Re: Inconsistencies in commit-graph technical docs.

[Derrick Stolee <stolee@gmail.com>]

On 6/28/2018 1:11 AM, Grant Welch wrote:
> I recently read the "Supercharging the Git Commit Graph blog by
> Derrick Stolee. I found the article interesting and wanted to verify
> the performance numbers for myself. Then that led me to want to know
> more about the implementation, so I read the technical design notes in
> commit-graph.txt, and then I jumped into the format documentation in
> commit-graph-format.txt.
>
> Along the way, I noticed a few issues. They might just be errors in
> the documentation, but I figured it was worth documenting my entire
> process just to be sure.
>
> "Supercharging the Git Commit Graph", by Derrick Stolee:
>    https://blogs.msdn.microsoft.com/devops/2018/06/25/supercharging-the-git-commit-graph/
>
> # TL;DR
>
> I found a few discrepencies between the documentation in
> commit-graph-format.txt and the results that I observed for myself.
>
> 1. The "Commit Data" chunk signature is documented to be 'CGET', but
> it should be 'CDAT'.
>
> commit-graph.c:18
>    #define GRAPH_CHUNKID_DATA 0x43444154 /* "CDAT" */

Thanks for catching this! Thankfully, this is an easy fix, as we only 
need to update the documentation.

> 2. The "no parent" value is documented to be 0xffffffff, but is
> actually 0x70000000.
>
> commit-graph.c:34
>    #define GRAPH_PARENT_NONE 0x70000000

This is a more important mistake, as it affects the data that was 
written in the commit-graph file.

Part of the problem is that leading hex digit of 0x7 which in binary is 
0b0111. We already designed a limit of at most 2^{31}-1 (~2.1 billion) 
commits in the commit-graph because of the way we track octopus edges, 
but this mistake has cost us more: we cannot store more than ~1.8 
billion commits.

I'm sorry for this mixup, mostly because it is aesthetically unpleasant. 
Those extra 300 million commits mean less to me than having a clean file 
format.

> 3. The "generation" field isn't set on any of the commits. (I don't
> know what to make of this.)

This is a difference between 2.18 and current 'master', which merged 
ds/generation-numbers. Commit-graphs written with Git 2.18 have all 
generation numbers listed as GENERATION_NUMBER_ZERO (0), which lets 
future versions know that the generation number is not computed yet, so 
the next commit-graph write will compute the correct generation number.

I'll send a patch soon fixing these doc issues.

Thanks,
-Stolee