Re: [PATCH 1/4] Makefile: don't re-define PERL_DEFINES

From: Junio C Hamano <gitster@pobox.com>
Date: May 6, 2021, 01:05 UTC
Message-ID: <xmqq4kfg3c4v.fsf@gitster.g>
In-Reply-To: <YJKm0dnwHBwQuTi+@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 22 quoted lines

> I did have one question:
>
>> diff --git a/Makefile b/Makefile
>> index 93664d67146..1d4c02e59d9 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -2270,9 +2270,10 @@ perl_localedir_SQ = $(localedir_SQ)
>>  
>>  ifndef NO_PERL
>>  PERL_HEADER_TEMPLATE = perl/header_templates/fixed_prefix.template.pl
>> -PERL_DEFINES = $(PERL_PATH_SQ):$(PERLLIB_EXTRA_SQ):$(perllibdir_SQ)
>> -
>> -PERL_DEFINES := $(PERL_PATH_SQ) $(PERLLIB_EXTRA_SQ) $(perllibdir_SQ)
>> +PERL_DEFINES :=
>> +PERL_DEFINES += $(PERL_PATH_SQ)
>> +PERL_DEFINES += $(PERLLIB_EXTRA_SQ)
>> +PERL_DEFINES += $(perllibdir_SQ)
>>  PERL_DEFINES += $(RUNTIME_PREFIX)
>
> I don't think we generally use simply-expanded variables in our Makefile
> unless there's a reason. Do we actually need it here? Obviously not new
> in your patch, but just a curiosity I noticed while reading it.

Splitting the appending into multiple lines does make sense, and is in line with what 07d90ead (Makefile: add Perl runtime prefix support, 2018-04-10) introduced the "first create a space separated list and then redefine that same variable with spaces replaced with colons" strategy to reach the final value (i.e. colon separated tokens that lets us notice when build options changed) for.

As to the simply-expanded vs recursively-expanded variable, there is aneed to use former, which comes from what the original commit 07d90ead did outside the context of this patch, which is:

    PERL_DEFINES := $(subst $(space),:,$(PERL_DEFINES))
    GIT-PERL-DEFINES: FORCE
            @FLAGS='$(PERL_DEFINES)'; \
                if test x"$$FLAGS" != x"`cat $@ 2>/dev/null`" ; then \
                    echo >&2 "    * new perl-specific parameters"; \
                    echo "$$FLAGS" >$@; \
                fi

That is, up to this point PERL_DEFINES accumulate various build-time settings with += (i.e. space separated tokens), and at this point finally it is turned into a colon separated tokens, which cannot be written with a recursively expanded variable.

But I tend to agree that you do not have to := clear the list in this patch.