[0.57.2] run_target with a missing quoted command started requiring the binary to exists

[keszybz]

Describe the bug
A clear and concise description of what the bug is.

We have the following code in systemd:

run_target(
        'doc-sync',
        depends : man_pages + html_pages,
        command : ['rsync', '-rlv', ...])

This target is only used when manually invoked. So far, the presence of rsync was not probed, so it was OK if rsync was not installed (until the target was executed). This changed in 0.57.2 (0.57.1 was still OK). So for example CI builds started failing. Please consider reverting the change, it doesn't seem to be something suitable for a point release.

project('a', 'c',
        version : '0')

run_target('run-foobar',
           command : ['foobar', '!'])

With meson-0.57.2-1.fc35.noarch, this fails.

[keszybz]

Oh, this has another impact:

run_target(
        'fuzzers',
        depends : fuzzer_exes,
        command : ['true'])

(The "true" is a placeholder, required because run_target requires a command to be specified.)

This now started printing:

Program true found: YES (/usr/bin/true)

[xclaesse]

This is probably caused by #8526.

I think it's the right thing to do, but I agree it probably should not be in a point release. That change was needed to fix another bug, and we probably overlooked the fact that the program was not required before.

If your target is optional, do something like:

rsync = find_program('rsync', required: false)
if rsync.found()
  run_target(..., command: [rsync, ...])
endif

[keszybz]

rsync = find_program('rsync', required: false)
if rsync.found()
  run_target(..., command: [rsync, ...])
endif

Yep, systemd/systemd#19297 does exactly this.

[eli-schwartz]

find_program() supports disabler: true

[keszybz]

find_program() supports disabler: true

Oh, you mean that we could do

rsync = find_program('rsync', required: false)
run_target(..., command: [rsync, ...])

In systemd we still support meson-0.46 sadly…

[eli-schwartz]

Oh well, leave it as a # TODO. :)