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/pull.112.git.gitgitgadget@xxxxxxxxx/ [2] https://public-inbox.org/git/87h8acivkh.fsf@xxxxxxxxxxxxxxxxxxx/
