warnings.pl: again, don't update the timestamp unless we need to

Don't update the timestamp unless we really have to do so.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

Author: H. Peter Anvin (Intel)

asm/warnings.pl

@@ -279,17 +279,20 @@ sub sort_warnings {
 close($out);
 
 # Write data to file if and only if it has changed
-# Windows requires append mode here
-open($out, '+>>', $outfile)
-    or die "$0: cannot open output file $outfile: $!\n";
-my $datalen = length($outdata);
-my $oldlen = read($out, my $oldoutdata, $datalen+1);
-if (!defined($oldlen) || $oldlen != $datalen ||
-    !($oldoutdata eq $outdata)) {
-    # Data changed, must rewrite
-    truncate($out, 0);
-    seek($out, 0, SEEK_SET)
-	or die "$0: cannot rewind output file $outfile: $!\n";
-    print $out $outdata;
+# For some systems, even if we don't write, opening for append
+# apparently touches the timestamp, so we need to read and write
+# as separate operations.
+if (open(my $out, '<', $outfile)) {
+    my $datalen = length($outdata);
+    my $oldlen = read($out, my $oldoutdata, $datalen+1);
+    close($out);
+    exit 0 if (defined($oldlen) && $oldlen == $datalen &&
+	       ($oldoutdata eq $outdata));
 }
+
+# Data changed, must rewrite
+open(my $out, '>', $outfile)
+    or die "$0: cannot open output file $outfile: $!\n";
+
+print $out $outdata;
 close($out);