Adding upstream version 1:2.39.2.upstream/1%2.39.2upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2090 insertions, 0 deletions

diff --git a/midx.c b/midx.c
new file mode 100644
index 0000000..7cfad04
--- /dev/null
+++ b/midx.c
@@ -0,0 +1,2090 @@
+#include "cache.h"
+#include "config.h"
+#include "csum-file.h"
+#include "dir.h"
+#include "lockfile.h"
+#include "packfile.h"
+#include "object-store.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"
+
+#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_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_LARGE_OFFSET_NEEDED 0x80000000
+
+#define PACK_EXPIRED UINT_MAX
+
+const unsigned char *get_midx_checksum(struct multi_pack_index *m)
+{
+	return m->data + m->data_len - the_hash_algo->rawsz;
+}
+
+void get_midx_filename(struct strbuf *out, const char *object_dir)
+{
+	strbuf_addf(out, "%s/pack/multi-pack-index", object_dir);
+}
+
+void get_midx_rev_filename(struct strbuf *out, struct multi_pack_index *m)
+{
+	get_midx_filename(out, m->object_dir);
+	strbuf_addf(out, "-%s.rev", hash_to_hex(get_midx_checksum(m)));
+}
+
+static int midx_read_oid_fanout(const unsigned char *chunk_start,
+				size_t chunk_size, void *data)
+{
+	struct multi_pack_index *m = data;
+	m->chunk_oid_fanout = (uint32_t *)chunk_start;
+
+	if (chunk_size != 4 * 256) {
+		error(_("multi-pack-index OID fanout is of the wrong size"));
+		return 1;
+	}
+	return 0;
+}
+
+struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local)
+{
+	struct multi_pack_index *m = NULL;
+	int fd;
+	struct stat st;
+	size_t midx_size;
+	void *midx_map = NULL;
+	uint32_t hash_version;
+	struct strbuf midx_name = STRBUF_INIT;
+	uint32_t i;
+	const char *cur_pack_name;
+	struct chunkfile *cf = NULL;
+
+	get_midx_filename(&midx_name, object_dir);
+
+	fd = git_open(midx_name.buf);
+
+	if (fd < 0)
+		goto cleanup_fail;
+	if (fstat(fd, &st)) {
+		error_errno(_("failed to read %s"), midx_name.buf);
+		goto cleanup_fail;
+	}
+
+	midx_size = xsize_t(st.st_size);
+
+	if (midx_size < MIDX_MIN_SIZE) {
+		error(_("multi-pack-index file %s is too small"), midx_name.buf);
+		goto cleanup_fail;
+	}
+
+	strbuf_release(&midx_name);
+
+	midx_map = xmmap(NULL, midx_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	close(fd);
+
+	FLEX_ALLOC_STR(m, object_dir, object_dir);
+	m->data = midx_map;
+	m->data_len = midx_size;
+	m->local = local;
+
+	m->signature = get_be32(m->data);
+	if (m->signature != MIDX_SIGNATURE)
+		die(_("multi-pack-index signature 0x%08x does not match signature 0x%08x"),
+		      m->signature, MIDX_SIGNATURE);
+
+	m->version = m->data[MIDX_BYTE_FILE_VERSION];
+	if (m->version != MIDX_VERSION)
+		die(_("multi-pack-index version %d not recognized"),
+		      m->version);
+
+	hash_version = m->data[MIDX_BYTE_HASH_VERSION];
+	if (hash_version != oid_version(the_hash_algo)) {
+		error(_("multi-pack-index hash version %u does not match version %u"),
+		      hash_version, oid_version(the_hash_algo));
+		goto cleanup_fail;
+	}
+	m->hash_len = the_hash_algo->rawsz;
+
+	m->num_chunks = m->data[MIDX_BYTE_NUM_CHUNKS];
