Update cache files atomically.

[Ralf Wildenhues]

When several configure scripts run in parallel (in different build
directories) and share the same config.cache file, there is a race
condition when saving the cache.  I don't know of a package that does
this, and the AC_CONFIG_SUBDIRS code doesn't provide this functionality,
but FWIW here's code that would fix it, by letting the last saver win.

I've had this on my disk for a long time; the list archives may serve
just as well.  I don't know whether there is any benefit in using it.

Cheers,
Ralf

    Update cache files atomically.

    * lib/autoconf/general.m4 (AC_CACHE_SAVE): Update $cache_file
    atomically.
    * NEWS: Update.

diff --git a/NEWS b/NEWS
index 0a2fbd4..ca04535 100644
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,8 @@ GNU Autoconf NEWS - User visible changes.
 ** The following m4sugar macros are new:
    m4_default_quoted
 
+** config.cache files are updated atomically.
+
 
 * Major changes in Autoconf 2.63 (2008-09-09) [stable]
   Released by Eric Blake, based on git versions 2.62.*.
diff --git a/lib/autoconf/general.m4 b/lib/autoconf/general.m4
index 986b7af..a8437b6 100644
--- a/lib/autoconf/general.m4
+++ b/lib/autoconf/general.m4
@@ -1950,9 +1950,14 @@ _AC_CACHE_DUMP() |
      :end'] >>confcache
 if diff "$cache_file" confcache >/dev/null 2>&1; then :; else
   if test -w "$cache_file"; then
-    test "x$cache_file" != "x/dev/null" &&
+    if test "x$cache_file" != "x/dev/null"; then
       AC_MSG_NOTICE([updating cache $cache_file])
-    cat confcache >$cache_file
+      if test -f "$cache_file"; then
+       mv -f confcache "$cache_file"
+      else
+       cat confcache >"$cache_file"
+      fi
+    fi
   else
     AC_MSG_NOTICE([not updating unwritable cache $cache_file])
   fi