commit-graph: Add a way to write commit-graph files

This change adds the git_commit_graph_writer_* functions to allow to
write and create `commit-graph` files from `.idx`/`.pack` files or
`git_revwalk`s.

Part of: #5757

5 files changed, 829 insertions, 2 deletions

diff --git a/include/git2/sys/commit_graph.h b/include/git2/sys/commit_graph.h
index 038c9b739..01cf8d1bf 100644
--- a/include/git2/sys/commit_graph.h
+++ b/include/git2/sys/commit_graph.h
@@ -40,6 +40,136 @@ GIT_EXTERN(int) git_commit_graph_open(git_commit_graph **cgraph_out, const char
  */
 GIT_EXTERN(void) git_commit_graph_free(git_commit_graph *cgraph);
 
-GIT_END_DECL
+/**
+ * Create a new writer for `commit-graph` files.
+ *
+ * @param out Location to store the writer pointer.
+ * @param objects_info_dir The `objects/info` directory.
+ * The `commit-graph` file will be written in this directory.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_new(
+		git_commit_graph_writer **out,
+		const char *objects_info_dir);
+
+/**
+ * Free the commit-graph writer and its resources.
+ *
+ * @param w The writer to free. If NULL no action is taken.
+ */
+GIT_EXTERN(void) git_commit_graph_writer_free(git_commit_graph_writer *w);
+
+/**
+ * Add an `.idx` file (associated to a packfile) to the writer.
+ *
+ * @param w The writer.
+ * @param repo The repository that owns the `.idx` file.
+ * @param idx_path The path of an `.idx` file.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_add_index_file(
+		git_commit_graph_writer *w,
+		git_repository *repo,
+		const char *idx_path);
+
+/**
+ * Add a revwalk to the writer. This will add all the commits from the revwalk
+ * to the commit-graph.
+ *
+ * @param w The writer.
+ * @param walk The git_revwalk.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_add_revwalk(
+		git_commit_graph_writer *w,
+		git_revwalk *walk);
+
 
+/**
+ * The strategy to use when adding a new set of commits to a pre-existing
+ * commit-graph chain.
+ */
+typedef enum {
+	/**
+	 * Do not split commit-graph files. The other split strategy-related option
+	 * fields are ignored.
+	 */
+	GIT_COMMIT_GRAPH_SPLIT_STRATEGY_SINGLE_FILE = 0,
+} git_commit_graph_split_strategy_t;
+
+/**
+ * Options structure for
+ * `git_commit_graph_writer_commit`/`git_commit_graph_writer_dump`.
+ *
+ * Initialize with `GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT`. Alternatively, you
+ * can use `git_commit_graph_writer_options_init`.
+ */
+typedef struct {
+	unsigned int version;
+
+	/**
+	 * The strategy to use when adding new commits to a pre-existing commit-graph
+	 * chain.
+	 */
+	git_commit_graph_split_strategy_t split_strategy;
+
+	/**
+	 * The number of commits in level N is less than X times the number of
+	 * commits in level N + 1.
+	 */
+	float size_multiple;
+
+	/**
+	 * The number of commits in level N + 1 is more than C commits.
+	 */
+	size_t max_commits;
+} git_commit_graph_writer_options;
+
+#define GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION 1
+#define GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT                                                       \
+	{                                                                                          \
+		GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION,                                           \
+		GIT_COMMIT_GRAPH_SPLIT_STRATEGY_SINGLE_FILE, 2.0f, 64000                           \
+	}
+
+/**
+ * Initialize git_commit_graph_writer_options structure
+ *
+ * Initializes a `git_commit_graph_writer_options` with default values. Equivalent to
+ * creating an instance with `GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT`.
+ *
+ * @param opts The `git_commit_graph_writer_options` struct to initialize.
+ * @param version The struct version; pass `GIT_COMMIT_GRAPH_WRITER_OPTIONS_VERSION`.
+ * @return Zero on success; -1 on failure.
+ */
+GIT_EXTERN(int) git_commit_graph_writer_options_init(
+	git_commit_graph_writer_options *opts,
+	unsigned int version);
+
+/**
+ * Write a `commit-graph` file to a file.
+ *
+ * @param w The writer
+ * @param opts Pointer to git_commit_graph_writer_options struct.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_commit(
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts);
+
+/**
+ * Dump the contents of the `commit-graph` to an in-memory buffer.
+ *
+ * @param buffer Buffer where to store the contents of the `commit-graph`.
+ * @param w The writer.
+ * @param opts Pointer to git_commit_graph_writer_options struct.
+ * @return 0 or an error code
+ */
+GIT_EXTERN(int) git_commit_graph_writer_dump(
+		git_buf *buffer,
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts);
+
+/** @} */
+GIT_END_DECL
 #endif
diff --git a/include/git2/types.h b/include/git2/types.h
index 562eb8e5f..4d5c72324 100644
--- a/include/git2/types.h
+++ b/include/git2/types.h
@@ -96,6 +96,9 @@ typedef struct git_odb_stream git_odb_stream;
 /** A stream to write a packfile to the ODB */
 typedef struct git_odb_writepack git_odb_writepack;
 
