thin: implement get_thin_device_id

get_thin_device_id() returns the device_id of a locally activated thin
logical volume.  This function is used to check if a thin snapshot merge
has completed.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 lib/metadata/thin_manip.c |   55 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 2 deletions(-)

Index: lvm2/lib/metadata/thin_manip.c
===================================================================
--- lvm2.orig/lib/metadata/thin_manip.c
+++ lvm2/lib/metadata/thin_manip.c
@@ -20,6 +20,7 @@
 #include "lv_alloc.h"
 #include "defaults.h"
 #include "display.h"
+#include "commands/toolcontext.h"
 
 /*
  * thin snapshots don't have intermediate snapshot segments, so we
@@ -935,7 +936,57 @@ int vg_remove_pool_metadata_spare(struct
  */
 int get_thin_device_id(struct logical_volume *lv, uint32_t *device_id)
 {
-	*device_id = find_merging_snapshot(lv)->device_id;
+	struct dm_task *dmt;
+	const char *uuid;
+	void *next = NULL;
+	uint64_t start, length;
+	char *target_type, *params;
+	char pool_dev[80], extra[80];
+	int r;
 
-	return 1;
+	if (!lv_is_active_locally(lv)) {
+		log_error("LV is not active locally");
+		return 0;
+	}
+
+	if (!(dmt = dm_task_create(DM_DEVICE_TABLE))) {
+		log_error("Failed to create device-mapper task struct");
+		return 0;
+	}
+
+	if (!(uuid = build_dm_uuid(lv->vg->cmd->mem, lv->lvid.s, lv_layer(lv))))
+		return_0;
+
+	if (!dm_task_set_uuid(dmt, uuid))
+		return_0;
+
+	if (!(r = dm_task_run(dmt)))
+		goto_out;
+
+	if (dm_get_next_target(dmt, next, &start, &length,
+			       &target_type, &params)) {
+		/* thin target is only ever in a singleton table */
+		r = 0;
+		goto_out;
+	}
+	if (strncmp(target_type, "thin", 4)) {
+		r = 0;
+		goto_out;
+	}
+
+	/*
+	 * Get the device_id from @params:
+	 * <pool dev> <dev id> [<external origin dev>]
+	 */
+	if (sscanf(params, "%s %u %s", pool_dev, device_id, extra) < 2) {
+		log_error("Invalid thin target table params: %s", params);
+		r = 0;
+		goto out;
+	}
+
+	r = 1;
+out:
+	dm_task_destroy(dmt);
+
+	return r;
 }