+
+	m->num_packs = get_be32(m->data + MIDX_BYTE_NUM_PACKS);
+
+	cf = init_chunkfile(NULL);
+
+	if (read_table_of_contents(cf, m->data, midx_size,
+				   MIDX_HEADER_SIZE, m->num_chunks))
+		goto cleanup_fail;
+
+	if (pair_chunk(cf, MIDX_CHUNKID_PACKNAMES, &m->chunk_pack_names) == CHUNK_NOT_FOUND)
+		die(_("multi-pack-index missing required pack-name chunk"));
+	if (read_chunk(cf, MIDX_CHUNKID_OIDFANOUT, midx_read_oid_fanout, m) == CHUNK_NOT_FOUND)
+		die(_("multi-pack-index missing required OID fanout chunk"));
+	if (pair_chunk(cf, MIDX_CHUNKID_OIDLOOKUP, &m->chunk_oid_lookup) == CHUNK_NOT_FOUND)
+		die(_("multi-pack-index missing required OID lookup chunk"));
+	if (pair_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS, &m->chunk_object_offsets) == CHUNK_NOT_FOUND)
+		die(_("multi-pack-index missing required object offsets chunk"));
+
+	pair_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets);
+
+	if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
+		pair_chunk(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex);
+
+	m->num_objects = ntohl(m->chunk_oid_fanout[255]);
+
+	CALLOC_ARRAY(m->pack_names, m->num_packs);
+	CALLOC_ARRAY(m->packs, m->num_packs);
+
+	cur_pack_name = (const char *)m->chunk_pack_names;
+	for (i = 0; i < m->num_packs; i++) {
+		m->pack_names[i] = cur_pack_name;
+
+		cur_pack_name += strlen(cur_pack_name) + 1;
+
+		if (i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0)
+			die(_("multi-pack-index pack names out of order: '%s' before '%s'"),
+			      m->pack_names[i - 1],
+			      m->pack_names[i]);
+	}
+
+	trace2_data_intmax("midx", the_repository, "load/num_packs", m->num_packs);
+	trace2_data_intmax("midx", the_repository, "load/num_objects", m->num_objects);
+
+	free_chunkfile(cf);
+	return m;
+
+cleanup_fail:
+	free(m);
+	strbuf_release(&midx_name);
+	free_chunkfile(cf);
+	if (midx_map)
+		munmap(midx_map, midx_size);
+	if (0 <= fd)
+		close(fd);
+	return NULL;
+}
+
+void close_midx(struct multi_pack_index *m)
+{
+	uint32_t i;
+
+	if (!m)
+		return;
+
+	close_midx(m->next);
+
+	munmap((unsigned char *)m->data, m->data_len);
+
+	for (i = 0; i < m->num_packs; i++) {
+		if (m->packs[i])
+			m->packs[i]->multi_pack_index = 0;
+	}
+	FREE_AND_NULL(m->packs);
+	FREE_AND_NULL(m->pack_names);
+	free(m);
+}
+
+int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id)
+{
+	struct strbuf pack_name = STRBUF_INIT;
+	struct packed_git *p;
+
+	if (pack_int_id >= m->num_packs)
+		die(_("bad pack-int-id: %u (%u total packs)"),
+		    pack_int_id, m->num_packs);
+
+	if (m->packs[pack_int_id])
+		return 0;
+
+	strbuf_addf(&pack_name, "%s/pack/%s", m->object_dir,
+		    m->pack_names[pack_int_id]);
+
+	p = add_packed_git(pack_name.buf, pack_name.len, m->local);
+	strbuf_release(&pack_name);
+
+	if (!p)
+		return 1;
+
+	p->multi_pack_index = 1;
+	m->packs[pack_int_id] = p;
+	install_packed_git(r, p);
+	list_add_tail(&p->mru, &r->objects->packed_git_mru);
+
+	return 0;
+}
+
+int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result)
+{
