Subject: Re: [PATCH v2 1/5] Makefile: rename objects in-place, don't clobber

From: Junio C Hamano <[email-masked]>
Date: Mon, 29 Mar 2021 11:21:52 -0700

Ævar Arnfjörð Bjarmason  <[email-masked]> writes:

> Per the log of changes to the Makfile and Junio's recent comment about
> [1] why that pattern got introduced it was for a different reason
> entirely, i.e. ("[]" edits are mine, for brevity):
>
>     [T]hat age old convention [...] is spelled [as]:
>
>     	thing:
>     		rm -f thing thing+
>     		prepare contents for thing >thing+

Did I say that?  I recall I specifically avoided the "redirection"
because this is *NOT* shell-script only principle.  If a command is
so broken that "cmd -o thing" that fails to work correctly leaves a
broken output in thing, then the pattern should be applied and made
to "cmd -o thing+ && mv thing+ thing".

On the other hand, if "cmd" refrains from leaving a half-baked
result in "-o thing" (and I believe $(CC) is well-behaved even on
AIX), I do not think it is a good idea to use that pattern.  One
version of AIX may refuse to overwrite a file in use because
creat("thing") that is necessary for "-o thing" may fail while
"thing" is in use), but another system may refuse to rename over a
file in use (i.e. "-o thing+" into a brand new "thing+" may be OK
but the final "mv thing+ thing" may fail).  So pretending that it is
a cure for broken use case is cluttering Makefile for no real
benefit and leading readers into confused thinking.

>     		mv thing+ thing
>
>     It protects us from a failure mode where "prepare contents for
>     thing" step is broken and leaves a "thing" that does not work, but
>     confuses make that make does not need to rebuild it, if you wrote it
>     as such:
>
>     	thing:
>     		prepare contents for thing >thing
>
>     [It might leave behind a corrupt 'thing'.] In any case, it is not
>     "we are trying to make thing available while it is being
>     rewritten" at all.
>
> That makes perfect sense for shellscripts, but as this change shows
> there's other good reasons to use this age old convention that weren't
> considered at the time.

So, no, the age old convention may have considered and does not
apply to such a use case.

>  git$X: git.o GIT-LDFLAGS $(BUILTIN_OBJS) $(GITLIBS)
> -	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
> -		$(filter %.o,$^) $(LIBS)
> +	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@+ $(ALL_LDFLAGS) \
> +		$(filter %.o,$^) $(LIBS) && \
> +	mv $@+ $@

Really, does anybody else use "$(CC) -o $@" in such a way in their
Makefile?  Having to do this smells simply crazy (I am not saying
you are crazy---the platform that forces you to write such a thing
is crazy).

So, while I do not think the end result would break the build (other
than it probably would leave crufts "make clean" would not notice
behind when interrupted in the middle), I am moderately negative on
this change.

Thanks.