1 files changed, 899 insertions, 290 deletions
diff --git a/commit-graph.c b/commit-graph.c
index 4c6127088f..1752341098 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -1,49 +1,55 @@
 #include "cache.h"
 #include "config.h"
+#include "dir.h"
 #include "git-compat-util.h"
 #include "lockfile.h"
 #include "pack.h"
 #include "packfile.h"
 #include "commit.h"
 #include "object.h"
+#include "refs.h"
 #include "revision.h"
 #include "sha1-lookup.h"
 #include "commit-graph.h"
 #include "object-store.h"
+#include "alloc.h"
+#include "hashmap.h"
+#include "replace-object.h"
+#include "progress.h"
 
 #define GRAPH_SIGNATURE 0x43475048 /* "CGPH" */
 #define GRAPH_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
 #define GRAPH_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
 #define GRAPH_CHUNKID_DATA 0x43444154 /* "CDAT" */
-#define GRAPH_CHUNKID_LARGEEDGES 0x45444745 /* "EDGE" */
+#define GRAPH_CHUNKID_EXTRAEDGES 0x45444745 /* "EDGE" */
 
-#define GRAPH_DATA_WIDTH 36
+#define GRAPH_DATA_WIDTH (the_hash_algo->rawsz + 16)
 
 #define GRAPH_VERSION_1 0x1
 #define GRAPH_VERSION GRAPH_VERSION_1
 
-#define GRAPH_OID_VERSION_SHA1 1
-#define GRAPH_OID_LEN_SHA1 GIT_SHA1_RAWSZ
-#define GRAPH_OID_VERSION GRAPH_OID_VERSION_SHA1
-#define GRAPH_OID_LEN GRAPH_OID_LEN_SHA1
-
-#define GRAPH_OCTOPUS_EDGES_NEEDED 0x80000000
-#define GRAPH_PARENT_MISSING 0x7fffffff
+#define GRAPH_EXTRA_EDGES_NEEDED 0x80000000
 #define GRAPH_EDGE_LAST_MASK 0x7fffffff
 #define GRAPH_PARENT_NONE 0x70000000
 
 #define GRAPH_LAST_EDGE 0x80000000
 
+#define GRAPH_HEADER_SIZE 8
 #define GRAPH_FANOUT_SIZE (4 * 256)
 #define GRAPH_CHUNKLOOKUP_WIDTH 12
-#define GRAPH_MIN_SIZE (5 * GRAPH_CHUNKLOOKUP_WIDTH + GRAPH_FANOUT_SIZE + \
-			GRAPH_OID_LEN + 8)
+#define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * GRAPH_CHUNKLOOKUP_WIDTH \
+			+ GRAPH_FANOUT_SIZE + the_hash_algo->rawsz)
 
 char *get_commit_graph_filename(const char *obj_dir)
 {
 	return xstrfmt("%s/info/commit-graph", obj_dir);
 }
 
+static uint8_t oid_version(void)
+{
+	return 1;
+}
+
 static struct commit_graph *alloc_commit_graph(void)
 {
 	struct commit_graph *g = xcalloc(1, sizeof(*g));
@@ -52,59 +58,137 @@ static struct commit_graph *alloc_commit_graph(void)
 	return g;
 }
 
-struct commit_graph *load_commit_graph_one(const char *graph_file)
+extern int read_replace_refs;
+
+static int commit_graph_compatible(struct repository *r)
+{
+	if (!r->gitdir)
+		return 0;
+
+	if (read_replace_refs) {
+		prepare_replace_object(r);
+		if (hashmap_get_size(&r->objects->replace_map->map))
+			return 0;
+	}
+
+	prepare_commit_graft(r);
+	if (r->parsed_objects && r->parsed_objects->grafts_nr)
+		return 0;
+	if (is_repository_shallow(r))
+		return 0;
+
+	return 1;
+}
+
+int open_commit_graph(const char *graph_file, int *fd, struct stat *st)
+{
+	*fd = git_open(graph_file);
+	if (*fd < 0)
+		return 0;
+	if (fstat(*fd, st)) {
+		close(*fd);
+		return 0;
+	}
+	return 1;
+}
+
+struct commit_graph *load_commit_graph_one_fd_st(int fd, struct stat *st)
 {
 	void *graph_map;
-	const unsigned char *data, *chunk_lookup;
 	size_t graph_size;
-	struct stat st;
+	struct commit_graph *ret;
+
+	graph_size = xsize_t(st->st_size);
+
+	if (graph_size < GRAPH_MIN_SIZE) {
+		close(fd);
+		error(_("commit-graph file is too small"));
+		return NULL;
+	}
+	graph_map = xmmap(NULL, graph_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	ret = parse_commit_graph(graph_map, fd, graph_size);
+
+	if (!ret) {
+		munmap(graph_map, graph_size);
+		close(fd);
+	}
+
+	return ret;
+}
+
+static int verify_commit_graph_lite(struct commit_graph *g)
+{
+	/*
+	 * Basic validation shared between parse_commit_graph()
+	 * which'll be called every time the graph is used, and the
+	 * much more expensive verify_commit_graph() used by
+	 * "commit-graph verify".
+	 *
+	 * There should only be very basic checks here to ensure that
+	 * we don't e.g. segfault in fill_commit_in_graph(), but
+	 * because this is a very hot codepath nothing that e.g. loops
+	 * over g->num_commits, or runs a checksum on the commit-graph
+	 * itself.
+	 */
+	if (!g->chunk_oid_fanout) {
+		error("commit-graph is missing the OID Fanout chunk");
+		return 1;
+	}
+	if (!g->chunk_oid_lookup) {
+		error("commit-graph is missing the OID Lookup chunk");
+		return 1;
+	}
+	if (!g->chunk_commit_data) {
+		error("commit-graph is missing the Commit Data chunk");
+		return 1;
+	}
+
+	return 0;
+}
+
+struct commit_graph *parse_commit_graph(void *graph_map, int fd,
+					size_t graph_size)
+{
+	const unsigned char *data, *chunk_lookup;
 	uint32_t i;
 	struct commit_graph *graph;
-	int fd = git_open(graph_file);
 	uint64_t last_chunk_offset;
 	uint32_t last_chunk_id;
 	uint32_t graph_signature;
 	unsigned char graph_version, hash_version;
 
-	if (fd < 0)
+	if (!graph_map)
 		return NULL;
-	if (fstat(fd, &st)) {
-		close(fd);
+
+	if (graph_size < GRAPH_MIN_SIZE)
 		return NULL;
-	}
-	graph_size = xsize_t(st.st_size);
 
-	if (graph_size < GRAPH_MIN_SIZE) {
-		close(fd);
-		die("graph file %s is too small", graph_file);
-	}
-	graph_map = xmmap(NULL, graph_size, PROT_READ, MAP_PRIVATE, fd, 0);
 	data = (const unsigned char *)graph_map;
 
 	graph_signature = get_be32(data);
 	if (graph_signature != GRAPH_SIGNATURE) {
-		error("graph signature %X does not match signature %X",
+		error(_("commit-graph signature %X does not match signature %X"),
 		      graph_signature, GRAPH_SIGNATURE);
-		goto cleanup_fail;
+		return NULL;
 	}
 
 	graph_version = *(unsigned char*)(data + 4);
 	if (graph_version != GRAPH_VERSION) {
-		error("graph version %X does not match version %X",
+		error(_("commit-graph version %X does not match version %X"),
 		      graph_version, GRAPH_VERSION);
-		goto cleanup_fail;
+		return NULL;
 	}
 
 	hash_version = *(unsigned char*)(data + 5);
-	if (hash_version != GRAPH_OID_VERSION) {
-		error("hash version %X does not match version %X",
-		      hash_version, GRAPH_OID_VERSION);
-		goto cleanup_fail;
+	if (hash_version != oid_version()) {
+		error(_("commit-graph hash version %X does not match version %X"),
+		      hash_version, oid_version());
+		return NULL;
 	}
 
 	graph = alloc_commit_graph();
 
