This commit adds functions to libzfs to check if one dataset is related

to another.

The `related_dataset()` function checks if the second provided dataset
is the same as or a child dataset of the first provided dataset. The
function returns a `boolean_t` value (`B_TRUE` if related).

The `zfs_related_dataset()` helper function takes a `zfs_handle_t` and
checks if the provided handle is related to the provided dataset. The
function returns a `boolean_t` value (`B_TRUE` if related).

libzfs_dataset.c:
- `related_dataset()`
- `zfs_related_dataset()`

libzfs.h:
- `zfs_related_dataset()`

Signed-off-by: QORTEC <lowell.bv@gmail.com>

include/libzfs.h

@@ -887,6 +887,7 @@ _LIBZFS_H int zfs_name_valid(const char *, zfs_type_t);
 _LIBZFS_H zfs_handle_t *zfs_path_to_zhandle(libzfs_handle_t *, const char *,
     zfs_type_t);
 _LIBZFS_H int zfs_parent_name(zfs_handle_t *, char *, size_t);
+_LIBZFS_H boolean_t zfs_dataset_related(zfs_handle_t *, const char *);
 _LIBZFS_H boolean_t zfs_dataset_exists(libzfs_handle_t *, const char *,
     zfs_type_t);
 _LIBZFS_H int zfs_spa_version(zfs_handle_t *, int *);

lib/libzfs/libzfs_dataset.c

@@ -3446,6 +3446,35 @@ is_descendant(const char *ds1, const char *ds2)
 	return (ds2[d1len] == '/' && (strncmp(ds1, ds2, d1len) == 0));
 }
 
+/*
+ * Is one dataset is related to another, including self-relation?
+ *
+ * Needs to handle these cases:
+ * Dataset 1	"a/foo"		"a/foo"		"a/foo"		"a/foo"
+ * Dataset 2	"a/foo"		"a/fo"		"a/foobar"	"a/foo/bar"
+ * Related?	Yes.		No.		No.		Yes.
+ */
+static boolean_t
+dataset_related(const char *ds1, const char *ds2)
+{
+	/* ds1 and ds2 are identical */
+	if (strcmp(ds1, ds2) == 0)
+		return (B_TRUE);
+
+	/* ds2 is a descendant of ds1 */
+	if (is_descendant(ds1, ds2))
+		return (B_TRUE);
+
+	/* dataset are not related. */
+	return (B_FALSE);
+}
+
+boolean_t
+zfs_dataset_related(zfs_handle_t *zhp, const char *parent)
+{
+	return (dataset_related(parent, zfs_get_name(zhp)));
+}
+
 /*
  * Given a complete name, return just the portion that refers to the parent.
  * Will return -1 if there is no parent (path is just the name of the