make: pass make jobserver to cargo

[kaspar030]

Contribution description

For cargo to honour our make's jobserver, it's recipe line needs to be prefixed with +.
Make >= 4.4 currently also needs the --jobserver-style=pipe argument. (Our CI is using 4.3).

Testing procedure

make -Ctests/rust-hello-world clean all -j2

On master, observe as many rustc processes (e.g., in top) as there are CPUs. With this PR, there should only be two.

(should your system have make 4.4, try make --jobserver-style=pipe -Ctests/rust-hello-world clean all -j2)

Issues/PRs references

Fixes #19045.

[riot-ci]

Murdock results

✔️ PASSED

6fb226d make: pass make jobserver to cargo

Success	Failures	Total	Runtime
6796	0	6796	07m:36s

Artifacts

• Documentation preview

[chrysn]

While I generally subscribe to "Comments should assume the user knows the language", not only is this the first time I see + in many years of Makefile use, but I also haven't even managed to find the documentation for them (although the page on .ONESHELL (TIL) mentions them as "special prefix characters". Thus, I'd welcome a comment being added that explains what's happening here.

Given that + also makes the line exempt from make --just-print, do we need to pass some --dry-run to cargo when an actual --just-print / -n is active? Or do we just not support that mode of Make?

[chrysn]

And: Can this go away again when #19045 is fixed? (If so, the comment should say so too).

[kaspar030]

Thus, I'd welcome a comment being added that explains what's happening here.

I added one.

Here's an explanation of the jobserver:

https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html

And: Can this go away again when #19045 is fixed? (If so, the comment should say so too).

Nope. If the "+" is not used, make passes invalid fds, hinting the jobserver is unavailable / should not be used.

Given that + also makes the line exempt from make --just-print, do we need to pass some --dry-run to cargo when an actual --just-print / -n is active? Or do we just not support that mode of Make?

hm, make -n fails horribly for me with/without this change. I'd say we don't support that mode.
By spec, jobserver clients should look for "-n" in the makeflags and do the right thing.

[chrysn]

Works as advertised. Thanks for fixing this!

[kaspar030]

I added one.

btw, that makefile is IMO a nice example of why make is not the nicest language to write build systems in. It does what needs to be done, but it gets ugly...

[kaspar030]

bors merge

[bors]

🕐 Waiting for PR status (GitHub check) to be set, probably by CI. Bors will automatically try to run when all required PR statuses are set.

[bors]

Build succeeded:

• Murdock
• static-tests
• tools-build-success