+	return bsearch_hash(oid->hash, m->chunk_oid_fanout, m->chunk_oid_lookup,
+			    the_hash_algo->rawsz, result);
+}
+
+struct object_id *nth_midxed_object_oid(struct object_id *oid,
+					struct multi_pack_index *m,
+					uint32_t n)
+{
+	if (n >= m->num_objects)
+		return NULL;
+
+	oidread(oid, m->chunk_oid_lookup + m->hash_len * n);
+	return oid;
+}
+
+off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
+{
+	const unsigned char *offset_data;
+	uint32_t offset32;
+
+	offset_data = m->chunk_object_offsets + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH;
+	offset32 = get_be32(offset_data + sizeof(uint32_t));
+
+	if (m->chunk_large_offsets && offset32 & MIDX_LARGE_OFFSET_NEEDED) {
+		if (sizeof(off_t) < sizeof(uint64_t))
+			die(_("multi-pack-index stores a 64-bit offset, but off_t is too small"));
+
+		offset32 ^= MIDX_LARGE_OFFSET_NEEDED;
+		return get_be64(m->chunk_large_offsets + sizeof(uint64_t) * offset32);
+	}
+
+	return offset32;
+}
+
+uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
+{
+	return get_be32(m->chunk_object_offsets +
+			(off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
+}
+
+int fill_midx_entry(struct repository *r,
+		    const struct object_id *oid,
+		    struct pack_entry *e,
+		    struct multi_pack_index *m)
+{
+	uint32_t pos;
+	uint32_t pack_int_id;
+	struct packed_git *p;
+
+	if (!bsearch_midx(oid, m, &pos))
+		return 0;
+
+	if (pos >= m->num_objects)
+		return 0;
+
+	pack_int_id = nth_midxed_pack_int_id(m, pos);
+
+	if (prepare_midx_pack(r, m, pack_int_id))
+		return 0;
+	p = m->packs[pack_int_id];
+
+	/*
+	* We are about to tell the caller where they can locate the
+	* requested object.  We better make sure the packfile is
+	* still here and can be accessed before supplying that
+	* answer, as it may have been deleted since the MIDX was
+	* loaded!
+	*/
+	if (!is_pack_valid(p))
+		return 0;
+
+	if (oidset_size(&p->bad_objects) &&
+	    oidset_contains(&p->bad_objects, oid))
+		return 0;
+
+	e->offset = nth_midxed_offset(m, pos);
+	e->p = p;
+
+	return 1;
+}
+
+/* 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)
+{
+	/* Skip past any initial matching prefix. */
+	while (*idx_name && *idx_name == *idx_or_pack_name) {
+		idx_name++;
+		idx_or_pack_name++;
+	}
+
+	/*
+	 * If we didn't match completely, we may have matched "pack-1234." and
+	 * be left with "idx" and "pack" respectively, which is also OK. We do
+	 * not have to check for "idx" and "idx", because that would have been
+	 * a complete match (and in that case these strcmps will be false, but
+	 * we'll correctly return 0 from the final strcmp() below.
+	 *
+	 * Technically this matches "fooidx" and "foopack", but we'd never have
+	 * such names in the first place.
+	 */
+	if (!strcmp(idx_name, "idx") && !strcmp(idx_or_pack_name, "pack"))
+		return 0;
+
+	/*
+	 * This not only checks for a complete match, but also orders based on
+	 * the first non-identical character, which means our ordering will
+	 * match a raw strcmp(). That makes it OK to use this to binary search
+	 * a naively-sorted list.
