Re: [PATCH 4/6] introduce a commit metapack

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 29, 2013, 17:38 UTC
Message-ID: <7vy5fbq48t.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20130129091610.GD9999@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 12 quoted lines

> +int commit_metapack(unsigned char *sha1,
> +		    uint32_t *timestamp,
> +		    unsigned char **tree,
> +		    unsigned char **parent1,
> +		    unsigned char **parent2)
> +{
> +	struct commit_metapack *p;
> +
> +	prepare_commit_metapacks();
> +	for (p = commit_metapacks; p; p = p->next) {
> +		unsigned char *data;
> +		int pos = sha1_entry_pos(p->index, 20, 0, 0, p->nr, p->nr, sha1);

This is a tangent, but isn't it about time to rip out the check for GIT_USE_LOOKUP in find_pack_entry_one(), I wonder.

Show 23 quoted lines

> +	prepare_commit_metapacks();
> +	for (p = commit_metapacks; p; p = p->next) {
> +		unsigned char *data;
> +		int pos = sha1_entry_pos(p->index, 20, 0, 0, p->nr, p->nr, sha1);
> +		if (pos < 0)
> +			continue;
> +
> +		/* timestamp(4) + tree(20) + parents(40) */
> +		data = p->data + 64 * pos;
> +		*timestamp = *(uint32_t *)data;
> +		*timestamp = ntohl(*timestamp);
> +		data += 4;
> +		*tree = data;
> +		data += 20;
> +		*parent1 = data;
> +		data += 20;
> +		*parent2 = data;
> +
> +		return 0;
> +	}
> +
> +	return -1;
> +}

I am torn on this one.

These cached properties of a single commit will not change no matter which pack it appears in, and it feels logically wrong, especially when you record these object names in the full SHA-1 form, to tie a "commit metapack" to a pack. Logically there needs only one commit metapack that describes all the commits known to the repository when the metapack was created.

In order to reduce the disk footprint and I/O cost, the future direction for this mechanism may want to point into an existing store of SHA-1 hashes with a shorter file offset, and the .idx file could be such a store, and in order to move in that direction, you cannot avoid tying a metapack to a pack.

Show 26 quoted lines

> +static void get_commits(struct metapack_writer *mw,
> +			const unsigned char *sha1,
> +			void *data)
> +{
> +	struct commit_list ***tail = data;
> +	enum object_type type = sha1_object_info(sha1, NULL);
> +	struct commit *c;
> +
> +	if (type != OBJ_COMMIT)
> +		return;
> +
> +	c = lookup_commit(sha1);
> +	if (!c || parse_commit(c))
> +		die("unable to read commit %s", sha1_to_hex(sha1));
> +
> +	/*
> +	 * Our fixed-size parent list cannot represent root commits, nor
> +	 * octopus merges. Just skip those commits, as we can fallback
> +	 * in those rare cases to reading the actual commit object.
> +	 */
> +	if (!c->parents ||
> +	    (c->parents && c->parents->next && c->parents->next->next))
> +		return;
> +
> +	*tail = &commit_list_insert(c, *tail)->next;
> +}

It feels somewhat wasteful to:

 - use commit_list for this, rather than an array of commit
   objects.  If you have a rough estimate of the number of commits
   in the pack, you could just preallocate a single array and use
   ALLOC_GROW() on it, no?

 - iterate over the .idx file and run sha1_object_info() and
   parse_commit() on many objects in the SHA-1 order.  Iterating in
   the way builtin/pack-objects.c::get_object_details() does avoids
   jumping around in existing packfiles, which may be more
   efficient, no?

Show 13 quoted lines

> +void commit_metapack_write(const char *idx)
> +{
> +	struct metapack_writer mw;
> +	struct commit_list *commits = NULL, *p;
> +	struct commit_list **tail = &commits;
> +	uint32_t nr = 0;
> +
> +	metapack_writer_init(&mw, idx, "commits", 1);
> +
> +	/* Figure out how many eligible commits we've got in this pack. */
> +	metapack_writer_foreach(&mw, get_commits, &tail);
> +	for (p = commits; p; p = p->next)
> +		nr++;