pseudo-merge: implement support for finding existing merges

This patch implements support for reusing existing pseudo-merge commits
when writing bitmaps when there is an existing pseudo-merge bitmap which
has exactly the same set of parents as one that we are about to write.

Note that unstable pseudo-merges are likely to change between
consecutive repacks, and so are generally poor candidates for reuse.
However, stable pseudo-merges (see the configuration option
'bitmapPseudoMerge.<name>.stableThreshold') are by definition unlikely
to change between runs (as they represent long-running branches).

Because there is no index from a *set* of pseudo-merge parents to a
matching pseudo-merge bitmap, we have to construct the bitmap
corresponding to the set of parents for each pending pseudo-merge commit
and see if a matching bitmap exists.

This is technically quadratic in the number of pseudo-merges, but is OK
in practice for a couple of reasons:

  - non-matching pseudo-merge bitmaps are rejected quickly as soon as
    they differ in a single bit

  - already-matched pseudo-merge bitmaps are discarded from subsequent
    rounds of search

  - the number of pseudo-merges is generally small, even for large
    repositories

In order to do this, implement (a) a function that finds a matching
pseudo-merge given some uncompressed bitset describing its parents, (b)
a function that computes the bitset of parents for a given pseudo-merge
commit, and (c) call that function before computing the set of reachable
objects for some pending pseudo-merge.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap-write.c

@@ -19,6 +19,10 @@
 #include "tree-walk.h"
 #include "pseudo-merge.h"
 #include "oid-array.h"
+#include "config.h"
+#include "alloc.h"
+#include "refs.h"
+#include "strmap.h"
 
 struct bitmapped_commit {
 	struct commit *commit;
@@ -465,6 +469,7 @@ static int fill_bitmap_tree(struct bitmap_writer *writer,
 }
 
 static int reused_bitmaps_nr;
+static int reused_pseudo_merge_bitmaps_nr;
 
 static int fill_bitmap_commit(struct bitmap_writer *writer,
 			      struct bb_commit *ent,
@@ -490,7 +495,7 @@ static int fill_bitmap_commit(struct bitmap_writer *writer,
 			struct bitmap *remapped = bitmap_new();
 
 			if (commit->object.flags & BITMAP_PSEUDO_MERGE)
-				old = NULL;
+				old = pseudo_merge_bitmap_for_commit(old_bitmap, c);
 			else
 				old = bitmap_for_commit(old_bitmap, c);
 			/*
@@ -501,6 +506,9 @@ static int fill_bitmap_commit(struct bitmap_writer *writer,
 			if (old && !rebuild_bitmap(mapping, old, remapped)) {
 				bitmap_or(ent->bitmap, remapped);
 				bitmap_free(remapped);
+				if (commit->object.flags & BITMAP_PSEUDO_MERGE)
+					reused_pseudo_merge_bitmaps_nr++;
+				else
 					reused_bitmaps_nr++;
 				continue;
 			}
@@ -631,6 +639,9 @@ int bitmap_writer_build(struct bitmap_writer *writer,
 			    the_repository);
 	trace2_data_intmax("pack-bitmap-write", the_repository,
 			   "building_bitmaps_reused", reused_bitmaps_nr);
+	trace2_data_intmax("pack-bitmap-write", the_repository,
+			   "building_bitmaps_pseudo_merge_reused",
+			   reused_pseudo_merge_bitmaps_nr);
 
 	stop_progress(&writer->progress);
 

pack-bitmap.c

@@ -1316,6 +1316,37 @@ static struct bitmap *find_boundary_objects(struct bitmap_index *bitmap_git,
 	return cb.base;
 }
 
+struct ewah_bitmap *pseudo_merge_bitmap_for_commit(struct bitmap_index *bitmap_git,
+						   struct commit *commit)
+{
+	struct commit_list *p;
+	struct bitmap *parents;
+	struct pseudo_merge *match = NULL;
+
+	if (!bitmap_git->pseudo_merges.nr)
+		return NULL;
+
+	parents = bitmap_new();
+
+	for (p = commit->parents; p; p = p->next) {
+		int pos = bitmap_position(bitmap_git, &p->item->object.oid);
+		if (pos < 0 || pos >= bitmap_num_objects(bitmap_git))
+			goto done;
+
+		bitmap_set(parents, pos);
+	}
+
+	match = pseudo_merge_for_parents(&bitmap_git->pseudo_merges,
+						parents);
+
+done:
+	bitmap_free(parents);
+	if (match)
+		return pseudo_merge_bitmap(&bitmap_git->pseudo_merges, match);
+
+	return NULL;
+}
+
 static void unsatisfy_all_pseudo_merges(struct bitmap_index *bitmap_git)
 {
 	uint32_t i;
@@ -2809,6 +2840,7 @@ void free_bitmap_index(struct bitmap_index *b)
 		 */
 		close_midx_revindex(b->midx);
 	}
+	free_pseudo_merge_map(&b->pseudo_merges);
 	free(b);
 }
 

pack-bitmap.h

@@ -142,6 +142,8 @@ int rebuild_bitmap(const uint32_t *reposition,
 		   struct bitmap *dest);
 struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
 				      struct commit *commit);
+struct ewah_bitmap *pseudo_merge_bitmap_for_commit(struct bitmap_index *bitmap_git,
+						   struct commit *commit);
 void bitmap_writer_select_commits(struct bitmap_writer *writer,
 				  struct commit **indexed_commits,
 				  unsigned int indexed_commits_nr);

pseudo-merge.c

@@ -699,3 +699,58 @@ int cascade_pseudo_merges(const struct pseudo_merge_map *pm,
 