-	graph->hash_len = GRAPH_OID_LEN;
+	graph->hash_len = the_hash_algo->rawsz;
 	graph->num_chunks = *(unsigned char*)(data + 6);
 	graph->graph_fd = fd;
 	graph->data = graph_map;
@@ -114,16 +198,27 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 	last_chunk_offset = 8;
 	chunk_lookup = data + 8;
 	for (i = 0; i < graph->num_chunks; i++) {
-		uint32_t chunk_id = get_be32(chunk_lookup + 0);
-		uint64_t chunk_offset = get_be64(chunk_lookup + 4);
+		uint32_t chunk_id;
+		uint64_t chunk_offset;
 		int chunk_repeated = 0;
 
+		if (data + graph_size - chunk_lookup <
+		    GRAPH_CHUNKLOOKUP_WIDTH) {
+			error(_("commit-graph chunk lookup table entry missing; file may be incomplete"));
+			free(graph);
+			return NULL;
+		}
+
+		chunk_id = get_be32(chunk_lookup + 0);
+		chunk_offset = get_be64(chunk_lookup + 4);
+
 		chunk_lookup += GRAPH_CHUNKLOOKUP_WIDTH;
 
-		if (chunk_offset > graph_size - GIT_MAX_RAWSZ) {
-			error("improper chunk offset %08x%08x", (uint32_t)(chunk_offset >> 32),
+		if (chunk_offset > graph_size - the_hash_algo->rawsz) {
+			error(_("commit-graph improper chunk offset %08x%08x"), (uint32_t)(chunk_offset >> 32),
 			      (uint32_t)chunk_offset);
-			goto cleanup_fail;
+			free(graph);
+			return NULL;
 		}
 
 		switch (chunk_id) {
@@ -148,17 +243,18 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 				graph->chunk_commit_data = data + chunk_offset;
 			break;
 
-		case GRAPH_CHUNKID_LARGEEDGES:
-			if (graph->chunk_large_edges)
+		case GRAPH_CHUNKID_EXTRAEDGES:
+			if (graph->chunk_extra_edges)
 				chunk_repeated = 1;
 			else
-				graph->chunk_large_edges = data + chunk_offset;
+				graph->chunk_extra_edges = data + chunk_offset;
 			break;
 		}
 
 		if (chunk_repeated) {
-			error("chunk id %08x appears multiple times", chunk_id);
-			goto cleanup_fail;
+			error(_("commit-graph chunk id %08x appears multiple times"), chunk_id);
+			free(graph);
+			return NULL;
 		}
 
 		if (last_chunk_id == GRAPH_CHUNKID_OIDLOOKUP)
@@ -171,61 +267,104 @@ struct commit_graph *load_commit_graph_one(const char *graph_file)
 		last_chunk_offset = chunk_offset;
 	}
 
-	return graph;
+	if (verify_commit_graph_lite(graph)) {
+		free(graph);
+		return NULL;
+	}
 
-cleanup_fail:
-	munmap(graph_map, graph_size);
-	close(fd);
-	exit(1);
+	return graph;
 }
 
-/* global storage */
-static struct commit_graph *commit_graph = NULL;
+static struct commit_graph *load_commit_graph_one(const char *graph_file)
+{
+
+	struct stat st;
+	int fd;
+	int open_ok = open_commit_graph(graph_file, &fd, &st);
+
+	if (!open_ok)
+		return NULL;
+
+	return load_commit_graph_one_fd_st(fd, &st);
+}
 
-static void prepare_commit_graph_one(const char *obj_dir)
+static void prepare_commit_graph_one(struct repository *r, const char *obj_dir)
 {
 	char *graph_name;
 
-	if (commit_graph)
+	if (r->objects->commit_graph)
 		return;
 
 	graph_name = get_commit_graph_filename(obj_dir);
-	commit_graph = load_commit_graph_one(graph_name);
+	r->objects->commit_graph =
+		load_commit_graph_one(graph_name);
 
 	FREE_AND_NULL(graph_name);
 }
 
-static int prepare_commit_graph_run_once = 0;
-static void prepare_commit_graph(void)
+/*
+ * Return 1 if commit_graph is non-NULL, and 0 otherwise.
+ *
+ * On the first invocation, this function attemps to load the commit
+ * graph if the_repository is configured to have one.
+ */
+static int prepare_commit_graph(struct repository *r)
 {
-	struct alternate_object_database *alt;
-	char *obj_dir;
+	struct object_directory *odb;
+	int config_value;
+
+	if (git_env_bool(GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD, 0))
+		die("dying as requested by the '%s' variable on commit-graph load!",
+		    GIT_TEST_COMMIT_GRAPH_DIE_ON_LOAD);
+
+	if (r->objects->commit_graph_attempted)
+		return !!r->objects->commit_graph;
+	r->objects->commit_graph_attempted = 1;
+
+	if (!git_env_bool(GIT_TEST_COMMIT_GRAPH, 0) &&
+	    (repo_config_get_bool(r, "core.commitgraph", &config_value) ||
+	    !config_value))
+		/*
+		 * This repository is not configured to use commit graphs, so
+		 * do not load one. (But report commit_graph_attempted anyway
+		 * so that commit graph loading is not attempted again for this
+		 * repository.)
+		 */
+		return 0;
 
-	if (prepare_commit_graph_run_once)
-		return;
-	prepare_commit_graph_run_once = 1;
+	if (!commit_graph_compatible(r))
+		return 0;
 
-	obj_dir = get_object_directory();
-	prepare_commit_graph_one(obj_dir);
-	prepare_alt_odb(the_repository);
-	for (alt = the_repository->objects->alt_odb_list;
-	     !commit_graph && alt;
-	     alt = alt->next)
-		prepare_commit_graph_one(alt->path);
+	prepare_alt_odb(r);
+	for (odb = r->objects->odb;
+	     !r->objects->commit_graph && odb;
+	     odb = odb->next)
+		prepare_commit_graph_one(r, odb->path);
+	return !!r->objects->commit_graph;
 }
 
-static void close_commit_graph(void)
+int generation_numbers_enabled(struct repository *r)
 {
-	if (!commit_graph)
-		return;
+	uint32_t first_generation;
+	struct commit_graph *g;
+	if (!prepare_commit_graph(r))
+	       return 0;
 
-	if (commit_graph->graph_fd >= 0) {
-		munmap((void *)commit_graph->data, commit_graph->data_len);
-		commit_graph->data = NULL;
-		close(commit_graph->graph_fd);
-	}
+	g = r->objects->commit_graph;
+
+	if (!g->num_commits)
+		return 0;
+
+	first_generation = get_be32(g->chunk_commit_data +
+				    g->hash_len + 8) >> 2;
+
+	return !!first_generation;
+}
 
-	FREE_AND_NULL(commit_graph);
+void close_commit_graph(struct raw_object_store *o)
+{
+	free_commit_graph(o->commit_graph);
+	o->commit_graph = NULL;
 }
 
 static int bsearch_graph(struct commit_graph *g, struct object_id *oid, uint32_t *pos)
@@ -234,21 +373,40 @@ static int bsearch_graph(struct commit_graph *g, struct object_id *oid, uint32_t
 			    g->chunk_oid_lookup, g->hash_len, pos);
 }
 
