Fix zombie mirror poll daemon (bug 585328)

This patch fixes a problem where the mirror polling process may never complete.

If you convert from a linear to a mirror and then convert that mirror back to linear /while/ the previous (up)convert is taking place, the mirror polling process will never complete. This is because the function that polls the mirror for completion doesn't check if it is still polling a mirror and the copy_percent that it gets back from the linear device is certainly never 100%.

The fix is simply to check if the daemon is still looking at a mirror device - if not, return PROGRESS_CHECK_FAILED.

The user sees the following output from the first (up)convert if someone else sneaks in and does a down-convert shortly after their convert:
[root at bp-01 ~]# lvconvert -m1 vg/lv
vg/lv: Converted: 43.4%
ABORTING: Mirror percentage check failed.

Index: LVM2/tools/polldaemon.c
===================================================================
@@ -78,7 +78,8 @@ progress_t poll_mirror_progress(struct c
 	percent_range_t percent_range, overall_percent_range;
 	uint32_t event_nr = 0;
 
-	if (!lv_mirror_percent(cmd, lv, !parms->interval, &segment_percent,
+	if (!(lv->status & MIRRORED) ||
+	    !lv_mirror_percent(cmd, lv, !parms->interval, &segment_percent,
 			       &percent_range, &event_nr) ||
 	    (percent_range == PERCENT_INVALID)) {
 		log_error("ABORTING: Mirror percentage check failed.");

I think it's actually safer to fix this here at this point in time. Otherwise, lv_mirror_percent could start failing unexpectedly on some mis-behaving code and introduce new bugs.

Indeed. Please fix.

I would personally also change the code to print a different error if the mirror is no longer a mirror, which would be likely less confusing to the user.

Anyway, assuming the lv_is_mirrored change, OK.

Yours,
Petr.