Re: [PATCH v2 2/5] Makefile: rename scripts in-place, don't clobber

From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, Jeff King <peff@peff.net>,
	Johannes Schindelin <johannes.schindelin@gmx.de>,
	Jonathan Nieder <jrnieder@gmail.com>
Subject: Re: [PATCH v2 2/5] Makefile: rename scripts in-place, don't clobber
Date: Tue, 30 Mar 2021 01:28:01 +0200	[thread overview]
Message-ID: <87czvhmtfi.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <xmqqk0ppkdmd.fsf@gitster.g>

On Mon, Mar 29 2021, Junio C Hamano wrote:

> Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:
>
>> It'll also guard against any weird issues with e.g. a command that
>> wants to read GIT-PERL-DEFINES to generate one of the %.perl scripts
>> racing with either a concurrent instance of "make" that has partially
>> updated the file, or test-lib.sh dying with some particularly weird
>> error because GIT-BUILD-OPTIONS was partway through being updated when
>> it ran.
>
> If something like that happens, doesn't that indicate a bug in the
> dependency graph in the Makefile or the implementation of "make"?
> The generated file is depended on for the consumer to be able to use
> a non-stale version---so the consumer should not start before the
> generator finishes.

If everything we were building in the Makefile would be invoked via
other Makefile rules, yes. But if I run say (cd t && ./t0000-basic.sh)
I'm having test-lib.sh pick up one of those (possibly partial) files,
this guarantees they'll all be atomically updated.

> You may be able to hide the breakage coming from "partially written
> file is easily recognizable and the consumer would barf".  But I am
> afraid that you are introducing a problem harder to diagnose, i.e.
> "the consumer reads from a complete file so there is no syntax error
> or other things that easily tells you there is a breakage, but what
> is used is stale and not up to date".
>
> The same comment applies to this step.  I do not think it would
> break (other than adding intermediate crufts) the result if you
> generate into temporary and rename to final, but it is not clear
> to me what the point is in doing so.

I just think it makes sense to do this for consistency. So on "master"
we've got 65 hits for $@+ in the Makefile, at the end of this saga we've
got 100.

I think being consistent across the board makes things easier to read,
and in combination with the later ".DELETE_ON_ERROR" we can also drop
other copy/paste cruft.