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

[Jeff King <peff@peff.net>]

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.

-Peff