Re: [PATCH 3/4] http.c: avoid danging pointer to local variable `finished`

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Tue, May 24 2022, Johannes Schindelin wrote:

> Hi Junio,
>
> On Tue, 24 May 2022, Junio C Hamano wrote:
>
>> The "clear slot->finished", by the way, is what I think is the right
>> thing to do, especially that the objective is to squelch the false
>> positive warning from a new compiler.  If there is a way to annotate
>> the line for the compiler to tell it not to warn about it, that would
>> have been even better.
>
> We could do something like this:
>
> -- snip --
> diff --git a/http.c b/http.c
> index b08795715f8a..2ac8d51d3668 100644
> --- a/http.c
> +++ b/http.c
> @@ -1365,7 +1365,14 @@ void run_active_slot(struct active_request_slot *slot)
>  	struct timeval select_timeout;
>  	int finished = 0;
>
> +#if __GNUC__ >= 12
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wdangling-pointer"
> +#endif
>  	slot->finished = &finished;
> +#if __GNUC__ >= 12
> +#pragma GCC diagnostic pop
> +#endif
>  	while (!finished) {
>  		step_active_slots();
> -- snap --
>
> That's quite ugly, though. And what's worse, it is pretty unreadable, too.

Unfortunately that sort of thing is a logic error as clang, ICC and
probably others are on a mission to make __GNUC__ as useless as
possible:
https://stackoverflow.com/questions/38499462/how-to-tell-clang-to-stop-pretending-to-be-other-compilers

I think it *might* work in practice though, my local clang claims to be
gcc 4, so maybe everyone faking it stops at a low enough version?

But did you spot 9c539d1027d (config.mak.dev: alternative workaround to
gcc 12 warning in http.c, 2022-04-15)? We already disable this file-wide
in config.mak.dev, but I didn't check if the Windows code was using that
(or if you were targeting those without DEVELOPER=1).

We could move that to thake main Makefile, but then we'd have to call
detect-compiler there. I have some local patches to do something like
that if there's interest (rather, to bootstrap compilation by compiling
a C object and getting the macro values, instead of relying on that
shellscript).