[PATCH 01/17] path-walk: introduce an object walk by path

From

Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>

Date

Oct 8, 2024, 14:11 UTC

Message-ID

<98bdc94a773d797152bebfbfda88fa1bb0707821.1728396724.git.gitgitgadget@gmail.com>

In-Reply-To

<pull.1813.git.1728396723.gitgitgadget@gmail.com>

From: Derrick Stolee <stolee@gmail.com>

Signed-off-by: Derrick Stolee <stolee@gmail.com>
---
 Documentation/technical/api-path-walk.txt |  54 +++++
 Makefile                                  |   1 +
 path-walk.c                               | 241 ++++++++++++++++++++++
 path-walk.h                               |  43 ++++
 4 files changed, 339 insertions(+)
 create mode 100644 Documentation/technical/api-path-walk.txt
 create mode 100644 path-walk.c
 create mode 100644 path-walk.h

diff --git a/Documentation/technical/api-path-walk.txt b/Documentation/technical/api-path-walk.txt
new file mode 100644
index 00000000000..6472222ae6d
--- /dev/null
+++ b/Documentation/technical/api-path-walk.txt
@@ -0,0 +1,54 @@
+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.
+
+When walking a range of commits with some `UNINTERESTING` objects, the
+objects with the `UNINTERESTING` flag are included in these batches. In
+order to walk `UNINTERESTING` objects, the `--boundary` option must be
+used in the commit walk in order to visit `UNINTERESTING` commits.
+
+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.
++
+If you want the path-walk API to emit `UNINTERESTING` objects based on the
+commit walk's boundary, be sure to set `revs.boundary` so the boundary
+commits are emitted.
+
+Examples
+--------
+
+See example usages in future changes.
diff --git a/Makefile b/Makefile
index 7344a7f7257..d0d8d6888e3 100644
--- a/Makefile
+++ b/Makefile
@@ -1094,6 +1094,7 @@ 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
diff --git a/path-walk.c b/path-walk.c
new file mode 100644
index 00000000000..66840187e28
--- /dev/null
+++ b/path-walk.c
@@ -0,0 +1,241 @@
+/*
+ * path-walk.c: implementation for path-based walks of the object graph.
+ */
+#include "git-compat-util.h"
+#include "path-walk.h"
+#include "blob.h"
+#include "commit.h"
+#include "dir.h"
+#include "hashmap.h"
+#include "hex.h"
+#include "object.h"
+#include "oid-array.h"
+#include "revision.h"
+#include "string-list.h"
+#include "strmap.h"
+#include "trace2.h"
+#include "tree.h"
+#include "tree-walk.h"
+
+struct type_and_oid_list
+{
+	enum object_type type;
+	struct oid_array oids;
+};
+
+#define TYPE_AND_OID_LIST_INIT { \
+	.type = OBJ_NONE, 	 \
+	.oids = OID_ARRAY_INIT	 \
+}
+
+struct path_walk_context {
+	/**
+	 * Repeats of data in 'struct path_walk_info' for
+	 * access with fewer characters.
+	 */
+	struct repository *repo;
+	struct rev_info *revs;
+	struct path_walk_info *info;
+
+	/**
+	 * Map a path to a 'struct type_and_oid_list'
+	 * containing the objects discovered at that
+	 * path.
+	 */
+	struct strmap paths_to_lists;
+
+	/**
+	 * Store the current list of paths in a stack, to
+	 * facilitate depth-first-search without recursion.
+	 */
+	struct string_list path_stack;
+};
+
+static int add_children(struct path_walk_context *ctx,
+			const char *base_path,
+			struct object_id *oid)
+{
+	struct tree_desc desc;
+	struct name_entry entry;
+	struct strbuf path = STRBUF_INIT;
+	size_t base_len;
+	struct tree *tree = lookup_tree(ctx->repo, oid);
+
+	if (!tree) {
+		error(_("failed to walk children of tree %s: not found"),
+		      oid_to_hex(oid));
+		return -1;
+	} else if (parse_tree_gently(tree, 1)) {
+		die("bad tree object %s", oid_to_hex(oid));
+	}
+
+	strbuf_addstr(&path, base_path);
+	base_len = path.len;
+
+	parse_tree(tree);
+	init_tree_desc(&desc, &tree->object.oid, tree->buffer, tree->size);
+	while (tree_entry(&desc, &entry)) {
+		struct type_and_oid_list *list;
+		struct object *o;
+		/* Not actually true, but we will ignore submodules later. */
+		enum object_type type = S_ISDIR(entry.mode) ? OBJ_TREE : OBJ_BLOB;
+
+		/* Skip submodules. */
+		if (S_ISGITLINK(entry.mode))
+			continue;
+
+		if (type == OBJ_TREE) {
+			struct tree *child = lookup_tree(ctx->repo, &entry.oid);
+			o = child ? &child->object : NULL;
+		} else if (type == OBJ_BLOB) {
+			struct blob *child = lookup_blob(ctx->repo, &entry.oid);
+			o = child ? &child->object : NULL;
+		} else {
+			/* Wrong type? */
+			continue;
+		}
+
+		if (!o) /* report error?*/
+			continue;
+
+		/* Skip this object if already seen. */
+		if (o->flags & SEEN)
+			continue;
+		o->flags |= SEEN;
+
+		strbuf_setlen(&path, base_len);
+		strbuf_add(&path, entry.path, entry.pathlen);
+
+		/*
+		 * Trees will end with "/" for concatenation and distinction
+		 * from blobs at the same path.
+		 */
+		if (type == OBJ_TREE)
+			strbuf_addch(&path, '/');
+