+/** a writer for commit-graph files. */
+typedef struct git_commit_graph_writer git_commit_graph_writer;
+
 /** An open refs database handle. */
 typedef struct git_refdb git_refdb;
 
diff --git a/src/commit_graph.c b/src/commit_graph.c
index 556925600..3d1c03639 100644
--- a/src/commit_graph.c
+++ b/src/commit_graph.c
@@ -7,11 +7,19 @@
 
 #include "commit_graph.h"
 
+#include "array.h"
+#include "filebuf.h"
 #include "futils.h"
 #include "hash.h"
+#include "oidarray.h"
+#include "oidmap.h"
 #include "pack.h"
+#include "repository.h"
+#include "revwalk.h"
 
 #define GIT_COMMIT_GRAPH_MISSING_PARENT 0x70000000
+#define GIT_COMMIT_GRAPH_GENERATION_NUMBER_MAX 0x3FFFFFFF
+#define GIT_COMMIT_GRAPH_GENERATION_NUMBER_INFINITY 0xFFFFFFFF
 
 #define COMMIT_GRAPH_SIGNATURE 0x43475048 /* "CGPH" */
 #define COMMIT_GRAPH_VERSION 1
@@ -36,6 +44,63 @@ struct git_commit_graph_chunk {
 	size_t length;
 };
 
