Subject: Re: [PATCH v4] [GSOC]trailer: pass arg as positional parameter

From: Junio C Hamano <[email-masked]>
Date: Wed, 31 Mar 2021 11:19:59 -0700

ZheNing Hu <[email-masked]> writes:

> The configuration is like this:
> trailer.bug.key=BUG:
> trailer.bug.ifexists=add
> trailer.bug.cmd=echo "123"
>
> And use:
>
> $ git interpret-trailers --trailer="bug:456" --trailer="bug:789"<<-EOF
> EOF
>
> BUG: 123
> BUG: 123 456
> BUG: 123 789

I think that is quite expected.  You said the command to run is
'echo 123', and that is not "pick a directory $D on $PATH where
there is an executable '$D/echo 123' exists, and run that".  It
runs the given command with the shell, and in general that is
what we want for end-user supplied commands specified in the
configuration file [*1*].

So we form a shell command whose beginning is 'echo 123' and tuck
the argument after that command line, so it is understandable that
"echo 123 456" gets executed for "--trailer=bug:456".

I wasn't following the discussion between you and Christian closely
but I recall seeing him saying that the command is executed one
extra time without any arg before it is run for actual --trailer
requests with the value?  I am guessing that is where the first
output "BUG: 123" (without anything else) is coming from.

*1* Imagine .editor set to 'emacs -nw' or 'vim -f'; we do not want
    Git to find a directory on $PATH that has an executable whose
    name is 'emacs -nw' and run that file (i.e. give 'emacs -nw' as
    the first argument to execlp()).  Instead, you'd want to behave
    as if the user typed "emacs -nw", followed by any arguments we
    want to give to it (in .editor's case, the name of the file to
    be edited) properly quoted for the shell.

    And the way we do so is to form a moral equivalent of

	execlp("sh", "-c", "emacs -nw $@", ...);

    and put the arguments at the end where I wrote ... (we actually
    do so with execvp(), but illustrating with execlp() is easier to
    read and write---hence "a moral equivalent of").