libbtrfsutil: add subvolume iterator helpers

This is how we can implement stuff like `btrfs subvol list`. Rather than
producing the entire list upfront, the iterator approach uses less
memory in the common case where the whole list is not stored (O(max
subvolume path length)). It supports both pre-order traversal (useful
for, e.g, recursive snapshot) and post-order traversal (useful for
recursive delete).

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 97 insertions, 0 deletions

diff --git a/libbtrfsutil/btrfsutil.h b/libbtrfsutil/btrfsutil.h
index 76ab06b6..09a71c32 100644
--- a/libbtrfsutil/btrfsutil.h
+++ b/libbtrfsutil/btrfsutil.h
@@ -391,6 +391,103 @@ enum btrfs_util_error btrfs_util_create_subvolume_fd(int parent_fd,
 						     uint64_t *async_transid,
 						     struct btrfs_util_qgroup_inherit *qgroup_inherit);
 
+struct btrfs_util_subvolume_iterator;
+
+/**
+ * BTRFS_UTIL_SUBVOLUME_ITERATOR_POST_ORDER - Iterate post-order. The default
+ * behavior is pre-order, e.g., foo will be yielded before foo/bar. If this flag
+ * is specified, foo/bar will be yielded before foo.
+ */
+#define BTRFS_UTIL_SUBVOLUME_ITERATOR_POST_ORDER (1 << 0)
+#define BTRFS_UTIL_SUBVOLUME_ITERATOR_MASK ((1 << 1) - 1)
+
+/**
+ * btrfs_util_create_subvolume_iterator() - Create an iterator over subvolumes
+ * in a Btrfs filesystem.
+ * @path: Path in a Btrfs filesystem. This may be any path in the filesystem; it
+ * does not have to refer to a subvolume unless @top is zero.
+ * @top: List subvolumes beneath (but not including) the subvolume with this ID.
+ * If zero is given, the subvolume ID of @path is used. To list all subvolumes,
+ * pass %BTRFS_FS_TREE_OBJECTID (i.e., 5). The returned paths are relative to
+ * the subvolume with this ID.
+ * @flags: Bitmask of BTRFS_UTIL_SUBVOLUME_ITERATOR_* flags.
+ * @ret: Returned iterator.
+ *
+ * The returned iterator must be freed with
+ * btrfs_util_destroy_subvolume_iterator().
+ *
+ * Return: %BTRFS_UTIL_OK on success, non-zero error code on failure.
+ */
+enum btrfs_util_error btrfs_util_create_subvolume_iterator(const char *path,
+							   uint64_t top,
+							   int flags,
+							   struct btrfs_util_subvolume_iterator **ret);
+
+/**
+ * btrfs_util_create_subvolume_iterator_fd() - See
+ * btrfs_util_create_subvolume_iterator().
+ */
+enum btrfs_util_error btrfs_util_create_subvolume_iterator_fd(int fd,
+							      uint64_t top,
+							      int flags,
+							      struct btrfs_util_subvolume_iterator **ret);
+
+/**
+ * btrfs_util_destroy_subvolume_iterator() - Destroy a subvolume iterator
+ * previously created by btrfs_util_create_subvolume_iterator().
+ * @iter: Iterator to destroy.
+ */
+void btrfs_util_destroy_subvolume_iterator(struct btrfs_util_subvolume_iterator *iter);
+
+/**
+ * btrfs_util_subvolume_iterator_fd() - Get the file descriptor associated with
+ * a subvolume iterator.
+ * @iter: Iterator to get.
+ *
+ * This can be used to get the file descriptor opened by
+ * btrfs_util_create_subvolume_iterator() in order to use it for other
+ * functions.
+ *
+ * Return: File descriptor.
+ */
+int btrfs_util_subvolume_iterator_fd(const struct btrfs_util_subvolume_iterator *iter);
+
+/**
+ * btrfs_util_subvolume_iterator_next() - Get the next subvolume from a
+ * subvolume iterator.
+ * @iter: Subvolume iterator.
+ * @path_ret: Returned subvolume path, relative to the subvolume ID used to
+ * create the iterator. May be %NULL.
+ * Must be freed with free().
+ * @id_ret: Returned subvolume ID. May be %NULL.
+ *
+ * This requires appropriate privilege (CAP_SYS_ADMIN).
+ *
+ * Return: %BTRFS_UTIL_OK on success, %BTRFS_UTIL_ERROR_STOP_ITERATION if there
+ * are no more subvolumes, non-zero error code on failure.
+ */
+enum btrfs_util_error btrfs_util_subvolume_iterator_next(struct btrfs_util_subvolume_iterator *iter,
+							 char **path_ret,
+							 uint64_t *id_ret);
+
+/**
+ * btrfs_util_subvolume_iterator_next_info() - Get information about the next
+ * subvolume for a subvolume iterator.
+ * @iter: Subvolume iterator.
+ * @path_ret: See btrfs_util_subvolume_iterator_next().
+ * @subvol: Returned subvolume information.
+ *
+ * This convenience function basically combines
+ * btrfs_util_subvolume_iterator_next() and btrfs_util_subvolume_info().
+ *
+ * This requires appropriate privilege (CAP_SYS_ADMIN).
+ *
+ * Return: See btrfs_util_subvolume_iterator_next().
+ */
+enum btrfs_util_error btrfs_util_subvolume_iterator_next_info(struct btrfs_util_subvolume_iterator *iter,
+							      char **path_ret,
+							      struct btrfs_util_subvolume_info *subvol);
+
 /**
  * btrfs_util_create_qgroup_inherit() - Create a qgroup inheritance specifier
  * for btrfs_util_create_subvolume() or btrfs_util_create_snapshot().