Merge branch 'tb/pack-bitmap'

Various improvements to the codepath that writes out pack bitmaps.

* tb/pack-bitmap: (24 commits)
  pack-bitmap-write: better reuse bitmaps
  pack-bitmap-write: relax unique revwalk condition
  pack-bitmap-write: use existing bitmaps
  pack-bitmap: factor out 'add_commit_to_bitmap()'
  pack-bitmap: factor out 'bitmap_for_commit()'
  pack-bitmap-write: ignore BITMAP_FLAG_REUSE
  pack-bitmap-write: build fewer intermediate bitmaps
  pack-bitmap.c: check reads more aggressively when loading
  pack-bitmap-write: rename children to reverse_edges
  t5310: add branch-based checks
  commit: implement commit_list_contains()
  bitmap: implement bitmap_is_subset()
  pack-bitmap-write: fill bitmap with commit history
  pack-bitmap-write: pass ownership of intermediate bitmaps
  pack-bitmap-write: reimplement bitmap writing
  ewah: add bitmap_dup() function
  ewah: implement bitmap_or()
  ewah: make bitmap growth less aggressive
  ewah: factor out bitmap growth
  rev-list: die when --test-bitmap detects a mismatch
  ...

10 files changed, 578 insertions, 296 deletions

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 5617c01b5a..2a00358f34 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -1104,7 +1104,6 @@ static void write_pack_file(void)
 				stop_progress(&progress_state);
 
 				bitmap_writer_show_progress(progress);
-				bitmap_writer_reuse_bitmaps(&to_pack);
 				bitmap_writer_select_commits(indexed_commits, indexed_commits_nr, -1);
 				bitmap_writer_build(&to_pack);
 				bitmap_writer_finish(written_list, nr_written,
diff --git a/commit.c b/commit.c
index fe1fa3dc41..9a785bf906 100644
--- a/commit.c
+++ b/commit.c
@@ -544,6 +544,17 @@ struct commit_list *commit_list_insert(struct commit *item, struct commit_list *
 	return new_list;
 }
 
+int commit_list_contains(struct commit *item, struct commit_list *list)
+{
+	while (list) {
+		if (list->item == item)
+			return 1;
+		list = list->next;
+	}
+
+	return 0;
+}
+
 unsigned commit_list_count(const struct commit_list *l)
 {
 	unsigned c = 0;
diff --git a/commit.h b/commit.h
index 5467786c7b..742a6de460 100644
--- a/commit.h
+++ b/commit.h
@@ -167,6 +167,8 @@ int find_commit_subject(const char *commit_buffer, const char **subject);
 
 struct commit_list *commit_list_insert(struct commit *item,
 					struct commit_list **list);
+int commit_list_contains(struct commit *item,
+			 struct commit_list *list);
 struct commit_list **commit_list_append(struct commit *commit,
 					struct commit_list **next);
 unsigned commit_list_count(const struct commit_list *l);
diff --git a/ewah/bitmap.c b/ewah/bitmap.c
index d8cec585af..0d31cdc866 100644
--- a/ewah/bitmap.c
+++ b/ewah/bitmap.c
@@ -35,18 +35,26 @@ struct bitmap *bitmap_new(void)
 	return bitmap_word_alloc(32);
 }
 
+struct bitmap *bitmap_dup(const struct bitmap *src)
+{
+	struct bitmap *dst = bitmap_word_alloc(src->word_alloc);
+	COPY_ARRAY(dst->words, src->words, src->word_alloc);
+	return dst;
+}
+
+static void bitmap_grow(struct bitmap *self, size_t word_alloc)
+{
+	size_t old_size = self->word_alloc;
+	ALLOC_GROW(self->words, word_alloc, self->word_alloc);
+	memset(self->words + old_size, 0x0,
+	       (self->word_alloc - old_size) * sizeof(eword_t));
+}
+
 void bitmap_set(struct bitmap *self, size_t pos)
 {
 	size_t block = EWAH_BLOCK(pos);
 
-	if (block >= self->word_alloc) {
-		size_t old_size = self->word_alloc;
-		self->word_alloc = block ? block * 2 : 1;
-		REALLOC_ARRAY(self->words, self->word_alloc);
-		memset(self->words + old_size, 0x0,
-			(self->word_alloc - old_size) * sizeof(eword_t));
-	}
-
+	bitmap_grow(self, block + 1);
 	self->words[block] |= EWAH_MASK(pos);
 }
 
@@ -121,6 +129,15 @@ void bitmap_and_not(struct bitmap *self, struct bitmap *other)
 		self->words[i] &= ~other->words[i];
 }
 
+void bitmap_or(struct bitmap *self, const struct bitmap *other)
+{
+	size_t i;
+
+	bitmap_grow(self, other->word_alloc);
+	for (i = 0; i < other->word_alloc; i++)
+		self->words[i] |= other->words[i];
+}
+
 void bitmap_or_ewah(struct bitmap *self, struct ewah_bitmap *other)
 {
 	size_t original_size = self->word_alloc;
@@ -178,6 +195,27 @@ int bitmap_equals(struct bitmap *self, struct bitmap *other)
 	return 1;
 }
 
+int bitmap_is_subset(struct bitmap *self, struct bitmap *other)
+{
+	size_t common_size, i;
+
+	if (self->word_alloc < other->word_alloc)
+		common_size = self->word_alloc;
+	else {
+		common_size = other->word_alloc;
+		for (i = common_size; i < self->word_alloc; i++) {
+			if (self->words[i])
+				return 1;
+		}
+	}
+
+	for (i = 0; i < common_size; i++) {
+		if (self->words[i] & ~other->words[i])
+			return 1;
+	}
+	return 0;
+}
+
 void bitmap_reset(struct bitmap *bitmap)
 {
 	memset(bitmap->words, 0x0, bitmap->word_alloc * sizeof(eword_t));
diff --git a/ewah/ewah_bitmap.c b/ewah/ewah_bitmap.c
index d59b1afe3d..2a8c7c5c33 100644
--- a/ewah/ewah_bitmap.c
+++ b/ewah/ewah_bitmap.c
@@ -19,6 +19,7 @@
 #include "git-compat-util.h"
 #include "ewok.h"
 #include "ewok_rlw.h"
+#include "cache.h"
 
 static inline size_t min_size(size_t a, size_t b)
 {
@@ -33,20 +34,13 @@ static inline size_t max_size(size_t a, size_t b)
 static inline void buffer_grow(struct ewah_bitmap *self, size_t new_size)
 {
 	size_t rlw_offset = (uint8_t *)self->rlw - (uint8_t *)self->buffer;
-
-	if (self->alloc_size >= new_size)
-		return;
-
-	self->alloc_size = new_size;
-	REALLOC_ARRAY(self->buffer, self->alloc_size);
+	ALLOC_GROW(self->buffer, new_size, self->alloc_size);
 	self->rlw = self->buffer + (rlw_offset / sizeof(eword_t));
 }
 
 static inline void buffer_push(struct ewah_bitmap *self, eword_t value)
 {
-	if (self->buffer_size + 1 >= self->alloc_size)
-		buffer_grow(self, self->buffer_size * 3 / 2);
-
+	buffer_grow(self, self->buffer_size + 1);
 	self->buffer[self->buffer_size++] = value;
 }
 
