Re: Regression caused by Commit: 5bd7ad2b225b ("Preserve the real value of -jN in MAKEFLAGS using jobserver.")

[Paul Smith]

On Thu, 2017-09-28 at 16:28 +1000, NeilBrown wrote:
> One of my Makefiles contains
>   MAKEFLAGS += -j
> 
> Prior to the above commit, this would cause make to run as if I had
> run
>    make -j
> 
> Since that commit, it appears to be ignored.

This is a bug clearly.  I've been thinking about it today and I wonder
what behavior people would expect to see if there are mismatches
between the various ways -j can be specified.

If it's a top-level make (not a sub-make in a jobserver grouping) then 
I think the behavior is pretty straightforward based on normal make
priorities: if there's a command line -j setting use that; if not and
there's a MAKEFLAGS -j set in the makefile use that; if not and there's
a MAKEFLAGS -j set in the environment use that.

I don't think make should warn if the different sources have different
-j values.  You might set MAKEFLAGS=-j5 in your environment or makefile
but have "make -j1" to explicitly turn it off--you wouldn't want a
message about that IMO.

More interesting: what about a sub-make which is part of an existing
jobserver group?  Consider a sub-make invoked with -jN on the command
line; you have a makefile like this:

  $ cat cmd.mk
  subdir: ; $(MAKE) -C $@ -j2

If someone runs a simple "make -f cmd.mk", then you'd want the sub-make 
to be run with -j2 of course.  But what if you do this:

  $ make -f cmd.mk -j4

What happens today is that a _new_ jobserver instance with -j2 is
started for the sub-make.  This could mean we have 5 total jobs
running, even though we specified -j4 on the command line.

That seems defensible, although I don't know that it's correct.

Now... what about if a sub-make has a MAKEFLAGS variable assignment? 
Does that override an existing jobserver and cause the current make
instance to be the master of a new jobserver group, as if it were set
on the command line?

For example:

  $ cat sub.mk
  MAKEFLAGS += -j3
  all: ...

  $ cat cmd.mk
  MAKEFLAGS += -j2
  subdir: ; $(MAKE) -f sub.mk

  $ make -f cmd.mk -j4

Here we have a top-level make running the "cmd.mk" file, and a sub-make 
running the "sub.mk" file.

The top make runs with -j4, because the command line overrides the
MAKEFLAGS += -j2 in cmd.mk.

What about the MAKEFLAGS setting in the sub.mk sub-make?  Does it force
a new top-level jobserver with jobs set to 3, as if we ran $(MAKE) -j3
in the subdir rule?  Or is it ignored (with a warning) if the sub-make
is already part of a jobserver group?