linux-kernel - Re: [PATCH 11/23] kconfig: add 'shell-stdout' function

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Date:   Mon, 19 Feb 2018 19:54:44 +0100
From:   Ulf Magnusson <ulfalizer@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/23] kconfig: add 'shell-stdout' function

On Mon, Feb 19, 2018 at 10:01:49AM -0800, Linus Torvalds wrote:
> On Mon, Feb 19, 2018 at 9:44 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I do like your "success"/"stdout" more than "shell"/"shell-stdout",
> > because with that naming I don't get the feeling that one should
> > subsume the other.
> 
> Hmm. Thinking about it some more, I really would prefer just "$(shell
> ...)" everywhere.
> 
> But it would be nice if perhaps the error handling would match the
> context somehow.
> 
> I'm wondering if this might tie into the whole quoting discussion in
> the other thread.
> 
> Because the rule could be:
> 
> (a) unquoted $(shell ) is a bool, and failing is ok (and turns into
> y/n depending on whether successful or failing)
> 
> So
> 
>   config CC_IS_GCC
>           bool
>           default $(shell $CC --version | grep -q gcc)
> 
> works automatically.
> 
> (b) but with quoting, $(shell ) is a string, and failing is an error
> 
> So
> 
>   config GCC_VERSION
>           int
>           default "$(shell-stdout $srctree/scripts/gcc-version.sh $CC
> | sed 's/^0*//')" if CC_IS_GCC
>           default 0
> 
> would need those quotes, and if the shell-script returns a failure,
> we'd _abort_.
> 
> Which is actually what we want there.
> 
> Hmm? Is that too nasty?
> 
>                 Linus

One minor drawback would be slight kludginess if you want "n"/"y" put
into a string depending on the success of a command:

	default	"foo-$(shell cmd && echo y || echo n)"

As opposed to:

	default "foo-$(success cmd)"

I don't know if that's significant enough to matter in practice.

Keeping it objective, I can't see any major downsides, though I'd really
prefer to just have $() do string interpolation within "". That keeps
the implementation trivial and makes the behavior and limitations
obvious once you know that n/m/y is just shorthand for "n"/"m"/"y".

Cheers,
Ulf