Re: [PATCH] Correctly initialize 'installed_handlers'

From: Junio C Hamano <gitster@pobox.com>
Date: May 8, 2020, 15:36 UTC
Message-ID: <xmqqy2q2zbej.fsf@gitster.c.googlers.com>
In-Reply-To: <20200508083340.GA36538@Carlos-MBP>

Carlo Marcelo Arenas Belón <carenas@gmail.com> writes:

Show 7 quoted lines

> the way this is handled would seem to indicate otherwise
>
> if (!installed_handlers) {
>                 atexit(remove_tmp_objdir);
>                 sigchain_push_common(remove_tmp_objdir_on_signal);
>                 installed_handlers++;
> }

It is a curious piece of code.

The "prepare a file-scope static and do something and increment it when it is 0" pattern expects the function to be called many times and do the guarded thing only once. However, there is this code:

	if (the_tmp_objdir)
		BUG(...);

before we do anything else, and then before that "arrange to clean up, but do so just once" block, there is

	the_tmp_objdir = t;

where t is the pointer to a "struct tmp_objdir" instance. So one part of the function expects to be called at most once, while another part is prepared to be called more than once.

Almost all of this function is attributed to 2564d994 (tmp-objdir: introduce API for temporary object directories, 2016-10-03), so let's see if Peff remembers anything about this curiosity.

Thanks.