+	 */
+	return strcmp(idx_or_pack_name, idx_name);
+}
+
+int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
+{
+	uint32_t first = 0, last = m->num_packs;
+
+	while (first < last) {
+		uint32_t mid = first + (last - first) / 2;
+		const char *current;
+		int cmp;
+
+		current = m->pack_names[mid];
+		cmp = cmp_idx_or_pack_name(idx_or_pack_name, current);
+		if (!cmp)
+			return 1;
+		if (cmp > 0) {
+			first = mid + 1;
+			continue;
+		}
+		last = mid;
+	}
+
+	return 0;
+}
+
+int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
+{
+	struct multi_pack_index *m;
+	struct multi_pack_index *m_search;
+
+	prepare_repo_settings(r);
+	if (!r->settings.core_multi_pack_index)
+		return 0;
+
+	for (m_search = r->objects->multi_pack_index; m_search; m_search = m_search->next)
+		if (!strcmp(object_dir, m_search->object_dir))
+			return 1;
+
+	m = load_multi_pack_index(object_dir, local);
+
+	if (m) {
+		struct multi_pack_index *mp = r->objects->multi_pack_index;
+		if (mp) {
+			m->next = mp->next;
+			mp->next = m;
+		} else
+			r->objects->multi_pack_index = m;
+		return 1;
+	}
+
+	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;
+}
+
+struct pack_info {
+	uint32_t orig_pack_int_id;
+	char *pack_name;
+	struct packed_git *p;
+	unsigned expired : 1;
+};
+
+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;
+	uint32_t nr;
+	uint32_t alloc;
+	struct multi_pack_index *m;
+	struct progress *progress;
+	unsigned pack_paths_checked;
+
+	struct pack_midx_entry *entries;
+	uint32_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;
+
+	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);
+
+		ctx->info[ctx->nr].p = add_packed_git(full_path,
+						      full_path_len,
+						      0);
+
+		if (!ctx->info[ctx->nr].p) {
+			warning(_("failed to add packfile '%s'"),
+				full_path);
+			return;
+		}
+
+		if (open_pack_index(ctx->info[ctx->nr].p)) {
+			warning(_("failed to open pack-index '%s'"),
+				full_path);
+			close_pack(ctx->info[ctx->nr].p);
+			FREE_AND_NULL(ctx->info[ctx->nr].p);
+			return;
+		}
+
+		ctx->info[ctx->nr].pack_name = xstrdup(file_name);
+		ctx->info[ctx->nr].orig_pack_int_id = ctx->nr;
+		ctx->info[ctx->nr].expired = 0;
+		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;
+	uint32_t nr;
+	uint32_t alloc;
+};
+
+static void midx_fanout_grow(struct midx_fanout *fanout, uint32_t 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,
+						  uint32_t *nr_objects,
+						  int preferred_pack)
+{
+	uint32_t cur_fanout, cur_pack, cur_object;
+	uint32_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 += 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, *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_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++)
+		pack_order[i] = data[i].nr;
+	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)
+{
+	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 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);
+
+			ctx.info[ctx.nr].orig_pack_int_id = i;
+			ctx.info[ctx.nr].pack_name = xstrdup(ctx.m->pack_names[i]);
+			ctx.info[ctx.nr].p = ctx.m->packs[i];
+			ctx.info[ctx.nr].expired = 0;
+
+			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);
+				ctx.info[ctx.nr].p = ctx.m->packs[i];
+			}
+
+			ctx.nr++;
+		}
+	}
+
+	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) {
+		int found = 0;
+		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;
+				found = 1;
+				break;
+			}
+		}
+
+		if (!found)
+			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)
+			pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
+	}
+
+	/* 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,
+		  (size_t)ctx.entries_nr * the_hash_algo->rawsz,
+		  write_midx_oid_lookup);
+	add_chunk(cf, MIDX_CHUNKID_OBJECTOFFSETS,
+		  (size_t)ctx.entries_nr * MIDX_CHUNK_OFFSET_WIDTH,
+		  write_midx_object_offsets);
+
+	if (ctx.large_offsets_needed)
+		add_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS,
