Re: versioning config.cache

[Dave Hart]

Here's my latest revision of ntp_cacheversion.m4 with a bunch of shell
programming tweaks thanks to Eric, plus a change to not clear the
cache right after an empty cache file is created on the first run.

Thanks for the tips and comments.

Cheers,
Dave Hart

# NTP_CACHEVERSION(component, version)
# ------------------------------------
# compare this configure script's cache version stamp with the stamp
# saved by the prior run in config.cache.  If they differ, clear all
# cache variables to avoid using results cached with a script that
# is known to differ in a cache-invalidating manner.
#
# Note: use immediately following AC_INIT in configure.ac, as clearing
# all _cv_ variables only makes sense immediately after loading, before
# use or modification.
#
# It is assumed that parent configure.ac files which use
# AC_CONFIG_SUBDIR to invoke child configure.ac files have used
# NTP_CACHEVERSION if any children do.  The top-level configure script
# will clear a previous cache lacking any saved cache version number,
# while children do not.  The reason is the children can rely on the
# parent having cleared any cache variables predating this mechanism.
# Therefore the child can rely on the config.cache generated by the
# parent on the first run despite not finding its version stamp
# previously saved.

AC_DEFUN_ONCE([NTP_CACHEVERSION], [
    AC_BEFORE([$0], [AM_INIT_AUTOMAKE])dnl
    AC_BEFORE([$0], [AM_CONFIG_HEADER])dnl
    AC_BEFORE([$0], [AC_PROG_CC])dnl

    ntp_cache_flush=1
    case "$ntp_cv_[$1]_cache_version" in
     [$2])
        # same version, good
        ntp_cache_flush=0
        ;;
     '')
        # No cache, predates ntp_cv_$1_cache_version, or is empty.
        case "$cache_file" in
         ../*)
            # Parent configure will have cleared cache once already.
            # This will misfire if a top-level configure is invoked
            # with --config-cache= value beginning with '../', is
            # there a better way to detect this configure was
            # invoked by a parent directory configure?
            ntp_cache_flush=0
            ;;
         /dev/null)
            ntp_cache_flush=0
            ;;
         *)
            # This appears to be the top-level configure file.
            # Do not clear the cache immediately after it is created
            # empty as it is noisy.  Differentiate a newly-created
            # config.cache from one predating the cache version
            # mechanism by looking for the first cached variable set
            # by Autoconf
            case "$ac_cv_path_install" in
             '')
                # presumably empty config.cache file
                ntp_cache_flush=0
            esac
        esac
        ;;
     *)
        # configure cache version mismatches config.cache version
    esac
    case "$ntp_cache_flush" in
     1)
        # Clear all *_cv_* variables except our various components'
        # ntp_cv_*_cache_version vars.
        
        c_varname_list=`set |
                        sed -n -e 's/=.*$//' \
                               -e '/ntp_cv_.*_cache_version/d' \
                               -e '/_cv_/p'
                       `
        for c_varname in $c_varname_list
        do
            dnl use AS_UNSET([$c_varname]) eventually
            eval ${c_varname}=;  $as_unset $c_varname
        done
        
        dnl use AS_UNSET([c_varname_list c_varname]) eventually
        c_varname_list=;  $as_unset c_varname_list c_varname
        
        AC_MSG_NOTICE([$cache_file saved by another version, ignored.])
        AC_MSG_NOTICE([configure script cache version: [$2]])
        c_version="${ntp_cv_[$1]_cache_version:-(no version found)}"
        AC_MSG_NOTICE([$cache_file version: $c_version])
        $as_unset c_version
    esac

    # save configure version in config.cache for next time
    ntp_cv_[$1]_cache_version="[$2]"

    $as_unset ntp_cache_flush
])dnl