Question: wrap-program without #:sh can be ok even when cross-compiling?

[Maxime Devos]

About <https://issues.guix.gnu.org/47869>
(the ‘wrap-program #:sh’ patch series):

While looking at guix/build/gnu-build-system.scm,
I noticed the 'patch-shebangs' phase, which is run
after the install phase. IIUC, this phase is for
changing shebangs ...

 #!/usr/bin/stuff

to ...

 #!/gnu/store/...-stuff-1.0/bin/stuff

IIUC, this phase tries to find the interpreter
in 'inputs'. So, if the original interpreter
was for SYSTEM, the new interpreter is for TARGET.

As such, when cross-compiling, when bash is present
in the inputs, when the patch-shebangs phase isn't
disabled and we use wrap-program without a #:sh
argument, even though wrap-program will use an
interpreter for SYSTEM, the patch-shebangs will
automatically correct it for us to an interpreter
for TARGET.

So, is the patch series in bug#47869 still useful?

  * if, for some reason, patch-shebangs? is #f,
    then we need to explicitely #:sh set --> patch useful

    patch-shebangs? doesn't seem to be set to #f anywhere though.

  * some build systems might not have a patch-shebangs phase
    (I'm not aware of any such build systems though) --> patch useful

  * explicit is better than implicit [citation needed, see e.g. python]
    --> patch useful

    The extra verbosity seems acceptable in build systems
    (see e.g. usage of wrap-program in
    guix/build/glib-or-gtk-build-system.scm
    and guix/build/python-build-system.scm).

    Dunno about package definitions though.

So I'd would say yes! But should we explicitely set
  #:sh (search-input-file inputs "bin/bash")
in package definitions?

Cons for explicit / pros for implicit (relying on patch-shebangs):

  * explicit form is a bit verbose
  * in the transition, many, many package definitions need to be adjusted.

Pros for explicit / cons for explicit:

  * by explicitely writing #:sh (search-input-file inputs "bin/bash"),
    it should be clear that bash-minimal (or bash) needs to be added
    to the package inputs

    (Note that I intend to separate 'inputs' from 'native-inputs' in
    build phases even when compiling natively. Haven't gotten around to
    trying it though, seems complicated ...)

    Alternative: write a linter checking that wrap-program is only used
    if "bash" (or "bash-minimal") is in the package inputs (native-inputs
    doesn't count here).

  * Using the explicit form is always correct. The implicit #:sh (which 
"bin/bash")
    is not always corrected by the patch-shebangs phase.

    Note that when cross-compiling, not setting #:sh and when bash
    is absent from inputs, the patch-shebang phase merely emits a warning
    (which can easily get lost in the noise) and _not_ an error.

Thoughts?

Maxime.

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