+			(size_t)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,
+			  ctx.entries_nr * sizeof(uint32_t),
+			  write_midx_revindex);
+	}
+
+	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;
+			goto cleanup;
+		}
+	}
+	/*
+	 * 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;
+};
+
+static void clear_midx_file_ext(const char *full_path, size_t full_path_len,
+				const char *file_name, void *_data)
+{
+	struct clear_midx_data *data = _data;
+
+	if (!(starts_with(file_name, "multi-pack-index-") &&
+	      ends_with(file_name, data->ext)))
+		return;
+	if (data->keep && !strcmp(data->keep, file_name))
+		return;
+
+	if (unlink(full_path))
+		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)
+{
+	struct clear_midx_data data;
+	memset(&data, 0, sizeof(struct clear_midx_data));
+
+	if (keep_hash)
+		data.keep = xstrfmt("multi-pack-index-%s%s",
+				    hash_to_hex(keep_hash), ext);
+	data.ext = ext;
+
+	for_each_file_in_pack_dir(object_dir,
+				  clear_midx_file_ext,
+				  &data);
+
+	free(data.keep);
+}
+
+void clear_midx_file(struct repository *r)
+{
+	struct strbuf midx = STRBUF_INIT;
+
+	get_midx_filename(&midx, r->objects->odb->path);
+
+	if (r->objects && r->objects->multi_pack_index) {
+		close_midx(r->objects->multi_pack_index);
+		r->objects->multi_pack_index = NULL;
+	}
+
+	if (remove_path(midx.buf))
+		die(_("failed to clear multi-pack-index at %s"), midx.buf);
+
+	clear_midx_files_ext(r->objects->odb->path, ".bitmap", NULL);
+	clear_midx_files_ext(r->objects->odb->path, ".rev", NULL);
+
+	strbuf_release(&midx);
+}
+
+static int verify_midx_error;
+
+__attribute__((format (printf, 1, 2)))
+static void midx_report(const char *fmt, ...)
+{
+	va_list ap;
+	verify_midx_error = 1;
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	fprintf(stderr, "\n");
+	va_end(ap);
+}
+
+struct pair_pos_vs_id
+{
+	uint32_t pos;
+	uint32_t pack_int_id;
+};
+
+static int compare_pair_pos_vs_id(const void *_a, const void *_b)
+{
+	struct pair_pos_vs_id *a = (struct pair_pos_vs_id *)_a;
+	struct pair_pos_vs_id *b = (struct pair_pos_vs_id *)_b;
+
+	return b->pack_int_id - a->pack_int_id;
+}
+
+/*
+ * Limit calls to display_progress() for performance reasons.
+ * The interval here was arbitrarily chosen.
+ */
+#define SPARSE_PROGRESS_INTERVAL (1 << 12)
+#define midx_display_sparse_progress(progress, n) \
+	do { \
+		uint64_t _n = (n); \
+		if ((_n & (SPARSE_PROGRESS_INTERVAL - 1)) == 0) \
+			display_progress(progress, _n); \
+	} while (0)
+
+int verify_midx_file(struct repository *r, const char *object_dir, unsigned flags)
+{
+	struct pair_pos_vs_id *pairs = NULL;
+	uint32_t i;
+	struct progress *progress = NULL;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+	verify_midx_error = 0;
+
+	if (!m) {
+		int result = 0;
+		struct stat sb;
+		struct strbuf filename = STRBUF_INIT;
+
+		get_midx_filename(&filename, object_dir);
+
+		if (!stat(filename.buf, &sb)) {
+			error(_("multi-pack-index file exists, but failed to parse"));
+			result = 1;
+		}
+		strbuf_release(&filename);
+		return result;
+	}
+
+	if (!midx_checksum_valid(m))
+		midx_report(_("incorrect checksum"));
+
+	if (flags & MIDX_PROGRESS)
+		progress = start_delayed_progress(_("Looking for referenced packfiles"),
+					  m->num_packs);
+	for (i = 0; i < m->num_packs; i++) {
+		if (prepare_midx_pack(r, m, i))
+			midx_report("failed to load pack in position %d", i);
+
+		display_progress(progress, i + 1);
+	}
+	stop_progress(&progress);
+
+	for (i = 0; i < 255; i++) {
+		uint32_t oid_fanout1 = ntohl(m->chunk_oid_fanout[i]);
+		uint32_t oid_fanout2 = ntohl(m->chunk_oid_fanout[i + 1]);
+
+		if (oid_fanout1 > oid_fanout2)
+			midx_report(_("oid fanout out of order: fanout[%d] = %"PRIx32" > %"PRIx32" = fanout[%d]"),
+				    i, oid_fanout1, oid_fanout2, i + 1);
+	}
+
+	if (m->num_objects == 0) {
+		midx_report(_("the midx contains no oid"));
+		/*
+		 * Remaining tests assume that we have objects, so we can
+		 * return here.
