Re: Bug in gnulib-tools prevents bison from bootstrapping

[Noah Misch]

On Fri, Aug 31, 2007 at 06:50:07AM -0600, Eric Blake wrote:
> >>> In other words, during configuration, autoconf finds the first program on
> >>> your PATH named gm4, gnum4, and then m4, which also meets the minimum
> >>> requirements of m4 1.4.5 or later.

> The attached patch implements the above algorithm.  However, before I
> apply it, I wonder if it is time to make a documented public macro that
> makes this idiom kosher, rather than using an internal macro.

> Note that _AC_PATH_PROG_FEATURE_CHECK already has a bug in its current
> usage in programs.m4 - it is currently only used by AC_PROG_GREP and
> AC_PROG_SED, both of which invoke it inside AC_CACHE_CHECK.  Therefore,
> its attempt to do AC_SUBST([$1]) is useless - if you run ./configure -C
> with the program variable unset, the first run will set the variable, but
> the second run will leave it unset.  Fortunately, both AC_PROG_GREP and
> AC_PROG_SED (and my patch below) work around this bug, by re-calling
> AC_SUBST(variable) after the cache context ends.

I see no AC_SUBST(M4) in your patch, though.

> I'm thinking a public macro should look like:
> 
> # AC_PATH_PROG_FEATURE_CHECK(VARIABLE, PROGNAME-LIST, MSG,
> #                            FEATURE-TEST, [PATH=$PATH])
> # --------------------------------------------------------

The name 'AC_PATH_PROGS_FEATURE_CHECK' is more accurate: the macro follows
AC_PATH_PROGS more than AC_PATH_PROG.  The MSG argument is unusual in an
Autoconf macro; typically the calling code would do that itself, if needed.

> # Cache a search for a program, using MSG as in AC_CACHE_CHECK.
> # If VARIABLE is set, use it without any questions.  Otherwise,
> # call FEATURE_TEST repeatedly with $ac_path_VARIABLE set to the
> # name of a program in PROGNAME-LIST found in PATH.  FEATURE-TEST
> # must set $ac_cv_path_VARIABLE to the path of an acceptable
> # program, or else _AC_PATH_PROG_FEATURE_CHECK will report that
> # no acceptable program was found, and abort.

Aborting should not be the fixed failure resolution of a public, generic macro.
AC_PATH_PROGS has a VALUE-IF-NOT-FOUND argument; it is probably best to keep
that pattern.  Callers can test for that value and abort.  On the other hand,
but for current practice, an ACTION-IF-NOT-FOUND allows for more intuitive
control flow in the caller.

>   If a suitable
> # $ac_path_VARIABLE is found in the FEATURE-TEST macro, it can
> # set $ac_path_VARIABLE_found=':' to accept that value without
> # any further checks.  On success, VARIABLE will contain the
> # final program found, and will be AC_SUBST'd.