+typedef git_array_t(size_t) parent_index_array_t;
+
+struct packed_commit {
+	size_t index;
+	git_oid sha1;
+	git_oid tree_oid;
+	uint32_t generation;
+	git_time_t commit_time;
+	git_array_oid_t parents;
+	parent_index_array_t parent_indices;
+};
+
+static void packed_commit_free(struct packed_commit *p)
+{
+	if (!p)
+		return;
+
+	git_array_clear(p->parents);
+	git_array_clear(p->parent_indices);
+	git__free(p);
+}
+
+static struct packed_commit *packed_commit_new(git_commit *commit)
+{
+	unsigned int i, parentcount = git_commit_parentcount(commit);
+	struct packed_commit *p
+			= (struct packed_commit *)git__calloc(1, sizeof(struct packed_commit));
+	if (!p)
+		goto cleanup;
+
+	git_array_init_to_size(p->parents, parentcount);
+	if (parentcount && !p->parents.ptr)
+		goto cleanup;
+
+	if (git_oid_cpy(&p->sha1, git_commit_id(commit)) < 0)
+		goto cleanup;
+	if (git_oid_cpy(&p->tree_oid, git_commit_tree_id(commit)) < 0)
+		goto cleanup;
+	p->commit_time = git_commit_time(commit);
+
+	for (i = 0; i < parentcount; ++i) {
+		git_oid *parent_id = git_array_alloc(p->parents);
+		if (!parent_id)
+			goto cleanup;
+		if (git_oid_cpy(parent_id, git_commit_parent_id(commit, i)) < 0)
+			goto cleanup;
+	}
+
+	return p;
+
+cleanup:
+	packed_commit_free(p);
+	return NULL;
+}
+
+typedef int (*commit_graph_write_cb)(const char *buf, size_t size, void *cb_data);
+
 static int commit_graph_error(const char *message)
 {
 	git_error_set(GIT_ERROR_ODB, "invalid commit-graph file - %s", message);
@@ -401,7 +466,6 @@ static int git_commit_graph_entry_get_byindex(
 			extra_edge_list_pos++;
 			e->parent_count++;
 		}
-
 	}
 	git_oid_cpy(&e->sha1, &file->oid_lookup[pos]);
 	return 0;
@@ -547,3 +611,583 @@ void git_commit_graph_file_free(git_commit_graph_file *file)
 	git_commit_graph_file_close(file);
 	git__free(file);
 }
+
+static int packed_commit__cmp(const void *a_, const void *b_)
+{
+	const struct packed_commit *a = a_;
+	const struct packed_commit *b = b_;
+	return git_oid_cmp(&a->sha1, &b->sha1);
+}
+
+int git_commit_graph_writer_new(git_commit_graph_writer **out, const char *objects_info_dir)
+{
+	git_commit_graph_writer *w = git__calloc(1, sizeof(git_commit_graph_writer));
+	GIT_ERROR_CHECK_ALLOC(w);
+
+	if (git_buf_sets(&w->objects_info_dir, objects_info_dir) < 0) {
+		git__free(w);
+		return -1;
+	}
+
+	if (git_vector_init(&w->commits, 0, packed_commit__cmp) < 0) {
+		git_buf_dispose(&w->objects_info_dir);
+		git__free(w);
+		return -1;
+	}
+
+	*out = w;
+	return 0;
+}
+
+void git_commit_graph_writer_free(git_commit_graph_writer *w)
+{
+	struct packed_commit *packed_commit;
+	size_t i;
+
+	if (!w)
+		return;
+
+	git_vector_foreach (&w->commits, i, packed_commit)
+		packed_commit_free(packed_commit);
+	git_vector_free(&w->commits);
+	git_buf_dispose(&w->objects_info_dir);
+	git__free(w);
+}
+
+struct object_entry_cb_state {
+	git_repository *repo;
+	git_odb *db;
+	git_vector *commits;
+};
+
+static int object_entry__cb(const git_oid *id, void *data)
+{
+	struct object_entry_cb_state *state = (struct object_entry_cb_state *)data;
+	git_commit *commit = NULL;
+	struct packed_commit *packed_commit = NULL;
+	size_t header_len;
+	git_object_t header_type;
+	int error = 0;
+
+	error = git_odb_read_header(&header_len, &header_type, state->db, id);
+	if (error < 0)
+		return error;
+
+	if (header_type != GIT_OBJECT_COMMIT)
+		return 0;
+
+	error = git_commit_lookup(&commit, state->repo, id);
+	if (error < 0)
+		return error;
+
+	packed_commit = packed_commit_new(commit);
+	git_commit_free(commit);
+	GIT_ERROR_CHECK_ALLOC(packed_commit);
+
+	error = git_vector_insert(state->commits, packed_commit);
+	if (error < 0) {
+		packed_commit_free(packed_commit);
+		return error;
