-zero performance hit without it:

Test                         pks-reftx-hook-interleaving   no-cache
--------------------------------------------------------------------------------
1400.2: update-ref           2.82(2.13+0.81)               2.86(2.19+0.78) +1.4%
1400.3: update-ref --stdin   0.22(0.07+0.15)               0.22(0.07+0.15) +0.0%

Patrick

[1]: https://public-inbox.org/git/20200603165142.GA24049@syl.local/

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