Subject: Re: [PATCH v2 14/25] shallow.c: implement a generic shallow boundary finder based on rev-list

From: Junio C Hamano <[email-masked]>
Date: Mon, 08 Feb 2016 13:09:24 -0800

Nguyễn Thái Ngọc Duy  <[email-masked]> writes:

> Instead of a custom commit walker like get_shallow_commits(), this new
> function uses rev-list to mark NOT_SHALLOW to all reachable commits,
> except borders. The definition of reachable is to be defined by the
> protocol later. This makes it more flexible to define shallow boundary.
>
> Note: if a commit has one NOT_SHALLOW parent and one SHALLOW parent,
> then it's considered the boundary. Which means in the client side, this
> commit has _no_ parents. This could lead to surprising cuts if we're not
> careful.
>
> Another option is to include more commits and only mark commits whose
> all parents are SHALLOW as boundary.

The second and third are greek to me at this point ;-) but hopefully
they will become clear as we read on.

> +/*
> + * Given rev-list arguments, run rev-list. All reachable commits
> + * except border ones are marked with not_shallow_flag. Border commits
> + * are marked with shallow_flag. The list of border/shallow commits
> + * are also returned.
> + */
> +struct commit_list *get_shallow_commits_by_rev_list(int ac, const char **av,
> +						    int shallow_flag,
> +						    int not_shallow_flag)
> +{
> +	struct commit_list *result = NULL, *p;
> +	struct rev_info revs;
> +	unsigned int i, nr;
> +
> +	/*
> +	 * SHALLOW (excluded) and NOT_SHALLOW (included) should not be
> +	 * set at this point. But better be safe than sorry.
> +	 */
> +	nr = get_max_object_index();
> +	for (i = 0; i < nr; i++) {
> +		struct object *o = get_indexed_object(i);
> +		if (!o || o->type != OBJ_COMMIT)
> +			continue;
> +		o->flags &= ~(shallow_flag | not_shallow_flag);
> +	}

This is slightly different from clear_object_flags(), but I cannot
tell if it is intended, or if you forgot that the function exists.

> +	is_repository_shallow(); /* make sure shallows are read */
> +
> +	init_revisions(&revs, NULL);
> +	save_commit_buffer = 0;
> +	setup_revisions(ac, av, &revs, NULL);
> +
> +	/* Mark all reachable commits as NOT_SHALLOW */
> +	if (prepare_revision_walk(&revs))
> +		die("revision walk setup failed");
> +	traverse_commit_list(&revs, show_commit, NULL, &not_shallow_flag);
> +
> +	/*
> +	 * mark border commits SHALLOW + NOT_SHALLOW.
> +	 * We cannot clear NOT_SHALLOW right now. Imagine border
> +	 * commit A is processed first, then commit B, whose parent is
> +	 * A, later. If NOT_SHALLOW on A is cleared at step 1, B
> +	 * itself is considered border at step 2, which is incorrect.
> +	 */
> +	nr = get_max_object_index();
> +	for (i = 0; i < nr; i++) {

I'd really like not to see a loop over 0..get_max_object_index().
Are there many codepaths that peek into the in-core entire object
store already?  Would it work equally well to keep track of the
commits discovered in show_commit() to use as the set of commits
you need to visit in this second pass?

> +		struct object *o = get_indexed_object(i);
> +		struct commit *c = (struct commit *)o;
> +
> +		if (!o || o->type != OBJ_COMMIT ||
> +		    !(o->flags & not_shallow_flag))
> +			continue;
> +
> +		if (parse_commit(c))
> +			die("unable to parse commit %s",
> +			    oid_to_hex(&c->object.oid));
> +
> +		for (p = c->parents; p; p = p->next)
> +			if (!(p->item->object.flags & not_shallow_flag)) {
> +				o->flags |= shallow_flag;
> +				commit_list_insert(c, &result);
> +				break;
> +			}
> +	}
> +
> +	/*
> +	 * Now we can clean up NOT_SHALLOW on border commits. Having
> +	 * both flags set can confuse the caller.
> +	 */
> +	for (p = result; p; p = p->next) {
> +		struct object *ro = &p->item->object;

Why "ro" only in this third pass, unlike the other two passes that
said "o" which is in a sense more descriptive?

> +		if ((ro->flags & not_shallow_flag) &&
> +		    (ro->flags & shallow_flag))

If you introduce a "both_flags = shallow_flag | not_shallow_flag"
at the very beginning, this will become

	if (o->flags & both_flags)
        	o->flags &= ~not_shallow_flag;

which would probably be easier to read.  You can pass the same to
clear_object_flags() at the first pass.

> +			ro->flags &= ~not_shallow_flag;
> +	}
> +	return result;
> +}

Other than that, this step looks quite straight-forward to me.

Thanks.

> +
>  static void check_shallow_file_for_update(void)
>  {
>  	if (is_shallow == -1)