+	}
+
+	return 0;
+}
+
+int git_commit_graph_writer_add_index_file(
+		git_commit_graph_writer *w,
+		git_repository *repo,
+		const char *idx_path)
+{
+	int error;
+	struct git_pack_file *p = NULL;
+	struct object_entry_cb_state state = {
+			.repo = repo,
+			.db = NULL,
+			.commits = &w->commits,
+	};
+
+	error = git_repository_odb(&state.db, repo);
+	if (error < 0)
+		goto cleanup;
+
+	error = git_mwindow_get_pack(&p, idx_path);
+	if (error < 0)
+		goto cleanup;
+
+	error = git_pack_foreach_entry(p, object_entry__cb, &state);
+	if (error < 0)
+		goto cleanup;
+
+cleanup:
+	git_mwindow_put_pack(p);
+	git_odb_free(state.db);
+	return error;
+}
+
+int git_commit_graph_writer_add_revwalk(git_commit_graph_writer *w, git_revwalk *walk)
+{
+	int error;
+	git_oid id;
+	git_repository *repo = git_revwalk_repository(walk);
+	git_commit *commit;
+	struct packed_commit* packed_commit;
+
+	while ((git_revwalk_next(&id, walk)) == 0) {
+		error = git_commit_lookup(&commit, repo, &id);
+		if (error < 0)
+			return error;
+
+		packed_commit = packed_commit_new(commit);
+		git_commit_free(commit);
+		GIT_ERROR_CHECK_ALLOC(packed_commit);
+
+		error = git_vector_insert(&w->commits, packed_commit);
+		if (error < 0) {
+			packed_commit_free(packed_commit);
+			return error;
+		}
+	}
+
+	return 0;
+}
+
+enum generation_number_commit_state {
+	GENERATION_NUMBER_COMMIT_STATE_UNVISITED = 0,
+	GENERATION_NUMBER_COMMIT_STATE_ADDED = 1,
+	GENERATION_NUMBER_COMMIT_STATE_EXPANDED = 2,
+	GENERATION_NUMBER_COMMIT_STATE_VISITED = 3,
+};
+
+static int compute_generation_numbers(git_vector *commits)
+{
+	git_array_t(size_t) index_stack = GIT_ARRAY_INIT;
+	size_t i, j;
+	size_t *parent_idx;
+	enum generation_number_commit_state *commit_states = NULL;
+	struct packed_commit *child_packed_commit;
+	git_oidmap *packed_commit_map = NULL;
+	int error = 0;
+
+	/* First populate the parent indices fields */
+	error = git_oidmap_new(&packed_commit_map);
+	if (error < 0)
+		goto cleanup;
+	git_vector_foreach (commits, i, child_packed_commit) {
+		child_packed_commit->index = i;
+		error = git_oidmap_set(
+				packed_commit_map, &child_packed_commit->sha1, child_packed_commit);
+		if (error < 0)
+			goto cleanup;
+	}
+
+	git_vector_foreach (commits, i, child_packed_commit) {
+		size_t parent_i, *parent_idx_ptr;
+		struct packed_commit *parent_packed_commit;
+		git_oid *parent_id;
+		git_array_init_to_size(
+				child_packed_commit->parent_indices,
+				git_array_size(child_packed_commit->parents));
+		if (git_array_size(child_packed_commit->parents)
+		    && !child_packed_commit->parent_indices.ptr) {
+			error = -1;
+			goto cleanup;
+		}
+		git_array_foreach (child_packed_commit->parents, parent_i, parent_id) {
+			parent_packed_commit = git_oidmap_get(packed_commit_map, parent_id);
+			if (!parent_packed_commit) {
+				git_error_set(GIT_ERROR_ODB,
+					      "parent commit %s not found in commit graph",
+					      git_oid_tostr_s(parent_id));
+				error = GIT_ENOTFOUND;
+				goto cleanup;
+			}
+			parent_idx_ptr = git_array_alloc(child_packed_commit->parent_indices);
+			if (!parent_idx_ptr) {
+				error = -1;
+				goto cleanup;
+			}
+			*parent_idx_ptr = parent_packed_commit->index;
+		}
+	}
+
+	/*
+	 * We copy all the commits to the stack and then during visitation,
+	 * each node can be added up to two times to the stack.
+	 */
+	git_array_init_to_size(index_stack, 3 * git_vector_length(commits));
+	if (!index_stack.ptr) {
+		error = -1;
+		goto cleanup;
+	}
+
+	commit_states = (enum generation_number_commit_state *)git__calloc(
+			git_vector_length(commits), sizeof(enum generation_number_commit_state));
+	if (!commit_states) {
+		error = -1;
+		goto cleanup;
+	}
+
+	/*