-static struct commit_list **insert_parent_or_die(struct commit_graph *g,
+static struct commit_list **insert_parent_or_die(struct repository *r,
+						 struct commit_graph *g,
 						 uint64_t pos,
 						 struct commit_list **pptr)
 {
 	struct commit *c;
 	struct object_id oid;
+
+	if (pos >= g->num_commits)
+		die("invalid parent position %"PRIu64, pos);
+
 	hashcpy(oid.hash, g->chunk_oid_lookup + g->hash_len * pos);
-	c = lookup_commit(&oid);
+	c = lookup_commit(r, &oid);
 	if (!c)
-		die("could not find commit %s", oid_to_hex(&oid));
+		die(_("could not find commit %s"), oid_to_hex(&oid));
 	c->graph_pos = pos;
 	return &commit_list_insert(c, pptr)->next;
 }
 
-static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uint32_t pos)
+static void fill_commit_graph_info(struct commit *item, struct commit_graph *g, uint32_t pos)
+{
+	const unsigned char *commit_data = g->chunk_commit_data + GRAPH_DATA_WIDTH * pos;
+	item->graph_pos = pos;
+	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
+}
+
+static inline void set_commit_tree(struct commit *c, struct tree *t)
+{
+	c->maybe_tree = t;
+}
+
+static int fill_commit_in_graph(struct repository *r,
+				struct commit *item,
+				struct commit_graph *g, uint32_t pos)
 {
 	uint32_t edge_value;
 	uint32_t *parent_data_ptr;
@@ -259,32 +417,34 @@ static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uin
 	item->object.parsed = 1;
 	item->graph_pos = pos;
 
-	item->maybe_tree = NULL;
+	set_commit_tree(item, NULL);
 
 	date_high = get_be32(commit_data + g->hash_len + 8) & 0x3;
 	date_low = get_be32(commit_data + g->hash_len + 12);
 	item->date = (timestamp_t)((date_high << 32) | date_low);
 
+	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
+
 	pptr = &item->parents;
 
 	edge_value = get_be32(commit_data + g->hash_len);
 	if (edge_value == GRAPH_PARENT_NONE)
 		return 1;
-	pptr = insert_parent_or_die(g, edge_value, pptr);
+	pptr = insert_parent_or_die(r, g, edge_value, pptr);
 
 	edge_value = get_be32(commit_data + g->hash_len + 4);
 	if (edge_value == GRAPH_PARENT_NONE)
 		return 1;
-	if (!(edge_value & GRAPH_OCTOPUS_EDGES_NEEDED)) {
-		pptr = insert_parent_or_die(g, edge_value, pptr);
+	if (!(edge_value & GRAPH_EXTRA_EDGES_NEEDED)) {
+		pptr = insert_parent_or_die(r, g, edge_value, pptr);
 		return 1;
 	}
 
-	parent_data_ptr = (uint32_t*)(g->chunk_large_edges +
+	parent_data_ptr = (uint32_t*)(g->chunk_extra_edges +
 			  4 * (uint64_t)(edge_value & GRAPH_EDGE_LAST_MASK));
 	do {
 		edge_value = get_be32(parent_data_ptr);
-		pptr = insert_parent_or_die(g,
+		pptr = insert_parent_or_die(r, g,
 					    edge_value & GRAPH_EDGE_LAST_MASK,
 					    pptr);
 		parent_data_ptr++;
@@ -293,59 +453,110 @@ static int fill_commit_in_graph(struct commit *item, struct commit_graph *g, uin
 	return 1;
 }
 
-int parse_commit_in_graph(struct commit *item)
+static int find_commit_in_graph(struct commit *item, struct commit_graph *g, uint32_t *pos)
 {
-	if (!core_commit_graph)
-		return 0;
-	if (item->object.parsed)
+	if (item->graph_pos != COMMIT_NOT_FROM_GRAPH) {
+		*pos = item->graph_pos;
 		return 1;
+	} else {
+		return bsearch_graph(g, &(item->object.oid), pos);
+	}
+}
 
-	prepare_commit_graph();
-	if (commit_graph) {
-		uint32_t pos;
-		int found;
-		if (item->graph_pos != COMMIT_NOT_FROM_GRAPH) {
-			pos = item->graph_pos;
-			found = 1;
-		} else {
-			found = bsearch_graph(commit_graph, &(item->object.oid), &pos);
-		}
+static int parse_commit_in_graph_one(struct repository *r,
+				     struct commit_graph *g,
+				     struct commit *item)
+{
+	uint32_t pos;
 
-		if (found)
-			return fill_commit_in_graph(item, commit_graph, pos);
-	}
+	if (item->object.parsed)
+		return 1;
+
+	if (find_commit_in_graph(item, g, &pos))
+		return fill_commit_in_graph(r, item, g, pos);
 
 	return 0;
 }
 
-static struct tree *load_tree_for_commit(struct commit_graph *g, struct commit *c)
+int parse_commit_in_graph(struct repository *r, struct commit *item)
+{
+	if (!prepare_commit_graph(r))
+		return 0;
+	return parse_commit_in_graph_one(r, r->objects->commit_graph, item);
+}
+
+void load_commit_graph_info(struct repository *r, struct commit *item)
+{
+	uint32_t pos;
+	if (!prepare_commit_graph(r))
+		return;
+	if (find_commit_in_graph(item, r->objects->commit_graph, &pos))
+		fill_commit_graph_info(item, r->objects->commit_graph, pos);
+}
+
+static struct tree *load_tree_for_commit(struct repository *r,
+					 struct commit_graph *g,
+					 struct commit *c)
 {
 	struct object_id oid;
 	const unsigned char *commit_data = g->chunk_commit_data +
 					   GRAPH_DATA_WIDTH * (c->graph_pos);
 
 	hashcpy(oid.hash, commit_data);
-	c->maybe_tree = lookup_tree(&oid);
+	set_commit_tree(c, lookup_tree(r, &oid));
 
 	return c->maybe_tree;
 }
 
-struct tree *get_commit_tree_in_graph(const struct commit *c)
+static struct tree *get_commit_tree_in_graph_one(struct repository *r,
+						 struct commit_graph *g,
+						 const struct commit *c)
 {
 	if (c->maybe_tree)
 		return c->maybe_tree;
 	if (c->graph_pos == COMMIT_NOT_FROM_GRAPH)
-		BUG("get_commit_tree_in_graph called from non-commit-graph commit");
+		BUG("get_commit_tree_in_graph_one called from non-commit-graph commit");
 
-	return load_tree_for_commit(commit_graph, (struct commit *)c);
+	return load_tree_for_commit(r, g, (struct commit *)c);
 }
 
+struct tree *get_commit_tree_in_graph(struct repository *r, const struct commit *c)
+{
+	return get_commit_tree_in_graph_one(r, r->objects->commit_graph, c);
+}
+
+struct packed_commit_list {
+	struct commit **list;
+	int nr;
+	int alloc;
+};
+
+struct packed_oid_list {
+	struct object_id *list;
+	int nr;
+	int alloc;
+};
+
+struct write_commit_graph_context {
+	struct repository *r;
+	const char *obj_dir;
+	char *graph_name;
+	struct packed_oid_list oids;
+	struct packed_commit_list commits;
+	int num_extra_edges;
+	unsigned long approx_nr_objects;
+	struct progress *progress;
+	int progress_done;
+	uint64_t progress_cnt;
+	unsigned append:1,
+		 report_progress:1;
+};
+
 static void write_graph_chunk_fanout(struct hashfile *f,
-				     struct commit **commits,
-				     int nr_commits)
+				     struct write_commit_graph_context *ctx)
 {
 	int i, count = 0;
-	struct commit **list = commits;
+	struct commit **list = ctx->commits.list;
 
