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

[Derrick Stolee <stolee@gmail.com>]

n 4/25/2018 10:35 PM, Junio C Hamano wrote:
> Derrick Stolee <dstolee@microsoft.com> writes:
>
>> @@ -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);
>> +
>>   		packedDate[1] = htonl((*list)->date);
>>   		hashwrite(f, packedDate, 8);
> The ones that have infinity are written as zero here.  The code that
> reads the generation field off of a file in fill_commit_graph_info()
> and fill_commit_in_graph() both leave such a record in file as-is,
> so the reader of what we write out will think it is _ZERO, not _INF.
>
> Not that it matters, as it seems that most of the code being added
> by this series treat _ZERO and _INF more or less interchangeably.
> But it does raise another question, i.e. do we need both _ZERO and
> _INF, or is it sufficient to have just a single _UNKNOWN?

This code is confusing. The 'if' condition is useless, since at this 
point every commit should be finite (since we computed generation 
numbers for everyone). We should just write the value always.

For the sake of discussion, the value _INFINITY means not in the graph 
and _ZERO means in the graph without a computed generation number. It's 
a small distinction, but it gives a single boundary to use for 
reachability queries that test generation number.

>
>> @@ -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;
>> +
>> +	for (i = 0; i < nr_commits; i++) {
>> +		if (commits[i]->generation != GENERATION_NUMBER_INFINITY &&
>> +		    commits[i]->generation != GENERATION_NUMBER_ZERO)
>> +			continue;
>> +
>> +		commit_list_insert(commits[i], &list);
>> +		while (list) {
>> +			struct commit *current = list->item;
>> +			struct commit_list *parent;
>> +			int all_parents_computed = 1;
>> +			uint32_t max_generation = 0;
>> +
>> +			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;
>> +				} else if (parent->item->generation > max_generation) {
>> +					max_generation = parent->item->generation;
>> +				}
>> +			}
>> +
>> +			if (all_parents_computed) {
>> +				current->generation = max_generation + 1;
>> +				pop_commit(&list);
>> +			}
> If we haven't computed all parents' generations yet,
> current->generation is undefined (or at least "left as
> initialized"), so it does not make much sense to attempt to clip it
> at _MAX at this point.  At leat not yet.
>
> IOW, shouldn't the following two lines be inside the "we now know
> genno of all parents, so we can compute genno for commit" block
> above?

You're right! Good catch. This code sets every merge commit to _MAX. It 
should be in the block above.

>
>> +			if (current->generation > GENERATION_NUMBER_MAX)
>> +				current->generation = GENERATION_NUMBER_MAX;
>> +		}
>> +	}
>> +}
>> +
>>   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);
>> +
>>   	graph_name = get_commit_graph_filename(obj_dir);
>>   	fd = hold_lock_file_for_update(&lk, graph_name, 0);