@@ -137,8 +131,7 @@ void ewah_add_dirty_words(
 
 		rlw_set_literal_words(self->rlw, literals + can_add);
 
-		if (self->buffer_size + can_add >= self->alloc_size)
-			buffer_grow(self, (self->buffer_size + can_add) * 3 / 2);
+		buffer_grow(self, self->buffer_size + can_add);
 
 		if (negate) {
 			size_t i;
diff --git a/ewah/ewok.h b/ewah/ewok.h
index 011852bef1..66920965da 100644
--- a/ewah/ewok.h
+++ b/ewah/ewok.h
@@ -173,13 +173,14 @@ struct bitmap {
 
 struct bitmap *bitmap_new(void);
 struct bitmap *bitmap_word_alloc(size_t word_alloc);
+struct bitmap *bitmap_dup(const struct bitmap *src);
 void bitmap_set(struct bitmap *self, size_t pos);
 void bitmap_unset(struct bitmap *self, size_t pos);
 int bitmap_get(struct bitmap *self, size_t pos);
 void bitmap_reset(struct bitmap *self);
 void bitmap_free(struct bitmap *self);
 int bitmap_equals(struct bitmap *self, struct bitmap *other);
-int bitmap_is_subset(struct bitmap *self, struct bitmap *super);
+int bitmap_is_subset(struct bitmap *self, struct bitmap *other);
 
 struct ewah_bitmap * bitmap_to_ewah(struct bitmap *bitmap);
 struct bitmap *ewah_to_bitmap(struct ewah_bitmap *ewah);
diff --git a/pack-bitmap-write.c b/pack-bitmap-write.c
index 5e998bdaa7..cc5ead9990 100644
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@@ -12,6 +12,7 @@
 #include "sha1-lookup.h"
 #include "pack-objects.h"
 #include "commit-reach.h"
+#include "prio-queue.h"
 
 struct bitmapped_commit {
 	struct commit *commit;
@@ -29,7 +30,6 @@ struct bitmap_writer {
 	struct ewah_bitmap *tags;
 
 	kh_oid_map_t *bitmaps;
-	kh_oid_map_t *reused;
 	struct packing_data *to_pack;
 
 	struct bitmapped_commit *selected;
@@ -110,10 +110,8 @@ void bitmap_writer_build_type_index(struct packing_data *to_pack,
 /**
  * Compute the actual bitmaps
  */
-static struct object **seen_objects;
-static unsigned int seen_objects_nr, seen_objects_alloc;
 
-static inline void push_bitmapped_commit(struct commit *commit, struct ewah_bitmap *reused)
+static inline void push_bitmapped_commit(struct commit *commit)
 {
 	if (writer.selected_nr >= writer.selected_alloc) {
 		writer.selected_alloc = (writer.selected_alloc + 32) * 2;
@@ -121,27 +119,12 @@ static inline void push_bitmapped_commit(struct commit *commit, struct ewah_bitm
 	}
 
 	writer.selected[writer.selected_nr].commit = commit;
-	writer.selected[writer.selected_nr].bitmap = reused;
+	writer.selected[writer.selected_nr].bitmap = NULL;
 	writer.selected[writer.selected_nr].flags = 0;
 
 	writer.selected_nr++;
 }
 
-static inline void mark_as_seen(struct object *object)
-{
-	ALLOC_GROW(seen_objects, seen_objects_nr + 1, seen_objects_alloc);
-	seen_objects[seen_objects_nr++] = object;
-}
-
-static inline void reset_all_seen(void)
-{
-	unsigned int i;
-	for (i = 0; i < seen_objects_nr; ++i) {
-		seen_objects[i]->flags &= ~(SEEN | ADDED | SHOWN);
-	}
-	seen_objects_nr = 0;
-}
-
 static uint32_t find_object_pos(const struct object_id *oid)
 {
 	struct object_entry *entry = packlist_find(writer.to_pack, oid);
@@ -154,60 +137,6 @@ static uint32_t find_object_pos(const struct object_id *oid)
 	return oe_in_pack_pos(writer.to_pack, entry);
 }
 
-static void show_object(struct object *object, const char *name, void *data)
-{
-	struct bitmap *base = data;
-	bitmap_set(base, find_object_pos(&object->oid));
-	mark_as_seen(object);
-}
-
-static void show_commit(struct commit *commit, void *data)
-{
-	mark_as_seen((struct object *)commit);
-}
-
-static int
-add_to_include_set(struct bitmap *base, struct commit *commit)
-{
-	khiter_t hash_pos;
-	uint32_t bitmap_pos = find_object_pos(&commit->object.oid);
-
-	if (bitmap_get(base, bitmap_pos))
-		return 0;
-
-	hash_pos = kh_get_oid_map(writer.bitmaps, commit->object.oid);
-	if (hash_pos < kh_end(writer.bitmaps)) {
-		struct bitmapped_commit *bc = kh_value(writer.bitmaps, hash_pos);
-		bitmap_or_ewah(base, bc->bitmap);
-		return 0;
-	}
-
-	bitmap_set(base, bitmap_pos);
-	return 1;
-}
-
-static int
-should_include(struct commit *commit, void *_data)
-{
-	struct bitmap *base = _data;
-
-	if (!add_to_include_set(base, commit)) {
-		struct commit_list *parent = commit->parents;
-
-		mark_as_seen((struct object *)commit);
-
-		while (parent) {
-			parent->item->object.flags |= SEEN;
-			mark_as_seen((struct object *)parent->item);
-			parent = parent->next;
-		}
-
-		return 0;
-	}
-
-	return 1;
-}
-
 static void compute_xor_offsets(void)
 {
 	static const int MAX_XOR_OFFSET_SEARCH = 10;
@@ -248,79 +177,326 @@ static void compute_xor_offsets(void)
 	}
 }
 