 	/*
 	 * Write the first-level table (the list is sorted,
@@ -353,9 +564,10 @@ static void write_graph_chunk_fanout(struct hashfile *f,
 	 * having to do eight extra binary search iterations).
 	 */
 	for (i = 0; i < 256; i++) {
-		while (count < nr_commits) {
+		while (count < ctx->commits.nr) {
 			if ((*list)->object.oid.hash[0] != i)
 				break;
+			display_progress(ctx->progress, ++ctx->progress_cnt);
 			count++;
 			list++;
 		}
@@ -365,12 +577,14 @@ static void write_graph_chunk_fanout(struct hashfile *f,
 }
 
 static void write_graph_chunk_oids(struct hashfile *f, int hash_len,
-				   struct commit **commits, int nr_commits)
+				   struct write_commit_graph_context *ctx)
 {
-	struct commit **list = commits;
+	struct commit **list = ctx->commits.list;
 	int count;
-	for (count = 0; count < nr_commits; count++, list++)
+	for (count = 0; count < ctx->commits.nr; count++, list++) {
+		display_progress(ctx->progress, ++ctx->progress_cnt);
 		hashwrite(f, (*list)->object.oid.hash, (int)hash_len);
+	}
 }
 
 static const unsigned char *commit_to_sha1(size_t index, void *table)
@@ -380,18 +594,19 @@ static const unsigned char *commit_to_sha1(size_t index, void *table)
 }
 
 static void write_graph_chunk_data(struct hashfile *f, int hash_len,
-				   struct commit **commits, int nr_commits)
+				   struct write_commit_graph_context *ctx)
 {
-	struct commit **list = commits;
-	struct commit **last = commits + nr_commits;
+	struct commit **list = ctx->commits.list;
+	struct commit **last = ctx->commits.list + ctx->commits.nr;
 	uint32_t num_extra_edges = 0;
 
 	while (list < last) {
 		struct commit_list *parent;
 		int edge_value;
 		uint32_t packedDate[2];
+		display_progress(ctx->progress, ++ctx->progress_cnt);
 
-		parse_commit(*list);
+		parse_commit_no_graph(*list);
 		hashwrite(f, get_commit_tree_oid(*list)->hash, hash_len);
 
 		parent = (*list)->parents;
@@ -400,12 +615,14 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 			edge_value = GRAPH_PARENT_NONE;
 		else {
 			edge_value = sha1_pos(parent->item->object.oid.hash,
-					      commits,
-					      nr_commits,
+					      ctx->commits.list,
+					      ctx->commits.nr,
 					      commit_to_sha1);
 
 			if (edge_value < 0)
-				edge_value = GRAPH_PARENT_MISSING;
+				BUG("missing parent %s for commit %s",
+				    oid_to_hex(&parent->item->object.oid),
+				    oid_to_hex(&(*list)->object.oid));
 		}
 
 		hashwrite_be32(f, edge_value);
@@ -416,19 +633,21 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 		if (!parent)
 			edge_value = GRAPH_PARENT_NONE;
 		else if (parent->next)
-			edge_value = GRAPH_OCTOPUS_EDGES_NEEDED | num_extra_edges;
+			edge_value = GRAPH_EXTRA_EDGES_NEEDED | num_extra_edges;
 		else {
 			edge_value = sha1_pos(parent->item->object.oid.hash,
-					      commits,
-					      nr_commits,
+					      ctx->commits.list,
+					      ctx->commits.nr,
 					      commit_to_sha1);
 			if (edge_value < 0)
-				edge_value = GRAPH_PARENT_MISSING;
+				BUG("missing parent %s for commit %s",
+				    oid_to_hex(&parent->item->object.oid),
+				    oid_to_hex(&(*list)->object.oid));
 		}
 
 		hashwrite_be32(f, edge_value);
 
-		if (edge_value & GRAPH_OCTOPUS_EDGES_NEEDED) {
+		if (edge_value & GRAPH_EXTRA_EDGES_NEEDED) {
 			do {
 				num_extra_edges++;
 				parent = parent->next;
@@ -440,6 +659,8 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 		else
 			packedDate[0] = 0;
 
+		packedDate[0] |= htonl((*list)->generation << 2);
+
 		packedDate[1] = htonl((*list)->date);
 		hashwrite(f, packedDate, 8);
 
@@ -447,16 +668,18 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
 	}
 }
 
-static void write_graph_chunk_large_edges(struct hashfile *f,
-					  struct commit **commits,
-					  int nr_commits)
+static void write_graph_chunk_extra_edges(struct hashfile *f,
+					  struct write_commit_graph_context *ctx)
 {
-	struct commit **list = commits;
-	struct commit **last = commits + nr_commits;
+	struct commit **list = ctx->commits.list;
+	struct commit **last = ctx->commits.list + ctx->commits.nr;
 	struct commit_list *parent;
 
 	while (list < last) {
 		int num_parents = 0;
+
+		display_progress(ctx->progress, ++ctx->progress_cnt);
+
 		for (parent = (*list)->parents; num_parents < 3 && parent;
 		     parent = parent->next)
 			num_parents++;
@@ -469,12 +692,14 @@ static void write_graph_chunk_large_edges(struct hashfile *f,
 		/* Since num_parents > 2, this initializer is safe. */
 		for (parent = (*list)->parents->next; parent; parent = parent->next) {
 			int edge_value = sha1_pos(parent->item->object.oid.hash,
-						  commits,
-						  nr_commits,
+						  ctx->commits.list,
+						  ctx->commits.nr,
 						  commit_to_sha1);
 
 			if (edge_value < 0)
-				edge_value = GRAPH_PARENT_MISSING;
+				BUG("missing parent %s for commit %s",
+				    oid_to_hex(&parent->item->object.oid),
+				    oid_to_hex(&(*list)->object.oid));
 			else if (!parent->next)
 				edge_value |= GRAPH_LAST_EDGE;
 
@@ -492,250 +717,354 @@ static int commit_compare(const void *_a, const void *_b)
 	return oidcmp(a, b);
 }
 
-struct packed_commit_list {
-	struct commit **list;
-	int nr;
-	int alloc;
-};
-
-struct packed_oid_list {
-	struct object_id *list;
-	int nr;
-	int alloc;
-};
-
 static int add_packed_commits(const struct object_id *oid,
 			      struct packed_git *pack,
 			      uint32_t pos,
 			      void *data)
 {
-	struct packed_oid_list *list = (struct packed_oid_list*)data;
+	struct write_commit_graph_context *ctx = (struct write_commit_graph_context*)data;
 	enum object_type type;
 	off_t offset = nth_packed_object_offset(pack, pos);
 	struct object_info oi = OBJECT_INFO_INIT;
 
+	if (ctx->progress)
+		display_progress(ctx->progress, ++ctx->progress_done);
+
 	oi.typep = &type;
-	if (packed_object_info(the_repository, pack, offset, &oi) < 0)
-		die("unable to get type of object %s", oid_to_hex(oid));
+	if (packed_object_info(ctx->r, pack, offset, &oi) < 0)
+		die(_("unable to get type of object %s"), oid_to_hex(oid));
 
 	if (type != OBJ_COMMIT)
 		return 0;
 
