Re: [PATCH] report which $PATH entry had trouble running execvp(3)

[Jeff King <peff@peff.net>]

On Tue, Apr 19, 2011 at 09:01:21PM -0700, Junio C Hamano wrote:

> You can add your own custom subcommand 'frotz' to the system by adding
> 'git-frotz' in a directory somewhere in your $PATH environment variable.
> When you ask "git frotz" from the command line, "git-frotz" is run via
> execvp(3).
> [...]
> we do not report 'git-frotz' in which directory we had trouble with.
> We could do better if we implemented the command search behaviour of
> execvp(3) ourselves.

I like the idea of giving the user more information about which
git-frotz was the problem. Usually there is just one, and pointing them
to it saves them time.

But what about the case of

  mkdir one two
  touch one/frotz two/frotz
  PATH=one:two:$PATH

We would report two/frotz, but might it be even better to say "we found
2 frotzes, but neither of them were executable"?

I don't know if it is worth the effort for such a weird corner case.

> Three plausible scenarios that the execvp(3) would fail for us are:
> 
>  * The first 'git-frotz' found in a directory on $PATH was not a proper
>    executable binary, and we got "Exec format error" (ENOEXEC);

What about the magic "unknown things get executed as shell scripts"
behavior that is implemented by libc's execvp? Your patch has a
regression for:

  echo "git log --with-some-options" >local/bin/git-frotz
  chmod +x local/bin/git-frotz
  git frotz

I have always found that behavior slightly insane, but it is
well-established, and your sane_execvp breaks anybody who is depending
on it.

> @@ -278,7 +324,7 @@ fail_pipe:
>  		} else if (cmd->use_shell) {
>  			execv_shell_cmd(cmd->argv);
>  		} else {
> -			execvp(cmd->argv[0], (char *const*) cmd->argv);
> +			cmd->argv[0] = sane_execvp(cmd->argv[0], cmd->argv);
>  		}
>  		/*
>  		 * Do not check for cmd->silent_exec_failure; the parent

This is inside "#ifndef WIN32". Presumably people on Windows want it,
too.  In fact, they already have their own execvp in compat/mingw.c. It
might make sense to bring the implementations together. Or perhaps not.
Theirs is quite different; it does a search of PATH itself, looking for
executables (and magically appending ".exe"), and then exec's the
result. On the other hand, doing that PATH lookup, deciding you have
something, and _then_ exec'ing can be convenient. IIRC, there are a few
warts in the git wrapper that could be improved by doing that, but I
don't recall the specifics anymore (maybe something like handling the
pager between the momemnt when we decide a command exists and when we
exec?).

-Peff