Subject: Re: [PATCH v4 03/10] commit-graph: compute generation numbers

From: Jakub Narebski <[email-masked]>
Date: Sun, 29 Apr 2018 11:08:13 +0200

Derrick Stolee <[email-masked]> writes:

> While preparing commits to be written into a commit-graph file, compute
> the generation numbers using a depth-first strategy.

Sidenote: for generation numbers it does not matter if we use
depth-first or breadth-first strategy, but it is more natural to use
depth-first search because generation numbers need post-order processing
(parents before child).

>
> The only commits that are walked in this depth-first search are those
> without a precomputed generation number. Thus, computation time will be
> relative to the number of new commits to the commit-graph file.

A question: what happens if the existing commit graph is from older
version of git and has _ZERO for generation numbers?

Answer: I see that we treat both _INFINITY (not in commit-graph) and
_ZERO (in commit graph but not computed) as not computed generation
numbers.  All right.

>
> If a computed generation number would exceed GENERATION_NUMBER_MAX, then
> use GENERATION_NUMBER_MAX instead.

All right, though I guess this would remain theoretical for a long
while.

We don't have any way of testing this, at least not without recompiling
Git with lower value of GENERATION_NUMBER_MAX -- which means not
automatically, isn't it?

>
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  commit-graph.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 9ad21c3ffb..047fa9fca5 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -439,6 +439,9 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
>  		else
>  			packedDate[0] = 0;
>  
> +		if ((*list)->generation != GENERATION_NUMBER_INFINITY)
> +			packedDate[0] |= htonl((*list)->generation << 2);
> +

If we stumble upon commit marked as "not in commit-graph" while writing
commit graph, it is a BUG(), isn't it?

(Problem noticed by Junio.)

It is a bit strange to me that the code uses get_be32 for reading, but
htonl for writing.  Is Git tested on non little-endian machines, like
big-endian ppc64 or s390x, or on mixed-endian machines (or
selectable-endian machines with data endianness set to non
little-endian, like ia64)?  If not, could we use for example openSUSE
Build Service (https://build.opensuse.org/) for this?

>  		packedDate[1] = htonl((*list)->date);
>  		hashwrite(f, packedDate, 8);
>  
> @@ -571,6 +574,46 @@ static void close_reachable(struct packed_oid_list *oids)
>  	}
>  }
>  
> +static void compute_generation_numbers(struct commit** commits,
> +				       int nr_commits)
> +{
> +	int i;
> +	struct commit_list *list = NULL;

All right, commit_list will work as stack.

> +
> +	for (i = 0; i < nr_commits; i++) {
> +		if (commits[i]->generation != GENERATION_NUMBER_INFINITY &&
> +		    commits[i]->generation != GENERATION_NUMBER_ZERO)
> +			continue;

All right, we consider _INFINITY and _SERO as not computed.  If
generation number is computed (by 'recursion' or from commit graph), we
(re)use it.  This means that generation number calculation is
incremental, as intended -- good.

> +
> +		commit_list_insert(commits[i], &list);

Start depth-first walks from commits given.

> +		while (list) {
> +			struct commit *current = list->item;
> +			struct commit_list *parent;
> +			int all_parents_computed = 1;

Here all_parents_computed is a boolean flag.  I see that it is easier to
start with assumption that all parents will have computed generation
numbers.

> +			uint32_t max_generation = 0;

The generation number value of 0 functions as sentinel; generation
numbers start from 1.  Not that it matters much, as lowest possible
generation number is 1, and we could have started from that value.

> +
> +			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;

If some parent doesn't have generation number calculated, we add it to
stack (and break out of loop because it is depth-first walk), and mark
this situation.  All right.

> +				} else if (parent->item->generation > max_generation) {
> +					max_generation = parent->item->generation;

Otherwise, update max_generation.  All right.

> +				}
> +			}
> +
> +			if (all_parents_computed) {
> +				current->generation = max_generation + 1;
> +				pop_commit(&list);
> +			}
> +
> +			if (current->generation > GENERATION_NUMBER_MAX)
> +				current->generation = GENERATION_NUMBER_MAX;

This conditional should be inside all_parents_computed test, for example
like this:

  +			if (all_parents_computed) {
  +				current->generation = max_generation + 1;
  +				if (current->generation > GENERATION_NUMBER_MAX)
  +					current->generation = GENERATION_NUMBER_MAX;
  +
  +				pop_commit(&list);
  +			}

(Noticed by Junio.)

Sidenote: when we revisit the commit, returning from depth-first walk of
one of its parents, we calculate max_generation from scratch again.
This does not matter for performance, as it's just data access and
calculating maximum - any workaround to not restart those calculations
would take more time and memory.  And it's simple.

> +		}
> +	}
> +}
> +
>  void write_commit_graph(const char *obj_dir,
>  			const char **pack_indexes,
>  			int nr_packs,
> @@ -694,6 +737,8 @@ void write_commit_graph(const char *obj_dir,
>  	if (commits.nr >= GRAPH_PARENT_MISSING)
>  		die(_("too many commits to write graph"));
>  
> +	compute_generation_numbers(commits.list, commits.nr);
> +

Nice and simple.  All right.

I guess that we do not pass "struct packed_commit_list commits" as
argument to compute_generation_numbers instead of "struct commit**
commits.list" and "int commits.nr" to compute_generation_numbers() to
keep the latter nice and generic?

>  	graph_name = get_commit_graph_filename(obj_dir);
>  	fd = hold_lock_file_for_update(&lk, graph_name, 0);

Best,
-- 
Jakub Narębski