-	ALLOC_GROW(list->list, list->nr + 1, list->alloc);
-	oidcpy(&(list->list[list->nr]), oid);
-	list->nr++;
+	ALLOC_GROW(ctx->oids.list, ctx->oids.nr + 1, ctx->oids.alloc);
+	oidcpy(&(ctx->oids.list[ctx->oids.nr]), oid);
+	ctx->oids.nr++;
 
 	return 0;
 }
 
-static void add_missing_parents(struct packed_oid_list *oids, struct commit *commit)
+static void add_missing_parents(struct write_commit_graph_context *ctx, struct commit *commit)
 {
 	struct commit_list *parent;
 	for (parent = commit->parents; parent; parent = parent->next) {
 		if (!(parent->item->object.flags & UNINTERESTING)) {
-			ALLOC_GROW(oids->list, oids->nr + 1, oids->alloc);
-			oidcpy(&oids->list[oids->nr], &(parent->item->object.oid));
-			oids->nr++;
+			ALLOC_GROW(ctx->oids.list, ctx->oids.nr + 1, ctx->oids.alloc);
+			oidcpy(&ctx->oids.list[ctx->oids.nr], &(parent->item->object.oid));
+			ctx->oids.nr++;
 			parent->item->object.flags |= UNINTERESTING;
 		}
 	}
 }
 
-static void close_reachable(struct packed_oid_list *oids)
+static void close_reachable(struct write_commit_graph_context *ctx)
 {
 	int i;
 	struct commit *commit;
 
-	for (i = 0; i < oids->nr; i++) {
-		commit = lookup_commit(&oids->list[i]);
+	if (ctx->report_progress)
+		ctx->progress = start_delayed_progress(
+					_("Loading known commits in commit graph"),
+					ctx->oids.nr);
+	for (i = 0; i < ctx->oids.nr; i++) {
+		display_progress(ctx->progress, i + 1);
+		commit = lookup_commit(ctx->r, &ctx->oids.list[i]);
 		if (commit)
 			commit->object.flags |= UNINTERESTING;
 	}
+	stop_progress(&ctx->progress);
 
 	/*
-	 * As this loop runs, oids->nr may grow, but not more
+	 * As this loop runs, ctx->oids.nr may grow, but not more
 	 * than the number of missing commits in the reachable
 	 * closure.
 	 */
-	for (i = 0; i < oids->nr; i++) {
-		commit = lookup_commit(&oids->list[i]);
-
-		if (commit && !parse_commit(commit))
-			add_missing_parents(oids, commit);
+	if (ctx->report_progress)
+		ctx->progress = start_delayed_progress(
+					_("Expanding reachable commits in commit graph"),
+					ctx->oids.nr);
+	for (i = 0; i < ctx->oids.nr; i++) {
+		display_progress(ctx->progress, i + 1);
+		commit = lookup_commit(ctx->r, &ctx->oids.list[i]);
+
+		if (commit && !parse_commit_no_graph(commit))
+			add_missing_parents(ctx, commit);
 	}
+	stop_progress(&ctx->progress);
 
-	for (i = 0; i < oids->nr; i++) {
-		commit = lookup_commit(&oids->list[i]);
+	if (ctx->report_progress)
+		ctx->progress = start_delayed_progress(
+					_("Clearing commit marks in commit graph"),
+					ctx->oids.nr);
+	for (i = 0; i < ctx->oids.nr; i++) {
+		display_progress(ctx->progress, i + 1);
+		commit = lookup_commit(ctx->r, &ctx->oids.list[i]);
 
 		if (commit)
 			commit->object.flags &= ~UNINTERESTING;
 	}
+	stop_progress(&ctx->progress);
 }
 
-void write_commit_graph(const char *obj_dir,
-			const char **pack_indexes,
-			int nr_packs,
-			const char **commit_hex,
-			int nr_commits,
-			int append)
+static void compute_generation_numbers(struct write_commit_graph_context *ctx)
 {
-	struct packed_oid_list oids;
-	struct packed_commit_list commits;
-	struct hashfile *f;
-	uint32_t i, count_distinct = 0;
-	char *graph_name;
-	int fd;
-	struct lock_file lk = LOCK_INIT;
-	uint32_t chunk_ids[5];
-	uint64_t chunk_offsets[5];
-	int num_chunks;
-	int num_extra_edges;
-	struct commit_list *parent;
-
-	oids.nr = 0;
-	oids.alloc = approximate_object_count() / 4;
+	int i;
+	struct commit_list *list = NULL;
+
+	if (ctx->report_progress)
+		ctx->progress = start_progress(
+					_("Computing commit graph generation numbers"),
+					ctx->commits.nr);
+	for (i = 0; i < ctx->commits.nr; i++) {
+		display_progress(ctx->progress, i + 1);
+		if (ctx->commits.list[i]->generation != GENERATION_NUMBER_INFINITY &&
+		    ctx->commits.list[i]->generation != GENERATION_NUMBER_ZERO)
+			continue;
 
-	if (append) {
-		prepare_commit_graph_one(obj_dir);
-		if (commit_graph)
-			oids.alloc += commit_graph->num_commits;
-	}
+		commit_list_insert(ctx->commits.list[i], &list);
+		while (list) {
+			struct commit *current = list->item;
+			struct commit_list *parent;
+			int all_parents_computed = 1;
+			uint32_t max_generation = 0;
+
+			for (parent = current->parents; parent; parent = parent->next) {
+				if (parent->item->generation == GENERATION_NUMBER_INFINITY ||
+				    parent->item->generation == GENERATION_NUMBER_ZERO) {
+					all_parents_computed = 0;
+					commit_list_insert(parent->item, &list);
+					break;
+				} else if (parent->item->generation > max_generation) {
+					max_generation = parent->item->generation;
+				}
+			}
 
-	if (oids.alloc < 1024)
-		oids.alloc = 1024;
-	ALLOC_ARRAY(oids.list, oids.alloc);
+			if (all_parents_computed) {
+				current->generation = max_generation + 1;
+				pop_commit(&list);
 
-	if (append && commit_graph) {
-		for (i = 0; i < commit_graph->num_commits; i++) {
-			const unsigned char *hash = commit_graph->chunk_oid_lookup +
-				commit_graph->hash_len * i;
-			hashcpy(oids.list[oids.nr++].hash, hash);
+				if (current->generation > GENERATION_NUMBER_MAX)
+					current->generation = GENERATION_NUMBER_MAX;
+			}
 		}
 	}
+	stop_progress(&ctx->progress);
+}
 
-	if (pack_indexes) {
-		struct strbuf packname = STRBUF_INIT;
-		int dirlen;
-		strbuf_addf(&packname, "%s/pack/", obj_dir);
-		dirlen = packname.len;
-		for (i = 0; i < nr_packs; i++) {
-			struct packed_git *p;
-			strbuf_setlen(&packname, dirlen);
-			strbuf_addstr(&packname, pack_indexes[i]);
-			p = add_packed_git(packname.buf, packname.len, 1);
-			if (!p)
-				die("error adding pack %s", packname.buf);
-			if (open_pack_index(p))
-				die("error opening index for %s", packname.buf);
-			for_each_object_in_pack(p, add_packed_commits, &oids);
-			close_pack(p);
-		}
-		strbuf_release(&packname);
-	}
+static int add_ref_to_list(const char *refname,
+			   const struct object_id *oid,
+			   int flags, void *cb_data)
+{
+	struct string_list *list = (struct string_list *)cb_data;
+
+	string_list_append(list, oid_to_hex(oid));
+	return 0;
+}
 
-	if (commit_hex) {
-		for (i = 0; i < nr_commits; i++) {
-			const char *end;
-			struct object_id oid;
-			struct commit *result;
+int write_commit_graph_reachable(const char *obj_dir, unsigned int flags)
+{
+	struct string_list list = STRING_LIST_INIT_DUP;
+	int result;
 
