Make 'zpool labelclear -f' work on offlined disks

This patch allows you to clear the label on offlined disks in an active
pool with `-f`.  Previously, labelclear wouldn't let you do that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes openzfs#12511

cmd/zpool/zpool_main.c

@@ -1231,6 +1231,26 @@ zpool_do_remove(int argc, char **argv)
 	return (ret);
 }
 
+/*
+ * Return 1 if a vdev is active (being used in a pool)
+ * Return 0 if a vdev is inactive (offlined or faulted, or not in active pool)
+ *
+ * This is useful for checking if a disk in an active pool is offlined or
+ * faulted.
+ */
+static int
+vdev_is_active(char *vdev_path)
+{
+	int fd;
+	fd = open(vdev_path, O_EXCL);
+	if (fd < 0) {
+		return (1);   /* cant open O_EXCL - disk is active */
+	}
+
+	close(fd);
+	return (0);   /* disk is inactive in the pool */
+}
+
 /*
  * zpool labelclear [-f] <vdev>
  *
@@ -1340,9 +1360,23 @@ zpool_do_labelclear(int argc, char **argv)
 	case POOL_STATE_ACTIVE:
 	case POOL_STATE_SPARE:
 	case POOL_STATE_L2CACHE:
+		/*
+		 * We allow the user to call 'zpool offline -f'
+		 * on an offlined disk in an active pool. We can check if
+		 * the disk is online by calling vdev_is_active().
+		 */
+		if (force && !vdev_is_active(vdev))
+			break;
+
 		(void) fprintf(stderr, gettext(
-		    "%s is a member (%s) of pool \"%s\"\n"),
+		    "%s is a member (%s) of pool \"%s\""),
 		    vdev, zpool_pool_state_to_name(state), name);
+
+		if (force) {
+			(void) fprintf(stderr, gettext(
+			    ". Offline the disk first to clear its label."));
+		}
+		printf("\n");
 		ret = 1;
 		goto errout;