-void bitmap_writer_build(struct packing_data *to_pack)
-{
-	static const double REUSE_BITMAP_THRESHOLD = 0.2;
+struct bb_commit {
+	struct commit_list *reverse_edges;
+	struct bitmap *commit_mask;
+	struct bitmap *bitmap;
+	unsigned selected:1,
+		 maximal:1;
+	unsigned idx; /* within selected array */
+};
 
-	int i, reuse_after, need_reset;
-	struct bitmap *base = bitmap_new();
-	struct rev_info revs;
+define_commit_slab(bb_data, struct bb_commit);
 
-	writer.bitmaps = kh_init_oid_map();
-	writer.to_pack = to_pack;
+struct bitmap_builder {
+	struct bb_data data;
+	struct commit **commits;
+	size_t commits_nr, commits_alloc;
+};
 
-	if (writer.show_progress)
-		writer.progress = start_progress("Building bitmaps", writer.selected_nr);
+static void bitmap_builder_init(struct bitmap_builder *bb,
+				struct bitmap_writer *writer,
+				struct bitmap_index *old_bitmap)
+{
+	struct rev_info revs;
+	struct commit *commit;
+	struct commit_list *reusable = NULL;
+	struct commit_list *r;
+	unsigned int i, num_maximal = 0;
 
-	repo_init_revisions(to_pack->repo, &revs, NULL);
-	revs.tag_objects = 1;
-	revs.tree_objects = 1;
-	revs.blob_objects = 1;
-	revs.no_walk = 0;
+	memset(bb, 0, sizeof(*bb));
+	init_bb_data(&bb->data);
 
-	revs.include_check = should_include;
 	reset_revision_walk();
+	repo_init_revisions(writer->to_pack->repo, &revs, NULL);
+	revs.topo_order = 1;
+	revs.first_parent_only = 1;
+
+	for (i = 0; i < writer->selected_nr; i++) {
+		struct commit *c = writer->selected[i].commit;
+		struct bb_commit *ent = bb_data_at(&bb->data, c);
+
+		ent->selected = 1;
+		ent->maximal = 1;
+		ent->idx = i;
+
+		ent->commit_mask = bitmap_new();
+		bitmap_set(ent->commit_mask, i);
+
+		add_pending_object(&revs, &c->object, "");
+	}
+
+	if (prepare_revision_walk(&revs))
+		die("revision walk setup failed");
+
+	while ((commit = get_revision(&revs))) {
+		struct commit_list *p = commit->parents;
+		struct bb_commit *c_ent;
+
+		parse_commit_or_die(commit);
+
+		c_ent = bb_data_at(&bb->data, commit);
+
+		/*
+		 * If there is no commit_mask, there is no reason to iterate
+		 * over this commit; it is not selected (if it were, it would
+		 * not have a blank commit mask) and all its children have
+		 * existing bitmaps (see the comment starting with "This commit
+		 * has an existing bitmap" below), so it does not contribute
+		 * anything to the final bitmap file or its descendants.
+		 */
+		if (!c_ent->commit_mask)
+			continue;
+
+		if (old_bitmap && bitmap_for_commit(old_bitmap, commit)) {
+			/*
+			 * This commit has an existing bitmap, so we can
+			 * get its bits immediately without an object
+			 * walk. That is, it is reusable as-is and there is no
+			 * need to continue walking beyond it.
+			 *
+			 * Mark it as such and add it to bb->commits separately
+			 * to avoid allocating a position in the commit mask.
+			 */
+			commit_list_insert(commit, &reusable);
+			goto next;
+		}
+
+		if (c_ent->maximal) {
+			num_maximal++;
+			ALLOC_GROW(bb->commits, bb->commits_nr + 1, bb->commits_alloc);
+			bb->commits[bb->commits_nr++] = commit;
+		}
+
+		if (p) {
+			struct bb_commit *p_ent = bb_data_at(&bb->data, p->item);
+			int c_not_p, p_not_c;
+
+			if (!p_ent->commit_mask) {
+				p_ent->commit_mask = bitmap_new();
+				c_not_p = 1;
+				p_not_c = 0;
+			} else {
+				c_not_p = bitmap_is_subset(c_ent->commit_mask, p_ent->commit_mask);
+				p_not_c = bitmap_is_subset(p_ent->commit_mask, c_ent->commit_mask);
+			}
+
+			if (!c_not_p)
+				continue;
+
+			bitmap_or(p_ent->commit_mask, c_ent->commit_mask);
+
+			if (p_not_c)
+				p_ent->maximal = 1;
+			else {
+				p_ent->maximal = 0;
+				free_commit_list(p_ent->reverse_edges);
+				p_ent->reverse_edges = NULL;
+			}
 
-	reuse_after = writer.selected_nr * REUSE_BITMAP_THRESHOLD;
-	need_reset = 0;
+			if (c_ent->maximal) {
+				commit_list_insert(commit, &p_ent->reverse_edges);
+			} else {
+				struct commit_list *cc = c_ent->reverse_edges;
+
+				for (; cc; cc = cc->next) {
+					if (!commit_list_contains(cc->item, p_ent->reverse_edges))
+						commit_list_insert(cc->item, &p_ent->reverse_edges);
+				}
+			}
+		}
+
+next:
+		bitmap_free(c_ent->commit_mask);
+		c_ent->commit_mask = NULL;
+	}
+
+	for (r = reusable; r; r = r->next) {
+		ALLOC_GROW(bb->commits, bb->commits_nr + 1, bb->commits_alloc);
+		bb->commits[bb->commits_nr++] = r->item;
+	}
+
+	trace2_data_intmax("pack-bitmap-write", the_repository,
+			   "num_selected_commits", writer->selected_nr);
+	trace2_data_intmax("pack-bitmap-write", the_repository,
+			   "num_maximal_commits", num_maximal);
+
+	free_commit_list(reusable);
+}
+
+static void bitmap_builder_clear(struct bitmap_builder *bb)
+{
+	clear_bb_data(&bb->data);
+	free(bb->commits);
+	bb->commits_nr = bb->commits_alloc = 0;
+}
+
+static void fill_bitmap_tree(struct bitmap *bitmap,
+			     struct tree *tree)
+{
+	uint32_t pos;
+	struct tree_desc desc;
+	struct name_entry entry;
+
+	/*
+	 * If our bit is already set, then there is nothing to do. Both this
+	 * tree and all of its children will be set.
+	 */
+	pos = find_object_pos(&tree->object.oid);
+	if (bitmap_get(bitmap, pos))
+		return;
+	bitmap_set(bitmap, pos);
 
