midx-write: move writing-related functions from midx.c

Introduce a new midx-write.c source file, which holds all of the
functionality from the MIDX sub-system related to writing new MIDX files.

Similar to the relationship between "pack-bitmap.c" and
"pack-bitmap-write.c", this source file will hold code that is specific
to writing MIDX files as opposed to reading them (the latter will remain
in midx.c).

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

4 files changed, 1555 insertions, 1539 deletions

diff --git a/Makefile b/Makefile
index c43c1bd1a0..44b281bb63 100644
--- a/Makefile
+++ b/Makefile
@@ -1075,6 +1075,7 @@ LIB_OBJS += merge-ort-wrappers.o
 LIB_OBJS += merge-recursive.o
 LIB_OBJS += merge.o
 LIB_OBJS += midx.o
+LIB_OBJS += midx-write.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += negotiator/default.o
 LIB_OBJS += negotiator/noop.o
diff --git a/midx-write.c b/midx-write.c
new file mode 100644
index 0000000000..5242d2a724
--- /dev/null
+++ b/midx-write.c
@@ -0,0 +1,1521 @@
+#include "git-compat-util.h"
+#include "abspath.h"
+#include "config.h"
+#include "hex.h"
+#include "lockfile.h"
+#include "packfile.h"
+#include "object-file.h"
+#include "hash-lookup.h"
+#include "midx.h"
+#include "progress.h"
+#include "trace2.h"
+#include "run-command.h"
+#include "chunk-format.h"
+#include "pack-bitmap.h"
+#include "refs.h"
+#include "revision.h"
+#include "list-objects.h"
+
+#define PACK_EXPIRED UINT_MAX
+#define BITMAP_POS_UNKNOWN (~((uint32_t)0))
+#define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
+#define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
+
+extern int midx_checksum_valid(struct multi_pack_index *m);
+extern void clear_midx_files_ext(const char *object_dir, const char *ext,
+				 unsigned char *keep_hash);
+extern int cmp_idx_or_pack_name(const char *idx_or_pack_name,
+				const char *idx_name);
+
+static size_t write_midx_header(struct hashfile *f,
+				unsigned char num_chunks,
+				uint32_t num_packs)
+{
+	hashwrite_be32(f, MIDX_SIGNATURE);
+	hashwrite_u8(f, MIDX_VERSION);
+	hashwrite_u8(f, oid_version(the_hash_algo));
+	hashwrite_u8(f, num_chunks);
+	hashwrite_u8(f, 0); /* unused */
+	hashwrite_be32(f, num_packs);
+
+	return MIDX_HEADER_SIZE;
+}
+
+struct pack_info {
+	uint32_t orig_pack_int_id;
+	char *pack_name;
+	struct packed_git *p;
+
+	uint32_t bitmap_pos;
+	uint32_t bitmap_nr;
+
+	unsigned expired : 1;
+};
+
+static void fill_pack_info(struct pack_info *info,
+			   struct packed_git *p, const char *pack_name,
+			   uint32_t orig_pack_int_id)
+{
+	memset(info, 0, sizeof(struct pack_info));
+
+	info->orig_pack_int_id = orig_pack_int_id;
+	info->pack_name = xstrdup(pack_name);
+	info->p = p;
+	info->bitmap_pos = BITMAP_POS_UNKNOWN;
+}
+
+static int pack_info_compare(const void *_a, const void *_b)
+{
+	struct pack_info *a = (struct pack_info *)_a;
+	struct pack_info *b = (struct pack_info *)_b;
+	return strcmp(a->pack_name, b->pack_name);
+}
+
+static int idx_or_pack_name_cmp(const void *_va, const void *_vb)
+{
+	const char *pack_name = _va;
+	const struct pack_info *compar = _vb;
+
+	return cmp_idx_or_pack_name(pack_name, compar->pack_name);
+}
+
+struct write_midx_context {
+	struct pack_info *info;
+	size_t nr;
+	size_t alloc;
+	struct multi_pack_index *m;
+	struct progress *progress;
+	unsigned pack_paths_checked;
+
+	struct pack_midx_entry *entries;
+	size_t entries_nr;
+
+	uint32_t *pack_perm;
+	uint32_t *pack_order;
+	unsigned large_offsets_needed:1;
+	uint32_t num_large_offsets;
+
+	int preferred_pack_idx;
+
+	struct string_list *to_include;
+};
+
+static void add_pack_to_midx(const char *full_path, size_t full_path_len,
+			     const char *file_name, void *data)
+{
+	struct write_midx_context *ctx = data;
+	struct packed_git *p;
+
+	if (ends_with(file_name, ".idx")) {
+		display_progress(ctx->progress, ++ctx->pack_paths_checked);
+		/*
+		 * Note that at most one of ctx->m and ctx->to_include are set,
+		 * so we are testing midx_contains_pack() and
+		 * string_list_has_string() independently (guarded by the
+		 * appropriate NULL checks).
+		 *
+		 * We could support passing to_include while reusing an existing
+		 * MIDX, but don't currently since the reuse process drags
+		 * forward all packs from an existing MIDX (without checking
+		 * whether or not they appear in the to_include list).
+		 *
+		 * If we added support for that, these next two conditional
+		 * should be performed independently (likely checking
+		 * to_include before the existing MIDX).
+		 */
+		if (ctx->m && midx_contains_pack(ctx->m, file_name))
+			return;
+		else if (ctx->to_include &&
+			 !string_list_has_string(ctx->to_include, file_name))
+			return;
+
+		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
+
+		p = add_packed_git(full_path, full_path_len, 0);
+		if (!p) {
+			warning(_("failed to add packfile '%s'"),
+				full_path);
+			return;
+		}
+
+		if (open_pack_index(p)) {
+			warning(_("failed to open pack-index '%s'"),
+				full_path);
+			close_pack(p);
+			free(p);
+			return;
+		}
+
+		fill_pack_info(&ctx->info[ctx->nr], p, file_name, ctx->nr);
+		ctx->nr++;
+	}
+}
+
+struct pack_midx_entry {
+	struct object_id oid;
+	uint32_t pack_int_id;
+	time_t pack_mtime;
+	uint64_t offset;
+	unsigned preferred : 1;
+};
+
+static int midx_oid_compare(const void *_a, const void *_b)
+{
+	const struct pack_midx_entry *a = (const struct pack_midx_entry *)_a;
+	const struct pack_midx_entry *b = (const struct pack_midx_entry *)_b;
+	int cmp = oidcmp(&a->oid, &b->oid);
+
+	if (cmp)
+		return cmp;
+
+	/* Sort objects in a preferred pack first when multiple copies exist. */
+	if (a->preferred > b->preferred)
+		return -1;
+	if (a->preferred < b->preferred)
+		return 1;
+
+	if (a->pack_mtime > b->pack_mtime)
+		return -1;
+	else if (a->pack_mtime < b->pack_mtime)
+		return 1;
+
+	return a->pack_int_id - b->pack_int_id;
+}
+
+static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
+				      struct pack_midx_entry *e,
+				      uint32_t pos)
+{
+	if (pos >= m->num_objects)
+		return 1;
+
+	nth_midxed_object_oid(&e->oid, m, pos);
+	e->pack_int_id = nth_midxed_pack_int_id(m, pos);
+	e->offset = nth_midxed_offset(m, pos);
+
+	/* consider objects in midx to be from "old" packs */
+	e->pack_mtime = 0;
+	return 0;
+}
+
+static void fill_pack_entry(uint32_t pack_int_id,
+			    struct packed_git *p,
+			    uint32_t cur_object,
+			    struct pack_midx_entry *entry,
+			    int preferred)
+{
+	if (nth_packed_object_id(&entry->oid, p, cur_object) < 0)
+		die(_("failed to locate object %d in packfile"), cur_object);
+
+	entry->pack_int_id = pack_int_id;
+	entry->pack_mtime = p->mtime;
+
+	entry->offset = nth_packed_object_offset(p, cur_object);
+	entry->preferred = !!preferred;
+}
+
+struct midx_fanout {
+	struct pack_midx_entry *entries;
+	size_t nr, alloc;
+};
+
+static void midx_fanout_grow(struct midx_fanout *fanout, size_t nr)
+{
+	if (nr < fanout->nr)
+		BUG("negative growth in midx_fanout_grow() (%"PRIuMAX" < %"PRIuMAX")",
+		    (uintmax_t)nr, (uintmax_t)fanout->nr);
+	ALLOC_GROW(fanout->entries, nr, fanout->alloc);
+}
+
+static void midx_fanout_sort(struct midx_fanout *fanout)
+{
+	QSORT(fanout->entries, fanout->nr, midx_oid_compare);
+}
+
+static void midx_fanout_add_midx_fanout(struct midx_fanout *fanout,
+					struct multi_pack_index *m,
+					uint32_t cur_fanout,
+					int preferred_pack)
+{
+	uint32_t start = 0, end;
+	uint32_t cur_object;
+
+	if (cur_fanout)
+		start = ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
+	end = ntohl(m->chunk_oid_fanout[cur_fanout]);
+
+	for (cur_object = start; cur_object < end; cur_object++) {
+		if ((preferred_pack > -1) &&
+		    (preferred_pack == nth_midxed_pack_int_id(m, cur_object))) {
+			/*
+			 * Objects from preferred packs are added
+			 * separately.
+			 */
+			continue;
+		}
+
+		midx_fanout_grow(fanout, fanout->nr + 1);
+		nth_midxed_pack_midx_entry(m,
+					   &fanout->entries[fanout->nr],
+					   cur_object);
+		fanout->entries[fanout->nr].preferred = 0;
+		fanout->nr++;
+	}
+}
+
+static void midx_fanout_add_pack_fanout(struct midx_fanout *fanout,
+					struct pack_info *info,
+					uint32_t cur_pack,
+					int preferred,
+					uint32_t cur_fanout)
+{
+	struct packed_git *pack = info[cur_pack].p;
+	uint32_t start = 0, end;
+	uint32_t cur_object;
+
+	if (cur_fanout)
+		start = get_pack_fanout(pack, cur_fanout - 1);
+	end = get_pack_fanout(pack, cur_fanout);
+
+	for (cur_object = start; cur_object < end; cur_object++) {
+		midx_fanout_grow(fanout, fanout->nr + 1);
+		fill_pack_entry(cur_pack,
+				info[cur_pack].p,
+				cur_object,
+				&fanout->entries[fanout->nr],
+				preferred);
+		fanout->nr++;
+	}
+}
+
+/*
+ * It is possible to artificially get into a state where there are many
+ * duplicate copies of objects. That can create high memory pressure if
+ * we are to create a list of all objects before de-duplication. To reduce
+ * this memory pressure without a significant performance drop, automatically
+ * group objects by the first byte of their object id. Use the IDX fanout
+ * tables to group the data, copy to a local array, then sort.
+ *
+ * Copy only the de-duplicated entries (selected by most-recent modified time
+ * of a packfile containing the object).
+ */
+static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
+						  struct pack_info *info,
+						  uint32_t nr_packs,
+						  size_t *nr_objects,
+						  int preferred_pack)
+{
+	uint32_t cur_fanout, cur_pack, cur_object;
+	size_t alloc_objects, total_objects = 0;
+	struct midx_fanout fanout = { 0 };
+	struct pack_midx_entry *deduplicated_entries = NULL;
+	uint32_t start_pack = m ? m->num_packs : 0;
+
+	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
+		total_objects = st_add(total_objects,
+				       info[cur_pack].p->num_objects);
+
+	/*
+	 * As we de-duplicate by fanout value, we expect the fanout
+	 * slices to be evenly distributed, with some noise. Hence,
+	 * allocate slightly more than one 256th.
+	 */
+	alloc_objects = fanout.alloc = total_objects > 3200 ? total_objects / 200 : 16;
+
+	ALLOC_ARRAY(fanout.entries, fanout.alloc);
+	ALLOC_ARRAY(deduplicated_entries, alloc_objects);
+	*nr_objects = 0;
+
+	for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
+		fanout.nr = 0;
+
+		if (m)
+			midx_fanout_add_midx_fanout(&fanout, m, cur_fanout,
+						    preferred_pack);
+
+		for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++) {
+			int preferred = cur_pack == preferred_pack;
+			midx_fanout_add_pack_fanout(&fanout,
+						    info, cur_pack,
+						    preferred, cur_fanout);
+		}
+
+		if (-1 < preferred_pack && preferred_pack < start_pack)
+			midx_fanout_add_pack_fanout(&fanout, info,
