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

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 29, 2021, 18:40 UTC
Message-ID: <xmqqk0ppkdmd.fsf@gitster.g>
In-Reply-To: <patch-2.6-49dbd54d0ff-20210329T161723Z-avarab@gmail.com>

Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:

Show 6 quoted lines

> 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.

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.