+	 * Perform a Post-Order traversal so that all parent nodes are fully
+	 * visited before the child node.
+	 */
+	git_vector_foreach (commits, i, child_packed_commit)
+		*(size_t *)git_array_alloc(index_stack) = i;
+
+	while (git_array_size(index_stack)) {
+		i = *git_array_pop(index_stack);
+		child_packed_commit = git_vector_get(commits, i);
+
+		if (commit_states[i] == GENERATION_NUMBER_COMMIT_STATE_VISITED) {
+			/* This commit has already been fully visited. */
+			continue;
+		}
+		if (commit_states[i] == GENERATION_NUMBER_COMMIT_STATE_EXPANDED) {
+			/* All of the commits parents have been visited. */
+			child_packed_commit->generation = 0;
+			git_array_foreach (child_packed_commit->parent_indices, j, parent_idx) {
+				struct packed_commit *parent = git_vector_get(commits, *parent_idx);
+				if (child_packed_commit->generation < parent->generation)
+					child_packed_commit->generation = parent->generation;
+			}
+			if (child_packed_commit->generation
+			    < GIT_COMMIT_GRAPH_GENERATION_NUMBER_MAX) {
+				++child_packed_commit->generation;
+			}
+			commit_states[i] = GENERATION_NUMBER_COMMIT_STATE_VISITED;
+			continue;
+		}
+
+		/*
+		 * This is the first time we see this commit. We need
+		 * to visit all its parents before we can fully visit
+		 * it.
+		 */
+		if (git_array_size(child_packed_commit->parent_indices) == 0) {
+			/*
+			 * Special case: if the commit has no parents, there's
+			 * no need to add it to the stack just to immediately
+			 * remove it.
+			 */
+			commit_states[i] = GENERATION_NUMBER_COMMIT_STATE_VISITED;
+			child_packed_commit->generation = 1;
+			continue;
+		}
+
+		/*
+		 * Add this current commit again so that it is visited
+		 * again once all its children have been visited.
+		 */
+		*(size_t *)git_array_alloc(index_stack) = i;
+		git_array_foreach (child_packed_commit->parent_indices, j, parent_idx) {
+			if (commit_states[*parent_idx]
+			    != GENERATION_NUMBER_COMMIT_STATE_UNVISITED) {
+				/* This commit has already been considered. */
+				continue;
+			}
+
+			commit_states[*parent_idx] = GENERATION_NUMBER_COMMIT_STATE_ADDED;
+			*(size_t *)git_array_alloc(index_stack) = *parent_idx;
+		}
+		commit_states[i] = GENERATION_NUMBER_COMMIT_STATE_EXPANDED;
+	}
+
+cleanup:
+	git_oidmap_free(packed_commit_map);
+	git__free(commit_states);
+	git_array_clear(index_stack);
+
+	return error;
+}
+
+static int write_offset(off64_t offset, commit_graph_write_cb write_cb, void *cb_data)
+{
+	int error;
+	uint32_t word;
+
+	word = htonl((uint32_t)((offset >> 32) & 0xffffffffu));
+	error = write_cb((const char *)&word, sizeof(word), cb_data);
+	if (error < 0)
+		return error;
+	word = htonl((uint32_t)((offset >> 0) & 0xffffffffu));
+	error = write_cb((const char *)&word, sizeof(word), cb_data);
+	if (error < 0)
+		return error;
+
+	return 0;
+}
+
+static int
+write_chunk_header(int chunk_id, off64_t offset, commit_graph_write_cb write_cb, void *cb_data)
+{
+	uint32_t word = htonl(chunk_id);
+	int error = write_cb((const char *)&word, sizeof(word), cb_data);
+	if (error < 0)
+		return error;
+	return write_offset(offset, write_cb, cb_data);
+}
+
+static int commit_graph_write_buf(const char *buf, size_t size, void *data)
+{
+	git_buf *b = (git_buf *)data;
+	return git_buf_put(b, buf, size);
+}
+
+struct commit_graph_write_hash_context {
+	commit_graph_write_cb write_cb;
+	void *cb_data;
+	git_hash_ctx *ctx;
+};
+
+static int commit_graph_write_hash(const char *buf, size_t size, void *data)
+{
+	struct commit_graph_write_hash_context *ctx
+			= (struct commit_graph_write_hash_context *)data;
+	int error;
+
+	error = git_hash_update(ctx->ctx, buf, size);
+	if (error < 0)
+		return error;
+