+						    preferred_pack, 1,
+						    cur_fanout);
+
+		midx_fanout_sort(&fanout);
+
+		/*
+		 * The batch is now sorted by OID and then mtime (descending).
+		 * Take only the first duplicate.
+		 */
+		for (cur_object = 0; cur_object < fanout.nr; cur_object++) {
+			if (cur_object && oideq(&fanout.entries[cur_object - 1].oid,
+						&fanout.entries[cur_object].oid))
+				continue;
+
+			ALLOC_GROW(deduplicated_entries, st_add(*nr_objects, 1),
+				   alloc_objects);
+			memcpy(&deduplicated_entries[*nr_objects],
+			       &fanout.entries[cur_object],
+			       sizeof(struct pack_midx_entry));
+			(*nr_objects)++;
+		}
+	}
+
+	free(fanout.entries);
+	return deduplicated_entries;
+}
+
+static int write_midx_pack_names(struct hashfile *f, void *data)
+{
+	struct write_midx_context *ctx = data;
+	uint32_t i;
+	unsigned char padding[MIDX_CHUNK_ALIGNMENT];
+	size_t written = 0;
+
+	for (i = 0; i < ctx->nr; i++) {
+		size_t writelen;
+
+		if (ctx->info[i].expired)
+			continue;
+
+		if (i && strcmp(ctx->info[i].pack_name, ctx->info[i - 1].pack_name) <= 0)
+			BUG("incorrect pack-file order: %s before %s",
+			    ctx->info[i - 1].pack_name,
+			    ctx->info[i].pack_name);
+
+		writelen = strlen(ctx->info[i].pack_name) + 1;
+		hashwrite(f, ctx->info[i].pack_name, writelen);
+		written += writelen;
+	}
+
+	/* add padding to be aligned */
+	i = MIDX_CHUNK_ALIGNMENT - (written % MIDX_CHUNK_ALIGNMENT);
+	if (i < MIDX_CHUNK_ALIGNMENT) {
+		memset(padding, 0, sizeof(padding));
+		hashwrite(f, padding, i);
+	}
+
+	return 0;
+}
+
+static int write_midx_bitmapped_packs(struct hashfile *f, void *data)
+{
+	struct write_midx_context *ctx = data;
+	size_t i;
+
+	for (i = 0; i < ctx->nr; i++) {
+		struct pack_info *pack = &ctx->info[i];
+		if (pack->expired)
+			continue;
+
+		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN && pack->bitmap_nr)
+			BUG("pack '%s' has no bitmap position, but has %d bitmapped object(s)",
+			    pack->pack_name, pack->bitmap_nr);
+
+		hashwrite_be32(f, pack->bitmap_pos);
+		hashwrite_be32(f, pack->bitmap_nr);
+	}
+	return 0;
+}
+
+static int write_midx_oid_fanout(struct hashfile *f,
+				 void *data)
+{
+	struct write_midx_context *ctx = data;
+	struct pack_midx_entry *list = ctx->entries;
+	struct pack_midx_entry *last = ctx->entries + ctx->entries_nr;
+	uint32_t count = 0;
+	uint32_t i;
+
+	/*
+	* Write the first-level table (the list is sorted,
+	* but we use a 256-entry lookup to be able to avoid
+	* having to do eight extra binary search iterations).
+	*/
+	for (i = 0; i < 256; i++) {
+		struct pack_midx_entry *next = list;
+
+		while (next < last && next->oid.hash[0] == i) {
+			count++;
+			next++;
+		}
+
+		hashwrite_be32(f, count);
+		list = next;
+	}
+
+	return 0;
+}
+
+static int write_midx_oid_lookup(struct hashfile *f,
+				 void *data)
+{
+	struct write_midx_context *ctx = data;
+	unsigned char hash_len = the_hash_algo->rawsz;
+	struct pack_midx_entry *list = ctx->entries;
+	uint32_t i;
+
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *obj = list++;
+
+		if (i < ctx->entries_nr - 1) {
+			struct pack_midx_entry *next = list;
+			if (oidcmp(&obj->oid, &next->oid) >= 0)
+				BUG("OIDs not in order: %s >= %s",
+				    oid_to_hex(&obj->oid),
+				    oid_to_hex(&next->oid));
+		}
+
+		hashwrite(f, obj->oid.hash, (int)hash_len);
+	}
+
+	return 0;
+}
+
+static int write_midx_object_offsets(struct hashfile *f,
+				     void *data)
+{
+	struct write_midx_context *ctx = data;
+	struct pack_midx_entry *list = ctx->entries;
+	uint32_t i, nr_large_offset = 0;
+
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *obj = list++;
+
+		if (ctx->pack_perm[obj->pack_int_id] == PACK_EXPIRED)
+			BUG("object %s is in an expired pack with int-id %d",
+			    oid_to_hex(&obj->oid),
+			    obj->pack_int_id);
+
+		hashwrite_be32(f, ctx->pack_perm[obj->pack_int_id]);
+
+		if (ctx->large_offsets_needed && obj->offset >> 31)
+			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
+		else if (!ctx->large_offsets_needed && obj->offset >> 32)
+			BUG("object %s requires a large offset (%"PRIx64") but the MIDX is not writing large offsets!",
+			    oid_to_hex(&obj->oid),
+			    obj->offset);
+		else
+			hashwrite_be32(f, (uint32_t)obj->offset);
+	}
+
+	return 0;
+}
+
+static int write_midx_large_offsets(struct hashfile *f,
+				    void *data)
+{
+	struct write_midx_context *ctx = data;
+	struct pack_midx_entry *list = ctx->entries;
+	struct pack_midx_entry *end = ctx->entries + ctx->entries_nr;
+	uint32_t nr_large_offset = ctx->num_large_offsets;
+
+	while (nr_large_offset) {
+		struct pack_midx_entry *obj;
+		uint64_t offset;
+
+		if (list >= end)
+			BUG("too many large-offset objects");
+
+		obj = list++;
+		offset = obj->offset;
+
+		if (!(offset >> 31))
+			continue;
+
+		hashwrite_be64(f, offset);
+
+		nr_large_offset--;
+	}
+
+	return 0;
+}
+
+static int write_midx_revindex(struct hashfile *f,
+			       void *data)
+{
+	struct write_midx_context *ctx = data;
+	uint32_t i;
+
+	for (i = 0; i < ctx->entries_nr; i++)
+		hashwrite_be32(f, ctx->pack_order[i]);
+
+	return 0;
+}
+
+struct midx_pack_order_data {
+	uint32_t nr;
+	uint32_t pack;
+	off_t offset;
+};
+
+static int midx_pack_order_cmp(const void *va, const void *vb)
+{
+	const struct midx_pack_order_data *a = va, *b = vb;
+	if (a->pack < b->pack)
+		return -1;
+	else if (a->pack > b->pack)
+		return 1;
+	else if (a->offset < b->offset)
+		return -1;
+	else if (a->offset > b->offset)
+		return 1;
+	else
+		return 0;
+}
+
+static uint32_t *midx_pack_order(struct write_midx_context *ctx)
+{
+	struct midx_pack_order_data *data;
+	uint32_t *pack_order;
+	uint32_t i;
+
+	trace2_region_enter("midx", "midx_pack_order", the_repository);
+
+	ALLOC_ARRAY(data, ctx->entries_nr);
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *e = &ctx->entries[i];
+		data[i].nr = i;
+		data[i].pack = ctx->pack_perm[e->pack_int_id];
+		if (!e->preferred)
+			data[i].pack |= (1U << 31);
+		data[i].offset = e->offset;
+	}
+
+	QSORT(data, ctx->entries_nr, midx_pack_order_cmp);
+
+	ALLOC_ARRAY(pack_order, ctx->entries_nr);
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *e = &ctx->entries[data[i].nr];
+		struct pack_info *pack = &ctx->info[ctx->pack_perm[e->pack_int_id]];
+		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
+			pack->bitmap_pos = i;
+		pack->bitmap_nr++;
+		pack_order[i] = data[i].nr;
+	}
+	for (i = 0; i < ctx->nr; i++) {
+		struct pack_info *pack = &ctx->info[ctx->pack_perm[i]];
+		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
+			pack->bitmap_pos = 0;
+	}
+	free(data);
+
+	trace2_region_leave("midx", "midx_pack_order", the_repository);
+
+	return pack_order;
+}
+
+static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
+				     struct write_midx_context *ctx)
+{
+	struct strbuf buf = STRBUF_INIT;
+	const char *tmp_file;
+
+	trace2_region_enter("midx", "write_midx_reverse_index", the_repository);
+
+	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
+
+	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
+					midx_hash, WRITE_REV);
+
+	if (finalize_object_file(tmp_file, buf.buf))
+		die(_("cannot store reverse index file"));
+
+	strbuf_release(&buf);
+
+	trace2_region_leave("midx", "write_midx_reverse_index", the_repository);
+}
+
+static void prepare_midx_packing_data(struct packing_data *pdata,
+				      struct write_midx_context *ctx)
+{
+	uint32_t i;
+
+	trace2_region_enter("midx", "prepare_midx_packing_data", the_repository);
+
+	memset(pdata, 0, sizeof(struct packing_data));
+	prepare_packing_data(the_repository, pdata);
+
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *from = &ctx->entries[ctx->pack_order[i]];
+		struct object_entry *to = packlist_alloc(pdata, &from->oid);
+
+		oe_set_in_pack(pdata, to,
+			       ctx->info[ctx->pack_perm[from->pack_int_id]].p);
+	}
+
+	trace2_region_leave("midx", "prepare_midx_packing_data", the_repository);
+}
+
+static int add_ref_to_pending(const char *refname,
+			      const struct object_id *oid,
+			      int flag, void *cb_data)
+{
+	struct rev_info *revs = (struct rev_info*)cb_data;
+	struct object_id peeled;
+	struct object *object;
+
+	if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
+		warning("symbolic ref is dangling: %s", refname);
+		return 0;
+	}
+
+	if (!peel_iterated_oid(oid, &peeled))
+		oid = &peeled;
+
+	object = parse_object_or_die(oid, refname);
+	if (object->type != OBJ_COMMIT)
+		return 0;
+
+	add_pending_object(revs, object, "");
+	if (bitmap_is_preferred_refname(revs->repo, refname))
+		object->flags |= NEEDS_BITMAP;
+	return 0;
+}
+
+struct bitmap_commit_cb {
+	struct commit **commits;
+	size_t commits_nr, commits_alloc;
+
+	struct write_midx_context *ctx;
+};
+
+static const struct object_id *bitmap_oid_access(size_t index,
+						 const void *_entries)
+{
+	const struct pack_midx_entry *entries = _entries;
+	return &entries[index].oid;
+}
+
+static void bitmap_show_commit(struct commit *commit, void *_data)
+{
+	struct bitmap_commit_cb *data = _data;
+	int pos = oid_pos(&commit->object.oid, data->ctx->entries,
+			  data->ctx->entries_nr,
+			  bitmap_oid_access);
+	if (pos < 0)
+		return;
+
+	ALLOC_GROW(data->commits, data->commits_nr + 1, data->commits_alloc);
+	data->commits[data->commits_nr++] = commit;
+}
+
+static int read_refs_snapshot(const char *refs_snapshot,
+			      struct rev_info *revs)
+{
+	struct strbuf buf = STRBUF_INIT;
+	struct object_id oid;
+	FILE *f = xfopen(refs_snapshot, "r");
+
+	while (strbuf_getline(&buf, f) != EOF) {
+		struct object *object;
+		int preferred = 0;
+		char *hex = buf.buf;
+		const char *end = NULL;
+
+		if (buf.len && *buf.buf == '+') {
+			preferred = 1;
+			hex = &buf.buf[1];
+		}
+
+		if (parse_oid_hex(hex, &oid, &end) < 0)
+			die(_("could not parse line: %s"), buf.buf);
+		if (*end)
+			die(_("malformed line: %s"), buf.buf);
+
+		object = parse_object_or_die(&oid, NULL);
+		if (preferred)
+			object->flags |= NEEDS_BITMAP;
+
+		add_pending_object(revs, object, "");
+	}
+
+	fclose(f);
+	strbuf_release(&buf);
+	return 0;
+}
+static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr_p,
+						    const char *refs_snapshot,
+						    struct write_midx_context *ctx)
+{
+	struct rev_info revs;
+	struct bitmap_commit_cb cb = {0};
+
+	trace2_region_enter("midx", "find_commits_for_midx_bitmap",
+			    the_repository);
+
+	cb.ctx = ctx;
+
+	repo_init_revisions(the_repository, &revs, NULL);
+	if (refs_snapshot) {
+		read_refs_snapshot(refs_snapshot, &revs);
+	} else {
+		setup_revisions(0, NULL, &revs, NULL);
+		for_each_ref(add_ref_to_pending, &revs);
+	}
+
+	/*
+	 * Skipping promisor objects here is intentional, since it only excludes