+		if (!(list = strmap_get(&ctx->paths_to_lists, path.buf))) {
+			CALLOC_ARRAY(list, 1);
+			list->type = type;
+			strmap_put(&ctx->paths_to_lists, path.buf, list);
+			string_list_append(&ctx->path_stack, path.buf);
+		}
+		oid_array_append(&list->oids, &entry.oid);
+	}
+
+	free_tree_buffer(tree);
+	strbuf_release(&path);
+	return 0;
+}
+
+/*
+ * For each path in paths_to_explore, walk the trees another level
+ * and add any found blobs to the batch (but only if they exist and
+ * haven't been added yet).
+ */
+static int walk_path(struct path_walk_context *ctx,
+		     const char *path)
+{
+	struct type_and_oid_list *list;
+	int ret = 0;
+
+	list = strmap_get(&ctx->paths_to_lists, path);
+
+	/* Evaluate function pointer on this data. */
+	ret = ctx->info->path_fn(path, &list->oids, list->type,
+				 ctx->info->path_fn_data);
+
+	/* Expand data for children. */
+	if (list->type == OBJ_TREE) {
+		for (size_t i = 0; i < list->oids.nr; i++) {
+			ret |= add_children(ctx,
+					    path,
+					    &list->oids.oid[i]);
+		}
+	}
+
+	oid_array_clear(&list->oids);
+	strmap_remove(&ctx->paths_to_lists, path, 1);
+	return ret;
+}
+
+static void clear_strmap(struct strmap *map)
+{
+	struct hashmap_iter iter;
+	struct strmap_entry *e;
+
+	hashmap_for_each_entry(&map->map, &iter, e, ent) {
+		struct type_and_oid_list *list = e->value;
+		oid_array_clear(&list->oids);
+	}
+	strmap_clear(map, 1);
+	strmap_init(map);
+}
+
+/**
+ * Given the configuration of 'info', walk the commits based on 'info->revs' and
+ * call 'info->path_fn' on each discovered path.
+ *
+ * Returns nonzero on an error.
+ */
+int walk_objects_by_path(struct path_walk_info *info)
+{
+	const char *root_path = "";
+	int ret = 0;
+	size_t commits_nr = 0, paths_nr = 0;
+	struct commit *c;
+	struct type_and_oid_list *root_tree_list;
+	struct path_walk_context ctx = {
+		.repo = info->revs->repo,
+		.revs = info->revs,
+		.info = info,
+		.path_stack = STRING_LIST_INIT_DUP,
+		.paths_to_lists = STRMAP_INIT
+	};
+
+	trace2_region_enter("path-walk", "commit-walk", info->revs->repo);
+
+	/* Insert a single list for the root tree into the paths. */
+	CALLOC_ARRAY(root_tree_list, 1);
+	root_tree_list->type = OBJ_TREE;
+	strmap_put(&ctx.paths_to_lists, root_path, root_tree_list);
+
+	if (prepare_revision_walk(info->revs))
+		die(_("failed to setup revision walk"));
+
+	while ((c = get_revision(info->revs))) {
+		struct object_id *oid = get_commit_tree_oid(c);
+		struct tree *t = lookup_tree(info->revs->repo, oid);
+		commits_nr++;
+
+		if (t) {
+			if (t->object.flags & SEEN)
+				continue;
+			t->object.flags |= SEEN;
+			oid_array_append(&root_tree_list->oids, oid);
+		} else {
+			warning("could not find tree %s", oid_to_hex(oid));
+		}
+	}
+
+	trace2_data_intmax("path-walk", ctx.repo, "commits", commits_nr);
+	trace2_region_leave("path-walk", "commit-walk", info->revs->repo);
+
+	string_list_append(&ctx.path_stack, root_path);
+
+	trace2_region_enter("path-walk", "path-walk", info->revs->repo);
+	while (!ret && ctx.path_stack.nr) {
+		char *path = ctx.path_stack.items[ctx.path_stack.nr - 1].string;
+		ctx.path_stack.nr--;
+		paths_nr++;
+
+		ret = walk_path(&ctx, path);
+
+		free(path);
+	}
+	trace2_data_intmax("path-walk", ctx.repo, "paths", paths_nr);
+	trace2_region_leave("path-walk", "path-walk", info->revs->repo);
+
+	clear_strmap(&ctx.paths_to_lists);
+	string_list_clear(&ctx.path_stack, 0);
+	return ret;
+}
diff --git a/path-walk.h b/path-walk.h
new file mode 100644
index 00000000000..c9e94a98bc8
--- /dev/null
+++ b/path-walk.h
@@ -0,0 +1,43 @@
+/*
+ * path-walk.h : Methods and structures for walking the object graph in batches
+ * by the paths that can reach those objects.
+ */
+#include "object.h" /* Required for 'enum object_type'. */
+
+struct rev_info;
+struct oid_array;
+
+/**
+ * The type of a function pointer for the method that is called on a list of
+ * objects reachable at a given path.
+ */
+typedef int (*path_fn)(const char *path,
+		       struct oid_array *oids,
+		       enum object_type type,
+		       void *data);
+
+struct path_walk_info {
+	/**
+	 * revs provides the definitions for the commit walk, including
+	 * which commits are UNINTERESTING or not.
+	 */
+	struct rev_info *revs;
+
+	/**
+	 * The caller wishes to execute custom logic on objects reachable at a
+	 * given path. Every reachable object will be visited exactly once, and
+	 * the first path to see an object wins. This may not be a stable choice.
+	 */
+	path_fn path_fn;
+	void *path_fn_data;
+};
+
+#define PATH_WALK_INFO_INIT { 0 }
+
+/**
+ * Given the configuration of 'info', walk the commits based on 'info->revs' and
+ * call 'info->path_fn' on each discovered path.
+ *
+ * Returns nonzero on an error.
+ */
+int walk_objects_by_path(struct path_walk_info *info);

-- 
gitgitgadget