+	return ctx->write_cb(buf, size, ctx->cb_data);
+}
+
+static void packed_commit_free_dup(void *packed_commit)
+{
+	packed_commit_free(packed_commit);
+}
+
+static int
+commit_graph_write(git_commit_graph_writer *w, commit_graph_write_cb write_cb, void *cb_data)
+{
+	int error = 0;
+	size_t i;
+	struct packed_commit *packed_commit;
+	struct git_commit_graph_header hdr = {
+			.signature = htonl(COMMIT_GRAPH_SIGNATURE),
+			.version = COMMIT_GRAPH_VERSION,
+			.object_id_version = COMMIT_GRAPH_OBJECT_ID_VERSION,
+			.chunks = 0,
+			.base_graph_files = 0,
+	};
+	uint32_t oid_fanout_count;
+	uint32_t extra_edge_list_count;
+	uint32_t oid_fanout[256];
+	off64_t offset;
+	git_buf oid_lookup = GIT_BUF_INIT,
+		commit_data = GIT_BUF_INIT, extra_edge_list = GIT_BUF_INIT;
+	git_oid cgraph_checksum = {{0}};
+	git_hash_ctx ctx;
+	struct commit_graph_write_hash_context hash_cb_data = {
+			.write_cb = write_cb,
+			.cb_data = cb_data,
+			.ctx = &ctx,
+	};
+
+	error = git_hash_ctx_init(&ctx);
+	if (error < 0)
+		return error;
+	cb_data = &hash_cb_data;
+	write_cb = commit_graph_write_hash;
+
+	/* Sort the commits. */
+	git_vector_sort(&w->commits);
+	git_vector_uniq(&w->commits, packed_commit_free_dup);
+	error = compute_generation_numbers(&w->commits);
+	if (error < 0)
+		goto cleanup;
+
+	/* Fill the OID Fanout table. */
+	oid_fanout_count = 0;
+	for (i = 0; i < 256; i++) {
+		while (oid_fanout_count < git_vector_length(&w->commits)
+		       && ((struct packed_commit *)git_vector_get(&w->commits, oid_fanout_count))
+						       ->sha1.id[0]
+				       <= i)
+			++oid_fanout_count;
+		oid_fanout[i] = htonl(oid_fanout_count);
+	}
+
+	/* Fill the OID Lookup table. */
+	git_vector_foreach (&w->commits, i, packed_commit) {
+		error = git_buf_put(
+				&oid_lookup, (const char *)&packed_commit->sha1, sizeof(git_oid));
+		if (error < 0)
+			goto cleanup;
+	}
+
+	/* Fill the Commit Data and Extra Edge List tables. */
+	extra_edge_list_count = 0;
+	git_vector_foreach (&w->commits, i, packed_commit) {
+		uint64_t commit_time;
+		uint32_t generation;
+		uint32_t word;
+		unsigned int parentcount = (unsigned int)git_array_size(packed_commit->parents);
+
+		error
+				= git_buf_put(&commit_data,
+					      (const char *)&packed_commit->tree_oid,
+					      sizeof(git_oid));
+		if (error < 0)
+			goto cleanup;
+
+		if (parentcount == 0)
+			word = htonl(GIT_COMMIT_GRAPH_MISSING_PARENT);
+		else
+			word = htonl((uint32_t)*git_array_get(packed_commit->parent_indices, 0));
+		error = git_buf_put(&commit_data, (const char *)&word, sizeof(word));
+		if (error < 0)
+			goto cleanup;
+
+		if (parentcount < 2)
+			word = htonl(GIT_COMMIT_GRAPH_MISSING_PARENT);
+		else if (parentcount == 2)
+			word = htonl((uint32_t)*git_array_get(packed_commit->parent_indices, 1));
+		else
+			word = htonl(0x80000000u | extra_edge_list_count);
+		error = git_buf_put(&commit_data, (const char *)&word, sizeof(word));
+		if (error < 0)
+			goto cleanup;
+
+		if (parentcount > 2) {
+			unsigned int parent_i;
+			for (parent_i = 1; parent_i < parentcount; ++parent_i) {
+				word = htonl((uint32_t)(
+						*git_array_get(packed_commit->parent_indices, parent_i)
+						| (parent_i + 1 == parentcount ? 0x80000000u : 0)));
+				error
+						= git_buf_put(&extra_edge_list,
+							      (const char *)&word,
+							      sizeof(word));
+				if (error < 0)
+					goto cleanup;
+			}
+			extra_edge_list_count += parentcount - 1;
+		}
+
+		generation = packed_commit->generation;
+		commit_time = (uint64_t)packed_commit->commit_time;
+		if (generation > GIT_COMMIT_GRAPH_GENERATION_NUMBER_MAX)
+			generation = GIT_COMMIT_GRAPH_GENERATION_NUMBER_MAX;
+		word = ntohl((uint32_t)((generation << 2) | ((commit_time >> 32ull) & 0x3ull)));