+	 * them from the list of reachable commits that we want to select from
+	 * when computing the selection of MIDX'd commits to receive bitmaps.
+	 *
+	 * Reachability bitmaps do require that their objects be closed under
+	 * reachability, but fetching any objects missing from promisors at this
+	 * point is too late. But, if one of those objects can be reached from
+	 * an another object that is included in the bitmap, then we will
+	 * complain later that we don't have reachability closure (and fail
+	 * appropriately).
+	 */
+	fetch_if_missing = 0;
+	revs.exclude_promisor_objects = 1;
+
+	if (prepare_revision_walk(&revs))
+		die(_("revision walk setup failed"));
+
+	traverse_commit_list(&revs, bitmap_show_commit, NULL, &cb);
+	if (indexed_commits_nr_p)
+		*indexed_commits_nr_p = cb.commits_nr;
+
+	release_revisions(&revs);
+
+	trace2_region_leave("midx", "find_commits_for_midx_bitmap",
+			    the_repository);
+
+	return cb.commits;
+}
+
+static int write_midx_bitmap(const char *midx_name,
+			     const unsigned char *midx_hash,
+			     struct packing_data *pdata,
+			     struct commit **commits,
+			     uint32_t commits_nr,
+			     uint32_t *pack_order,
+			     unsigned flags)
+{
+	int ret, i;
+	uint16_t options = 0;
+	struct pack_idx_entry **index;
+	char *bitmap_name = xstrfmt("%s-%s.bitmap", midx_name,
+					hash_to_hex(midx_hash));
+
+	trace2_region_enter("midx", "write_midx_bitmap", the_repository);
+
+	if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
+		options |= BITMAP_OPT_HASH_CACHE;
+
+	if (flags & MIDX_WRITE_BITMAP_LOOKUP_TABLE)
+		options |= BITMAP_OPT_LOOKUP_TABLE;
+
+	/*
+	 * Build the MIDX-order index based on pdata.objects (which is already
+	 * in MIDX order; c.f., 'midx_pack_order_cmp()' for the definition of
+	 * this order).
+	 */
+	ALLOC_ARRAY(index, pdata->nr_objects);
+	for (i = 0; i < pdata->nr_objects; i++)
+		index[i] = &pdata->objects[i].idx;
+
+	bitmap_writer_show_progress(flags & MIDX_PROGRESS);
+	bitmap_writer_build_type_index(pdata, index, pdata->nr_objects);
+
+	/*
+	 * bitmap_writer_finish expects objects in lex order, but pack_order
+	 * gives us exactly that. use it directly instead of re-sorting the
+	 * array.
+	 *
+	 * This changes the order of objects in 'index' between
+	 * bitmap_writer_build_type_index and bitmap_writer_finish.
+	 *
+	 * The same re-ordering takes place in the single-pack bitmap code via
+	 * write_idx_file(), which is called by finish_tmp_packfile(), which
+	 * happens between bitmap_writer_build_type_index() and
+	 * bitmap_writer_finish().
+	 */
+	for (i = 0; i < pdata->nr_objects; i++)
+		index[pack_order[i]] = &pdata->objects[i].idx;
+
+	bitmap_writer_select_commits(commits, commits_nr, -1);
+	ret = bitmap_writer_build(pdata);
+	if (ret < 0)
+		goto cleanup;
+
+	bitmap_writer_set_checksum(midx_hash);
+	bitmap_writer_finish(index, pdata->nr_objects, bitmap_name, options);
+
+cleanup:
+	free(index);
+	free(bitmap_name);
+
+	trace2_region_leave("midx", "write_midx_bitmap", the_repository);
+
+	return ret;
+}
+
+static struct multi_pack_index *lookup_multi_pack_index(struct repository *r,
+							const char *object_dir)
+{
+	struct multi_pack_index *result = NULL;
+	struct multi_pack_index *cur;
+	char *obj_dir_real = real_pathdup(object_dir, 1);
+	struct strbuf cur_path_real = STRBUF_INIT;
+
+	/* Ensure the given object_dir is local, or a known alternate. */
+	find_odb(r, obj_dir_real);
+
+	for (cur = get_multi_pack_index(r); cur; cur = cur->next) {
+		strbuf_realpath(&cur_path_real, cur->object_dir, 1);
+		if (!strcmp(obj_dir_real, cur_path_real.buf)) {
+			result = cur;
+			goto cleanup;
+		}
+	}
+
+cleanup:
+	free(obj_dir_real);
+	strbuf_release(&cur_path_real);
+	return result;
+}
+
+static int write_midx_internal(const char *object_dir,
+			       struct string_list *packs_to_include,
+			       struct string_list *packs_to_drop,
+			       const char *preferred_pack_name,
+			       const char *refs_snapshot,
+			       unsigned flags)
+{
+	struct strbuf midx_name = STRBUF_INIT;
+	unsigned char midx_hash[GIT_MAX_RAWSZ];
+	uint32_t i;
+	struct hashfile *f = NULL;
+	struct lock_file lk;
+	struct write_midx_context ctx = { 0 };
+	int bitmapped_packs_concat_len = 0;
+	int pack_name_concat_len = 0;
+	int dropped_packs = 0;
+	int result = 0;
+	struct chunkfile *cf;
+
+	trace2_region_enter("midx", "write_midx_internal", the_repository);
+
+	get_midx_filename(&midx_name, object_dir);
+	if (safe_create_leading_directories(midx_name.buf))
+		die_errno(_("unable to create leading directories of %s"),
+			  midx_name.buf);
+
+	if (!packs_to_include) {
+		/*
+		 * Only reference an existing MIDX when not filtering which
+		 * packs to include, since all packs and objects are copied
+		 * blindly from an existing MIDX if one is present.
+		 */
+		ctx.m = lookup_multi_pack_index(the_repository, object_dir);
+	}
+
+	if (ctx.m && !midx_checksum_valid(ctx.m)) {
+		warning(_("ignoring existing multi-pack-index; checksum mismatch"));
+		ctx.m = NULL;
+	}
+
+	ctx.nr = 0;
+	ctx.alloc = ctx.m ? ctx.m->num_packs : 16;
+	ctx.info = NULL;
+	ALLOC_ARRAY(ctx.info, ctx.alloc);
+
+	if (ctx.m) {
+		for (i = 0; i < ctx.m->num_packs; i++) {
+			ALLOC_GROW(ctx.info, ctx.nr + 1, ctx.alloc);
+
+			if (flags & MIDX_WRITE_REV_INDEX) {
+				/*
+				 * If generating a reverse index, need to have
+				 * packed_git's loaded to compare their
+				 * mtimes and object count.
+				 */
+				if (prepare_midx_pack(the_repository, ctx.m, i)) {
+					error(_("could not load pack"));
+					result = 1;
+					goto cleanup;
+				}
+
+				if (open_pack_index(ctx.m->packs[i]))
+					die(_("could not open index for %s"),
+					    ctx.m->packs[i]->pack_name);
+			}
+
+			fill_pack_info(&ctx.info[ctx.nr++], ctx.m->packs[i],
+				       ctx.m->pack_names[i], i);
+		}
+	}
+
+	ctx.pack_paths_checked = 0;
+	if (flags & MIDX_PROGRESS)
+		ctx.progress = start_delayed_progress(_("Adding packfiles to multi-pack-index"), 0);
+	else
+		ctx.progress = NULL;
+
+	ctx.to_include = packs_to_include;
+
+	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
+	stop_progress(&ctx.progress);
+
+	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
+	    !(packs_to_include || packs_to_drop)) {
+		struct bitmap_index *bitmap_git;
+		int bitmap_exists;
+		int want_bitmap = flags & MIDX_WRITE_BITMAP;
+
+		bitmap_git = prepare_midx_bitmap_git(ctx.m);
+		bitmap_exists = bitmap_git && bitmap_is_midx(bitmap_git);
+		free_bitmap_index(bitmap_git);
+
+		if (bitmap_exists || !want_bitmap) {
+			/*
+			 * The correct MIDX already exists, and so does a
+			 * corresponding bitmap (or one wasn't requested).
+			 */
+			if (!want_bitmap)
+				clear_midx_files_ext(object_dir, ".bitmap",
+						     NULL);
+			goto cleanup;
+		}
+	}
+
+	if (preferred_pack_name) {
+		ctx.preferred_pack_idx = -1;
+
+		for (i = 0; i < ctx.nr; i++) {
+			if (!cmp_idx_or_pack_name(preferred_pack_name,
+						  ctx.info[i].pack_name)) {
+				ctx.preferred_pack_idx = i;
+				break;
+			}
+		}
+
+		if (ctx.preferred_pack_idx == -1)
+			warning(_("unknown preferred pack: '%s'"),
+				preferred_pack_name);
+	} else if (ctx.nr &&
+		   (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP))) {
+		struct packed_git *oldest = ctx.info[ctx.preferred_pack_idx].p;
+		ctx.preferred_pack_idx = 0;
+
+		if (packs_to_drop && packs_to_drop->nr)
+			BUG("cannot write a MIDX bitmap during expiration");
+
+		/*
+		 * set a preferred pack when writing a bitmap to ensure that
+		 * the pack from which the first object is selected in pseudo
+		 * pack-order has all of its objects selected from that pack
+		 * (and not another pack containing a duplicate)
+		 */
+		for (i = 1; i < ctx.nr; i++) {
+			struct packed_git *p = ctx.info[i].p;
+
+			if (!oldest->num_objects || p->mtime < oldest->mtime) {
+				oldest = p;
+				ctx.preferred_pack_idx = i;
+			}
+		}
+
+		if (!oldest->num_objects) {
+			/*
+			 * If all packs are empty; unset the preferred index.
+			 * This is acceptable since there will be no duplicate
+			 * objects to resolve, so the preferred value doesn't
+			 * matter.
+			 */
+			ctx.preferred_pack_idx = -1;
+		}
+	} else {
+		/*
+		 * otherwise don't mark any pack as preferred to avoid
+		 * interfering with expiration logic below
+		 */
+		ctx.preferred_pack_idx = -1;
+	}
+
+	if (ctx.preferred_pack_idx > -1) {
+		struct packed_git *preferred = ctx.info[ctx.preferred_pack_idx].p;
+		if (!preferred->num_objects) {
+			error(_("cannot select preferred pack %s with no objects"),
+			      preferred->pack_name);
+			result = 1;
+			goto cleanup;
+		}
+	}
+
+	ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr,
+					 ctx.preferred_pack_idx);
+
+	ctx.large_offsets_needed = 0;
+	for (i = 0; i < ctx.entries_nr; i++) {
+		if (ctx.entries[i].offset > 0x7fffffff)
+			ctx.num_large_offsets++;
+		if (ctx.entries[i].offset > 0xffffffff)
+			ctx.large_offsets_needed = 1;
+	}
+
+	QSORT(ctx.info, ctx.nr, pack_info_compare);
+
+	if (packs_to_drop && packs_to_drop->nr) {
+		int drop_index = 0;
+		int missing_drops = 0;
+
+		for (i = 0; i < ctx.nr && drop_index < packs_to_drop->nr; i++) {
+			int cmp = strcmp(ctx.info[i].pack_name,
+					 packs_to_drop->items[drop_index].string);
+
+			if (!cmp) {
+				drop_index++;
+				ctx.info[i].expired = 1;
+			} else if (cmp > 0) {
+				error(_("did not see pack-file %s to drop"),
+				      packs_to_drop->items[drop_index].string);
+				drop_index++;
+				missing_drops++;
+				i--;
+			} else {
+				ctx.info[i].expired = 0;
+			}
+		}
+
+		if (missing_drops) {
+			result = 1;
+			goto cleanup;
+		}
+	}
+
+	/*
+	 * pack_perm stores a permutation between pack-int-ids from the
+	 * previous multi-pack-index to the new one we are writing:
+	 *
+	 * pack_perm[old_id] = new_id
+	 */
+	ALLOC_ARRAY(ctx.pack_perm, ctx.nr);
+	for (i = 0; i < ctx.nr; i++) {
+		if (ctx.info[i].expired) {
+			dropped_packs++;
+			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = PACK_EXPIRED;
+		} else {
+			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = i - dropped_packs;
+		}
+	}
+
+	for (i = 0; i < ctx.nr; i++) {
+		if (ctx.info[i].expired)
+			continue;
+		pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
+		bitmapped_packs_concat_len += 2 * sizeof(uint32_t);
+	}
+
+	/* Check that the preferred pack wasn't expired (if given). */
+	if (preferred_pack_name) {
+		struct pack_info *preferred = bsearch(preferred_pack_name,
+						      ctx.info, ctx.nr,
+						      sizeof(*ctx.info),
+						      idx_or_pack_name_cmp);
+		if (preferred) {
+			uint32_t perm = ctx.pack_perm[preferred->orig_pack_int_id];
+			if (perm == PACK_EXPIRED)
+				warning(_("preferred pack '%s' is expired"),
+					preferred_pack_name);
+		}
+	}
+
+	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
