snapshot: zero cow header for read-only snapshot

When read-only snapshot was created, tool was skipping header
initialization of cow device.  If it happened device has been
already containing header from some previous snapshot, it's
been 'reused' for a newly created snapshot instead of being cleared.

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.106 - 
 ====================================
+  Zero snapshot COW header when creating read-only snapshot.
   Comment out config lines in dumpconfig output without default values defined.
   Improve detection of clustered mirror support.
   Enhance raid code with feature flags, for now checks for raid10.

test/shell/snapshot-usage.sh

@@ -131,4 +131,15 @@ vgremove -ff $vg1
 
 fi
 
+lvremove -f $vg
+
+# Check snapshot really deletes COW header for read-only snapshot
+aux lvmconf "allocation/wipe_signatures_when_zeroing_new_lvs = 1"
+lvcreate -L10 -n $lv1 $vg
+lvcreate -s -L10 -n snap $vg/$lv1
+# Populate snapshot with some filesystem signatures
+mkfs.ext4 "$DM_DEV_DIR/$vg/snap"
+lvremove -f $vg/snap
+lvcreate -s -pr -l12 -n snap $vg/$lv1
+
 vgremove -ff $vg

tools/lvcreate.c

@@ -769,8 +769,12 @@ static int _read_activation_params(struct lvcreate_params *lp,
 	lp->permission = arg_uint_value(cmd, permission_ARG,
 					LVM_READ | LVM_WRITE);
 
-	/* Must not zero/wipe read only volume */
-	if (!(lp->permission & LVM_WRITE)) {
+	if (lp->snapshot) {
+		/* Snapshot has to zero COW header */
+		lp->zero = 1;
+		lp->wipe_signatures = 0;
+	} else if (!(lp->permission & LVM_WRITE)) {
+		/* Must not zero/wipe read only volume */
 		lp->zero = 0;
 		lp->wipe_signatures = 0;
 	}