bug#14196: Problem with invoking "missing" in directory with "(" in name (Automake 1.11.6, Autoconf 2.68

[Jim Meyering]

On Fri, Sep 4, 2020 at 4:38 PM Zack Weinberg <zackw@panix.com> wrote:
> OK, after a quick investigation, the failure happens at configure time
> but the problem code belongs to automake.  Specifically,
> AM_MISSING_HAS_RUN. I think the tidiest fix is to quote the value of
> $am_aux_dir/missing unconditionally:
>
> diff --git a/m4/missing.m4 b/m4/missing.m4
> index 6f742fb20..56c84fbf5 100644
> --- a/m4/missing.m4
> +++ b/m4/missing.m4
> @@ -21,12 +21,7 @@ AC_DEFUN([AM_MISSING_HAS_RUN],
>  [AC_REQUIRE([AM_AUX_DIR_EXPAND])dnl
>  AC_REQUIRE_AUX_FILE([missing])dnl
>  if test x"${MISSING+set}" != xset; then
> -  case $am_aux_dir in
> -  *\ * | *\    *)
> -    MISSING="\${SHELL} \"$am_aux_dir/missing\"" ;;
> -  *)
> -    MISSING="\${SHELL} $am_aux_dir/missing" ;;
> -  esac
> +  MISSING="\${SHELL} \"$am_aux_dir/missing\"" ;;
>  fi
>  # Use eval to expand $SHELL
>  if eval "$MISSING --is-lightweight"; then
>
> This doesn't handle the possibility of $am_aux_dir containing
> backslash or double quote characters, but that's ok because
> AM_SANITY_CHECK will already have refused to run configure with either
> the current working directory or the source directory named like that.
> (An alternative patch would be to add ( ) to the set of characters
> rejected by AM_SANITY_CHECK.)

Thanks for investigating and patching.
I think this patch is the right way to go.
Otherwise, I suspect we'd have to make AM_SANITY_CHECK disallow not
only parentheses, but also at least these !?*[]

While looking at this, I wondered... why are the inner quotes double quotes?
I.e., we might as well use single quotes. Slightly cleaner, too, since
no backslash is required for them:
+ MISSING="\${SHELL} '$am_aux_dir/missing'" ;;