+		 */
+		goto cleanup;
+	}
+
+	if (flags & MIDX_PROGRESS)
+		progress = start_sparse_progress(_("Verifying OID order in multi-pack-index"),
+						 m->num_objects - 1);
+	for (i = 0; i < m->num_objects - 1; i++) {
+		struct object_id oid1, oid2;
+
+		nth_midxed_object_oid(&oid1, m, i);
+		nth_midxed_object_oid(&oid2, m, i + 1);
+
+		if (oidcmp(&oid1, &oid2) >= 0)
+			midx_report(_("oid lookup out of order: oid[%d] = %s >= %s = oid[%d]"),
+				    i, oid_to_hex(&oid1), oid_to_hex(&oid2), i + 1);
+
+		midx_display_sparse_progress(progress, i + 1);
+	}
+	stop_progress(&progress);
+
+	/*
+	 * Create an array mapping each object to its packfile id.  Sort it
+	 * to group the objects by packfile.  Use this permutation to visit
+	 * each of the objects and only require 1 packfile to be open at a
+	 * time.
+	 */
+	ALLOC_ARRAY(pairs, m->num_objects);
+	for (i = 0; i < m->num_objects; i++) {
+		pairs[i].pos = i;
+		pairs[i].pack_int_id = nth_midxed_pack_int_id(m, i);
+	}
+
+	if (flags & MIDX_PROGRESS)
+		progress = start_sparse_progress(_("Sorting objects by packfile"),
+						 m->num_objects);
+	display_progress(progress, 0); /* TODO: Measure QSORT() progress */
+	QSORT(pairs, m->num_objects, compare_pair_pos_vs_id);
+	stop_progress(&progress);
+
+	if (flags & MIDX_PROGRESS)
+		progress = start_sparse_progress(_("Verifying object offsets"), m->num_objects);
+	for (i = 0; i < m->num_objects; i++) {
+		struct object_id oid;
+		struct pack_entry e;
+		off_t m_offset, p_offset;
+
+		if (i > 0 && pairs[i-1].pack_int_id != pairs[i].pack_int_id &&
+		    m->packs[pairs[i-1].pack_int_id])
+		{
+			close_pack_fd(m->packs[pairs[i-1].pack_int_id]);
+			close_pack_index(m->packs[pairs[i-1].pack_int_id]);
+		}
+
+		nth_midxed_object_oid(&oid, m, pairs[i].pos);
+
+		if (!fill_midx_entry(r, &oid, &e, m)) {
+			midx_report(_("failed to load pack entry for oid[%d] = %s"),
+				    pairs[i].pos, oid_to_hex(&oid));
+			continue;
+		}
+
+		if (open_pack_index(e.p)) {
+			midx_report(_("failed to load pack-index for packfile %s"),
+				    e.p->pack_name);
+			break;
+		}
+
+		m_offset = e.offset;
+		p_offset = find_pack_entry_one(oid.hash, e.p);
+
+		if (m_offset != p_offset)
+			midx_report(_("incorrect object offset for oid[%d] = %s: %"PRIx64" != %"PRIx64),
+				    pairs[i].pos, oid_to_hex(&oid), m_offset, p_offset);
+
+		midx_display_sparse_progress(progress, i + 1);
+	}
+	stop_progress(&progress);
+
+cleanup:
+	free(pairs);
+	close_midx(m);
+
+	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 = (size_t)(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 strbuf base_name = STRBUF_INIT;
+	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");
+
+	strbuf_addstr(&base_name, object_dir);
+	strbuf_addstr(&base_name, "/pack/pack");
+	strvec_push(&cmd.args, base_name.buf);
+
+	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");
+
+	strbuf_release(&base_name);
+
+	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;
+}