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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.
`pl`::
This pattern list pointer allows focusing the path-walk search to
a set of patterns, only emitting paths that match the given
patterns. See linkgit:gitignore[5] or
linkgit:git-sparse-checkout[1] for details about pattern lists.
When the pattern list uses cone-mode patterns, then the path-walk
API can prune the set of paths it walks to improve performance.
Examples
--------
See example usages in:
`t/helper/test-path-walk.c`,
`builtin/backfill.c`
`builtin/pack-objects.c`