+		error = git_buf_put(&commit_data, (const char *)&word, sizeof(word));
+		if (error < 0)
+			goto cleanup;
+		word = ntohl((uint32_t)(commit_time & 0xffffffffull));
+		error = git_buf_put(&commit_data, (const char *)&word, sizeof(word));
+		if (error < 0)
+			goto cleanup;
+	}
+
+	/* Write the header. */
+	hdr.chunks = 3;
+	if (git_buf_len(&extra_edge_list) > 0)
+		hdr.chunks++;
+	error = write_cb((const char *)&hdr, sizeof(hdr), cb_data);
+	if (error < 0)
+		goto cleanup;
+
+	/* Write the chunk headers. */
+	offset = sizeof(hdr) + (hdr.chunks + 1) * 12;
+	error = write_chunk_header(COMMIT_GRAPH_OID_FANOUT_ID, offset, write_cb, cb_data);
+	if (error < 0)
+		goto cleanup;
+	offset += sizeof(oid_fanout);
+	error = write_chunk_header(COMMIT_GRAPH_OID_LOOKUP_ID, offset, write_cb, cb_data);
+	if (error < 0)
+		goto cleanup;
+	offset += git_buf_len(&oid_lookup);
+	error = write_chunk_header(COMMIT_GRAPH_COMMIT_DATA_ID, offset, write_cb, cb_data);
+	if (error < 0)
+		goto cleanup;
+	offset += git_buf_len(&commit_data);
+	if (git_buf_len(&extra_edge_list) > 0) {
+		error = write_chunk_header(
+				COMMIT_GRAPH_EXTRA_EDGE_LIST_ID, offset, write_cb, cb_data);
+		if (error < 0)
+			goto cleanup;
+		offset += git_buf_len(&extra_edge_list);
+	}
+	error = write_chunk_header(0, offset, write_cb, cb_data);
+	if (error < 0)
+		goto cleanup;
+
+	/* Write all the chunks. */
+	error = write_cb((const char *)oid_fanout, sizeof(oid_fanout), cb_data);
+	if (error < 0)
+		goto cleanup;
+	error = write_cb(git_buf_cstr(&oid_lookup), git_buf_len(&oid_lookup), cb_data);
+	if (error < 0)
+		goto cleanup;
+	error = write_cb(git_buf_cstr(&commit_data), git_buf_len(&commit_data), cb_data);
+	if (error < 0)
+		goto cleanup;
+	error
+			= write_cb(git_buf_cstr(&extra_edge_list),
+				   git_buf_len(&extra_edge_list),
+				   cb_data);
+	if (error < 0)
+		goto cleanup;
+
+	/* Finalize the checksum and write the trailer. */
+	error = git_hash_final(&cgraph_checksum, &ctx);
+	if (error < 0)
+		goto cleanup;
+	error = write_cb((const char *)&cgraph_checksum, sizeof(cgraph_checksum), cb_data);
+	if (error < 0)
+		goto cleanup;
+
+cleanup:
+	git_buf_dispose(&oid_lookup);
+	git_buf_dispose(&commit_data);
+	git_buf_dispose(&extra_edge_list);
+	git_hash_ctx_cleanup(&ctx);
+	return error;
+}
+
+static int commit_graph_write_filebuf(const char *buf, size_t size, void *data)
+{
+	git_filebuf *f = (git_filebuf *)data;
+	return git_filebuf_write(f, buf, size);
+}
+
+int git_commit_graph_writer_commit(
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts)
+{
+	int error;
+	int filebuf_flags = GIT_FILEBUF_DO_NOT_BUFFER;
+	git_buf commit_graph_path = GIT_BUF_INIT;
+	git_filebuf output = GIT_FILEBUF_INIT;
+
+	GIT_UNUSED(opts);
+
+	error = git_buf_joinpath(
+			&commit_graph_path, git_buf_cstr(&w->objects_info_dir), "commit-graph");
+	if (error < 0)
+		return error;
+
+	if (git_repository__fsync_gitdir)
+		filebuf_flags |= GIT_FILEBUF_FSYNC;
+	error = git_filebuf_open(&output, git_buf_cstr(&commit_graph_path), filebuf_flags, 0644);
+	git_buf_dispose(&commit_graph_path);
+	if (error < 0)
+		return error;
+
+	error = commit_graph_write(w, commit_graph_write_filebuf, &output);
+	if (error < 0) {
+		git_filebuf_cleanup(&output);
+		return error;
+	}
+
+	return git_filebuf_commit(&output);
+}
+
+int git_commit_graph_writer_dump(
+		git_buf *cgraph,
+		git_commit_graph_writer *w,
+		git_commit_graph_writer_options *opts)
+{
+	GIT_UNUSED(opts);
+	return commit_graph_write(w, commit_graph_write_buf, cgraph);
+}
diff --git a/src/commit_graph.h b/src/commit_graph.h
index e5e3ea1fd..9d0a827de 100644
--- a/src/commit_graph.h
+++ b/src/commit_graph.h
@@ -14,6 +14,7 @@
 #include "git2/sys/commit_graph.h"
 