-			if (commit_hex[i] && parse_oid_hex(commit_hex[i], &oid, &end))
-				continue;
+	for_each_ref(add_ref_to_list, &list);
+	result = write_commit_graph(obj_dir, NULL, &list,
+				    flags);
 
-			result = lookup_commit_reference_gently(&oid, 1);
+	string_list_clear(&list, 0);
+	return result;
+}
 
-			if (result) {
-				ALLOC_GROW(oids.list, oids.nr + 1, oids.alloc);
-				oidcpy(&oids.list[oids.nr], &(result->object.oid));
-				oids.nr++;
-			}
+static int fill_oids_from_packs(struct write_commit_graph_context *ctx,
+				struct string_list *pack_indexes)
+{
+	uint32_t i;
+	struct strbuf progress_title = STRBUF_INIT;
+	struct strbuf packname = STRBUF_INIT;
+	int dirlen;
+
+	strbuf_addf(&packname, "%s/pack/", ctx->obj_dir);
+	dirlen = packname.len;
+	if (ctx->report_progress) {
+		strbuf_addf(&progress_title,
+			    Q_("Finding commits for commit graph in %d pack",
+			       "Finding commits for commit graph in %d packs",
+			       pack_indexes->nr),
+			    pack_indexes->nr);
+		ctx->progress = start_delayed_progress(progress_title.buf, 0);
+		ctx->progress_done = 0;
+	}
+	for (i = 0; i < pack_indexes->nr; i++) {
+		struct packed_git *p;
+		strbuf_setlen(&packname, dirlen);
+		strbuf_addstr(&packname, pack_indexes->items[i].string);
+		p = add_packed_git(packname.buf, packname.len, 1);
+		if (!p) {
+			error(_("error adding pack %s"), packname.buf);
+			return -1;
+		}
+		if (open_pack_index(p)) {
+			error(_("error opening index for %s"), packname.buf);
+			return -1;
 		}
+		for_each_object_in_pack(p, add_packed_commits, ctx,
+					FOR_EACH_OBJECT_PACK_ORDER);
+		close_pack(p);
+		free(p);
 	}
 
-	if (!pack_indexes && !commit_hex)
-		for_each_packed_object(add_packed_commits, &oids, 0);
+	stop_progress(&ctx->progress);
+	strbuf_reset(&progress_title);
+	strbuf_release(&packname);
+
+	return 0;
+}
+
+static void fill_oids_from_commit_hex(struct write_commit_graph_context *ctx,
+				      struct string_list *commit_hex)
+{
+	uint32_t i;
+	struct strbuf progress_title = STRBUF_INIT;
+
+	if (ctx->report_progress) {
+		strbuf_addf(&progress_title,
+			    Q_("Finding commits for commit graph from %d ref",
+			       "Finding commits for commit graph from %d refs",
+			       commit_hex->nr),
+			    commit_hex->nr);
+		ctx->progress = start_delayed_progress(
+					progress_title.buf,
+					commit_hex->nr);
+	}
+	for (i = 0; i < commit_hex->nr; i++) {
+		const char *end;
+		struct object_id oid;
+		struct commit *result;
+
+		display_progress(ctx->progress, i + 1);
+		if (commit_hex->items[i].string &&
+		    parse_oid_hex(commit_hex->items[i].string, &oid, &end))
+			continue;
 
-	close_reachable(&oids);
+		result = lookup_commit_reference_gently(ctx->r, &oid, 1);
+
+		if (result) {
+			ALLOC_GROW(ctx->oids.list, ctx->oids.nr + 1, ctx->oids.alloc);
+			oidcpy(&ctx->oids.list[ctx->oids.nr], &(result->object.oid));
+			ctx->oids.nr++;
+		}
+	}
+	stop_progress(&ctx->progress);
+	strbuf_release(&progress_title);
+}
 
-	QSORT(oids.list, oids.nr, commit_compare);
+static void fill_oids_from_all_packs(struct write_commit_graph_context *ctx)
+{
+	if (ctx->report_progress)
+		ctx->progress = start_delayed_progress(
+			_("Finding commits for commit graph among packed objects"),
+			ctx->approx_nr_objects);
+	for_each_packed_object(add_packed_commits, ctx,
+			       FOR_EACH_OBJECT_PACK_ORDER);
+	if (ctx->progress_done < ctx->approx_nr_objects)
+		display_progress(ctx->progress, ctx->approx_nr_objects);
+	stop_progress(&ctx->progress);
+}
 
-	count_distinct = 1;
-	for (i = 1; i < oids.nr; i++) {
-		if (oidcmp(&oids.list[i-1], &oids.list[i]))
+static uint32_t count_distinct_commits(struct write_commit_graph_context *ctx)
+{
+	uint32_t i, count_distinct = 1;
+
+	if (ctx->report_progress)
+		ctx->progress = start_delayed_progress(
+			_("Counting distinct commits in commit graph"),
+			ctx->oids.nr);
+	display_progress(ctx->progress, 0); /* TODO: Measure QSORT() progress */
+	QSORT(ctx->oids.list, ctx->oids.nr, commit_compare);
+
+	for (i = 1; i < ctx->oids.nr; i++) {
+		display_progress(ctx->progress, i + 1);
+		if (!oideq(&ctx->oids.list[i - 1], &ctx->oids.list[i]))
 			count_distinct++;
 	}
+	stop_progress(&ctx->progress);
 
-	if (count_distinct >= GRAPH_PARENT_MISSING)
-		die(_("the commit graph format cannot write %d commits"), count_distinct);
+	return count_distinct;
+}
 
-	commits.nr = 0;
-	commits.alloc = count_distinct;
-	ALLOC_ARRAY(commits.list, commits.alloc);
+static void copy_oids_to_commits(struct write_commit_graph_context *ctx)
+{
+	uint32_t i;
+	struct commit_list *parent;
 
-	num_extra_edges = 0;
-	for (i = 0; i < oids.nr; i++) {
+	ctx->num_extra_edges = 0;
+	if (ctx->report_progress)
+		ctx->progress = start_delayed_progress(
+			_("Finding extra edges in commit graph"),
+			ctx->oids.nr);
+	for (i = 0; i < ctx->oids.nr; i++) {
 		int num_parents = 0;
-		if (i > 0 && !oidcmp(&oids.list[i-1], &oids.list[i]))
+		display_progress(ctx->progress, i + 1);
+		if (i > 0 && oideq(&ctx->oids.list[i - 1], &ctx->oids.list[i]))
 			continue;
 
-		commits.list[commits.nr] = lookup_commit(&oids.list[i]);
-		parse_commit(commits.list[commits.nr]);
+		ctx->commits.list[ctx->commits.nr] = lookup_commit(ctx->r, &ctx->oids.list[i]);
+		parse_commit_no_graph(ctx->commits.list[ctx->commits.nr]);
 
-		for (parent = commits.list[commits.nr]->parents;
+		for (parent = ctx->commits.list[ctx->commits.nr]->parents;
 		     parent; parent = parent->next)
 			num_parents++;
 
 		if (num_parents > 2)
-			num_extra_edges += num_parents - 1;
+			ctx->num_extra_edges += num_parents - 1;
 
-		commits.nr++;
+		ctx->commits.nr++;
 	}
-	num_chunks = num_extra_edges ? 4 : 3;
-
-	if (commits.nr >= GRAPH_PARENT_MISSING)
-		die(_("too many commits to write graph"));
-
-	graph_name = get_commit_graph_filename(obj_dir);
-	fd = hold_lock_file_for_update(&lk, graph_name, 0);
-
-	if (fd < 0) {
-		struct strbuf folder = STRBUF_INIT;
-		strbuf_addstr(&folder, graph_name);
-		strbuf_setlen(&folder, strrchr(folder.buf, '/') - folder.buf);
-
-		if (mkdir(folder.buf, 0777) < 0)
-			die_errno(_("cannot mkdir %s"), folder.buf);
-		strbuf_release(&folder);
-
-		fd = hold_lock_file_for_update(&lk, graph_name, LOCK_DIE_ON_ERROR);
+	stop_progress(&ctx->progress);
+}
 
