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>

*** 1720,1729 ****
--- 1720,1730 ----
  }
  
  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)
  {
*** 1791,1803 ****
--- 1792,1808 ----
  {
          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));
  
*** 1809,1837 ****
          dsl_dataset_rele(clone, FTAG);
          dsl_dataset_rele(ds, FTAG);
  }
  
  /*
!  * 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_arg_t ddra;
  
          ddra.ddra_fsname = fsname;
          ddra.ddra_owner = owner;
  
          return (dsl_sync_task(fsname, dsl_dataset_rollback_check,
!             dsl_dataset_rollback_sync, (void *)&ddra, 1));
  }
  
  struct promotenode {
          list_node_t link;
          dsl_dataset_t *ds;
--- 1814,1846 ----
          dsl_dataset_rele(clone, FTAG);
          dsl_dataset_rele(ds, FTAG);
  }
  
  /*
!  * 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, 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, &ddra, 1));
  }
  
  struct promotenode {
          list_node_t link;
          dsl_dataset_t *ds;