Re: [sr #110215] AC_EGREP_HEADER appears to be broken in master

Hi Ross,

On 2020-03-24, Ross Burton <[email protected]> wrote:
[...]
> AC_CHECK_FUNC(statvfs,[HAVE_STATVFS=yes])
> dnl Arg, linux and bsd put their statfs function in different places
> if test x"$HAVE_STATVFS" != x"yes"; then
>    AC_EGREP_HEADER(statfs,sys/vfs.h,[AC_DEFINE(HAVE_VFS_H)],[
>
> AC_EGREP_HEADER(statfs,sys/mount.h,[AC_DEFINE(HAVE_MOUNT_H)],[AC_MSG_ERROR(failed:
> Need statvfs)])
>    ])
> fi
> (and later looks for h_errno then fails)
>
> And the generated code does this:
>
> if test x"$HAVE_STATVFS" != x"yes"; then
> { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for grep that
> handles long lines and
> -e" >&5
> printf %s "checking for grep that handles long lines and -e... " >&6; }
> ...
>
> So the search for egrep only happens if statvfs isn't found by
> AC_CHECK_FUNC.  My understanding is that this isn't how AC_REQUIRE
> should be working, right?

No, this is exactly how AC_REQUIRE is supposed to be working.  The configure
script has a bug.  Now I'm not sure why this fails for you only with latest
autoconf, because AC_REQUIRE has worked like this for a very long time.  It
might have worked by luck because some earlier macro previously used
AC_PROG_EGREP, and maybe it no longer does.

The result of expanding AC_REQUIRE'd macros are hoisted to the outermost
macro expansion (although note that only macros defined with AC_DEFUN
and friends will participate in this process).

So if you have an open-coded shell "if" outside of any macro, then
within the if body, you expand a macro that uses AC_REQUIRE, then the
AC_REQUIRE'd macro gets expanded _within the if_.

This is almost never the desired behaviour because autoconf will
consider the macro expanded and will not expand the dependency at other
AC_REQUIRE sites, leading to the problem you see.

There are several options for configure authors can to avoid this
problem, including:

 (1) You can use AS_IF, which will cause m4 to hoist the dependency
     expansion outside of the conditional.
 (2) You can explicitly call all dependencies such as AC_PROG_EGREP
     outside of the if.

Obviously (2) is only possible if you know the dependencies in advance
(not so good for externally-updated macros) so I suggest using AS_IF.

Hope that helps,
  Nick