-	for (i = writer.selected_nr - 1; i >= 0; --i) {
-		struct bitmapped_commit *stored;
-		struct object *object;
+	if (parse_tree(tree) < 0)
+		die("unable to load tree object %s",
+		    oid_to_hex(&tree->object.oid));
+	init_tree_desc(&desc, tree->buffer, tree->size);
 
-		khiter_t hash_pos;
-		int hash_ret;
+	while (tree_entry(&desc, &entry)) {
+		switch (object_type(entry.mode)) {
+		case OBJ_TREE:
+			fill_bitmap_tree(bitmap,
+					 lookup_tree(the_repository, &entry.oid));
+			break;
+		case OBJ_BLOB:
+			bitmap_set(bitmap, find_object_pos(&entry.oid));
+			break;
+		default:
+			/* Gitlink, etc; not reachable */
+			break;
+		}
+	}
 
-		stored = &writer.selected[i];
-		object = (struct object *)stored->commit;
+	free_tree_buffer(tree);
+}
 
-		if (stored->bitmap == NULL) {
-			if (i < writer.selected_nr - 1 &&
-			    (need_reset ||
-			     !in_merge_bases(writer.selected[i + 1].commit,
-					     stored->commit))) {
-			    bitmap_reset(base);
-			    reset_all_seen();
+static void fill_bitmap_commit(struct bb_commit *ent,
+			       struct commit *commit,
+			       struct prio_queue *queue,
+			       struct prio_queue *tree_queue,
+			       struct bitmap_index *old_bitmap,
+			       const uint32_t *mapping)
+{
+	if (!ent->bitmap)
+		ent->bitmap = bitmap_new();
+
+	prio_queue_put(queue, commit);
+
+	while (queue->nr) {
+		struct commit_list *p;
+		struct commit *c = prio_queue_get(queue);
+
+		if (old_bitmap && mapping) {
+			struct ewah_bitmap *old = bitmap_for_commit(old_bitmap, c);
+			/*
+			 * If this commit has an old bitmap, then translate that
+			 * bitmap and add its bits to this one. No need to walk
+			 * parents or the tree for this commit.
+			 */
+			if (old && !rebuild_bitmap(mapping, old, ent->bitmap))
+				continue;
+		}
+
+		/*
+		 * Mark ourselves and queue our tree. The commit
+		 * walk ensures we cover all parents.
+		 */
+		bitmap_set(ent->bitmap, find_object_pos(&c->object.oid));
+		prio_queue_put(tree_queue, get_commit_tree(c));
+
+		for (p = c->parents; p; p = p->next) {
+			int pos = find_object_pos(&p->item->object.oid);
+			if (!bitmap_get(ent->bitmap, pos)) {
+				bitmap_set(ent->bitmap, pos);
+				prio_queue_put(queue, p->item);
 			}
+		}
+	}
 
-			add_pending_object(&revs, object, "");
-			revs.include_check_data = base;
+	while (tree_queue->nr)
+		fill_bitmap_tree(ent->bitmap, prio_queue_get(tree_queue));
+}
 
-			if (prepare_revision_walk(&revs))
-				die("revision walk setup failed");
+static void store_selected(struct bb_commit *ent, struct commit *commit)
+{
+	struct bitmapped_commit *stored = &writer.selected[ent->idx];
+	khiter_t hash_pos;
+	int hash_ret;
 
-			traverse_commit_list(&revs, show_commit, show_object, base);
+	stored->bitmap = bitmap_to_ewah(ent->bitmap);
 
-			object_array_clear(&revs.pending);
+	hash_pos = kh_put_oid_map(writer.bitmaps, commit->object.oid, &hash_ret);
+	if (hash_ret == 0)
+		die("Duplicate entry when writing index: %s",
+		    oid_to_hex(&commit->object.oid));
+	kh_value(writer.bitmaps, hash_pos) = stored;
+}
 
-			stored->bitmap = bitmap_to_ewah(base);
-			need_reset = 0;
-		} else
-			need_reset = 1;
+void bitmap_writer_build(struct packing_data *to_pack)
+{
+	struct bitmap_builder bb;
+	size_t i;
+	int nr_stored = 0; /* for progress */
+	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
+	struct prio_queue tree_queue = { NULL };
+	struct bitmap_index *old_bitmap;
+	uint32_t *mapping;
 
-		if (i >= reuse_after)
-			stored->flags |= BITMAP_FLAG_REUSE;
+	writer.bitmaps = kh_init_oid_map();
+	writer.to_pack = to_pack;
 
-		hash_pos = kh_put_oid_map(writer.bitmaps, object->oid, &hash_ret);
-		if (hash_ret == 0)
-			die("Duplicate entry when writing index: %s",
-			    oid_to_hex(&object->oid));
+	if (writer.show_progress)
+		writer.progress = start_progress("Building bitmaps", writer.selected_nr);
+	trace2_region_enter("pack-bitmap-write", "building_bitmaps_total",
+			    the_repository);
+
+	old_bitmap = prepare_bitmap_git(to_pack->repo);
+	if (old_bitmap)
+		mapping = create_bitmap_mapping(old_bitmap, to_pack);
+	else
+		mapping = NULL;
+
+	bitmap_builder_init(&bb, &writer, old_bitmap);
+	for (i = bb.commits_nr; i > 0; i--) {
+		struct commit *commit = bb.commits[i-1];
+		struct bb_commit *ent = bb_data_at(&bb.data, commit);
+		struct commit *child;
+		int reused = 0;
+
+		fill_bitmap_commit(ent, commit, &queue, &tree_queue,
+				   old_bitmap, mapping);
+
+		if (ent->selected) {
+			store_selected(ent, commit);
+			nr_stored++;
+			display_progress(writer.progress, nr_stored);
+		}
 
-		kh_value(writer.bitmaps, hash_pos) = stored;
-		display_progress(writer.progress, writer.selected_nr - i);
+		while ((child = pop_commit(&ent->reverse_edges))) {
+			struct bb_commit *child_ent =
+				bb_data_at(&bb.data, child);
+
+			if (child_ent->bitmap)
+				bitmap_or(child_ent->bitmap, ent->bitmap);
+			else if (reused)
+				child_ent->bitmap = bitmap_dup(ent->bitmap);
+			else {
+				child_ent->bitmap = ent->bitmap;
+				reused = 1;
+			}
+		}
+		if (!reused)
+			bitmap_free(ent->bitmap);
+		ent->bitmap = NULL;
 	}
+	clear_prio_queue(&queue);
+	clear_prio_queue(&tree_queue);
+	bitmap_builder_clear(&bb);
+	free(mapping);
+
+	trace2_region_leave("pack-bitmap-write", "building_bitmaps_total",
+			    the_repository);
 