+		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
+					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
+
+	hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
+	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
+
+	if (ctx.nr - dropped_packs == 0) {
+		error(_("no pack files to index."));
+		result = 1;
+		goto cleanup;
+	}
+
+	if (!ctx.entries_nr) {
+		if (flags & MIDX_WRITE_BITMAP)
+			warning(_("refusing to write multi-pack .bitmap without any objects"));
+		flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
+	}
+
+	cf = init_chunkfile(f);
+
+	add_chunk(cf, MIDX_CHUNKID_PACKNAMES, pack_name_concat_len,
+		  write_midx_pack_names);
+	add_chunk(cf, MIDX_CHUNKID_OIDFANOUT, MIDX_CHUNK_FANOUT_SIZE,
+		  write_midx_oid_fanout);
+	add_chunk(cf, MIDX_CHUNKID_OIDLOOKUP,
+		  st_mult(ctx.entries_nr, the_hash_algo->rawsz),
+		  write_midx_oid_lookup);
+	add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
+		  st_mult(ctx.entries_nr, MIDX_CHUNK_OFFSET_WIDTH),
+		  write_midx_object_offsets);
+
+	if (ctx.large_offsets_needed)
+		add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
+			st_mult(ctx.num_large_offsets,
+				MIDX_CHUNK_LARGE_OFFSET_WIDTH),
+			write_midx_large_offsets);
+
+	if (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP)) {
+		ctx.pack_order = midx_pack_order(&ctx);
+		add_chunk(cf, MIDX_CHUNKID_REVINDEX,
+			  st_mult(ctx.entries_nr, sizeof(uint32_t)),
+			  write_midx_revindex);
+		add_chunk(cf, MIDX_CHUNKID_BITMAPPEDPACKS,
+			  bitmapped_packs_concat_len,
+			  write_midx_bitmapped_packs);
+	}
+
+	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
+	write_chunkfile(cf, &ctx);
+
+	finalize_hashfile(f, midx_hash, FSYNC_COMPONENT_PACK_METADATA,
+			  CSUM_FSYNC | CSUM_HASH_IN_STREAM);
+	free_chunkfile(cf);
+
+	if (flags & MIDX_WRITE_REV_INDEX &&
+	    git_env_bool("GIT_TEST_MIDX_WRITE_REV", 0))
+		write_midx_reverse_index(midx_name.buf, midx_hash, &ctx);
+
+	if (flags & MIDX_WRITE_BITMAP) {
+		struct packing_data pdata;
+		struct commit **commits;
+		uint32_t commits_nr;
+
+		if (!ctx.entries_nr)
+			BUG("cannot write a bitmap without any objects");
+
+		prepare_midx_packing_data(&pdata, &ctx);
+
+		commits = find_commits_for_midx_bitmap(&commits_nr, refs_snapshot, &ctx);
+
+		/*
+		 * The previous steps translated the information from
+		 * 'entries' into information suitable for constructing
+		 * bitmaps. We no longer need that array, so clear it to
+		 * reduce memory pressure.
+		 */
+		FREE_AND_NULL(ctx.entries);
+		ctx.entries_nr = 0;
+
+		if (write_midx_bitmap(midx_name.buf, midx_hash, &pdata,
+				      commits, commits_nr, ctx.pack_order,
+				      flags) < 0) {
+			error(_("could not write multi-pack bitmap"));
+			result = 1;
+			clear_packing_data(&pdata);
+			free(commits);
+			goto cleanup;
+		}
+
+		clear_packing_data(&pdata);
+		free(commits);
+	}
+	/*
+	 * NOTE: Do not use ctx.entries beyond this point, since it might
+	 * have been freed in the previous if block.
+	 */
+
+	if (ctx.m)
+		close_object_store(the_repository->objects);
+
+	if (commit_lock_file(&lk) < 0)
+		die_errno(_("could not write multi-pack-index"));
+
+	clear_midx_files_ext(object_dir, ".bitmap", midx_hash);
+	clear_midx_files_ext(object_dir, ".rev", midx_hash);
+
+cleanup:
+	for (i = 0; i < ctx.nr; i++) {
+		if (ctx.info[i].p) {
+			close_pack(ctx.info[i].p);
+			free(ctx.info[i].p);
+		}
+		free(ctx.info[i].pack_name);
+	}
+
+	free(ctx.info);
+	free(ctx.entries);
+	free(ctx.pack_perm);
+	free(ctx.pack_order);
+	strbuf_release(&midx_name);
+
+	trace2_region_leave("midx", "write_midx_internal", the_repository);
+
+	return result;
+}
+
+int write_midx_file(const char *object_dir,
+		    const char *preferred_pack_name,
+		    const char *refs_snapshot,
+		    unsigned flags)
+{
+	return write_midx_internal(object_dir, NULL, NULL, preferred_pack_name,
+				   refs_snapshot, flags);
+}
+
+int write_midx_file_only(const char *object_dir,
+			 struct string_list *packs_to_include,
+			 const char *preferred_pack_name,
+			 const char *refs_snapshot,
+			 unsigned flags)
+{
+	return write_midx_internal(object_dir, packs_to_include, NULL,
+				   preferred_pack_name, refs_snapshot, flags);
+}
+
+int expire_midx_packs(struct repository *r, const char *object_dir, unsigned flags)
+{
+	uint32_t i, *count, result = 0;
+	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
+	struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
+	struct progress *progress = NULL;
+
+	if (!m)
+		return 0;
+
+	CALLOC_ARRAY(count, m->num_packs);
+
+	if (flags & MIDX_PROGRESS)
+		progress = start_delayed_progress(_("Counting referenced objects"),
+					  m->num_objects);
+	for (i = 0; i < m->num_objects; i++) {
+		int pack_int_id = nth_midxed_pack_int_id(m, i);
+		count[pack_int_id]++;
+		display_progress(progress, i + 1);
+	}
+	stop_progress(&progress);
+
+	if (flags & MIDX_PROGRESS)
+		progress = start_delayed_progress(_("Finding and deleting unreferenced packfiles"),
+					  m->num_packs);
+	for (i = 0; i < m->num_packs; i++) {
+		char *pack_name;
+		display_progress(progress, i + 1);
+
+		if (count[i])
+			continue;
+
+		if (prepare_midx_pack(r, m, i))
+			continue;
+
+		if (m->packs[i]->pack_keep || m->packs[i]->is_cruft)
+			continue;
+
+		pack_name = xstrdup(m->packs[i]->pack_name);
+		close_pack(m->packs[i]);
+
+		string_list_insert(&packs_to_drop, m->pack_names[i]);
+		unlink_pack_path(pack_name, 0);
+		free(pack_name);
+	}
+	stop_progress(&progress);
+
+	free(count);
+
+	if (packs_to_drop.nr)
+		result = write_midx_internal(object_dir, NULL, &packs_to_drop, NULL, NULL, flags);
+
+	string_list_clear(&packs_to_drop, 0);
+
+	return result;
+}
+
+struct repack_info {
+	timestamp_t mtime;
+	uint32_t referenced_objects;
+	uint32_t pack_int_id;
+};
+
+static int compare_by_mtime(const void *a_, const void *b_)
+{
+	const struct repack_info *a, *b;
+
+	a = (const struct repack_info *)a_;
+	b = (const struct repack_info *)b_;
+
+	if (a->mtime < b->mtime)
+		return -1;
+	if (a->mtime > b->mtime)
+		return 1;
+	return 0;
+}
+
+static int fill_included_packs_all(struct repository *r,
+				   struct multi_pack_index *m,
+				   unsigned char *include_pack)
+{
+	uint32_t i, count = 0;
+	int pack_kept_objects = 0;
+
+	repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);
+
+	for (i = 0; i < m->num_packs; i++) {
+		if (prepare_midx_pack(r, m, i))
+			continue;
+		if (!pack_kept_objects && m->packs[i]->pack_keep)
+			continue;
+		if (m->packs[i]->is_cruft)
+			continue;
+
+		include_pack[i] = 1;
+		count++;
+	}
+
+	return count < 2;
+}
+
+static int fill_included_packs_batch(struct repository *r,
+				     struct multi_pack_index *m,
+				     unsigned char *include_pack,
+				     size_t batch_size)
+{
+	uint32_t i, packs_to_repack;
+	size_t total_size;
+	struct repack_info *pack_info;
+	int pack_kept_objects = 0;
+
+	CALLOC_ARRAY(pack_info, m->num_packs);
+
+	repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);
+
+	for (i = 0; i < m->num_packs; i++) {
+		pack_info[i].pack_int_id = i;
+
+		if (prepare_midx_pack(r, m, i))
+			continue;
+
+		pack_info[i].mtime = m->packs[i]->mtime;
+	}
+
+	for (i = 0; i < m->num_objects; i++) {
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+		pack_info[pack_int_id].referenced_objects++;
+	}
+
+	QSORT(pack_info, m->num_packs, compare_by_mtime);
+
+	total_size = 0;
+	packs_to_repack = 0;
+	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
+		int pack_int_id = pack_info[i].pack_int_id;
+		struct packed_git *p = m->packs[pack_int_id];
+		size_t expected_size;
+
+		if (!p)
+			continue;
+		if (!pack_kept_objects && p->pack_keep)
+			continue;
+		if (p->is_cruft)
+			continue;
+		if (open_pack_index(p) || !p->num_objects)
+			continue;
+
+		expected_size = st_mult(p->pack_size,
+					pack_info[i].referenced_objects);
+		expected_size /= p->num_objects;
+
+		if (expected_size >= batch_size)
+			continue;
+
+		packs_to_repack++;
+		total_size += expected_size;
+		include_pack[pack_int_id] = 1;
+	}
+
+	free(pack_info);
+
+	if (packs_to_repack < 2)
+		return 1;
+
+	return 0;
+}
+
+int midx_repack(struct repository *r, const char *object_dir, size_t batch_size, unsigned flags)
+{
+	int result = 0;
+	uint32_t i;
+	unsigned char *include_pack;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	FILE *cmd_in;
+	struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
+
+	/*
+	 * When updating the default for these configuration
+	 * variables in builtin/repack.c, these must be adjusted
+	 * to match.
+	 */
+	int delta_base_offset = 1;
+	int use_delta_islands = 0;
+
+	if (!m)
+		return 0;
+
+	CALLOC_ARRAY(include_pack, m->num_packs);
+
+	if (batch_size) {
+		if (fill_included_packs_batch(r, m, include_pack, batch_size))
+			goto cleanup;
+	} else if (fill_included_packs_all(r, m, include_pack))
+		goto cleanup;
+
+	repo_config_get_bool(r, "repack.usedeltabaseoffset", &delta_base_offset);
+	repo_config_get_bool(r, "repack.usedeltaislands", &use_delta_islands);
+
+	strvec_push(&cmd.args, "pack-objects");
+
+	strvec_pushf(&cmd.args, "%s/pack/pack", object_dir);
+
+	if (delta_base_offset)
+		strvec_push(&cmd.args, "--delta-base-offset");
+	if (use_delta_islands)
+		strvec_push(&cmd.args, "--delta-islands");
+
+	if (flags & MIDX_PROGRESS)
+		strvec_push(&cmd.args, "--progress");
+	else
+		strvec_push(&cmd.args, "-q");
+
+	cmd.git_cmd = 1;
+	cmd.in = cmd.out = -1;
+
+	if (start_command(&cmd)) {
+		error(_("could not start pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	cmd_in = xfdopen(cmd.in, "w");
+
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+
+		if (!include_pack[pack_int_id])
+			continue;
+
+		nth_midxed_object_oid(&oid, m, i);
+		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
+	}
+	fclose(cmd_in);
+
+	if (finish_command(&cmd)) {
+		error(_("could not finish pack-objects"));
+		result = 1;
+		goto cleanup;
+	}
+
+	result = write_midx_internal(object_dir, NULL, NULL, NULL, NULL, flags);
+
+cleanup:
+	free(include_pack);
+	return result;
+}
diff --git a/midx.c b/midx.c
index 41521e019c..ae3b49166c 100644
--- a/midx.c
+++ b/midx.c
@@ -1,52 +1,22 @@
 #include "git-compat-util.h"
-#include "abspath.h"
 #include "config.h"
-#include "csum-file.h"
 #include "dir.h"
-#include "gettext.h"
 #include "hex.h"
-#include "lockfile.h"
 #include "packfile.h"
 #include "object-file.h"
-#include "object-store-ll.h"
 #include "hash-lookup.h"
 #include "midx.h"
 #include "progress.h"
 #include "trace2.h"
-#include "run-command.h"
-#include "repository.h"
 #include "chunk-format.h"
-#include "pack.h"
 #include "pack-bitmap.h"
-#include "refs.h"
-#include "revision.h"
-#include "list-objects.h"
 #include "pack-revindex.h"
 
