Merge branch 'ds/path-walk-1' into jch

Introduce a new API to visit objects in batches based on a common
path, or by type.

Comments?

* ds/path-walk-1:
  path-walk: mark trees and blobs as UNINTERESTING
  path-walk: visit tags and cached objects
  path-walk: allow consumer to specify object types
  t6601: add helper for testing path-walk API
  test-lib-functions: add test_cmp_sorted
  path-walk: introduce an object walk by path

Documentation/technical/api-path-walk.txt

@@ -0,0 +1,63 @@
+Path-Walk API
+=============
+
+The path-walk API is used to walk reachable objects, but to visit objects
+in batches based on a common path they appear in, or by type.
+
+For example, all reachable commits are visited in a group. All tags are
+visited in a group. Then, all root trees are visited. At some point, all
+blobs reachable via a path `my/dir/to/A` are visited. When there are
+multiple paths possible to reach the same object, then only one of those
+paths is used to visit the object.
+
+Basics
+------
+
+To use the path-walk API, include `path-walk.h` and call
+`walk_objects_by_path()` with a customized `path_walk_info` struct. The
+struct is used to set all of the options for how the walk should proceed.
+Let's dig into the different options and their use.
+
+`path_fn` and `path_fn_data`::
+	The most important option is the `path_fn` option, which is a
+	function pointer to the callback that can execute logic on the
+	object IDs for objects grouped by type and path. This function
+	also receives a `data` value that corresponds to the
+	`path_fn_data` member, for providing custom data structures to
+	this callback function.
+
+`revs`::
+	To configure the exact details of the reachable set of objects,
+	use the `revs` member and initialize it using the revision
+	machinery in `revision.h`. Initialize `revs` using calls such as
+	`setup_revisions()` or `parse_revision_opt()`. Do not call
+	`prepare_revision_walk()`, as that will be called within
+	`walk_objects_by_path()`.
++
+It is also important that you do not specify the `--objects` flag for the
+`revs` struct. The revision walk should only be used to walk commits, and
+the objects will be walked in a separate way based on those starting
+commits.
+
+`commits`, `blobs`, `trees`, `tags`::
+	By default, these members are enabled and signal that the path-walk
+	API should call the `path_fn` on objects of these types. Specialized
+	applications could disable some options to make it simpler to walk
+	the objects or to have fewer calls to `path_fn`.
++
+While it is possible to walk only commits in this way, consumers would be
+better off using the revision walk API instead.
+
+`prune_all_uninteresting`::
+	By default, all reachable paths are emitted by the path-walk API.
+	This option allows consumers to declare that they are not
+	interested in paths where all included objects are marked with the
+	`UNINTERESTING` flag. This requires using the `boundary` option in
+	the revision walk so that the walk emits commits marked with the
+	`UNINTERESTING` flag.
+
+Examples
+--------
+
+See example usages in:
+	`t/helper/test-path-walk.c`

Makefile

@@ -822,6 +822,7 @@ TEST_BUILTINS_OBJS += test-parse-options.o
 TEST_BUILTINS_OBJS += test-parse-pathspec-file.o
 TEST_BUILTINS_OBJS += test-partial-clone.o
 TEST_BUILTINS_OBJS += test-path-utils.o
+TEST_BUILTINS_OBJS += test-path-walk.o
 TEST_BUILTINS_OBJS += test-pcre2-config.o
 TEST_BUILTINS_OBJS += test-pkt-line.o
 TEST_BUILTINS_OBJS += test-proc-receive.o
@@ -1099,6 +1100,7 @@ LIB_OBJS += parse-options.o
 LIB_OBJS += patch-delta.o
 LIB_OBJS += patch-ids.o
 LIB_OBJS += path.o
+LIB_OBJS += path-walk.o
 LIB_OBJS += pathspec.o
 LIB_OBJS += pkt-line.o
 LIB_OBJS += preload-index.o