Re: [PATCH v2 04/14] commit-graph: implement construct_commit_graph()

[Jonathan Tan <jonathantanmy@google.com>]

On Tue, 30 Jan 2018 16:39:33 -0500
Derrick Stolee <stolee@gmail.com> wrote:

> +#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" */

Could all these just be string constants? sha1write can handle them well
enough.

> +static void write_graph_chunk_fanout(struct sha1file *f,
> +				     struct commit **commits,
> +				     int nr_commits)
> +{
> +	uint32_t i, count = 0;
> +	struct commit **list = commits;
> +	struct commit **last = commits + nr_commits;
> +
> +	/*
> +	 * 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++) {
> +		uint32_t swap_count;
> +
> +		while (list < last) {
> +			if ((*list)->object.oid.hash[0] != i)
> +				break;
> +			count++;
> +			list++;
> +		}
> +
> +		swap_count = htonl(count);
> +		sha1write(f, &swap_count, 4);

You can use sha1write_be32() instead of swapping.

> +static void write_graph_chunk_large_edges(struct sha1file *f,
> +					  struct commit **commits,
> +					  int nr_commits)
> +{
> +	struct commit **list = commits;
> +	struct commit **last = commits + nr_commits;
> +	struct commit_list *parent;
> +
> +	while (list < last) {
> +		int num_parents = 0;
> +		for (parent = (*list)->parents; num_parents < 3 && parent;
> +		     parent = parent->next)
> +			num_parents++;
> +
> +		if (num_parents <= 2) {
> +			list++;
> +			continue;
> +		}
> +
> +		for (parent = (*list)->parents; parent; parent = parent->next) {
> +			uint32_t int_id, swap_int_id;
> +			uint32_t last_edge = 0;
> +
> +			if (parent == (*list)->parents)
> +				continue;

Probably better to just initialize "parent = (*list)->parents->next".
Also probably best to add a comment describing why you are doing this
(e.g. "The first parent is already in the main commit table; the large
edges table only contains the second parent onwards").

> +struct packed_commit_list {
> +	struct commit **list;
> +	int num;
> +	int size;
> +};
> +
> +struct packed_oid_list {
> +	struct object_id **list;
> +	int num;
> +	int size;
> +};

What are num and size? If they're nr and alloc, maybe use those names
instead.

> +static int if_packed_commit_add_to_list(const struct object_id *oid,
> +					struct packed_git *pack,
> +					uint32_t pos,
> +					void *data)
> +{
> +	struct packed_oid_list *list = (struct packed_oid_list*)data;
> +	enum object_type type;
> +	unsigned long size;
> +	void *inner_data;
> +	off_t offset = nth_packed_object_offset(pack, pos);
> +	inner_data = unpack_entry(pack, offset, &type, &size);
> +
> +	if (inner_data)
> +		free(inner_data);
> +
> +	if (type != OBJ_COMMIT)
> +		return 0;
> +
> +	ALLOC_GROW(list->list, list->num + 1, list->size);
> +	list->list[list->num] = (struct object_id *)malloc(sizeof(struct object_id));

No need to cast return value of malloc. Also, use xmalloc?

> +struct object_id *construct_commit_graph(const char *pack_dir)
> +{
> +	struct packed_oid_list oids;
> +	struct packed_commit_list commits;
> +	struct commit_graph_header hdr;
> +	struct sha1file *f;
> +	int i, count_distinct = 0;
> +	struct strbuf tmp_file = STRBUF_INIT;
> +	unsigned char final_hash[GIT_MAX_RAWSZ];
> +	char *graph_name;
> +	int fd;
> +	uint32_t chunk_ids[5];
> +	uint64_t chunk_offsets[5];
> +	int num_long_edges;
> +	struct object_id *f_hash;
> +	char *fname;
> +	struct commit_list *parent;
> +
> +	oids.num = 0;
> +	oids.size = 1024;
> +	ALLOC_ARRAY(oids.list, oids.size);
> +	for_each_packed_object(if_packed_commit_add_to_list, &oids, 0);
> +	QSORT(oids.list, oids.num, commit_compare);
> +
> +	count_distinct = 1;
> +	for (i = 1; i < oids.num; i++) {
> +		if (oidcmp(oids.list[i-1], oids.list[i]))
> +			count_distinct++;
> +	}
> +
> +	commits.num = 0;
> +	commits.size = count_distinct;
> +	ALLOC_ARRAY(commits.list, commits.size);
> +
> +	num_long_edges = 0;
> +	for (i = 0; i < oids.num; i++) {
> +		int num_parents = 0;
> +		if (i > 0 && !oidcmp(oids.list[i-1], oids.list[i]))
> +			continue;
> +
> +		commits.list[commits.num] = lookup_commit(oids.list[i]);
> +		parse_commit(commits.list[commits.num]);
> +
> +		for (parent = commits.list[commits.num]->parents;
> +		     parent; parent = parent->next)
> +			num_parents++;
> +
> +		if (num_parents > 2)
> +			num_long_edges += num_parents - 1;
> +
> +		commits.num++;
> +	}
> +
> +	strbuf_addstr(&tmp_file, pack_dir);
> +	strbuf_addstr(&tmp_file, "/tmp_graph_XXXXXX");
> +
> +	fd = git_mkstemp_mode(tmp_file.buf, 0444);
> +	if (fd < 0)
> +		die_errno("unable to create '%s'", tmp_file.buf);
> +
> +	graph_name = strbuf_detach(&tmp_file, NULL);
> +	f = sha1fd(fd, graph_name);
> +
> +	hdr.graph_signature = htonl(GRAPH_SIGNATURE);
> +	hdr.graph_version = GRAPH_VERSION;
> +	hdr.hash_version = GRAPH_OID_VERSION;
> +	hdr.hash_len = GRAPH_OID_LEN;
> +	hdr.num_chunks = 4;
> +
> +	assert(sizeof(hdr) == 8);
> +	sha1write(f, &hdr, sizeof(hdr));

Instead of assembling these into a data structure, could you just use
individual calls to sha1write_be32 and sha1write_u8? Same comment
throughout this function.

> diff --git a/commit-graph.h b/commit-graph.h
> new file mode 100644
> index 0000000000..7b3469a7df
> --- /dev/null
> +++ b/commit-graph.h
> @@ -0,0 +1,20 @@
> +#ifndef COMMIT_GRAPH_H
> +#define COMMIT_GRAPH_H
> +
> +#include "git-compat-util.h"
> +#include "commit.h"
> +
> +extern char* get_commit_graph_filename_hash(const char *pack_dir,
> +					    struct object_id *hash);
> +
> +struct commit_graph_header {
> +	uint32_t graph_signature;
> +	unsigned char graph_version;
> +	unsigned char hash_version;
> +	unsigned char hash_len;
> +	unsigned char num_chunks;
> +};

This seems like it should be an internal detail of commit-graph.c.
(Also, as commented above, this struct might not be necessary at all.)