Use dsl_dataset_snap_lookup()

Retire the dmu_snapshot_id() function which was introduced in the
initial .zfs control directory implementation.  There is already
an existing dsl_dataset_snap_lookup() which does exactly what we
need, and the dmu_snapshot_id() function as implemented is racy.

https://github.com/zfsonlinux/zfs/issues/1215#issuecomment-12579879

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1238

include/sys/dmu.h

@@ -731,7 +731,7 @@ extern uint64_t dmu_objset_syncprop(objset_t *os);
 extern uint64_t dmu_objset_logbias(objset_t *os);
 extern int dmu_snapshot_list_next(objset_t *os, int namelen, char *name,
     uint64_t *id, uint64_t *offp, boolean_t *case_conflict);
-extern int dmu_snapshot_id(objset_t *os, const char *snapname, uint64_t *idp);
+extern int dmu_snapshot_lookup(objset_t *os, const char *name, uint64_t *val);
 extern int dmu_snapshot_realname(objset_t *os, char *name, char *real,
     int maxlen, boolean_t *conflict);
 extern int dmu_dir_list_next(objset_t *os, int namelen, char *name,

include/sys/dsl_dataset.h

@@ -251,6 +251,8 @@ int dsl_dataset_block_kill(dsl_dataset_t *ds, const blkptr_t *bp,
 boolean_t dsl_dataset_block_freeable(dsl_dataset_t *ds, const blkptr_t *bp,
     uint64_t blk_birth);
 uint64_t dsl_dataset_prev_snap_txg(dsl_dataset_t *ds);
+int dsl_dataset_snap_lookup(dsl_dataset_t *ds, const char *name,
+    uint64_t *value);
 
 void dsl_dataset_dirty(dsl_dataset_t *ds, dmu_tx_t *tx);
 void dsl_dataset_stats(dsl_dataset_t *os, nvlist_t *nv);

module/zfs/dmu_objset.c

@@ -1573,39 +1573,10 @@ dmu_snapshot_list_next(objset_t *os, int namelen, char *name,
 	return (0);
 }
 
-/*
- * Determine the objset id for a given snapshot name.
- */
 int
-dmu_snapshot_id(objset_t *os, const char *snapname, uint64_t *idp)
+dmu_snapshot_lookup(objset_t *os, const char *name, uint64_t *value)
 {
-	dsl_dataset_t *ds = os->os_dsl_dataset;
+	return dsl_dataset_snap_lookup(os->os_dsl_dataset, name, value);
-	zap_cursor_t cursor;
-	zap_attribute_t attr;
-	int error;
-
-	if (ds->ds_phys->ds_snapnames_zapobj == 0)
-		return (ENOENT);
-
-	zap_cursor_init(&cursor, ds->ds_dir->dd_pool->dp_meta_objset,
-	    ds->ds_phys->ds_snapnames_zapobj);
-
-	error = zap_cursor_move_to_key(&cursor, snapname, MT_EXACT);
-	if (error) {
-		zap_cursor_fini(&cursor);
-		return (error);
-	}
-
-	error = zap_cursor_retrieve(&cursor, &attr);
-	if (error) {
-		zap_cursor_fini(&cursor);
-		return (error);
-	}
-
-	*idp = attr.za_first_integer;
-	zap_cursor_fini(&cursor);
-
-	return (0);
 }
 
 int

module/zfs/dsl_dataset.c

@@ -314,7 +314,7 @@ dsl_dataset_get_snapname(dsl_dataset_t *ds)
 	return (err);
 }
 
-static int
+int
 dsl_dataset_snap_lookup(dsl_dataset_t *ds, const char *name, uint64_t *value)
 {
 	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;

module/zfs/zfs_ctldir.c

@@ -443,7 +443,7 @@ zfsctl_snapdir_lookup(struct inode *dip, char *name, struct inode **ipp,
 
 	ZFS_ENTER(zsb);
 
-	error = dmu_snapshot_id(zsb->z_os, name, &id);
+	error = dmu_snapshot_lookup(zsb->z_os, name, &id);
 	if (error) {
 		ZFS_EXIT(zsb);
 		return (error);
@@ -909,7 +909,7 @@ zfsctl_lookup_objset(struct super_block *sb, uint64_t objsetid, zfs_sb_t **zsbp)
 	 */
 	sep = avl_first(&zsb->z_ctldir_snaps);
 	while (sep != NULL) {
-		error = dmu_snapshot_id(zsb->z_os, sep->se_name, &id);
+		error = dmu_snapshot_lookup(zsb->z_os, sep->se_name, &id);
 		if (error)
 			goto out;