-#define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
-#define MIDX_VERSION 1
-#define MIDX_BYTE_FILE_VERSION 4
-#define MIDX_BYTE_HASH_VERSION 5
-#define MIDX_BYTE_NUM_CHUNKS 6
-#define MIDX_BYTE_NUM_PACKS 8
-#define MIDX_HEADER_SIZE 12
-#define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + the_hash_algo->rawsz)
-
-#define MIDX_CHUNK_ALIGNMENT 4
-#define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
-#define MIDX_CHUNKID_BITMAPPEDPACKS 0x42544d50 /* "BTMP" */
-#define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
-#define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
-#define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
-#define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
-#define MIDX_CHUNKID_REVINDEX 0x52494458 /* "RIDX" */
-#define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
-#define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
-#define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
-#define MIDX_CHUNK_BITMAPPED_PACKS_WIDTH (2 * sizeof(uint32_t))
-#define MIDX_LARGE_OFFSET_NEEDED 0x80000000
-
-#define PACK_EXPIRED UINT_MAX
+int midx_checksum_valid(struct multi_pack_index *m);
+void clear_midx_files_ext(const char *object_dir, const char *ext,
+			  unsigned char *keep_hash);
+int cmp_idx_or_pack_name(const char *idx_or_pack_name,
+			 const char *idx_name);
 