-	bitmap_free(base);
 	stop_progress(&writer.progress);
 
 	compute_xor_offsets();
@@ -360,35 +536,6 @@ static int date_compare(const void *_a, const void *_b)
 	return (long)b->date - (long)a->date;
 }
 
-void bitmap_writer_reuse_bitmaps(struct packing_data *to_pack)
-{
-	struct bitmap_index *bitmap_git;
-	if (!(bitmap_git = prepare_bitmap_git(to_pack->repo)))
-		return;
-
-	writer.reused = kh_init_oid_map();
-	rebuild_existing_bitmaps(bitmap_git, to_pack, writer.reused,
-				 writer.show_progress);
-	/*
-	 * NEEDSWORK: rebuild_existing_bitmaps() makes writer.reused reference
-	 * some bitmaps in bitmap_git, so we can't free the latter.
-	 */
-}
-
-static struct ewah_bitmap *find_reused_bitmap(const struct object_id *oid)
-{
-	khiter_t hash_pos;
-
-	if (!writer.reused)
-		return NULL;
-
-	hash_pos = kh_get_oid_map(writer.reused, *oid);
-	if (hash_pos >= kh_end(writer.reused))
-		return NULL;
-
-	return kh_value(writer.reused, hash_pos);
-}
-
 void bitmap_writer_select_commits(struct commit **indexed_commits,
 				  unsigned int indexed_commits_nr,
 				  int max_bitmaps)
@@ -402,12 +549,11 @@ void bitmap_writer_select_commits(struct commit **indexed_commits,
 
 	if (indexed_commits_nr < 100) {
 		for (i = 0; i < indexed_commits_nr; ++i)
-			push_bitmapped_commit(indexed_commits[i], NULL);
+			push_bitmapped_commit(indexed_commits[i]);
 		return;
 	}
 
 	for (;;) {
-		struct ewah_bitmap *reused_bitmap = NULL;
 		struct commit *chosen = NULL;
 
 		next = next_commit_index(i);
@@ -422,15 +568,13 @@ void bitmap_writer_select_commits(struct commit **indexed_commits,
 
 		if (next == 0) {
 			chosen = indexed_commits[i];
-			reused_bitmap = find_reused_bitmap(&chosen->object.oid);
 		} else {
 			chosen = indexed_commits[i + next];
 
 			for (j = 0; j <= next; ++j) {
 				struct commit *cm = indexed_commits[i + j];
 
-				reused_bitmap = find_reused_bitmap(&cm->object.oid);
-				if (reused_bitmap || (cm->object.flags & NEEDS_BITMAP) != 0) {
+				if ((cm->object.flags & NEEDS_BITMAP) != 0) {
 					chosen = cm;
 					break;
 				}
@@ -440,7 +584,7 @@ void bitmap_writer_select_commits(struct commit **indexed_commits,
 			}
 		}
 
-		push_bitmapped_commit(chosen, reused_bitmap);
+		push_bitmapped_commit(chosen);
 
 		i += next + 1;
 		display_progress(writer.progress, i);
diff --git a/pack-bitmap.c b/pack-bitmap.c
index 4077e731e8..d88745fb02 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -138,9 +138,10 @@ static struct ewah_bitmap *read_bitmap_1(struct bitmap_index *index)
 static int load_bitmap_header(struct bitmap_index *index)
 {
 	struct bitmap_disk_header *header = (void *)index->map;
+	size_t header_size = sizeof(*header) - GIT_MAX_RAWSZ + the_hash_algo->rawsz;
 
-	if (index->map_size < sizeof(*header) + the_hash_algo->rawsz)
-		return error("Corrupted bitmap index (missing header data)");
+	if (index->map_size < header_size + the_hash_algo->rawsz)
+		return error("Corrupted bitmap index (too small)");
 
 	if (memcmp(header->magic, BITMAP_IDX_SIGNATURE, sizeof(BITMAP_IDX_SIGNATURE)) != 0)
 		return error("Corrupted bitmap index file (wrong header)");
@@ -152,19 +153,23 @@ static int load_bitmap_header(struct bitmap_index *index)
 	/* Parse known bitmap format options */
 	{
 		uint32_t flags = ntohs(header->options);
+		size_t cache_size = st_mult(index->pack->num_objects, sizeof(uint32_t));
+		unsigned char *index_end = index->map + index->map_size - the_hash_algo->rawsz;
 
 		if ((flags & BITMAP_OPT_FULL_DAG) == 0)
 			return error("Unsupported options for bitmap index file "
 				"(Git requires BITMAP_OPT_FULL_DAG)");
 
 		if (flags & BITMAP_OPT_HASH_CACHE) {
-			unsigned char *end = index->map + index->map_size - the_hash_algo->rawsz;
-			index->hashes = ((uint32_t *)end) - index->pack->num_objects;
+			if (cache_size > index_end - index->map - header_size)
+				return error("corrupted bitmap index file (too short to fit hash cache)");
+			index->hashes = (void *)(index_end - cache_size);
+			index_end -= cache_size;
 		}
 	}
 
 	index->entry_count = ntohl(header->entry_count);
-	index->map_pos += sizeof(*header) - GIT_MAX_RAWSZ + the_hash_algo->rawsz;
+	index->map_pos += header_size;
 	return 0;
 }
 
@@ -224,11 +229,16 @@ static int load_bitmap_entries_v1(struct bitmap_index *index)
 		uint32_t commit_idx_pos;
 		struct object_id oid;
 
+		if (index->map_size - index->map_pos < 6)
+			return error("corrupt ewah bitmap: truncated header for entry %d", i);
+
 		commit_idx_pos = read_be32(index->map, &index->map_pos);
 		xor_offset = read_u8(index->map, &index->map_pos);
 		flags = read_u8(index->map, &index->map_pos);
 
-		nth_packed_object_id(&oid, index->pack, commit_idx_pos);
+		if (nth_packed_object_id(&oid, index->pack, commit_idx_pos) < 0)
+			return error("corrupt ewah bitmap: commit index %u out of range",
+				     (unsigned)commit_idx_pos);
 
 		bitmap = read_bitmap_1(index);
 		if (!bitmap)
@@ -370,6 +380,16 @@ struct include_data {
 	struct bitmap *seen;
 };
 
+struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
+				      struct commit *commit)
+{
+	khiter_t hash_pos = kh_get_oid_map(bitmap_git->bitmaps,
+					   commit->object.oid);
+	if (hash_pos >= kh_end(bitmap_git->bitmaps))
+		return NULL;
+	return lookup_stored_bitmap(kh_value(bitmap_git->bitmaps, hash_pos));
+}
+
 static inline int bitmap_position_extended(struct bitmap_index *bitmap_git,
 					   const struct object_id *oid)
 {
@@ -455,10 +475,10 @@ static void show_commit(struct commit *commit, void *data)
 
 static int add_to_include_set(struct bitmap_index *bitmap_git,
 			      struct include_data *data,
-			      const struct object_id *oid,
+			      struct commit *commit,
 			      int bitmap_pos)
 {
-	khiter_t hash_pos;
+	struct ewah_bitmap *partial;
 
 	if (data->seen && bitmap_get(data->seen, bitmap_pos))
 		return 0;
@@ -466,10 +486,9 @@ static int add_to_include_set(struct bitmap_index *bitmap_git,
 	if (bitmap_get(data->base, bitmap_pos))
 		return 0;
 
-	hash_pos = kh_get_oid_map(bitmap_git->bitmaps, *oid);
-	if (hash_pos < kh_end(bitmap_git->bitmaps)) {
-		struct stored_bitmap *st = kh_value(bitmap_git->bitmaps, hash_pos);
-		bitmap_or_ewah(data->base, lookup_stored_bitmap(st));
+	partial = bitmap_for_commit(bitmap_git, commit);
+	if (partial) {
+		bitmap_or_ewah(data->base, partial);
 		return 0;
 	}
 
@@ -488,8 +507,7 @@ static int should_include(struct commit *commit, void *_data)
 						  (struct object *)commit,
 						  NULL);
 
-	if (!add_to_include_set(data->bitmap_git, data, &commit->object.oid,
-				bitmap_pos)) {
+	if (!add_to_include_set(data->bitmap_git, data, commit, bitmap_pos)) {
 		struct commit_list *parent = commit->parents;
 
 		while (parent) {
@@ -503,6 +521,23 @@ static int should_include(struct commit *commit, void *_data)
 	return 1;
 }
 
+static int add_commit_to_bitmap(struct bitmap_index *bitmap_git,
+				struct bitmap **base,
+				struct commit *commit)
+{
+	struct ewah_bitmap *or_with = bitmap_for_commit(bitmap_git, commit);
+
+	if (!or_with)
+		return 0;
+
+	if (*base == NULL)
+		*base = ewah_to_bitmap(or_with);
+	else
+		bitmap_or_ewah(*base, or_with);
+
+	return 1;
+}
+
 static struct bitmap *find_objects(struct bitmap_index *bitmap_git,
 				   struct rev_info *revs,
 				   struct object_list *roots,
@@ -526,21 +561,10 @@ static struct bitmap *find_objects(struct bitmap_index *bitmap_git,
 		struct object *object = roots->item;
 		roots = roots->next;
 
-		if (object->type == OBJ_COMMIT) {
-			khiter_t pos = kh_get_oid_map(bitmap_git->bitmaps, object->oid);
-
-			if (pos < kh_end(bitmap_git->bitmaps)) {
-				struct stored_bitmap *st = kh_value(bitmap_git->bitmaps, pos);
-				struct ewah_bitmap *or_with = lookup_stored_bitmap(st);
-
-				if (base == NULL)
-					base = ewah_to_bitmap(or_with);
-				else
-					bitmap_or_ewah(base, or_with);
-
-				object->flags |= SEEN;
-				continue;
-			}
+		if (object->type == OBJ_COMMIT &&
+		    add_commit_to_bitmap(bitmap_git, &base, (struct commit *)object)) {
+			object->flags |= SEEN;
+			continue;
 		}
 
 		object_list_insert(object, &not_mapped);
@@ -1272,10 +1296,10 @@ void test_bitmap_walk(struct rev_info *revs)
 {
 	struct object *root;
 	struct bitmap *result = NULL;
-	khiter_t pos;
 	size_t result_popcnt;
 	struct bitmap_test_data tdata;
 	struct bitmap_index *bitmap_git;
+	struct ewah_bitmap *bm;
 
 	if (!(bitmap_git = prepare_bitmap_git(revs->repo)))
 		die("failed to load bitmap indexes");
@@ -1287,12 +1311,9 @@ void test_bitmap_walk(struct rev_info *revs)
 		bitmap_git->version, bitmap_git->entry_count);
 
 	root = revs->pending.objects[0].item;
-	pos = kh_get_oid_map(bitmap_git->bitmaps, root->oid);
-
-	if (pos < kh_end(bitmap_git->bitmaps)) {
-		struct stored_bitmap *st = kh_value(bitmap_git->bitmaps, pos);
-		struct ewah_bitmap *bm = lookup_stored_bitmap(st);
+	bm = bitmap_for_commit(bitmap_git, (struct commit *)root);
 
+	if (bm) {
 		fprintf(stderr, "Found bitmap for %s. %d bits / %08x checksum\n",
 			oid_to_hex(&root->oid), (int)bm->bit_size, ewah_checksum(bm));
 
@@ -1323,14 +1344,14 @@ void test_bitmap_walk(struct rev_info *revs)
 	if (bitmap_equals(result, tdata.base))
 		fprintf(stderr, "OK!\n");
 	else
-		fprintf(stderr, "Mismatch!\n");
+		die("mismatch in bitmap results");
 
 	free_bitmap_index(bitmap_git);
 }
 
-static int rebuild_bitmap(uint32_t *reposition,
-			  struct ewah_bitmap *source,
-			  struct bitmap *dest)
+int rebuild_bitmap(const uint32_t *reposition,
+		   struct ewah_bitmap *source,
+		   struct bitmap *dest)
 {
 	uint32_t pos = 0;
 	struct ewah_iterator it;
@@ -1359,19 +1380,11 @@ static int rebuild_bitmap(uint32_t *reposition,
 	return 0;
 }
 
-int rebuild_existing_bitmaps(struct bitmap_index *bitmap_git,
-			     struct packing_data *mapping,
-			     kh_oid_map_t *reused_bitmaps,
-			     int show_progress)
+uint32_t *create_bitmap_mapping(struct bitmap_index *bitmap_git,
+				struct packing_data *mapping)
 {
 	uint32_t i, num_objects;
 	uint32_t *reposition;
-	struct bitmap *rebuild;
-	struct stored_bitmap *stored;
-	struct progress *progress = NULL;
-
-	khiter_t hash_pos;
-	int hash_ret;
 
 	num_objects = bitmap_git->pack->num_objects;
 	reposition = xcalloc(num_objects, sizeof(uint32_t));
@@ -1389,33 +1402,7 @@ int rebuild_existing_bitmaps(struct bitmap_index *bitmap_git,
 			reposition[i] = oe_in_pack_pos(mapping, oe) + 1;
 	}
 