 	return ret;
 }
+
+struct pseudo_merge *pseudo_merge_for_parents(const struct pseudo_merge_map *pm,
+					      struct bitmap *parents)
+{
+	struct pseudo_merge *match = NULL;
+	size_t i;
+
+	if (!pm->nr)
+		return NULL;
+
+	/*
+	 * NOTE: this loop is quadratic in the worst-case (where no
+	 * matching pseudo-merge bitmaps are found), but in practice
+	 * this is OK for a few reasons:
+	 *
+	 *   - Rejecting pseudo-merge bitmaps that do not match the
+	 *     given commit is done quickly (i.e. `bitmap_equals_ewah()`
+	 *     returns early when we know the two bitmaps aren't equal.
+	 *
+	 *   - Already matched pseudo-merge bitmaps (which we track with
+	 *     the `->satisfied` bit here) are skipped as potential
+	 *     candidates.
+	 *
+	 *   - The number of pseudo-merges should be small (in the
+	 *     hundreds for most repositories).
+	 *
+	 * If in the future this semi-quadratic behavior does become a
+	 * problem, another approach would be to keep track of which
+	 * pseudo-merges are still "viable" after enumerating the
+	 * pseudo-merge commit's parents:
+	 *
+	 *   - A pseudo-merge bitmap becomes non-viable when the bit(s)
+	 *     corresponding to one or more parent(s) of the given
+	 *     commit are not set in a candidate pseudo-merge's commits
+	 *     bitmap.
+	 *
+	 *   - After processing all bits, enumerate the remaining set of
+	 *     viable pseudo-merge bitmaps, and check that their
+	 *     popcount() matches the number of parents in the given
+	 *     commit.
+	 */
+	for (i = 0; i < pm->nr; i++) {
+		struct pseudo_merge *candidate = use_pseudo_merge(pm, &pm->v[i]);
+		if (!candidate || candidate->satisfied)
+			continue;
+		if (!bitmap_equals_ewah(parents, candidate->commits))
+			continue;
+
+		match = candidate;
+		match->satisfied = 1;
+		break;
+	}
+
+	return match;
+}

pseudo-merge.h

@@ -206,4 +206,11 @@ int cascade_pseudo_merges(const struct pseudo_merge_map *pm,
 			  struct bitmap *result,
 			  struct bitmap *roots);
 
+/*
+ * Returns a pseudo-merge which contains the exact set of commits
+ * listed in the "parents" bitamp, or NULL if none could be found.
+ */
+struct pseudo_merge *pseudo_merge_for_parents(const struct pseudo_merge_map *pm,
+					      struct bitmap *parents);
+
 #endif

t/t5333-pseudo-merge-bitmaps.sh

@@ -22,6 +22,10 @@ test_pseudo_merges_cascades () {
 	test_trace2_data bitmap pseudo_merges_cascades "$1"
 }
 
+test_pseudo_merges_reused () {
+	test_trace2_data pack-bitmap-write building_bitmaps_pseudo_merge_reused "$1"
+}
+
 tag_everything () {
 	git rev-list --all --no-object-names >in &&
 	perl -lne '
@@ -325,4 +329,65 @@ test_expect_success 'pseudo-merge overlap stale traversal' '
 	)
 '
 
+test_expect_success 'pseudo-merge reuse' '
+	git init pseudo-merge-reuse &&
+	(
+		cd pseudo-merge-reuse &&
+
+		stable="1641013200" && # 2022-01-01
+		unstable="1672549200" && # 2023-01-01
+
+		GIT_COMMITTER_DATE="$stable +0000" &&
+		export GIT_COMMITTER_DATE &&
+		test_commit_bulk --notick 128 &&
+		GIT_COMMITTER_DATE="$unstable +0000" &&
+		export GIT_COMMITTER_DATE &&
+		test_commit_bulk --notick 128 &&
+
+		tag_everything &&
+
+		git \
+			-c bitmapPseudoMerge.test.pattern="refs/tags/" \
+			-c bitmapPseudoMerge.test.maxMerges=1 \
+			-c bitmapPseudoMerge.test.threshold=now \
+			-c bitmapPseudoMerge.test.stableThreshold=$(($unstable - 1)) \
+			-c bitmapPseudoMerge.test.stableSize=512 \
+			repack -adb &&
+
+		test_pseudo_merges >merges &&
+		test_line_count = 2 merges &&
+
+		test_pseudo_merge_commits 0 >stable-oids.before &&
+		test_pseudo_merge_commits 1 >unstable-oids.before &&
+
+		: >trace2.txt &&
+		GIT_TRACE2_EVENT=$PWD/trace2.txt git \
+			-c bitmapPseudoMerge.test.pattern="refs/tags/" \
+			-c bitmapPseudoMerge.test.maxMerges=2 \
+			-c bitmapPseudoMerge.test.threshold=now \
+			-c bitmapPseudoMerge.test.stableThreshold=$(($unstable - 1)) \
+			-c bitmapPseudoMerge.test.stableSize=512 \
+			repack -adb &&
+
+		test_pseudo_merges_reused 1 <trace2.txt &&
+
+		test_pseudo_merges >merges &&
+		test_line_count = 3 merges &&
+
+		test_pseudo_merge_commits 0 >stable-oids.after &&
+		for i in 1 2
+		do
+			test_pseudo_merge_commits $i || return 1
+		done >unstable-oids.after &&
+
+		sort -u <stable-oids.before >expect &&
+		sort -u <stable-oids.after >actual &&
+		test_cmp expect actual &&
+
+		sort -u <unstable-oids.before >expect &&
+		sort -u <unstable-oids.after >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done