[PATCH v3 01/14] btrfs: Introduce btrfs_for_each_slot iterator macro

[Gabriel Niebler <gniebler@suse.com>]

There is a common pattern when searching for a key in btrfs:

* Call btrfs_search_slot to find the slot for the key
* Enter an endless loop:
	* If the found slot is larger than the no. of items in the current leaf,
	  check the next leaf
	* If it's still not found in the next leaf, terminate the loop
	* Otherwise do something with the found key
	* Increment the current slot and continue

To reduce code duplication, we can replace this code pattern with an iterator
macro, similar to the existing for_each_X macros found elsewhere in the kernel.
This also makes the code easier to understand for newcomers by putting a name
to the encapsulated functionality.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Gabriel Niebler <gniebler@suse.com>
---
 fs/btrfs/ctree.c | 33 +++++++++++++++++++++++++++++++++
 fs/btrfs/ctree.h | 24 ++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index a7db3f6f1b7b..d735bd472616 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2277,6 +2277,39 @@ int btrfs_search_backwards(struct btrfs_root *root, struct btrfs_key *key,
 	return ret;
 }
 
+/* Search for a valid slot for the given path.
+ *
+ * @root: The root node of the tree.
+ * @key: Will contain a valid item if found.
+ * @path: The starting point to validate the slot.
+ *
+ * Return 0 if the item is valid, 1 if not found and < 0 if error.
+ */
+int btrfs_get_next_valid_item(struct btrfs_root *root, struct btrfs_key *key,
+			      struct btrfs_path *path)
+{
+	while (1) {
+		int ret;
+		const int slot = path->slots[0];
+		const struct extent_buffer *leaf = path->nodes[0];
+		/* this is where we start walking through the path */
+		if (slot >= btrfs_header_nritems(leaf)) {
+			/*
+			 * if we've reached the last slot in this leaf we need
+			 * to go to the next leaf and reset the path
+			 */
+			ret = btrfs_next_leaf(root, path);
+			if (ret)
+				return ret;
+			continue;
+		}
+		/* store the found, valid item in key */
+		btrfs_item_key_to_cpu(leaf, key, slot);
+		break;
+	}
+	return 0;
+}
+
 /*
  * adjust the pointers going up the tree, starting at level
  * making sure the right key of each node is points to 'key'.
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 947f04789389..98091334b749 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2976,6 +2976,30 @@ int btrfs_next_old_leaf(struct btrfs_root *root, struct btrfs_path *path,
 int btrfs_search_backwards(struct btrfs_root *root, struct btrfs_key *key,
 			   struct btrfs_path *path);
 
+int btrfs_get_next_valid_item(struct btrfs_root *root, struct btrfs_key *key,
+			      struct btrfs_path *path);
+
+/* Search in @root for a given @key, and store the slot found in @found_key.
+ *
+ * @root: The root node of the tree.
+ * @key: The key we are looking for.
+ * @found_key: Will hold the found item.
+ * @path: Holds the current slot/leaf.
+ * @iter_ret: Contains the value returned from btrfs_search_slot or
+ *            btrfs_get_next_valid_item, whichever was executed last.
+ *
+ * The iter_ret is an output variable that will contain the return value of
+ * btrfs_search_slot, if it encountered an error, or the value returned from
+ * btrfs_get_next_valid_item, otherwise. That return value can be 0, if a valid
+ * slot was found, 1 if there were no more leaves, and <0 if there was an error.
+ */
+#define btrfs_for_each_slot(root, key, found_key, path, iter_ret)		\
+	for (iter_ret = btrfs_search_slot(NULL, root, key, path, 0, 0);		\
+		iter_ret >= 0 &&						\
+		(iter_ret = btrfs_get_next_valid_item(root, found_key, path)) == 0; \
+		path->slots[0]++						  \
+	)
+
 static inline int btrfs_next_old_item(struct btrfs_root *root,
 				      struct btrfs_path *p, u64 time_seq)
 {
-- 
2.35.1