Re: [PATCH v2 3/4] Makefiles: change search through $(MAKEFLAGS) for GNU make 4.4

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Dec 1, 2022, 12:37 UTC
Message-ID: <221201.86v8mvgsrj.gmgdl@evledraar.gmail.com>
In-Reply-To: <25c59966c83cdae078bdefa49f47ca8d3199475c.camel@gnu.org>

On Wed, Nov 30 2022, Paul Smith wrote:

Show 15 quoted lines

> On Wed, 2022-11-30 at 13:28 +0900, Junio C Hamano wrote:
>> I have to wonder how many projects they have broken with this change
>
> Some, but not that many.  Most projects don't try to investigate
> MAKEFLAGS, and of those that do many were already using the recommended
> method, because even prior to GNU make 4.4 it was possible for
> MAKEFLAGS to have stray "s" characters, in unusual situations (for
> example if MAKEFLAGS were set in the makefile).
>
> There were various bugs filed that various options could not be
> investigated from within makefiles and also that running make from
> within $(shell ...) didn't work right because MAKEFLAGS was not set.
>
> It was just a mess, trying to keep the value of MAKEFLAGS set to
> different values at different points in the processing of make.

It was definitely a bit of a hack on our part, but to be fair before
this 4.4 release doing it this way was recommended by the
documentation. I see you changed that recently, but maybe this on top
makes sense?
	
	diff --git a/doc/make.texi b/doc/make.texi
	index e3a3ade4..9e9a894e 100644
	--- a/doc/make.texi
	+++ b/doc/make.texi
	@@ -5069,7 +5069,7 @@ Variable @code{MAKEFILES}}.
	 @vindex MAKEFLAGS
	 Flags such as @samp{-s} and @samp{-k} are passed automatically to the
	 sub-@code{make} through the variable @code{MAKEFLAGS}.  This variable is
	-set up automatically by @code{make} to contain the flag letters that
	+set up automatically by @code{make} to contain the normalized flag letters that
	 @code{make} received.  Thus, if you do @w{@samp{make -ks}} then
	 @code{MAKEFLAGS} gets the value @samp{ks}.
	 
	@@ -5085,6 +5085,10 @@ option has both single-letter and long options, the single-letter option is
	 always preferred.  If there are no single-letter options on the command line,
	 then the value of @code{MAKEFLAGS} starts with a space.
	 
	+The value of @code{MAKEFLAGS} does not correspond to the order in which
	+command line options are provided. Both @w{@samp{make -sk}} and @w{@samp{make -sk}}
	+will result in a @code{MAKEFLAGS} value of @samp{ks}.
	+
	 @cindex command line variable definitions, and recursion
	 @cindex variables, command line, and recursion
	 @cindex recursion, and command line variable definitions
	@@ -12378,10 +12382,13 @@ influences such as interrupts (@code{SIGINT}), etc.  You may want to install
	 signal handlers to manage this write-back.
	 
	 @item
	-Your tool may also examine the first word of the @code{MAKEFLAGS} variable and
	+Your tool may also examine the first word of the @samp{-$(MAKEFLAGS)} expression and
	 look for the character @code{n}.  If this character is present then
	 @code{make} was invoked with the @samp{-n} option and your tool may want to
	 stop without performing any operations.
	+
	+Note that this is not equivalent to checking for the first word of
	+@code{MAKEFLAGS}.
	 @end itemize
	 
	 @node Windows Jobserver,  , POSIX Jobserver, Job Slots

I.e. that "Your tool" part seems to still be assuming 4.3 semantics.

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> Also, ensuring this trick for searching MAKEFLAGS continues to work
> would have meant strictly controlling what new options we could add to
> GNU make.  I haven't seen any other project use the filter-out --%
> trick that the Git makefiles do, but even with that it won't help if a
> new single-letter option that takes an argument is added.

I'd think it would probably make sense to promise that GNU make will never add such options, so that what's currently documented continues to work. I.e. it supports:

	--debug=all

Not these forms:

	-dwhy
	-d=why

But if we're on the topic: The only reason git's Makefile uses these is because it's trying to fake up some pretty verbose-but-not-too-verbose mode. You can see this in our tree at "shared.mak", the kernel does something similar.

For our Makefile this is pretty close to what we'd get from a simpler:

	make -B --debug=why -s |
        sed -E -n \
		-e "s/.*: update target '(.*).o' due to.*/   CC \\1.o/" \
                -e 's/due to.*//' \
                -e 'p'

I.e. when we have %.o" targets this is emitting " CC $@" lines, I've left matching the rest as an excercise for the reader, but it would be e.g. "GEN_PERL" or whatever for %.pm and so on.

I don't know what this would look like exactly, but it would be neat if GNU make supported some way to emit such friendly output in general. Something like a sprintf format where you'd have access to the sort of input that "due to" string gets internally (and perhaps a bit more, e.g. something indicating overall progress through the graph...).