 const unsigned char *get_midx_checksum(struct multi_pack_index *m)
 {
@@ -115,6 +85,8 @@ static int midx_read_object_offsets(const unsigned char *chunk_start,
 	return 0;
 }
 
+#define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + the_hash_algo->rawsz)
+
 struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local)
 {
 	struct multi_pack_index *m = NULL;
@@ -294,6 +266,8 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t
 	return 0;
 }
 
+#define MIDX_CHUNK_BITMAPPED_PACKS_WIDTH (2 * sizeof(uint32_t))
+
 int nth_bitmapped_pack(struct repository *r, struct multi_pack_index *m,
 		       struct bitmapped_pack *bp, uint32_t pack_int_id)
 {
@@ -400,8 +374,8 @@ int fill_midx_entry(struct repository *r,
 }
 
 /* Match "foo.idx" against either "foo.pack" _or_ "foo.idx". */
-static int cmp_idx_or_pack_name(const char *idx_or_pack_name,
-				const char *idx_name)
+int cmp_idx_or_pack_name(const char *idx_or_pack_name,
+			 const char *idx_name)
 {
 	/* Skip past any initial matching prefix. */
 	while (*idx_name && *idx_name == *idx_or_pack_name) {
@@ -508,1262 +482,11 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 	return 0;
 }
 
-static size_t write_midx_header(struct hashfile *f,
-				unsigned char num_chunks,
-				uint32_t num_packs)
-{
-	hashwrite_be32(f, MIDX_SIGNATURE);
-	hashwrite_u8(f, MIDX_VERSION);
-	hashwrite_u8(f, oid_version(the_hash_algo));
-	hashwrite_u8(f, num_chunks);
-	hashwrite_u8(f, 0); /* unused */
-	hashwrite_be32(f, num_packs);
-
-	return MIDX_HEADER_SIZE;
-}
-
-#define BITMAP_POS_UNKNOWN (~((uint32_t)0))
-
-struct pack_info {
-	uint32_t orig_pack_int_id;
-	char *pack_name;
-	struct packed_git *p;
-
-	uint32_t bitmap_pos;
-	uint32_t bitmap_nr;
-
-	unsigned expired : 1;
-};
-
-static void fill_pack_info(struct pack_info *info,
-			   struct packed_git *p, const char *pack_name,
-			   uint32_t orig_pack_int_id)
-{
-	memset(info, 0, sizeof(struct pack_info));
-
-	info->orig_pack_int_id = orig_pack_int_id;
-	info->pack_name = xstrdup(pack_name);
-	info->p = p;
-	info->bitmap_pos = BITMAP_POS_UNKNOWN;
-}
-
-static int pack_info_compare(const void *_a, const void *_b)
-{
-	struct pack_info *a = (struct pack_info *)_a;
-	struct pack_info *b = (struct pack_info *)_b;
-	return strcmp(a->pack_name, b->pack_name);
-}
-
-static int idx_or_pack_name_cmp(const void *_va, const void *_vb)
-{
-	const char *pack_name = _va;
-	const struct pack_info *compar = _vb;
-
-	return cmp_idx_or_pack_name(pack_name, compar->pack_name);
-}
-
-struct write_midx_context {
-	struct pack_info *info;
-	size_t nr;
-	size_t alloc;
-	struct multi_pack_index *m;
-	struct progress *progress;
-	unsigned pack_paths_checked;
-
-	struct pack_midx_entry *entries;
-	size_t entries_nr;
-
-	uint32_t *pack_perm;
-	uint32_t *pack_order;
-	unsigned large_offsets_needed:1;
-	uint32_t num_large_offsets;
-
-	int preferred_pack_idx;
-
-	struct string_list *to_include;
-};
-
-static void add_pack_to_midx(const char *full_path, size_t full_path_len,
-			     const char *file_name, void *data)
-{
-	struct write_midx_context *ctx = data;
-	struct packed_git *p;
-
-	if (ends_with(file_name, ".idx")) {
-		display_progress(ctx->progress, ++ctx->pack_paths_checked);
-		/*
-		 * Note that at most one of ctx->m and ctx->to_include are set,
-		 * so we are testing midx_contains_pack() and
-		 * string_list_has_string() independently (guarded by the
-		 * appropriate NULL checks).
-		 *
-		 * We could support passing to_include while reusing an existing
-		 * MIDX, but don't currently since the reuse process drags
-		 * forward all packs from an existing MIDX (without checking
-		 * whether or not they appear in the to_include list).
-		 *
-		 * If we added support for that, these next two conditional
-		 * should be performed independently (likely checking
-		 * to_include before the existing MIDX).
-		 */
-		if (ctx->m && midx_contains_pack(ctx->m, file_name))
-			return;
-		else if (ctx->to_include &&
-			 !string_list_has_string(ctx->to_include, file_name))
-			return;
-
-		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);
-
-		p = add_packed_git(full_path, full_path_len, 0);
-		if (!p) {
-			warning(_("failed to add packfile '%s'"),
-				full_path);
-			return;
-		}
-
-		if (open_pack_index(p)) {
-			warning(_("failed to open pack-index '%s'"),
-				full_path);
-			close_pack(p);
-			free(p);
-			return;
-		}
-
-		fill_pack_info(&ctx->info[ctx->nr], p, file_name, ctx->nr);
-		ctx->nr++;
-	}
-}
-
-struct pack_midx_entry {
-	struct object_id oid;
-	uint32_t pack_int_id;
-	time_t pack_mtime;
-	uint64_t offset;
-	unsigned preferred : 1;
-};
-
-static int midx_oid_compare(const void *_a, const void *_b)
-{
-	const struct pack_midx_entry *a = (const struct pack_midx_entry *)_a;
-	const struct pack_midx_entry *b = (const struct pack_midx_entry *)_b;
-	int cmp = oidcmp(&a->oid, &b->oid);
-
-	if (cmp)
-		return cmp;
-
-	/* Sort objects in a preferred pack first when multiple copies exist. */
-	if (a->preferred > b->preferred)
-		return -1;
-	if (a->preferred < b->preferred)
-		return 1;
-
-	if (a->pack_mtime > b->pack_mtime)
-		return -1;
-	else if (a->pack_mtime < b->pack_mtime)
-		return 1;
-
-	return a->pack_int_id - b->pack_int_id;
-}
-
-static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
-				      struct pack_midx_entry *e,
-				      uint32_t pos)
-{
-	if (pos >= m->num_objects)
-		return 1;
-
-	nth_midxed_object_oid(&e->oid, m, pos);
-	e->pack_int_id = nth_midxed_pack_int_id(m, pos);
-	e->offset = nth_midxed_offset(m, pos);
-
-	/* consider objects in midx to be from "old" packs */
-	e->pack_mtime = 0;
-	return 0;
-}
-
-static void fill_pack_entry(uint32_t pack_int_id,
-			    struct packed_git *p,
-			    uint32_t cur_object,
-			    struct pack_midx_entry *entry,
-			    int preferred)
-{
-	if (nth_packed_object_id(&entry->oid, p, cur_object) < 0)
-		die(_("failed to locate object %d in packfile"), cur_object);
-
-	entry->pack_int_id = pack_int_id;
-	entry->pack_mtime = p->mtime;
-
-	entry->offset = nth_packed_object_offset(p, cur_object);
-	entry->preferred = !!preferred;
-}
-
-struct midx_fanout {
-	struct pack_midx_entry *entries;
-	size_t nr, alloc;
-};
-
-static void midx_fanout_grow(struct midx_fanout *fanout, size_t nr)
-{
-	if (nr < fanout->nr)
-		BUG("negative growth in midx_fanout_grow() (%"PRIuMAX" < %"PRIuMAX")",
-		    (uintmax_t)nr, (uintmax_t)fanout->nr);
-	ALLOC_GROW(fanout->entries, nr, fanout->alloc);
-}
-
-static void midx_fanout_sort(struct midx_fanout *fanout)
-{
-	QSORT(fanout->entries, fanout->nr, midx_oid_compare);
-}
-
-static void midx_fanout_add_midx_fanout(struct midx_fanout *fanout,
-					struct multi_pack_index *m,
-					uint32_t cur_fanout,
-					int preferred_pack)
-{
-	uint32_t start = 0, end;
-	uint32_t cur_object;
-
-	if (cur_fanout)
-		start = ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
-	end = ntohl(m->chunk_oid_fanout[cur_fanout]);
-
-	for (cur_object = start; cur_object < end; cur_object++) {
-		if ((preferred_pack > -1) &&
-		    (preferred_pack == nth_midxed_pack_int_id(m, cur_object))) {
-			/*
-			 * Objects from preferred packs are added
-			 * separately.
-			 */
-			continue;
-		}
-
-		midx_fanout_grow(fanout, fanout->nr + 1);
-		nth_midxed_pack_midx_entry(m,
-					   &fanout->entries[fanout->nr],
-					   cur_object);
-		fanout->entries[fanout->nr].preferred = 0;
-		fanout->nr++;
-	}
-}
-
-static void midx_fanout_add_pack_fanout(struct midx_fanout *fanout,
-					struct pack_info *info,
-					uint32_t cur_pack,
-					int preferred,
-					uint32_t cur_fanout)
-{
-	struct packed_git *pack = info[cur_pack].p;
-	uint32_t start = 0, end;
-	uint32_t cur_object;
-
-	if (cur_fanout)
-		start = get_pack_fanout(pack, cur_fanout - 1);
-	end = get_pack_fanout(pack, cur_fanout);
-
-	for (cur_object = start; cur_object < end; cur_object++) {
-		midx_fanout_grow(fanout, fanout->nr + 1);
-		fill_pack_entry(cur_pack,
-				info[cur_pack].p,
-				cur_object,
-				&fanout->entries[fanout->nr],
-				preferred);
-		fanout->nr++;
-	}
-}
-
-/*
- * It is possible to artificially get into a state where there are many
- * duplicate copies of objects. That can create high memory pressure if
- * we are to create a list of all objects before de-duplication. To reduce
- * this memory pressure without a significant performance drop, automatically
- * group objects by the first byte of their object id. Use the IDX fanout
- * tables to group the data, copy to a local array, then sort.
- *
- * Copy only the de-duplicated entries (selected by most-recent modified time
- * of a packfile containing the object).
- */
-static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
-						  struct pack_info *info,
-						  uint32_t nr_packs,
-						  size_t *nr_objects,
-						  int preferred_pack)
-{
-	uint32_t cur_fanout, cur_pack, cur_object;
-	size_t alloc_objects, total_objects = 0;
-	struct midx_fanout fanout = { 0 };
-	struct pack_midx_entry *deduplicated_entries = NULL;
-	uint32_t start_pack = m ? m->num_packs : 0;
-
-	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
-		total_objects = st_add(total_objects,
-				       info[cur_pack].p->num_objects);
-
-	/*
-	 * As we de-duplicate by fanout value, we expect the fanout
-	 * slices to be evenly distributed, with some noise. Hence,
-	 * allocate slightly more than one 256th.
-	 */
-	alloc_objects = fanout.alloc = total_objects > 3200 ? total_objects / 200 : 16;
-
-	ALLOC_ARRAY(fanout.entries, fanout.alloc);
-	ALLOC_ARRAY(deduplicated_entries, alloc_objects);
-	*nr_objects = 0;
-
-	for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
-		fanout.nr = 0;
-
-		if (m)
-			midx_fanout_add_midx_fanout(&fanout, m, cur_fanout,
-						    preferred_pack);
-
-		for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++) {
-			int preferred = cur_pack == preferred_pack;
-			midx_fanout_add_pack_fanout(&fanout,
-						    info, cur_pack,
-						    preferred, cur_fanout);
-		}
-
-		if (-1 < preferred_pack && preferred_pack < start_pack)
-			midx_fanout_add_pack_fanout(&fanout, info,
-						    preferred_pack, 1,
-						    cur_fanout);
-
-		midx_fanout_sort(&fanout);
-
-		/*
-		 * The batch is now sorted by OID and then mtime (descending).
-		 * Take only the first duplicate.
-		 */
-		for (cur_object = 0; cur_object < fanout.nr; cur_object++) {
-			if (cur_object && oideq(&fanout.entries[cur_object - 1].oid,
-						&fanout.entries[cur_object].oid))
-				continue;
-
-			ALLOC_GROW(deduplicated_entries, st_add(*nr_objects, 1),
-				   alloc_objects);
-			memcpy(&deduplicated_entries[*nr_objects],
-			       &fanout.entries[cur_object],
-			       sizeof(struct pack_midx_entry));
-			(*nr_objects)++;
-		}
-	}
-
-	free(fanout.entries);
-	return deduplicated_entries;
-}
-
-static int write_midx_pack_names(struct hashfile *f, void *data)
-{
-	struct write_midx_context *ctx = data;
-	uint32_t i;
-	unsigned char padding[MIDX_CHUNK_ALIGNMENT];
-	size_t written = 0;
-
-	for (i = 0; i < ctx->nr; i++) {
-		size_t writelen;
-
-		if (ctx->info[i].expired)
-			continue;
-
-		if (i && strcmp(ctx->info[i].pack_name, ctx->info[i - 1].pack_name) <= 0)
-			BUG("incorrect pack-file order: %s before %s",
-			    ctx->info[i - 1].pack_name,
-			    ctx->info[i].pack_name);
-
-		writelen = strlen(ctx->info[i].pack_name) + 1;
-		hashwrite(f, ctx->info[i].pack_name, writelen);
-		written += writelen;
-	}
-
-	/* add padding to be aligned */
-	i = MIDX_CHUNK_ALIGNMENT - (written % MIDX_CHUNK_ALIGNMENT);
-	if (i < MIDX_CHUNK_ALIGNMENT) {
-		memset(padding, 0, sizeof(padding));
-		hashwrite(f, padding, i);
-	}
-
-	return 0;
-}
-
-static int write_midx_bitmapped_packs(struct hashfile *f, void *data)
-{
-	struct write_midx_context *ctx = data;
-	size_t i;
-
-	for (i = 0; i < ctx->nr; i++) {
-		struct pack_info *pack = &ctx->info[i];
-		if (pack->expired)
-			continue;
-
-		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN && pack->bitmap_nr)
-			BUG("pack '%s' has no bitmap position, but has %d bitmapped object(s)",
-			    pack->pack_name, pack->bitmap_nr);
-
-		hashwrite_be32(f, pack->bitmap_pos);
-		hashwrite_be32(f, pack->bitmap_nr);
-	}
-	return 0;
-}
-
-static int write_midx_oid_fanout(struct hashfile *f,
-				 void *data)
-{
-	struct write_midx_context *ctx = data;
-	struct pack_midx_entry *list = ctx->entries;
-	struct pack_midx_entry *last = ctx->entries + ctx->entries_nr;
-	uint32_t count = 0;
-	uint32_t i;
-
-	/*
-	* Write the first-level table (the list is sorted,
-	* but we use a 256-entry lookup to be able to avoid
-	* having to do eight extra binary search iterations).
-	*/
-	for (i = 0; i < 256; i++) {
-		struct pack_midx_entry *next = list;
-
-		while (next < last && next->oid.hash[0] == i) {
-			count++;
-			next++;
-		}
-
-		hashwrite_be32(f, count);
-		list = next;
-	}
-
-	return 0;
-}
-
-static int write_midx_oid_lookup(struct hashfile *f,
-				 void *data)
-{
-	struct write_midx_context *ctx = data;
-	unsigned char hash_len = the_hash_algo->rawsz;
-	struct pack_midx_entry *list = ctx->entries;
-	uint32_t i;
-
-	for (i = 0; i < ctx->entries_nr; i++) {
-		struct pack_midx_entry *obj = list++;
-
-		if (i < ctx->entries_nr - 1) {
-			struct pack_midx_entry *next = list;
-			if (oidcmp(&obj->oid, &next->oid) >= 0)
-				BUG("OIDs not in order: %s >= %s",
-				    oid_to_hex(&obj->oid),
-				    oid_to_hex(&next->oid));
-		}
-
-		hashwrite(f, obj->oid.hash, (int)hash_len);
-	}
-
-	return 0;
-}
-
-static int write_midx_object_offsets(struct hashfile *f,
-				     void *data)
-{
-	struct write_midx_context *ctx = data;
-	struct pack_midx_entry *list = ctx->entries;
-	uint32_t i, nr_large_offset = 0;
-
-	for (i = 0; i < ctx->entries_nr; i++) {
-		struct pack_midx_entry *obj = list++;
-
-		if (ctx->pack_perm[obj->pack_int_id] == PACK_EXPIRED)
-			BUG("object %s is in an expired pack with int-id %d",
-			    oid_to_hex(&obj->oid),
-			    obj->pack_int_id);
-
-		hashwrite_be32(f, ctx->pack_perm[obj->pack_int_id]);
-
-		if (ctx->large_offsets_needed && obj->offset >> 31)
-			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
-		else if (!ctx->large_offsets_needed && obj->offset >> 32)
-			BUG("object %s requires a large offset (%"PRIx64") but the MIDX is not writing large offsets!",
-			    oid_to_hex(&obj->oid),
-			    obj->offset);
-		else
-			hashwrite_be32(f, (uint32_t)obj->offset);
-	}
-
-	return 0;
-}
-
-static int write_midx_large_offsets(struct hashfile *f,
-				    void *data)
-{
-	struct write_midx_context *ctx = data;
-	struct pack_midx_entry *list = ctx->entries;
-	struct pack_midx_entry *end = ctx->entries + ctx->entries_nr;
-	uint32_t nr_large_offset = ctx->num_large_offsets;
-
-	while (nr_large_offset) {
-		struct pack_midx_entry *obj;
-		uint64_t offset;
-
-		if (list >= end)
-			BUG("too many large-offset objects");
-
-		obj = list++;
-		offset = obj->offset;
-
-		if (!(offset >> 31))
-			continue;
-
-		hashwrite_be64(f, offset);
-
-		nr_large_offset--;
-	}
-
-	return 0;
-}
-
-static int write_midx_revindex(struct hashfile *f,
-			       void *data)
-{
-	struct write_midx_context *ctx = data;
-	uint32_t i;
-
-	for (i = 0; i < ctx->entries_nr; i++)
-		hashwrite_be32(f, ctx->pack_order[i]);
-
-	return 0;
-}
-
-struct midx_pack_order_data {
-	uint32_t nr;
-	uint32_t pack;
-	off_t offset;
-};
-
-static int midx_pack_order_cmp(const void *va, const void *vb)
-{
-	const struct midx_pack_order_data *a = va, *b = vb;
-	if (a->pack < b->pack)
-		return -1;
-	else if (a->pack > b->pack)
-		return 1;
-	else if (a->offset < b->offset)
-		return -1;
-	else if (a->offset > b->offset)
-		return 1;
-	else
-		return 0;
-}
-
-static uint32_t *midx_pack_order(struct write_midx_context *ctx)
-{
-	struct midx_pack_order_data *data;
-	uint32_t *pack_order;
-	uint32_t i;
-
-	trace2_region_enter("midx", "midx_pack_order", the_repository);
-
-	ALLOC_ARRAY(data, ctx->entries_nr);
-	for (i = 0; i < ctx->entries_nr; i++) {
-		struct pack_midx_entry *e = &ctx->entries[i];
-		data[i].nr = i;
-		data[i].pack = ctx->pack_perm[e->pack_int_id];
-		if (!e->preferred)
-			data[i].pack |= (1U << 31);
-		data[i].offset = e->offset;
-	}
-
-	QSORT(data, ctx->entries_nr, midx_pack_order_cmp);
-
-	ALLOC_ARRAY(pack_order, ctx->entries_nr);
-	for (i = 0; i < ctx->entries_nr; i++) {
-		struct pack_midx_entry *e = &ctx->entries[data[i].nr];
-		struct pack_info *pack = &ctx->info[ctx->pack_perm[e->pack_int_id]];
-		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
-			pack->bitmap_pos = i;
-		pack->bitmap_nr++;
-		pack_order[i] = data[i].nr;
-	}
-	for (i = 0; i < ctx->nr; i++) {
-		struct pack_info *pack = &ctx->info[ctx->pack_perm[i]];
-		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
-			pack->bitmap_pos = 0;
-	}
-	free(data);
-
-	trace2_region_leave("midx", "midx_pack_order", the_repository);
-
-	return pack_order;
-}
-
-static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
-				     struct write_midx_context *ctx)
-{
-	struct strbuf buf = STRBUF_INIT;
-	const char *tmp_file;
-
-	trace2_region_enter("midx", "write_midx_reverse_index", the_repository);
-
-	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
-
-	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
-					midx_hash, WRITE_REV);
-
-	if (finalize_object_file(tmp_file, buf.buf))
-		die(_("cannot store reverse index file"));
-
-	strbuf_release(&buf);
-
-	trace2_region_leave("midx", "write_midx_reverse_index", the_repository);
-}
-
-static void clear_midx_files_ext(const char *object_dir, const char *ext,
-				 unsigned char *keep_hash);
-
-static int midx_checksum_valid(struct multi_pack_index *m)
+int midx_checksum_valid(struct multi_pack_index *m)
 {
 	return hashfile_checksum_valid(m->data, m->data_len);
 }
 
