Re: resumable git clone

ISTM an easier and possibly more backward compatible way to do this is to have receive-pack not just send a list of the refs it has, but also a list of hashes that are older that the sender can use for matching. For example for each ref something like ref~10, ref~20, ref~40, etc.. Or parents of recent merges, recent common ancestors, entries from the reflog, etc... I'm sure someone can think of something. Unless that would bloat the data for the common case. Perhaps a "receive-pack --try-harder".

I think the problem with partial pack files is due to git storing the current revisions as full files and storing old revisions as diffs w.r.t. them. Which makes it hard to make sense of a partial pack file because you actually need the current revisions to make sense of the old ones.

You could make it more reliable by, if send-pack would notice it is sending 1000 revisions, to split its job internally into chucks of 100 revisions, making it more likely a partial pack will be somewhat useful. In the end you're sending the same data, just in a different order, though there is some inefficiency.