revision: add --maximal option

When inspecting a range of commits from some set of starting references, it
is sometimes useful to learn which commits are not reachable from any other
commits in the selected range.

One such application is in the creation of a sequence of bundles for the
bundle URI feature. Creating a stack of bundles representing different
slices of time includes defining which references to include. If all
references are used, then this may be overwhelming or redundant. Instead,
selecting commits that are maximal to the range could help defining a
smaller reference set to use in the bundle header.

Add a new '--maximal' option to restrict the output of a revision range to
be only the commits that are not reachable from any other commit in the
range, based on the reachability definition of the walk.

This is accomplished by adding a new 28th bit flag, CHILD_VISITED, that is
set as we walk. This does extend the bit range in object.h, but using an
earlier bit may collide with another feature.

The tests demonstrate the behavior of the feature with a positive-only
range, ranges with negative references, and walk-modifying flags like
--first-parent and --exclude-first-parent-only.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee

Documentation/rev-list-options.adoc

@@ -444,6 +444,10 @@ The following options affect the way the simplification is performed:
 	times; if so, a commit is included if it is any of the commits
 	given or if it is an ancestor or descendant of one of them.
 
+`--maximal`::
+	Restrict the output commits to be those that are not reachable
+	from any other commits in the revision range.
+
 A more detailed explanation follows.
 
 Suppose you specified `foo` as the _<paths>_.  We shall call commits

object.h

@@ -64,7 +64,7 @@ void object_array_init(struct object_array *array);
 
 /*
  * object flag allocation:
- * revision.h:               0---------10         15               23------27
+ * revision.h:               0---------10         15               23--------28
  * fetch-pack.c:             01    67
  * negotiator/default.c:       2--5
  * walker.c:                 0-2
@@ -86,7 +86,7 @@ void object_array_init(struct object_array *array);
  * builtin/unpack-objects.c:                                 2021
  * pack-bitmap.h:                                              2122
  */
-#define FLAG_BITS  28
+#define FLAG_BITS  29
 
 #define TYPE_BITS 3
 

revision.c

@@ -1150,7 +1150,8 @@ static int process_parents(struct rev_info *revs, struct commit *commit,
 			struct commit *p = parent->item;
 			parent = parent->next;
 			if (p)
-				p->object.flags |= UNINTERESTING;
+				p->object.flags |= UNINTERESTING |
+						   CHILD_VISITED;
 			if (repo_parse_commit_gently(revs->repo, p, 1) < 0)
 				continue;
 			if (p->parents)
@@ -1204,7 +1205,7 @@ static int process_parents(struct rev_info *revs, struct commit *commit,
 			if (!*slot)
 				*slot = *revision_sources_at(revs->sources, commit);
 		}
-		p->object.flags |= pass_flags;
+		p->object.flags |= pass_flags | CHILD_VISITED;
 		if (!(p->object.flags & SEEN)) {
 			p->object.flags |= (SEEN | NOT_USER_GIVEN);
 			if (list)
@@ -2381,6 +2382,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->first_parent_only = 1;
 	} else if (!strcmp(arg, "--exclude-first-parent-only")) {
 		revs->exclude_first_parent_only = 1;
+	} else if (!strcmp(arg, "--maximal")) {
+		revs->maximal = 1;
 	} else if (!strcmp(arg, "--ancestry-path")) {
 		revs->ancestry_path = 1;
 		revs->simplify_history = 0;
@@ -4125,6 +4128,8 @@ enum commit_action get_commit_action(struct rev_info *revs, struct commit *commi
 {
 	if (commit->object.flags & SHOWN)
 		return commit_ignore;
+	if (revs->maximal && (commit->object.flags & CHILD_VISITED))
+		return commit_ignore;
 	if (revs->unpacked && has_object_pack(revs->repo, &commit->object.oid))
 		return commit_ignore;
 	if (revs->no_kept_objects) {

revision.h

@@ -52,7 +52,9 @@
 #define NOT_USER_GIVEN	(1u<<25)
 #define TRACK_LINEAR	(1u<<26)
 #define ANCESTRY_PATH	(1u<<27)
-#define ALL_REV_FLAGS	(((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR | PULL_MERGE)
+#define CHILD_VISITED	(1u<<28)
+#define ALL_REV_FLAGS	(((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR \
+				      | PULL_MERGE | CHILD_VISITED)
 
 #define DECORATE_SHORT_REFS	1
 #define DECORATE_FULL_REFS	2
@@ -198,6 +200,7 @@ struct rev_info {
 			cherry_mark:1,
 			bisect:1,
 			ancestry_path:1,
+			maximal:1,
 
 			/* True if --ancestry-path was specified without an
 			 * argument. The bottom revisions are implicitly

t/t6600-test-reach.sh

@@ -762,4 +762,79 @@ test_expect_success 'for-each-ref is-base: --sort' '
 		--sort=refname --sort=-is-base:commit-2-3
 '
 
+test_expect_success 'rev-list --maximal (all positive)' '
+	# Only one maximal.
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-4-2
+	refs/heads/commit-4-4
+	refs/heads/commit-8-4
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-8-4)
+	EOF
+	run_all_modes git rev-list --maximal --stdin &&
+
+	# All maximal.
+	cat >input <<-\EOF &&
+	refs/heads/commit-5-2
+	refs/heads/commit-4-3
+	refs/heads/commit-3-4
+	refs/heads/commit-2-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-5-2)
+	$(git rev-parse refs/heads/commit-4-3)
+	$(git rev-parse refs/heads/commit-3-4)
+	$(git rev-parse refs/heads/commit-2-5)
+	EOF
+	run_all_modes git rev-list --maximal --stdin &&
+
+	# Mix of both.
+	cat >input <<-\EOF &&
+	refs/heads/commit-5-2
+	refs/heads/commit-3-2
+	refs/heads/commit-2-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-5-2)
+	$(git rev-parse refs/heads/commit-2-5)
+	EOF
+	run_all_modes git rev-list --maximal --stdin
+'
+
+test_expect_success 'rev-list --maximal (range)' '
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-5
+	refs/heads/commit-6-4
+	^refs/heads/commit-4-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-6-4)
+	EOF
+	run_all_modes git rev-list --maximal --stdin &&
+
+	# first-parent changes reachability: the first parent
+	# reduces the second coordinate to 1 before reducing the
+	# first coordinate.
+	cat >input <<-\EOF &&
+	refs/heads/commit-1-1
+	refs/heads/commit-2-5
+	refs/heads/commit-6-4
+	^refs/heads/commit-4-5
+	EOF
+
+	cat >expect <<-EOF &&
+	$(git rev-parse refs/heads/commit-6-4)
+	$(git rev-parse refs/heads/commit-2-5)
+	EOF
+	run_all_modes git rev-list --maximal --stdin \
+		--first-parent --exclude-first-parent-only
+'
+
 test_done