Doubts about .WAIT and .NOTPARALLEL (was: GNU Make 4.4 released!)

[Alejandro Colomar]

Hi Paul,

On 10/31/22 08:01, Paul Smith wrote:

> * New feature: The .WAIT special target
>    If the .WAIT target appears between two prerequisites of a target, then
>    GNU Make will wait for all of the targets to the left of .WAIT in the list
>    to complete before starting any of the targets to the right of .WAIT.
>    This feature is available in some other versions of make, and it will be
>    required by an upcoming version of the POSIX standard for make.
>    Different patches were made by Alexey Neyman <alex.neyman@auriga.ru> (2005)
>    and Steffen Nurpmeso <steffen@sdaoden.eu> (2020) that were useful but the
>    result is a different implementation (closer to Alexey's idea).

I'm curious: what is .WAIT made for?  Isn't it just a workaround for broken 
dependencies?  I mean, considering the following example:

    all: one two .WAIT three
    one two three: ; @sleep 1; echo $@

It's the same as if it was written as

    all: one two three
    three: one two
    one two three: ; @sleep 1; echo $@

Isn't it?

> * New feature: .NOTPARALLEL accepts prerequisites
>    If the .NOTPARALLEL special target has prerequisites then all prerequisites
>    of those targets will be run serially (as if .WAIT was specified between
>    each prerequisite).

But my useful question comes with .NOTPARALLEL, which I think can be useful for 
something I asked some time ago, and can't be done with pre-4.4 make.

The documentation is a bit unclear on the exact details of this feature, by 
writing it in terms of .WAIT.

Let's say I have the following:

    all: a b c
    .NOTPARALLEL: b
    a: b c
    a b c: ; @sleep 1; echo $@

Is it guaranteed that b will be run either before or after c but never at the 
same time?  Please clarify the manual regarding this.  Maybe this example is 
good for the docs.

The usefulness of something like this is that b might run a command that asks 
for a password, and I want literraly _everything_ else on hold while a program 
is asking for a password.

Of course, all this is without recursive make, I know.

> * New feature: The --shuffle command line option
>    This option reorders goals and prerequisites to simulate non-determinism
>    that may be seen using parallel build.  Shuffle mode allows a form of "fuzz
>    testing" of parallel builds to verify that all prerequisites are correctly
>    described in the makefile.
>    Implementation provided by Sergei Trofimovich <siarheit@google.com>

I guess shuffle doesn't mess with .NOTPARALLEL or .WAIT.  Just to confirm.

> * New feature: The --jobserver-style command line option and named pipes
>    A new jobserver method is used on systems where mkfifo(3) is supported.
>    This solves a number of obscure issues related to using the jobserver
>    and recursive invocations of GNU Make.  This change means that sub-makes
>    will connect to the jobserver even if they are not marked as recursive.
>    It also means that other tools that want to participate in the jobserver
>    will need to be enhanced as described in the GNU Make manual.
>    You can force GNU Make to use the simple pipe-based jobserver (perhaps if
>    you are integrating with other tools or older versions of GNU Make) by
>    adding the '--jobserver-style=pipe' option to the command line of the
>    top-level invocation of GNU Make, or via MAKEFLAGS or GNUMAKEFLAGS.
>    To detect this change search for 'jobserver-fifo' in the .FEATURES variable.

Another curious question:  Does this fifo jobserver allow .NOTPARALLEL even with 
recursive makes?  I guess not, but I had to ask.

Cheers,

Alex
--
<http://www.alejandro-colomar.es/>

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