Subject: Re: Revision walking, commit dates, slop

From: Derrick Stolee <[email-masked]>
Date: Mon, 20 May 2019 07:20:01 -0400

On 5/20/2019 7:02 AM, Jakub Narebski wrote:
>
> Are there any blockers that prevent the switch to this
> "generation number v2"?
> 
> - Is it a problem with insufficient data to choose the correct numbering
>   as "generation number v2' (there can be only one)?
> - Is it a problem with selected "generation number v2" being
>   incompatibile with gen v2, and Git failing when new version of
>   commit-graph is used instead of softly just not using commit-graph?
> - Or is it something else?

The switch becomes a bit complicated.

First, the plan was to version the commit-graph file to v2, and that would
include a byte in the header for the "reachability index version" [1]. Since
older clients fail hard on a newer file version, we are switching to instead
including the reachability index version as a value in a more flexible 
"metadata chunk" [2]. Using the generation number column for the corrected
commit-date offsets (assuming we also guarantee the offset is strictly
increasing from parent to child), these new values will be backwards-
compatible _except_ for 'git commit-graph verify'.

Second, we need to pull the reachability index value into a commit slab.
The generation value is currently 32 bits, but we will expand that to
64 as it stores a timestamp. The commit struct is a bit bloated already,
so this will reduce the required memory space even when not using the
commit-graph. But, it requires some refactoring, including every place
where we pass a "min_generation" needs to change type and name.

Third and finally, we need to calculate the new values and change the
read logic to sum the offset and commit-date (when the metadata chunk
says we are using corrected commit date).

While none of this is _incredibly_ hard to do, it does require a bit
of care. It's on my list to get to at some point, but making the file
incremental is higher priority to me.

Thanks,
-Stolee

[1] https://public-inbox.org/git/[email-masked]/
[2] https://public-inbox.org/git/[email-masked]/