[23/26] fs: btrfs: Introduce lookup_data_extent() for later use

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 986b9347b012..3332055ff840 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -814,3 +814,104 @@  out:
 	free(dbuf);
 	return ret;
 }
+
+/*
+ * Get the first file extent that covers bytenr @file_offset.
+ *
+ * @file_offset must be aligned to sectorsize.
+ *
+ * return 0 for found, and path points to the file extent.
+ * return >0 for not found, and fill @next_offset.
+ * @next_offset can be 0 if there is no next file extent.
+ * return <0 for error.
+ */
+static int lookup_data_extent(struct btrfs_root *root, struct btrfs_path *path,
+			      u64 ino, u64 file_offset, u64 *next_offset)
+{
+	struct btrfs_key key;
+	struct btrfs_file_extent_item *fi;
+	u8 extent_type;
+	int ret = 0;
+
+	ASSERT(IS_ALIGNED(file_offset, root->fs_info->sectorsize));
+	key.objectid = ino;
+	key.type = BTRFS_EXTENT_DATA_KEY;
+	key.offset = file_offset;
+
+	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+	/* Error or we're already at the file extent */
+	if (ret <= 0)
+		return ret;
+	if (ret > 0) {
+		/* Check previous file extent */
+		ret = btrfs_previous_item(root, path, ino,
+					  BTRFS_EXTENT_DATA_KEY);
+		if (ret < 0)
+			return ret;
+		if (ret > 0)
+			goto check_next;
+	}
+	/* Now the key.offset must be smaller than @file_offset */
+	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+	if (key.objectid != ino ||
+	    key.type != BTRFS_EXTENT_DATA_KEY)
+		goto check_next;
+
+	fi = btrfs_item_ptr(path->nodes[0], path->slots[0],
+			    struct btrfs_file_extent_item);
+	extent_type = btrfs_file_extent_type(path->nodes[0], fi);
+	if (extent_type == BTRFS_FILE_EXTENT_INLINE) {
+		if (file_offset == 0)
+			return 0;
+		/* Inline extent should be the only extent, no next extent. */
+		*next_offset = 0;
+		return 1;
+	}
+
+	/* This file extent covers @file_offset */
+	if (key.offset <= file_offset && key.offset +
+	    btrfs_file_extent_num_bytes(path->nodes[0], fi) > file_offset)
+		return 0;
+check_next:
+	ret = btrfs_next_item(root, path);
+	if (ret < 0)
+		return ret;
+	if (ret > 0) {
+		*next_offset = 0;
+		return 1;
+	}
+
+	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+	fi = btrfs_item_ptr(path->nodes[0], path->slots[0],
+			    struct btrfs_file_extent_item);
+	/* Next next data extent */
+	if (key.objectid != ino ||
+	    key.type != BTRFS_EXTENT_DATA_KEY) {
+		*next_offset = 0;
+		return 1;
+	}
+	/* Current file extent already beyond @file_offset */
+	if (key.offset > file_offset) {
+		*next_offset = key.offset;
+		return 1;
+	}
+	/* This file extent covers @file_offset */
+	if (key.offset <= file_offset && key.offset +
+	    btrfs_file_extent_num_bytes(path->nodes[0], fi) > file_offset)
+		return 0;
+	/* This file extent ends before @file_offset, check next */
+	ret = btrfs_next_item(root, path);
+	if (ret < 0)
+		return ret;
+	if (ret > 0) {
+		*next_offset = 0;
+		return 1;
+	}
+	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
+	if (key.type != BTRFS_EXTENT_DATA_KEY || key.objectid != ino) {
+		*next_offset = 0;
+		return 1;
+	}
+	*next_offset = key.offset;
+	return 1;
+}