patchrefs: fix interleaving hook calls with reference-transaction hook

> 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.

> 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;

> 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.

> > 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%

> On Fri, Aug 07, 2020 at 11:04:12AM +0200, Patrick Steinhardt wrote:
> 
> > > 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%
> 
> I'm skeptical that those results are useful. In the first test, we're
> running update-ref 1000 times, so:
> 
>   - the cache shouldn't be helping at all, since we only have one ref to
>     update (well, I guess once for "prepare" and once for "commit", so
>     really it's saving one syscall total per process).
> 
>   - I'd expect a lot of noise because we're spending most of our time in
>     starting up the process
> 
> In the second test, we run 1000 ref operations per update-ref process.
> So we should be cutting down on our hook-lookup overhead by a factor of
> 1000. Yet it shows no improvement.
> 
> That implies you're just seeing noise. And indeed, with the patch below
> I get:
> 
> Test                         HEAD^             HEAD
> --------------------------------------------------------------------
> 1400.2: update-ref           1.93(1.57+0.42)   1.91(1.55+0.42) -1.0%
> 1400.3: update-ref --stdin   0.07(0.02+0.05)   0.07(0.02+0.05) +0.0%
> 
> Running it a second time gets me +0.5%. :)

> On Fri, Aug 07, 2020 at 05:32:39AM -0400, Jeff King wrote:
>
>> That implies you're just seeing noise. And indeed, with the patch below
>> I get:
>> 
>> Test                         HEAD^             HEAD
>> --------------------------------------------------------------------
>> 1400.2: update-ref           1.93(1.57+0.42)   1.91(1.55+0.42) -1.0%
>> 1400.3: update-ref --stdin   0.07(0.02+0.05)   0.07(0.02+0.05) +0.0%
>> 
>> Running it a second time gets me +0.5%. :)
>
> Yeah, it's also been my take that OS-level overhead is probably going to
> matter more than those access calls, and I argued such back when I
> proposed the hook. So I'm perfectly happy to see this caching mechanism
> go.

> >> Test                         HEAD^             HEAD
> >> --------------------------------------------------------------------
> >> 1400.2: update-ref           1.93(1.57+0.42)   1.91(1.55+0.42) -1.0%
> >> 1400.3: update-ref --stdin   0.07(0.02+0.05)   0.07(0.02+0.05) +0.0%
> >> 
> >> Running it a second time gets me +0.5%. :)
> >
> > Yeah, it's also been my take that OS-level overhead is probably going to
> > matter more than those access calls, and I argued such back when I
> > proposed the hook. So I'm perfectly happy to see this caching mechanism
> > go.
> 
> Is the above about negative cache?  IOW, does the above demonstrate
> that one extra access() to make sure there is no hook does not hurt
> us anything?

> Yeah, it's also been my take that OS-level overhead is probably going to
> matter more than those access calls, and I argued such back when I
> proposed the hook. So I'm perfectly happy to see this caching mechanism
> go.
> 
> Should I re-post a v2 with your patch and my test?

> On Fri, Aug 07, 2020 at 11:49:46AM +0200, Patrick Steinhardt wrote:
>
>> Yeah, it's also been my take that OS-level overhead is probably going to
>> matter more than those access calls, and I argued such back when I
>> proposed the hook. So I'm perfectly happy to see this caching mechanism
>> go.
>> 
>> Should I re-post a v2 with your patch and my test?
>
> Sure, that would be fine (to be clear, I'd also be OK with your original
> patch, too; it was mostly just a curiosity to me).