-	rebuild = bitmap_new();
-	i = 0;
-
-	if (show_progress)
-		progress = start_progress("Reusing bitmaps", 0);
-
-	kh_foreach_value(bitmap_git->bitmaps, stored, {
-		if (stored->flags & BITMAP_FLAG_REUSE) {
-			if (!rebuild_bitmap(reposition,
-					    lookup_stored_bitmap(stored),
-					    rebuild)) {
-				hash_pos = kh_put_oid_map(reused_bitmaps,
-							  stored->oid,
-							  &hash_ret);
-				kh_value(reused_bitmaps, hash_pos) =
-					bitmap_to_ewah(rebuild);
-			}
-			bitmap_reset(rebuild);
-			display_progress(progress, ++i);
-		}
-	});
-
-	stop_progress(&progress);
-
-	free(reposition);
-	bitmap_free(rebuild);
-	return 0;
+	return reposition;
 }
 
 void free_bitmap_index(struct bitmap_index *b)
diff --git a/pack-bitmap.h b/pack-bitmap.h
index 1203120c43..25dfcf5615 100644
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@@ -73,7 +73,13 @@ void bitmap_writer_set_checksum(unsigned char *sha1);
 void bitmap_writer_build_type_index(struct packing_data *to_pack,
 				    struct pack_idx_entry **index,
 				    uint32_t index_nr);
-void bitmap_writer_reuse_bitmaps(struct packing_data *to_pack);
+uint32_t *create_bitmap_mapping(struct bitmap_index *bitmap_git,
+				struct packing_data *mapping);
+int rebuild_bitmap(const uint32_t *reposition,
+		   struct ewah_bitmap *source,
+		   struct bitmap *dest);
+struct ewah_bitmap *bitmap_for_commit(struct bitmap_index *bitmap_git,
+				      struct commit *commit);
 void bitmap_writer_select_commits(struct commit **indexed_commits,
 		unsigned int indexed_commits_nr, int max_bitmaps);
 void bitmap_writer_build(struct packing_data *to_pack);
diff --git a/t/t5310-pack-bitmaps.sh b/t/t5310-pack-bitmaps.sh
index 1d40fcad39..3a2c9d2d8e 100755
--- a/t/t5310-pack-bitmaps.sh
+++ b/t/t5310-pack-bitmaps.sh
@@ -20,84 +20,172 @@ has_any () {
 	grep -Ff "$1" "$2"
 }
 
+# To ensure the logic for "maximal commits" is exercised, make
+# the repository a bit more complicated.
+#
+#    other                         second
+#      *                             *
+# (99 commits)                  (99 commits)
+#      *                             *
+#      |\                           /|
+#      | * octo-other  octo-second * |
+#      |/|\_________  ____________/|\|
+#      | \          \/  __________/  |
+#      |  | ________/\ /             |
+#      *  |/          * merge-right  *
+#      | _|__________/ \____________ |
+#      |/ |                         \|
+# (l1) *  * merge-left               * (r1)
+#      | / \________________________ |
+#      |/                           \|
+# (l2) *                             * (r2)
+#       \___________________________ |
+#                                   \|
+#                                    * (base)
+#
+# We only push bits down the first-parent history, which
+# makes some of these commits unimportant!
+#
+# The important part for the maximal commit algorithm is how
+# the bitmasks are extended. Assuming starting bit positions
+# for second (bit 0) and other (bit 1), the bitmasks at the
+# end should be:
+#
+#      second: 1       (maximal, selected)
+#       other: 01      (maximal, selected)
+#      (base): 11 (maximal)
+#
+# This complicated history was important for a previous
+# version of the walk that guarantees never walking a
+# commit multiple times. That goal might be important
+# again, so preserve this complicated case. For now, this
+# test will guarantee that the bitmaps are computed
+# correctly, even with the repeat calculations.
+
 test_expect_success 'setup repo with moderate-sized history' '
-	test_commit_bulk --id=file 100 &&
+	test_commit_bulk --id=file 10 &&
+	git branch -M second &&
 	git checkout -b other HEAD~5 &&
 	test_commit_bulk --id=side 10 &&
-	git checkout master &&
-	bitmaptip=$(git rev-parse master) &&
+
+	# add complicated history setup, including merges and
+	# ambiguous merge-bases
+
+	git checkout -b merge-left other~2 &&
+	git merge second~2 -m "merge-left" &&
+
+	git checkout -b merge-right second~1 &&
+	git merge other~1 -m "merge-right" &&
+
+	git checkout -b octo-second second &&
+	git merge merge-left merge-right -m "octopus-second" &&
+
+	git checkout -b octo-other other &&
+	git merge merge-left merge-right -m "octopus-other" &&
+
+	git checkout other &&
+	git merge octo-other -m "pull octopus" &&
+
+	git checkout second &&
+	git merge octo-second -m "pull octopus" &&
+
+	# Remove these branches so they are not selected
+	# as bitmap tips
+	git branch -D merge-left &&
+	git branch -D merge-right &&
+	git branch -D octo-other &&
+	git branch -D octo-second &&
+
+	# add padding to make these merges less interesting
+	# and avoid having them selected for bitmaps
+	test_commit_bulk --id=file 100 &&
+	git checkout other &&
+	test_commit_bulk --id=side 100 &&
+	git checkout second &&
+
+	bitmaptip=$(git rev-parse second) &&
 	blob=$(echo tagged-blob | git hash-object -w --stdin) &&
 	git tag tagged-blob $blob &&
 	git config repack.writebitmaps true
 '
 
 test_expect_success 'full repack creates bitmaps' '
-	git repack -ad &&
+	GIT_TRACE2_EVENT_NESTING=4 GIT_TRACE2_EVENT="$(pwd)/trace" \
+		git repack -ad &&
 	ls .git/objects/pack/ | grep bitmap >output &&
-	test_line_count = 1 output
+	test_line_count = 1 output &&
+	grep "\"key\":\"num_selected_commits\",\"value\":\"106\"" trace &&
+	grep "\"key\":\"num_maximal_commits\",\"value\":\"107\"" trace
 '
 
 test_expect_success 'rev-list --test-bitmap verifies bitmaps' '
 	git rev-list --test-bitmap HEAD
 '
 