-static void prepare_midx_packing_data(struct packing_data *pdata,
-				      struct write_midx_context *ctx)
-{
-	uint32_t i;
-
-	trace2_region_enter("midx", "prepare_midx_packing_data", the_repository);
-
-	memset(pdata, 0, sizeof(struct packing_data));
-	prepare_packing_data(the_repository, pdata);
-
-	for (i = 0; i < ctx->entries_nr; i++) {
-		struct pack_midx_entry *from = &ctx->entries[ctx->pack_order[i]];
-		struct object_entry *to = packlist_alloc(pdata, &from->oid);
-
-		oe_set_in_pack(pdata, to,
-			       ctx->info[ctx->pack_perm[from->pack_int_id]].p);
-	}
-
-	trace2_region_leave("midx", "prepare_midx_packing_data", the_repository);
-}
-
-static int add_ref_to_pending(const char *refname,
-			      const struct object_id *oid,
-			      int flag, void *cb_data)
-{
-	struct rev_info *revs = (struct rev_info*)cb_data;
-	struct object_id peeled;
-	struct object *object;
-
-	if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
-		warning("symbolic ref is dangling: %s", refname);
-		return 0;
-	}
-
-	if (!peel_iterated_oid(oid, &peeled))
-		oid = &peeled;
-
-	object = parse_object_or_die(oid, refname);
-	if (object->type != OBJ_COMMIT)
-		return 0;
-
-	add_pending_object(revs, object, "");
-	if (bitmap_is_preferred_refname(revs->repo, refname))
-		object->flags |= NEEDS_BITMAP;
-	return 0;
-}
-
-struct bitmap_commit_cb {
-	struct commit **commits;
-	size_t commits_nr, commits_alloc;
-
-	struct write_midx_context *ctx;
-};
-
-static const struct object_id *bitmap_oid_access(size_t index,
-						 const void *_entries)
-{
-	const struct pack_midx_entry *entries = _entries;
-	return &entries[index].oid;
-}
-
-static void bitmap_show_commit(struct commit *commit, void *_data)
-{
-	struct bitmap_commit_cb *data = _data;
-	int pos = oid_pos(&commit->object.oid, data->ctx->entries,
-			  data->ctx->entries_nr,
-			  bitmap_oid_access);
-	if (pos < 0)
-		return;
-
-	ALLOC_GROW(data->commits, data->commits_nr + 1, data->commits_alloc);
-	data->commits[data->commits_nr++] = commit;
-}
-
-static int read_refs_snapshot(const char *refs_snapshot,
-			      struct rev_info *revs)
-{
-	struct strbuf buf = STRBUF_INIT;
-	struct object_id oid;
-	FILE *f = xfopen(refs_snapshot, "r");
-
-	while (strbuf_getline(&buf, f) != EOF) {
-		struct object *object;
-		int preferred = 0;
-		char *hex = buf.buf;
-		const char *end = NULL;
-
-		if (buf.len && *buf.buf == '+') {
-			preferred = 1;
-			hex = &buf.buf[1];
-		}
-
-		if (parse_oid_hex(hex, &oid, &end) < 0)
-			die(_("could not parse line: %s"), buf.buf);
-		if (*end)
-			die(_("malformed line: %s"), buf.buf);
-
-		object = parse_object_or_die(&oid, NULL);
-		if (preferred)
-			object->flags |= NEEDS_BITMAP;
-
-		add_pending_object(revs, object, "");
-	}
-
-	fclose(f);
-	strbuf_release(&buf);
-	return 0;
-}
-
-static struct commit **find_commits_for_midx_bitmap(uint32_t *indexed_commits_nr_p,
-						    const char *refs_snapshot,
-						    struct write_midx_context *ctx)
-{
-	struct rev_info revs;
-	struct bitmap_commit_cb cb = {0};
-
-	trace2_region_enter("midx", "find_commits_for_midx_bitmap",
-			    the_repository);
-
-	cb.ctx = ctx;
-
-	repo_init_revisions(the_repository, &revs, NULL);
-	if (refs_snapshot) {
-		read_refs_snapshot(refs_snapshot, &revs);
-	} else {
-		setup_revisions(0, NULL, &revs, NULL);
-		for_each_ref(add_ref_to_pending, &revs);
-	}
-
-	/*
-	 * Skipping promisor objects here is intentional, since it only excludes
-	 * them from the list of reachable commits that we want to select from
-	 * when computing the selection of MIDX'd commits to receive bitmaps.
-	 *
-	 * Reachability bitmaps do require that their objects be closed under
-	 * reachability, but fetching any objects missing from promisors at this
-	 * point is too late. But, if one of those objects can be reached from
-	 * an another object that is included in the bitmap, then we will
-	 * complain later that we don't have reachability closure (and fail
-	 * appropriately).
-	 */
-	fetch_if_missing = 0;
-	revs.exclude_promisor_objects = 1;
-
-	if (prepare_revision_walk(&revs))
-		die(_("revision walk setup failed"));
-
-	traverse_commit_list(&revs, bitmap_show_commit, NULL, &cb);
-	if (indexed_commits_nr_p)
-		*indexed_commits_nr_p = cb.commits_nr;
-
-	release_revisions(&revs);
-
-	trace2_region_leave("midx", "find_commits_for_midx_bitmap",
-			    the_repository);
-
-	return cb.commits;
-}
-
-static int write_midx_bitmap(const char *midx_name,
-			     const unsigned char *midx_hash,
-			     struct packing_data *pdata,
-			     struct commit **commits,
-			     uint32_t commits_nr,
-			     uint32_t *pack_order,
-			     unsigned flags)
-{
-	int ret, i;
-	uint16_t options = 0;
-	struct pack_idx_entry **index;
-	char *bitmap_name = xstrfmt("%s-%s.bitmap", midx_name,
-					hash_to_hex(midx_hash));
-
-	trace2_region_enter("midx", "write_midx_bitmap", the_repository);
-
-	if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
-		options |= BITMAP_OPT_HASH_CACHE;
-
-	if (flags & MIDX_WRITE_BITMAP_LOOKUP_TABLE)
-		options |= BITMAP_OPT_LOOKUP_TABLE;
-
-	/*
-	 * Build the MIDX-order index based on pdata.objects (which is already
-	 * in MIDX order; c.f., 'midx_pack_order_cmp()' for the definition of
-	 * this order).
-	 */
-	ALLOC_ARRAY(index, pdata->nr_objects);
-	for (i = 0; i < pdata->nr_objects; i++)
-		index[i] = &pdata->objects[i].idx;
-
-	bitmap_writer_show_progress(flags & MIDX_PROGRESS);
-	bitmap_writer_build_type_index(pdata, index, pdata->nr_objects);
-
-	/*
-	 * bitmap_writer_finish expects objects in lex order, but pack_order
-	 * gives us exactly that. use it directly instead of re-sorting the
-	 * array.
-	 *
-	 * This changes the order of objects in 'index' between
-	 * bitmap_writer_build_type_index and bitmap_writer_finish.
-	 *
-	 * The same re-ordering takes place in the single-pack bitmap code via
-	 * write_idx_file(), which is called by finish_tmp_packfile(), which
-	 * happens between bitmap_writer_build_type_index() and
-	 * bitmap_writer_finish().
-	 */
-	for (i = 0; i < pdata->nr_objects; i++)
-		index[pack_order[i]] = &pdata->objects[i].idx;
-
-	bitmap_writer_select_commits(commits, commits_nr, -1);
-	ret = bitmap_writer_build(pdata);
-	if (ret < 0)
-		goto cleanup;
-
-	bitmap_writer_set_checksum(midx_hash);
-	bitmap_writer_finish(index, pdata->nr_objects, bitmap_name, options);
-
-cleanup:
-	free(index);
-	free(bitmap_name);
-
-	trace2_region_leave("midx", "write_midx_bitmap", the_repository);
-
-	return ret;
-}
-
-static struct multi_pack_index *lookup_multi_pack_index(struct repository *r,
-							const char *object_dir)
-{
-	struct multi_pack_index *result = NULL;
-	struct multi_pack_index *cur;
-	char *obj_dir_real = real_pathdup(object_dir, 1);
-	struct strbuf cur_path_real = STRBUF_INIT;
-
-	/* Ensure the given object_dir is local, or a known alternate. */
-	find_odb(r, obj_dir_real);
-
-	for (cur = get_multi_pack_index(r); cur; cur = cur->next) {
-		strbuf_realpath(&cur_path_real, cur->object_dir, 1);
-		if (!strcmp(obj_dir_real, cur_path_real.buf)) {
-			result = cur;
-			goto cleanup;
-		}
-	}
-
-cleanup:
-	free(obj_dir_real);
-	strbuf_release(&cur_path_real);
-	return result;
-}
-
-static int write_midx_internal(const char *object_dir,
-			       struct string_list *packs_to_include,
-			       struct string_list *packs_to_drop,
-			       const char *preferred_pack_name,
-			       const char *refs_snapshot,
-			       unsigned flags)
-{
-	struct strbuf midx_name = STRBUF_INIT;
-	unsigned char midx_hash[GIT_MAX_RAWSZ];
-	uint32_t i;
-	struct hashfile *f = NULL;
-	struct lock_file lk;
-	struct write_midx_context ctx = { 0 };
-	int bitmapped_packs_concat_len = 0;
-	int pack_name_concat_len = 0;
-	int dropped_packs = 0;
-	int result = 0;
-	struct chunkfile *cf;
-
-	trace2_region_enter("midx", "write_midx_internal", the_repository);
-
-	get_midx_filename(&midx_name, object_dir);
-	if (safe_create_leading_directories(midx_name.buf))
-		die_errno(_("unable to create leading directories of %s"),
-			  midx_name.buf);
-
-	if (!packs_to_include) {
-		/*
-		 * Only reference an existing MIDX when not filtering which
-		 * packs to include, since all packs and objects are copied
-		 * blindly from an existing MIDX if one is present.
-		 */
-		ctx.m = lookup_multi_pack_index(the_repository, object_dir);
-	}
-
-	if (ctx.m && !midx_checksum_valid(ctx.m)) {
-		warning(_("ignoring existing multi-pack-index; checksum mismatch"));
-		ctx.m = NULL;
-	}
-
-	ctx.nr = 0;
-	ctx.alloc = ctx.m ? ctx.m->num_packs : 16;
-	ctx.info = NULL;
-	ALLOC_ARRAY(ctx.info, ctx.alloc);
-
-	if (ctx.m) {
-		for (i = 0; i < ctx.m->num_packs; i++) {
-			ALLOC_GROW(ctx.info, ctx.nr + 1, ctx.alloc);
-
-			if (flags & MIDX_WRITE_REV_INDEX) {
-				/*
-				 * If generating a reverse index, need to have
-				 * packed_git's loaded to compare their
-				 * mtimes and object count.
-				 */
-				if (prepare_midx_pack(the_repository, ctx.m, i)) {
-					error(_("could not load pack"));
-					result = 1;
-					goto cleanup;
-				}
-
-				if (open_pack_index(ctx.m->packs[i]))
-					die(_("could not open index for %s"),
-					    ctx.m->packs[i]->pack_name);
-			}
-
-			fill_pack_info(&ctx.info[ctx.nr++], ctx.m->packs[i],
-				       ctx.m->pack_names[i], i);
-		}
-	}
-
-	ctx.pack_paths_checked = 0;
-	if (flags & MIDX_PROGRESS)
-		ctx.progress = start_delayed_progress(_("Adding packfiles to multi-pack-index"), 0);
-	else
-		ctx.progress = NULL;
-
-	ctx.to_include = packs_to_include;
-
-	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &ctx);
-	stop_progress(&ctx.progress);
-
-	if ((ctx.m && ctx.nr == ctx.m->num_packs) &&
-	    !(packs_to_include || packs_to_drop)) {
-		struct bitmap_index *bitmap_git;
-		int bitmap_exists;
-		int want_bitmap = flags & MIDX_WRITE_BITMAP;
-
-		bitmap_git = prepare_midx_bitmap_git(ctx.m);
-		bitmap_exists = bitmap_git && bitmap_is_midx(bitmap_git);
-		free_bitmap_index(bitmap_git);
-
-		if (bitmap_exists || !want_bitmap) {
-			/*
-			 * The correct MIDX already exists, and so does a
-			 * corresponding bitmap (or one wasn't requested).
-			 */
-			if (!want_bitmap)
-				clear_midx_files_ext(object_dir, ".bitmap",
-						     NULL);
-			goto cleanup;
-		}
-	}
-
-	if (preferred_pack_name) {
-		ctx.preferred_pack_idx = -1;
-
-		for (i = 0; i < ctx.nr; i++) {
-			if (!cmp_idx_or_pack_name(preferred_pack_name,
-						  ctx.info[i].pack_name)) {
-				ctx.preferred_pack_idx = i;
-				break;
-			}
-		}
-
-		if (ctx.preferred_pack_idx == -1)
-			warning(_("unknown preferred pack: '%s'"),
-				preferred_pack_name);
-	} else if (ctx.nr &&
-		   (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP))) {
-		struct packed_git *oldest = ctx.info[ctx.preferred_pack_idx].p;
-		ctx.preferred_pack_idx = 0;
-
-		if (packs_to_drop && packs_to_drop->nr)
-			BUG("cannot write a MIDX bitmap during expiration");
-
-		/*
-		 * set a preferred pack when writing a bitmap to ensure that
-		 * the pack from which the first object is selected in pseudo
-		 * pack-order has all of its objects selected from that pack
-		 * (and not another pack containing a duplicate)
-		 */
-		for (i = 1; i < ctx.nr; i++) {
-			struct packed_git *p = ctx.info[i].p;
-
-			if (!oldest->num_objects || p->mtime < oldest->mtime) {
-				oldest = p;
-				ctx.preferred_pack_idx = i;
-			}
-		}
-
-		if (!oldest->num_objects) {
-			/*
-			 * If all packs are empty; unset the preferred index.
-			 * This is acceptable since there will be no duplicate
-			 * objects to resolve, so the preferred value doesn't
-			 * matter.
-			 */
-			ctx.preferred_pack_idx = -1;
-		}
-	} else {
-		/*
-		 * otherwise don't mark any pack as preferred to avoid
-		 * interfering with expiration logic below
-		 */
-		ctx.preferred_pack_idx = -1;
-	}
-
-	if (ctx.preferred_pack_idx > -1) {
-		struct packed_git *preferred = ctx.info[ctx.preferred_pack_idx].p;
-		if (!preferred->num_objects) {
-			error(_("cannot select preferred pack %s with no objects"),
-			      preferred->pack_name);
-			result = 1;
-			goto cleanup;
-		}
-	}
-
-	ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr,
-					 ctx.preferred_pack_idx);
-
-	ctx.large_offsets_needed = 0;
-	for (i = 0; i < ctx.entries_nr; i++) {
-		if (ctx.entries[i].offset > 0x7fffffff)
-			ctx.num_large_offsets++;
-		if (ctx.entries[i].offset > 0xffffffff)
-			ctx.large_offsets_needed = 1;
-	}
-
-	QSORT(ctx.info, ctx.nr, pack_info_compare);
-
-	if (packs_to_drop && packs_to_drop->nr) {
-		int drop_index = 0;
-		int missing_drops = 0;
-
-		for (i = 0; i < ctx.nr && drop_index < packs_to_drop->nr; i++) {
-			int cmp = strcmp(ctx.info[i].pack_name,
-					 packs_to_drop->items[drop_index].string);
-
-			if (!cmp) {
-				drop_index++;
-				ctx.info[i].expired = 1;
-			} else if (cmp > 0) {
-				error(_("did not see pack-file %s to drop"),
-				      packs_to_drop->items[drop_index].string);
-				drop_index++;
-				missing_drops++;
-				i--;
-			} else {
-				ctx.info[i].expired = 0;
-			}
-		}
-
-		if (missing_drops) {
-			result = 1;
-			goto cleanup;
-		}
-	}
-
-	/*
-	 * pack_perm stores a permutation between pack-int-ids from the
-	 * previous multi-pack-index to the new one we are writing:
-	 *
-	 * pack_perm[old_id] = new_id
-	 */
-	ALLOC_ARRAY(ctx.pack_perm, ctx.nr);
-	for (i = 0; i < ctx.nr; i++) {
-		if (ctx.info[i].expired) {
-			dropped_packs++;
-			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = PACK_EXPIRED;
-		} else {
-			ctx.pack_perm[ctx.info[i].orig_pack_int_id] = i - dropped_packs;
-		}
-	}
-
-	for (i = 0; i < ctx.nr; i++) {
-		if (ctx.info[i].expired)
-			continue;
-		pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
-		bitmapped_packs_concat_len += 2 * sizeof(uint32_t);
-	}
-
-	/* Check that the preferred pack wasn't expired (if given). */
-	if (preferred_pack_name) {
-		struct pack_info *preferred = bsearch(preferred_pack_name,
-						      ctx.info, ctx.nr,
-						      sizeof(*ctx.info),
-						      idx_or_pack_name_cmp);
-		if (preferred) {
-			uint32_t perm = ctx.pack_perm[preferred->orig_pack_int_id];
-			if (perm == PACK_EXPIRED)
-				warning(_("preferred pack '%s' is expired"),
-					preferred_pack_name);
-		}
-	}
-
-	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
-		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
-					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
-
-	hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
-	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
-
-	if (ctx.nr - dropped_packs == 0) {
-		error(_("no pack files to index."));
-		result = 1;
-		goto cleanup;
-	}
-
-	if (!ctx.entries_nr) {
-		if (flags & MIDX_WRITE_BITMAP)
-			warning(_("refusing to write multi-pack .bitmap without any objects"));
-		flags &= ~(MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP);
-	}
-
-	cf = init_chunkfile(f);
-
-	add_chunk(cf, MIDX_CHUNKID_PACKNAMES, pack_name_concat_len,
-		  write_midx_pack_names);
-	add_chunk(cf, MIDX_CHUNKID_OIDFANOUT, MIDX_CHUNK_FANOUT_SIZE,
-		  write_midx_oid_fanout);
-	add_chunk(cf, MIDX_CHUNKID_OIDLOOKUP,
-		  st_mult(ctx.entries_nr, the_hash_algo->rawsz),
-		  write_midx_oid_lookup);
-	add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
-		  st_mult(ctx.entries_nr, MIDX_CHUNK_OFFSET_WIDTH),
-		  write_midx_object_offsets);
-
-	if (ctx.large_offsets_needed)
-		add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
-			st_mult(ctx.num_large_offsets,
-				MIDX_CHUNK_LARGE_OFFSET_WIDTH),
-			write_midx_large_offsets);
-
-	if (flags & (MIDX_WRITE_REV_INDEX | MIDX_WRITE_BITMAP)) {
-		ctx.pack_order = midx_pack_order(&ctx);
-		add_chunk(cf, MIDX_CHUNKID_REVINDEX,
-			  st_mult(ctx.entries_nr, sizeof(uint32_t)),
-			  write_midx_revindex);
-		add_chunk(cf, MIDX_CHUNKID_BITMAPPEDPACKS,
-			  bitmapped_packs_concat_len,
-			  write_midx_bitmapped_packs);
-	}
-
-	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
-	write_chunkfile(cf, &ctx);
-
-	finalize_hashfile(f, midx_hash, FSYNC_COMPONENT_PACK_METADATA,
-			  CSUM_FSYNC | CSUM_HASH_IN_STREAM);
-	free_chunkfile(cf);
-
-	if (flags & MIDX_WRITE_REV_INDEX &&
-	    git_env_bool("GIT_TEST_MIDX_WRITE_REV", 0))
-		write_midx_reverse_index(midx_name.buf, midx_hash, &ctx);
-
-	if (flags & MIDX_WRITE_BITMAP) {
-		struct packing_data pdata;
-		struct commit **commits;
-		uint32_t commits_nr;
-
-		if (!ctx.entries_nr)
-			BUG("cannot write a bitmap without any objects");
-
-		prepare_midx_packing_data(&pdata, &ctx);
-
-		commits = find_commits_for_midx_bitmap(&commits_nr, refs_snapshot, &ctx);
-
-		/*
-		 * The previous steps translated the information from
-		 * 'entries' into information suitable for constructing
-		 * bitmaps. We no longer need that array, so clear it to
-		 * reduce memory pressure.
-		 */
-		FREE_AND_NULL(ctx.entries);
-		ctx.entries_nr = 0;
-
-		if (write_midx_bitmap(midx_name.buf, midx_hash, &pdata,
-				      commits, commits_nr, ctx.pack_order,
-				      flags) < 0) {
-			error(_("could not write multi-pack bitmap"));
-			result = 1;
-			clear_packing_data(&pdata);
-			free(commits);
-			goto cleanup;
-		}
-
-		clear_packing_data(&pdata);
-		free(commits);
-	}
-	/*
-	 * NOTE: Do not use ctx.entries beyond this point, since it might
-	 * have been freed in the previous if block.
-	 */
-
-	if (ctx.m)
-		close_object_store(the_repository->objects);
-
-	if (commit_lock_file(&lk) < 0)
-		die_errno(_("could not write multi-pack-index"));
-
-	clear_midx_files_ext(object_dir, ".bitmap", midx_hash);
-	clear_midx_files_ext(object_dir, ".rev", midx_hash);
-
-cleanup:
-	for (i = 0; i < ctx.nr; i++) {
-		if (ctx.info[i].p) {
-			close_pack(ctx.info[i].p);
-			free(ctx.info[i].p);
-		}
-		free(ctx.info[i].pack_name);
-	}
-
-	free(ctx.info);
-	free(ctx.entries);
-	free(ctx.pack_perm);
-	free(ctx.pack_order);
-	strbuf_release(&midx_name);
-
-	trace2_region_leave("midx", "write_midx_internal", the_repository);
-
-	return result;
-}
-
-int write_midx_file(const char *object_dir,
-		    const char *preferred_pack_name,
-		    const char *refs_snapshot,
-		    unsigned flags)
-{
-	return write_midx_internal(object_dir, NULL, NULL, preferred_pack_name,
-				   refs_snapshot, flags);
-}
-
-int write_midx_file_only(const char *object_dir,
-			 struct string_list *packs_to_include,
-			 const char *preferred_pack_name,
-			 const char *refs_snapshot,
-			 unsigned flags)
-{
-	return write_midx_internal(object_dir, packs_to_include, NULL,
-				   preferred_pack_name, refs_snapshot, flags);
-}
-
 struct clear_midx_data {
 	char *keep;
 	const char *ext;
@@ -1784,8 +507,8 @@ static void clear_midx_file_ext(const char *full_path, size_t full_path_len UNUS
 		die_errno(_("failed to remove %s"), full_path);
 }
 
