3996 want a libzfs_core API to rollback to latest snapshot
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

@@ -1720,10 +1720,11 @@
 }
 
 typedef struct dsl_dataset_rollback_arg {
         const char *ddra_fsname;
         void *ddra_owner;
+        nvlist_t *ddra_result;
 } dsl_dataset_rollback_arg_t;
 
 static int
 dsl_dataset_rollback_check(void *arg, dmu_tx_t *tx)
 {

@@ -1791,13 +1792,17 @@
 {
         dsl_dataset_rollback_arg_t *ddra = arg;
         dsl_pool_t *dp = dmu_tx_pool(tx);
         dsl_dataset_t *ds, *clone;
         uint64_t cloneobj;
+        char namebuf[ZFS_MAXNAMELEN];
 
         VERIFY0(dsl_dataset_hold(dp, ddra->ddra_fsname, FTAG, &ds));
 
+        dsl_dataset_name(ds->ds_prev, namebuf);
+        fnvlist_add_string(ddra->ddra_result, "target", namebuf);
+
         cloneobj = dsl_dataset_create_sync(ds->ds_dir, "%rollback",
             ds->ds_prev, DS_CREATE_FLAG_NODIRTY, kcred, tx);
 
         VERIFY0(dsl_dataset_hold_obj(dp, cloneobj, FTAG, &clone));
 

@@ -1809,29 +1814,33 @@
         dsl_dataset_rele(clone, FTAG);
         dsl_dataset_rele(ds, FTAG);
 }
 
 /*
- * If owner != NULL:
+ * Rolls back the given filesystem or volume to the most recent snapshot.
+ * The name of the most recent snapshot will be returned under key "target"
+ * in the result nvlist.
  *
+ * If owner != NULL:
  * - The existing dataset MUST be owned by the specified owner at entry
  * - Upon return, dataset will still be held by the same owner, whether we
  *   succeed or not.
  *
  * This mode is required any time the existing filesystem is mounted.  See
  * notes above zfs_suspend_fs() for further details.
  */
 int
-dsl_dataset_rollback(const char *fsname, void *owner)
+dsl_dataset_rollback(const char *fsname, void *owner, nvlist_t *result)
 {
         dsl_dataset_rollback_arg_t ddra;
 
         ddra.ddra_fsname = fsname;
         ddra.ddra_owner = owner;
+        ddra.ddra_result = result;
 
         return (dsl_sync_task(fsname, dsl_dataset_rollback_check,
-            dsl_dataset_rollback_sync, (void *)&ddra, 1));
+            dsl_dataset_rollback_sync, &ddra, 1));
 }
 
 struct promotenode {
         list_node_t link;
         dsl_dataset_t *ds;