-		if (fd < 0)
-			die_errno("unable to create '%s'", graph_name);
+static int write_commit_graph_file(struct write_commit_graph_context *ctx)
+{
+	uint32_t i;
+	struct hashfile *f;
+	struct lock_file lk = LOCK_INIT;
+	uint32_t chunk_ids[5];
+	uint64_t chunk_offsets[5];
+	const unsigned hashsz = the_hash_algo->rawsz;
+	struct strbuf progress_title = STRBUF_INIT;
+	int num_chunks = ctx->num_extra_edges ? 4 : 3;
+
+	ctx->graph_name = get_commit_graph_filename(ctx->obj_dir);
+	if (safe_create_leading_directories(ctx->graph_name)) {
+		UNLEAK(ctx->graph_name);
+		error(_("unable to create leading directories of %s"),
+			ctx->graph_name);
+		return -1;
 	}
 
+	hold_lock_file_for_update(&lk, ctx->graph_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
 
 	hashwrite_be32(f, GRAPH_SIGNATURE);
 
 	hashwrite_u8(f, GRAPH_VERSION);
-	hashwrite_u8(f, GRAPH_OID_VERSION);
+	hashwrite_u8(f, oid_version());
 	hashwrite_u8(f, num_chunks);
 	hashwrite_u8(f, 0); /* unused padding byte */
 
 	chunk_ids[0] = GRAPH_CHUNKID_OIDFANOUT;
 	chunk_ids[1] = GRAPH_CHUNKID_OIDLOOKUP;
 	chunk_ids[2] = GRAPH_CHUNKID_DATA;
-	if (num_extra_edges)
-		chunk_ids[3] = GRAPH_CHUNKID_LARGEEDGES;
+	if (ctx->num_extra_edges)
+		chunk_ids[3] = GRAPH_CHUNKID_EXTRAEDGES;
 	else
 		chunk_ids[3] = 0;
 	chunk_ids[4] = 0;
 
 	chunk_offsets[0] = 8 + (num_chunks + 1) * GRAPH_CHUNKLOOKUP_WIDTH;
 	chunk_offsets[1] = chunk_offsets[0] + GRAPH_FANOUT_SIZE;
-	chunk_offsets[2] = chunk_offsets[1] + GRAPH_OID_LEN * commits.nr;
-	chunk_offsets[3] = chunk_offsets[2] + (GRAPH_OID_LEN + 16) * commits.nr;
-	chunk_offsets[4] = chunk_offsets[3] + 4 * num_extra_edges;
+	chunk_offsets[2] = chunk_offsets[1] + hashsz * ctx->commits.nr;
+	chunk_offsets[3] = chunk_offsets[2] + (hashsz + 16) * ctx->commits.nr;
+	chunk_offsets[4] = chunk_offsets[3] + 4 * ctx->num_extra_edges;
 
 	for (i = 0; i <= num_chunks; i++) {
 		uint32_t chunk_write[3];
@@ -746,16 +1075,296 @@ void write_commit_graph(const char *obj_dir,
 		hashwrite(f, chunk_write, 12);
 	}
 
-	write_graph_chunk_fanout(f, commits.list, commits.nr);
-	write_graph_chunk_oids(f, GRAPH_OID_LEN, commits.list, commits.nr);
-	write_graph_chunk_data(f, GRAPH_OID_LEN, commits.list, commits.nr);
-	write_graph_chunk_large_edges(f, commits.list, commits.nr);
-
-	close_commit_graph();
+	if (ctx->report_progress) {
+		strbuf_addf(&progress_title,
+			    Q_("Writing out commit graph in %d pass",
+			       "Writing out commit graph in %d passes",
+			       num_chunks),
+			    num_chunks);
+		ctx->progress = start_delayed_progress(
+			progress_title.buf,
+			num_chunks * ctx->commits.nr);
+	}
+	write_graph_chunk_fanout(f, ctx);
+	write_graph_chunk_oids(f, hashsz, ctx);
+	write_graph_chunk_data(f, hashsz, ctx);
+	if (ctx->num_extra_edges)
+		write_graph_chunk_extra_edges(f, ctx);
+	stop_progress(&ctx->progress);
+	strbuf_release(&progress_title);
+
+	close_commit_graph(ctx->r->objects);
 	finalize_hashfile(f, NULL, CSUM_HASH_IN_STREAM | CSUM_FSYNC);
 	commit_lock_file(&lk);
 