-rev_list_tests() {
-	state=$1
-
-	test_expect_success "counting commits via bitmap ($state)" '
-		git rev-list --count HEAD >expect &&
-		git rev-list --use-bitmap-index --count HEAD >actual &&
+rev_list_tests_head () {
+	test_expect_success "counting commits via bitmap ($state, $branch)" '
+		git rev-list --count $branch >expect &&
+		git rev-list --use-bitmap-index --count $branch >actual &&
 		test_cmp expect actual
 	'
 
-	test_expect_success "counting partial commits via bitmap ($state)" '
-		git rev-list --count HEAD~5..HEAD >expect &&
-		git rev-list --use-bitmap-index --count HEAD~5..HEAD >actual &&
+	test_expect_success "counting partial commits via bitmap ($state, $branch)" '
+		git rev-list --count $branch~5..$branch >expect &&
+		git rev-list --use-bitmap-index --count $branch~5..$branch >actual &&
 		test_cmp expect actual
 	'
 
-	test_expect_success "counting commits with limit ($state)" '
-		git rev-list --count -n 1 HEAD >expect &&
-		git rev-list --use-bitmap-index --count -n 1 HEAD >actual &&
+	test_expect_success "counting commits with limit ($state, $branch)" '
+		git rev-list --count -n 1 $branch >expect &&
+		git rev-list --use-bitmap-index --count -n 1 $branch >actual &&
 		test_cmp expect actual
 	'
 
-	test_expect_success "counting non-linear history ($state)" '
-		git rev-list --count other...master >expect &&
-		git rev-list --use-bitmap-index --count other...master >actual &&
+	test_expect_success "counting non-linear history ($state, $branch)" '
+		git rev-list --count other...second >expect &&
+		git rev-list --use-bitmap-index --count other...second >actual &&
 		test_cmp expect actual
 	'
 
-	test_expect_success "counting commits with limiting ($state)" '
-		git rev-list --count HEAD -- 1.t >expect &&
-		git rev-list --use-bitmap-index --count HEAD -- 1.t >actual &&
+	test_expect_success "counting commits with limiting ($state, $branch)" '
+		git rev-list --count $branch -- 1.t >expect &&
+		git rev-list --use-bitmap-index --count $branch -- 1.t >actual &&
 		test_cmp expect actual
 	'
 
-	test_expect_success "counting objects via bitmap ($state)" '
-		git rev-list --count --objects HEAD >expect &&
-		git rev-list --use-bitmap-index --count --objects HEAD >actual &&
+	test_expect_success "counting objects via bitmap ($state, $branch)" '
+		git rev-list --count --objects $branch >expect &&
+		git rev-list --use-bitmap-index --count --objects $branch >actual &&
 		test_cmp expect actual
 	'
 
-	test_expect_success "enumerate commits ($state)" '
-		git rev-list --use-bitmap-index HEAD >actual &&
-		git rev-list HEAD >expect &&
+	test_expect_success "enumerate commits ($state, $branch)" '
+		git rev-list --use-bitmap-index $branch >actual &&
+		git rev-list $branch >expect &&
 		test_bitmap_traversal --no-confirm-bitmaps expect actual
 	'
 
-	test_expect_success "enumerate --objects ($state)" '
-		git rev-list --objects --use-bitmap-index HEAD >actual &&
-		git rev-list --objects HEAD >expect &&
+	test_expect_success "enumerate --objects ($state, $branch)" '
+		git rev-list --objects --use-bitmap-index $branch >actual &&
+		git rev-list --objects $branch >expect &&
 		test_bitmap_traversal expect actual
 	'
 
-	test_expect_success "bitmap --objects handles non-commit objects ($state)" '
-		git rev-list --objects --use-bitmap-index HEAD tagged-blob >actual &&
+	test_expect_success "bitmap --objects handles non-commit objects ($state, $branch)" '
+		git rev-list --objects --use-bitmap-index $branch tagged-blob >actual &&
 		grep $blob actual
 	'
 }
 
+rev_list_tests () {
+	state=$1
+
+	for branch in "second" "other"
+	do
+		rev_list_tests_head
+	done
+}
+
 rev_list_tests 'full bitmap'
 
 test_expect_success 'clone from bitmapped repository' '
@@ -128,7 +216,7 @@ test_expect_success 'setup further non-bitmapped commits' '
 rev_list_tests 'partial bitmap'
 
 test_expect_success 'fetch (partial bitmap)' '
-	git --git-dir=clone.git fetch origin master:master &&
+	git --git-dir=clone.git fetch origin second:second &&
 	git rev-parse HEAD >expect &&
 	git --git-dir=clone.git rev-parse HEAD >actual &&
 	test_cmp expect actual
@@ -230,7 +318,7 @@ test_expect_success 'full repack, reusing previous bitmaps' '
 '
 
 test_expect_success 'fetch (full bitmap)' '
-	git --git-dir=clone.git fetch origin master:master &&
+	git --git-dir=clone.git fetch origin second:second &&
 	git rev-parse HEAD >expect &&
 	git --git-dir=clone.git rev-parse HEAD >actual &&
 	test_cmp expect actual
@@ -343,7 +431,20 @@ test_expect_success 'pack reuse respects --incremental' '
 	test_must_be_empty actual
 '
 
-test_expect_success 'truncated bitmap fails gracefully' '
+test_expect_success 'truncated bitmap fails gracefully (ewah)' '
+	test_config pack.writebitmaphashcache false &&
+	git repack -ad &&
+	git rev-list --use-bitmap-index --count --all >expect &&
+	bitmap=$(ls .git/objects/pack/*.bitmap) &&
+	test_when_finished "rm -f $bitmap" &&
+	test_copy_bytes 256 <$bitmap >$bitmap.tmp &&
+	mv -f $bitmap.tmp $bitmap &&
+	git rev-list --use-bitmap-index --count --all >actual 2>stderr &&
+	test_cmp expect actual &&
+	test_i18ngrep corrupt.ewah.bitmap stderr
+'
+
+test_expect_success 'truncated bitmap fails gracefully (cache)' '
 	git repack -ad &&
 	git rev-list --use-bitmap-index --count --all >expect &&
 	bitmap=$(ls .git/objects/pack/*.bitmap) &&
@@ -352,7 +453,7 @@ test_expect_success 'truncated bitmap fails gracefully' '
 	mv -f $bitmap.tmp $bitmap &&
 	git rev-list --use-bitmap-index --count --all >actual 2>stderr &&
 	test_cmp expect actual &&
-	test_i18ngrep corrupt stderr
+	test_i18ngrep corrupted.bitmap.index stderr
 '
 
 # have_delta <obj> <expected_base>