 #include "map.h"
+#include "vector.h"
 
 /**
  * A commit-graph file.
@@ -119,6 +120,20 @@ int git_commit_graph_get_file(git_commit_graph_file **file_out, git_commit_graph
 void git_commit_graph_refresh(git_commit_graph *cgraph);
 
 /*
+ * A writer for `commit-graph` files.
+ */
+struct git_commit_graph_writer {
+	/*
+	 * The path of the `objects/info` directory where the `commit-graph` will be
+	 * stored.
+	 */
+	git_buf objects_info_dir;
+
+	/* The list of packed commits. */
+	git_vector commits;
+};
+
+/*
  * Returns whether the git_commit_graph_file needs to be reloaded since the
  * contents of the commit-graph file have changed on disk.
  */
diff --git a/tests/graph/commit_graph.c b/tests/graph/commit_graph.c
index 2ff157eb4..5926dca6f 100644
--- a/tests/graph/commit_graph.c
+++ b/tests/graph/commit_graph.c
@@ -1,8 +1,10 @@
 #include "clar_libgit2.h"
 
 #include <git2.h>
+#include <git2/sys/commit_graph.h>
 
 #include "commit_graph.h"
+#include "futils.h"
 
 void test_graph_commit_graph__parse(void)
 {
@@ -88,3 +90,36 @@ void test_graph_commit_graph__parse_octopus_merge(void)
 	git_repository_free(repo);
 	git_buf_dispose(&commit_graph_path);
 }
+
+void test_graph_commit_graph__writer(void)
+{
+	git_repository *repo;
+	git_commit_graph_writer *w = NULL;
+	git_revwalk *walk;
+	git_commit_graph_writer_options opts = GIT_COMMIT_GRAPH_WRITER_OPTIONS_INIT;
+	git_buf cgraph = GIT_BUF_INIT, expected_cgraph = GIT_BUF_INIT, path = GIT_BUF_INIT;
+
+	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_path(repo), "objects/info"));
+	cl_git_pass(git_commit_graph_writer_new(&w, git_buf_cstr(&path)));
+
+	/* This is equivalent to `git commit-graph write --reachable`. */
+	cl_git_pass(git_revwalk_new(&walk, repo));
+	cl_git_pass(git_revwalk_push_glob(walk, "refs/*"));
+	cl_git_pass(git_commit_graph_writer_add_revwalk(w, walk));
+	git_revwalk_free(walk);
+
+	cl_git_pass(git_commit_graph_writer_dump(&cgraph, w, &opts));
+	cl_git_pass(git_buf_joinpath(&path, git_repository_path(repo), "objects/info/commit-graph"));
+	cl_git_pass(git_futils_readbuffer(&expected_cgraph, git_buf_cstr(&path)));
+
+	cl_assert_equal_i(git_buf_len(&cgraph), git_buf_len(&expected_cgraph));
+	cl_assert_equal_i(memcmp(git_buf_cstr(&cgraph), git_buf_cstr(&expected_cgraph), git_buf_len(&cgraph)), 0);
+
+	git_buf_dispose(&cgraph);
+	git_buf_dispose(&expected_cgraph);
+	git_buf_dispose(&path);
+	git_commit_graph_writer_free(w);
+	git_repository_free(repo);
+}