-	free(oids.list);
-	oids.alloc = 0;
-	oids.nr = 0;
+	return 0;
+}
+
+int write_commit_graph(const char *obj_dir,
+		       struct string_list *pack_indexes,
+		       struct string_list *commit_hex,
+		       unsigned int flags)
+{
+	struct write_commit_graph_context *ctx;
+	uint32_t i, count_distinct = 0;
+	int res = 0;
+
+	if (!commit_graph_compatible(the_repository))
+		return 0;
+
+	ctx = xcalloc(1, sizeof(struct write_commit_graph_context));
+	ctx->r = the_repository;
+	ctx->obj_dir = obj_dir;
+	ctx->append = flags & COMMIT_GRAPH_APPEND ? 1 : 0;
+	ctx->report_progress = flags & COMMIT_GRAPH_PROGRESS ? 1 : 0;
+
+	ctx->approx_nr_objects = approximate_object_count();
+	ctx->oids.alloc = ctx->approx_nr_objects / 32;
+
+	if (ctx->append) {
+		prepare_commit_graph_one(ctx->r, ctx->obj_dir);
+		if (ctx->r->objects->commit_graph)
+			ctx->oids.alloc += ctx->r->objects->commit_graph->num_commits;
+	}
+
+	if (ctx->oids.alloc < 1024)
+		ctx->oids.alloc = 1024;
+	ALLOC_ARRAY(ctx->oids.list, ctx->oids.alloc);
+
+	if (ctx->append && ctx->r->objects->commit_graph) {
+		struct commit_graph *g = ctx->r->objects->commit_graph;
+		for (i = 0; i < g->num_commits; i++) {
+			const unsigned char *hash = g->chunk_oid_lookup + g->hash_len * i;
+			hashcpy(ctx->oids.list[ctx->oids.nr++].hash, hash);
+		}
+	}
+
+	if (pack_indexes) {
+		if ((res = fill_oids_from_packs(ctx, pack_indexes)))
+			goto cleanup;
+	}
+
+	if (commit_hex)
+		fill_oids_from_commit_hex(ctx, commit_hex);
+
+	if (!pack_indexes && !commit_hex)
+		fill_oids_from_all_packs(ctx);
+
+	close_reachable(ctx);
+
+	count_distinct = count_distinct_commits(ctx);
+
+	if (count_distinct >= GRAPH_EDGE_LAST_MASK) {
+		error(_("the commit graph format cannot write %d commits"), count_distinct);
+		res = -1;
+		goto cleanup;
+	}
+
+	ctx->commits.alloc = count_distinct;
+	ALLOC_ARRAY(ctx->commits.list, ctx->commits.alloc);
+
+	copy_oids_to_commits(ctx);
+
+	if (ctx->commits.nr >= GRAPH_EDGE_LAST_MASK) {
+		error(_("too many commits to write graph"));
+		res = -1;
+		goto cleanup;
+	}
+
+	compute_generation_numbers(ctx);
+
+	res = write_commit_graph_file(ctx);
+
+cleanup:
+	free(ctx->graph_name);
+	free(ctx->commits.list);
+	free(ctx->oids.list);
+	free(ctx);
+
+	return res;
+}
+
+#define VERIFY_COMMIT_GRAPH_ERROR_HASH 2
+static int verify_commit_graph_error;
+
+static void graph_report(const char *fmt, ...)
+{
+	va_list ap;
+
+	verify_commit_graph_error = 1;
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	fprintf(stderr, "\n");
+	va_end(ap);
+}
+
+#define GENERATION_ZERO_EXISTS 1
+#define GENERATION_NUMBER_EXISTS 2
+
+int verify_commit_graph(struct repository *r, struct commit_graph *g)
+{
+	uint32_t i, cur_fanout_pos = 0;
+	struct object_id prev_oid, cur_oid, checksum;
+	int generation_zero = 0;
+	struct hashfile *f;
+	int devnull;
+	struct progress *progress = NULL;
+
+	if (!g) {
+		graph_report("no commit-graph file loaded");
+		return 1;
+	}
+
+	verify_commit_graph_error = verify_commit_graph_lite(g);
+	if (verify_commit_graph_error)
+		return verify_commit_graph_error;
+
+	devnull = open("/dev/null", O_WRONLY);
+	f = hashfd(devnull, NULL);
+	hashwrite(f, g->data, g->data_len - g->hash_len);
+	finalize_hashfile(f, checksum.hash, CSUM_CLOSE);
+	if (!hasheq(checksum.hash, g->data + g->data_len - g->hash_len)) {
+		graph_report(_("the commit-graph file has incorrect checksum and is likely corrupt"));
+		verify_commit_graph_error = VERIFY_COMMIT_GRAPH_ERROR_HASH;
+	}
+
+	for (i = 0; i < g->num_commits; i++) {
+		struct commit *graph_commit;
+
+		hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);
+
+		if (i && oidcmp(&prev_oid, &cur_oid) >= 0)
+			graph_report(_("commit-graph has incorrect OID order: %s then %s"),
+				     oid_to_hex(&prev_oid),
+				     oid_to_hex(&cur_oid));
+
+		oidcpy(&prev_oid, &cur_oid);
+
+		while (cur_oid.hash[0] > cur_fanout_pos) {
+			uint32_t fanout_value = get_be32(g->chunk_oid_fanout + cur_fanout_pos);
+
+			if (i != fanout_value)
+				graph_report(_("commit-graph has incorrect fanout value: fanout[%d] = %u != %u"),
+					     cur_fanout_pos, fanout_value, i);
+			cur_fanout_pos++;
+		}
+
+		graph_commit = lookup_commit(r, &cur_oid);
+		if (!parse_commit_in_graph_one(r, g, graph_commit))
+			graph_report(_("failed to parse commit %s from commit-graph"),
+				     oid_to_hex(&cur_oid));
+	}
+
+	while (cur_fanout_pos < 256) {
+		uint32_t fanout_value = get_be32(g->chunk_oid_fanout + cur_fanout_pos);
+
+		if (g->num_commits != fanout_value)
+			graph_report(_("commit-graph has incorrect fanout value: fanout[%d] = %u != %u"),
+				     cur_fanout_pos, fanout_value, i);
+
+		cur_fanout_pos++;
+	}
+
+	if (verify_commit_graph_error & ~VERIFY_COMMIT_GRAPH_ERROR_HASH)
+		return verify_commit_graph_error;
+
+	progress = start_progress(_("Verifying commits in commit graph"),
+				  g->num_commits);
+	for (i = 0; i < g->num_commits; i++) {
+		struct commit *graph_commit, *odb_commit;
+		struct commit_list *graph_parents, *odb_parents;
+		uint32_t max_generation = 0;
+
+		display_progress(progress, i + 1);
+		hashcpy(cur_oid.hash, g->chunk_oid_lookup + g->hash_len * i);
+
+		graph_commit = lookup_commit(r, &cur_oid);
+		odb_commit = (struct commit *)create_object(r, cur_oid.hash, alloc_commit_node(r));
+		if (parse_commit_internal(odb_commit, 0, 0)) {
+			graph_report(_("failed to parse commit %s from object database for commit-graph"),
+				     oid_to_hex(&cur_oid));
+			continue;
+		}
+
+		if (!oideq(&get_commit_tree_in_graph_one(r, g, graph_commit)->object.oid,
+			   get_commit_tree_oid(odb_commit)))
+			graph_report(_("root tree OID for commit %s in commit-graph is %s != %s"),
+				     oid_to_hex(&cur_oid),
+				     oid_to_hex(get_commit_tree_oid(graph_commit)),
+				     oid_to_hex(get_commit_tree_oid(odb_commit)));
+
+		graph_parents = graph_commit->parents;
+		odb_parents = odb_commit->parents;
+
+		while (graph_parents) {
+			if (odb_parents == NULL) {
+				graph_report(_("commit-graph parent list for commit %s is too long"),
+					     oid_to_hex(&cur_oid));
+				break;
+			}
+
+			if (!oideq(&graph_parents->item->object.oid, &odb_parents->item->object.oid))
+				graph_report(_("commit-graph parent for %s is %s != %s"),
+					     oid_to_hex(&cur_oid),
+					     oid_to_hex(&graph_parents->item->object.oid),
+					     oid_to_hex(&odb_parents->item->object.oid));
+
+			if (graph_parents->item->generation > max_generation)
+				max_generation = graph_parents->item->generation;
+
+			graph_parents = graph_parents->next;
+			odb_parents = odb_parents->next;
+		}
+
+		if (odb_parents != NULL)
+			graph_report(_("commit-graph parent list for commit %s terminates early"),
+				     oid_to_hex(&cur_oid));
+
+		if (!graph_commit->generation) {
+			if (generation_zero == GENERATION_NUMBER_EXISTS)
+				graph_report(_("commit-graph has generation number zero for commit %s, but non-zero elsewhere"),
+					     oid_to_hex(&cur_oid));
+			generation_zero = GENERATION_ZERO_EXISTS;
+		} else if (generation_zero == GENERATION_ZERO_EXISTS)
+			graph_report(_("commit-graph has non-zero generation number for commit %s, but zero elsewhere"),
+				     oid_to_hex(&cur_oid));
+
+		if (generation_zero == GENERATION_ZERO_EXISTS)
+			continue;
+
+		/*
+		 * If one of our parents has generation GENERATION_NUMBER_MAX, then
+		 * our generation is also GENERATION_NUMBER_MAX. Decrement to avoid
+		 * extra logic in the following condition.
+		 */
+		if (max_generation == GENERATION_NUMBER_MAX)
+			max_generation--;
+
+		if (graph_commit->generation != max_generation + 1)
+			graph_report(_("commit-graph generation for commit %s is %u != %u"),
+				     oid_to_hex(&cur_oid),
+				     graph_commit->generation,
+				     max_generation + 1);
+
+		if (graph_commit->date != odb_commit->date)
+			graph_report(_("commit date for commit %s in commit-graph is %"PRItime" != %"PRItime),
+				     oid_to_hex(&cur_oid),
+				     graph_commit->date,
+				     odb_commit->date);
+	}
+	stop_progress(&progress);
+
+	return verify_commit_graph_error;
+}
+
+void free_commit_graph(struct commit_graph *g)
+{
+	if (!g)
+		return;
+	if (g->graph_fd >= 0) {
+		munmap((void *)g->data, g->data_len);
+		g->data = NULL;
+		close(g->graph_fd);
+	}
+	free(g);
 }