Subject: Re: [PATCH] Makefile: make COMPUTE_HEADER_DEPENDENCIES=auto work with DEVOPTS=pedantic

From: Junio C Hamano <[email-masked]>
Date: Wed, 22 Sep 2021 09:08:00 -0700

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

> The "COMPUTE_HEADER_DEPENDENCIES" feature added in [1] was extended to
> use auto-detection in [2]. Then when -Wpedantic support was added to
> DEVOPTS in [3] we started passing -Wpedantic in combination with
> -Werror to the compiler here.
>
> This broke the auto-detection, but since we'd quieted it in [4] we
> didn't find out.

Are the references correct?  I am not seeing "quiet"ing in [4].  The
redirection 2>&1 to cram error messages also to $(dep_check), hence
making it impossible to match '0', was done in [2].

We did make the pedantic mode the default and pass both -pedantic
and -Wpedantic after [4].  Before we had only -pedantic.

> It was emitting all of this on STDERR under GCC:
>
>     /dev/null:1: error: ISO C forbids an empty translation unit
>     [-Werror=pedantic]
>     cc1: note: unrecognized command-line option
>     ‘-Wno-pedantic-ms-format’ may have been intended to silence
>     earlier diagnostics
>     cc1: all warnings being treated as errors
>
> Let's fix that bug by maintaining a NON_DEVELOPER_CFLAGS, it's like
> ALL_CFLAGS but without anything we add in config.mak.dev, and
> furthermore stop redirecting STDERR to /dev/null, this means that
> someone whose compiler doesn't support this will see this output, but
> also this new message:
>
>     Non-zero 1 exit with COMPUTE_HEADER_DEPENDENCIES=auto, set it to "yes" or "no" to quiet auto-detect

Hmmmmmph.  

I recentaly saw many .depend directories (not necessarily empty)
left after "make distclean". After building on one branch, I often
check out a different branch then run distclean on the new branch,
so leftover build artifacts are not necessarily a bug in our
Makefile, but the bug you found may explain it?

While I agree with your analysis of the problem, I cannot shake this
nagging feeling that the proposed solution is barking up a wrong
tree.  After all, -pedantic and any other option that lets the
compiler notice that it is being asked to compile an empty source
can come directly from the end user (e.g. CC="gcc -pedantic" or as
part of CFLAGS)---realization of which makes me wonder if it is
essential to compile /dev/null for this check, or any reasonably
syntactically correct program would do.

I wonder if the attached (with clean-up to remove the tracing cruft)
would show us a better direction.  It feeds a single line

	int dummy_for_dep_check;

C "program" from the standard input of the compiler to tackle the
"you are not supposed to be compiling an empty compilation unit"
problem in a more direct way.

 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git c/Makefile w/Makefile
index 9df565f27b..0593ab7287 100644
--- c/Makefile
+++ w/Makefile
@@ -1277,9 +1277,9 @@ COMPUTE_HEADER_DEPENDENCIES = auto
 endif
 
 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),auto)
-dep_check = $(shell $(CC) $(ALL_CFLAGS) \
+dep_check = $(shell echo >&2 doing dep check; echo int dummy_for_dep_check\; | $(CC) $(ALL_CFLAGS) \
 	-c -MF /dev/null -MQ /dev/null -MMD -MP \
-	-x c /dev/null -o /dev/null 2>&1; \
+	-x c - -o /dev/null || echo >&2 oops; \
 	echo $$?)
 ifeq ($(dep_check),0)
 override COMPUTE_HEADER_DEPENDENCIES = yes