-static void clear_midx_files_ext(const char *object_dir, const char *ext,
-				 unsigned char *keep_hash)
+void clear_midx_files_ext(const char *object_dir, const char *ext,
+			  unsigned char *keep_hash)
 {
 	struct clear_midx_data data;
 	memset(&data, 0, sizeof(struct clear_midx_data));
@@ -1988,251 +711,3 @@ cleanup:
 
 	return verify_midx_error;
 }
-
-int expire_midx_packs(struct repository *r, const char *object_dir, unsigned flags)
-{
-	uint32_t i, *count, result = 0;
-	struct string_list packs_to_drop = STRING_LIST_INIT_DUP;
-	struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
-	struct progress *progress = NULL;
-
-	if (!m)
-		return 0;
-
-	CALLOC_ARRAY(count, m->num_packs);
-
-	if (flags & MIDX_PROGRESS)
-		progress = start_delayed_progress(_("Counting referenced objects"),
-					  m->num_objects);
-	for (i = 0; i < m->num_objects; i++) {
-		int pack_int_id = nth_midxed_pack_int_id(m, i);
-		count[pack_int_id]++;
-		display_progress(progress, i + 1);
-	}
-	stop_progress(&progress);
-
-	if (flags & MIDX_PROGRESS)
-		progress = start_delayed_progress(_("Finding and deleting unreferenced packfiles"),
-					  m->num_packs);
-	for (i = 0; i < m->num_packs; i++) {
-		char *pack_name;
-		display_progress(progress, i + 1);
-
-		if (count[i])
-			continue;
-
-		if (prepare_midx_pack(r, m, i))
-			continue;
-
-		if (m->packs[i]->pack_keep || m->packs[i]->is_cruft)
-			continue;
-
-		pack_name = xstrdup(m->packs[i]->pack_name);
-		close_pack(m->packs[i]);
-
-		string_list_insert(&packs_to_drop, m->pack_names[i]);
-		unlink_pack_path(pack_name, 0);
-		free(pack_name);
-	}
-	stop_progress(&progress);
-
-	free(count);
-
-	if (packs_to_drop.nr)
-		result = write_midx_internal(object_dir, NULL, &packs_to_drop, NULL, NULL, flags);
-
-	string_list_clear(&packs_to_drop, 0);
-
-	return result;
-}
-
-struct repack_info {
-	timestamp_t mtime;
-	uint32_t referenced_objects;
-	uint32_t pack_int_id;
-};
-
-static int compare_by_mtime(const void *a_, const void *b_)
-{
-	const struct repack_info *a, *b;
-
-	a = (const struct repack_info *)a_;
-	b = (const struct repack_info *)b_;
-
-	if (a->mtime < b->mtime)
-		return -1;
-	if (a->mtime > b->mtime)
-		return 1;
-	return 0;
-}
-
-static int fill_included_packs_all(struct repository *r,
-				   struct multi_pack_index *m,
-				   unsigned char *include_pack)
-{
-	uint32_t i, count = 0;
-	int pack_kept_objects = 0;
-
-	repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);
-
-	for (i = 0; i < m->num_packs; i++) {
-		if (prepare_midx_pack(r, m, i))
-			continue;
-		if (!pack_kept_objects && m->packs[i]->pack_keep)
-			continue;
-		if (m->packs[i]->is_cruft)
-			continue;
-
-		include_pack[i] = 1;
-		count++;
-	}
-
-	return count < 2;
-}
-
-static int fill_included_packs_batch(struct repository *r,
-				     struct multi_pack_index *m,
-				     unsigned char *include_pack,
-				     size_t batch_size)
-{
-	uint32_t i, packs_to_repack;
-	size_t total_size;
-	struct repack_info *pack_info;
-	int pack_kept_objects = 0;
-
-	CALLOC_ARRAY(pack_info, m->num_packs);
-
-	repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);
-
-	for (i = 0; i < m->num_packs; i++) {
-		pack_info[i].pack_int_id = i;
-
-		if (prepare_midx_pack(r, m, i))
-			continue;
-
-		pack_info[i].mtime = m->packs[i]->mtime;
-	}
-
-	for (i = 0; i < m->num_objects; i++) {
-		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
-		pack_info[pack_int_id].referenced_objects++;
-	}
-
-	QSORT(pack_info, m->num_packs, compare_by_mtime);
-
-	total_size = 0;
-	packs_to_repack = 0;
-	for (i = 0; total_size < batch_size && i < m->num_packs; i++) {
-		int pack_int_id = pack_info[i].pack_int_id;
-		struct packed_git *p = m->packs[pack_int_id];
-		size_t expected_size;
-
-		if (!p)
-			continue;
-		if (!pack_kept_objects && p->pack_keep)
-			continue;
-		if (p->is_cruft)
-			continue;
-		if (open_pack_index(p) || !p->num_objects)
-			continue;
-
-		expected_size = st_mult(p->pack_size,
-					pack_info[i].referenced_objects);
-		expected_size /= p->num_objects;
-
-		if (expected_size >= batch_size)
-			continue;
-
-		packs_to_repack++;
-		total_size += expected_size;
-		include_pack[pack_int_id] = 1;
-	}
-
-	free(pack_info);
-
-	if (packs_to_repack < 2)
-		return 1;
-
-	return 0;
-}
-
-int midx_repack(struct repository *r, const char *object_dir, size_t batch_size, unsigned flags)
-{
-	int result = 0;
-	uint32_t i;
-	unsigned char *include_pack;
-	struct child_process cmd = CHILD_PROCESS_INIT;
-	FILE *cmd_in;
-	struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
-
-	/*
-	 * When updating the default for these configuration
-	 * variables in builtin/repack.c, these must be adjusted
-	 * to match.
-	 */
-	int delta_base_offset = 1;
-	int use_delta_islands = 0;
-
-	if (!m)
-		return 0;
-
-	CALLOC_ARRAY(include_pack, m->num_packs);
-
-	if (batch_size) {
-		if (fill_included_packs_batch(r, m, include_pack, batch_size))
-			goto cleanup;
-	} else if (fill_included_packs_all(r, m, include_pack))
-		goto cleanup;
-
-	repo_config_get_bool(r, "repack.usedeltabaseoffset", &delta_base_offset);
-	repo_config_get_bool(r, "repack.usedeltaislands", &use_delta_islands);
-
-	strvec_push(&cmd.args, "pack-objects");
-
-	strvec_pushf(&cmd.args, "%s/pack/pack", object_dir);
-
-	if (delta_base_offset)
-		strvec_push(&cmd.args, "--delta-base-offset");
-	if (use_delta_islands)
-		strvec_push(&cmd.args, "--delta-islands");
-
-	if (flags & MIDX_PROGRESS)
-		strvec_push(&cmd.args, "--progress");
-	else
-		strvec_push(&cmd.args, "-q");
-
-	cmd.git_cmd = 1;
-	cmd.in = cmd.out = -1;
-
-	if (start_command(&cmd)) {
-		error(_("could not start pack-objects"));
-		result = 1;
-		goto cleanup;
-	}
-
-	cmd_in = xfdopen(cmd.in, "w");
-
-	for (i = 0; i < m->num_objects; i++) {
-		struct object_id oid;
-		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
-
-		if (!include_pack[pack_int_id])
-			continue;
-
-		nth_midxed_object_oid(&oid, m, i);
-		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
-	}
-	fclose(cmd_in);
-
-	if (finish_command(&cmd)) {
-		error(_("could not finish pack-objects"));
-		result = 1;
-		goto cleanup;
-	}
-
-	result = write_midx_internal(object_dir, NULL, NULL, NULL, NULL, flags);
-
-cleanup:
-	free(include_pack);
-	return result;
-}
diff --git a/midx.h b/midx.h
index b374a7afaf..dc477dff44 100644
--- a/midx.h
+++ b/midx.h
@@ -8,6 +8,25 @@ struct pack_entry;
 struct repository;
 struct bitmapped_pack;
 
+#define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
+#define MIDX_VERSION 1
+#define MIDX_BYTE_FILE_VERSION 4
+#define MIDX_BYTE_HASH_VERSION 5
+#define MIDX_BYTE_NUM_CHUNKS 6
+#define MIDX_BYTE_NUM_PACKS 8
+#define MIDX_HEADER_SIZE 12
+
+#define MIDX_CHUNK_ALIGNMENT 4
+#define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
+#define MIDX_CHUNKID_BITMAPPEDPACKS 0x42544d50 /* "BTMP" */
+#define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
+#define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
+#define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
+#define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
+#define MIDX_CHUNKID_REVINDEX 0x52494458 /* "RIDX" */
+#define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
+#define MIDX_LARGE_OFFSET_NEEDED 0x80000000
+
 #define GIT_TEST_MULTI_PACK_INDEX "GIT_TEST_MULTI_PACK_INDEX"
 #define GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP \
 	"GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP"