Subject: Re: [PATCH v2 03/10] commit: add generation number to struct commmit

From: Derrick Stolee <[email-masked]>
Date: Wed, 11 Apr 2018 08:57:05 -0400

On 4/10/2018 10:31 PM, Junio C Hamano wrote:
> Derrick Stolee <[email-masked]> writes:
>
>> The generation number of a commit is defined recursively as follows:
>>
>> * If a commit A has no parents, then the generation number of A is one.
>> * If a commit A has parents, then the generation number of A is one
>>    more than the maximum generation number among the parents of A.
>>
>> Add a uint32_t generation field to struct commit so we can pass this
>> information to revision walks. We use two special values to signal
>> the generation number is invalid:
>>
>> GENERATION_NUMBER_ININITY 0xFFFFFFFF
>> GENERATION_NUMBER_ZERO 0
>>
>> The first (_INFINITY) means the generation number has not been loaded or
>> computed. The second (_ZERO) means the generation number was loaded
>> from a commit graph file that was stored before generation numbers
>> were computed.
> Should it also be possible for a caller to tell if a given commit
> has too deep a history, i.e. we do not know its generation number
> exactly, but we know it is larger than 1<<30?
>
> It seems that we only have a 30-bit field in the file, so wouldn't
> we need a special value defined in (e.g. "0") so that we can tell
> that the commit has such a large generation number?  E.g.
>
>> +	item->generation = get_be32(commit_data + g->hash_len + 8) >> 2;
> 	if (!item->generation)
> 		item->generation = GENERATION_NUMBER_OVERFLOW;
>
> when we read it from the file?
>
> We obviously need to do something similar when assigning a
> generation number to a child commit, perhaps like
>
> 	#define GENERATION_NUMBER_OVERFLOW (GENERATION_NUMBER_MAX + 1)
>
> 	commit->generation = 1; /* assume no parent */
> 	for (p = commit->parents; p; p++) {
> 		uint32_t gen = p->item->generation + 1;
>
> 		if (gen >= GENERATION_NUMBER_OVERFLOW) {
> 			commit->generation = GENERATION_NUMBER_OVERFLOW;
> 			break;
> 		} else if (commit->generation < gen)
> 			commit->generation = gen;
> 	}
>          
> or something?  And then on the writing side you'd encode too large a
> generation as '0'.

You raise a very good point. How about we do a slightly different 
arrangement for these overflow commits?

Instead of storing the commits in the commit-graph file as "0" (which 
currently means "written by a version of git that did not compute 
generation numbers") we could let GENERATION_NUMBER_MAX be the maximum 
generation of a commit in the commit-graph, and if a commit would have 
larger generation, we collapse it down to that value.

It slightly complicates the diagram I made in 
Documentation/technical/commit-graph.txt, but it was already a bit of a 
simplification. Here is an updated diagram, but likely we will want to 
limit discussion of the special-case GENERATION_NUMBER_MAX to the prose, 
since it is not a practical situation at the moment.

     +-----------------------------------------+
     | GENERATION_NUMBER_INFINITY = 0xFFFFFFFF |
     +-----------------------------------------+
       |    |            |      ^
       |    |            |      |
       |    |            +------+
       |    |         [gen(A) = gen(B)]
       |    V
       |  +------------------------------------+
       |  | GENERATION_NUMBER_MAX = 0x3FFFFFFF |
       |  +------------------------------------+
       |    |            |      ^
       |    |            |      |
       |    |            +------+
       |    |         [gen(A) = gen(B)]
       V    V
     +-------------------------------------+
     | 0 < commit->generation < 0x3FFFFFFF |
     +-------------------------------------+
         |            |      ^
         |            |      |
         |            +------+
         |        [gen(A) > gen(B)]
         V
     +-------------------------------------+
     | GENERATION_NUMBER_ZERO = 0          |
     +-------------------------------------+
              |      ^
              |      |
              +------+
              [gen(A) = gen(B)]

Thanks,
-Stolee