On Tue, Nov 21, 2023 at 2:46 PM Patrick Steinhardt <ps@xxxxxx> wrote: > To me it seems like deletions in this case only delete a particular log > entry instead of the complete log for a particular reference. And some > older discussion [1] seems to confirm my hunch that a complete reflog is > deleted not with `log_type = 0x0`, but instead by writing the null > object ID as new ID. No, writing a null OID (more precisely a transition from $SHA1 => $ZERO) means that a branch was at $SHA1, and then was deleted. The reflog continues to exist, and new entries may be added by reviving the branch. That would add a $ZERO => $NEWSHA transition, but the history of the branch prior to its deletion is retained. > # This behaviour is a bit more on the weird side. We delete the > # referee, and that causes the files backend to claim that the reflog > # for HEAD is gone, as well. The reflog still exists though, as > # demonstrated in the next command. > $ git update-ref -m delete-main -d refs/heads/main > $ git reflog show HEAD > fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree. This looks wrong to me. HEAD has a history, and that history didn't go away because the current branch happened to be deleted. > It kind of feels like the second step in the files backend where the > reflog is claimed to not exist is buggy -- I'd have expected to still right, I agree. > see the reflog, as the HEAD reference exists quite alright and has never > stopped to exist. And in the third step, I'd have expected to see three > reflog entries, including the deletion that is indeed still present in > the on-disk logfile. > > But with the reftable backend the problem becomes worse: we cannot even > represent the fact that the reflog still exists, but that the deletion > of the referee has caused the HEAD to point to the null OID, because the > null OID indicates complete deletion of the reflog. This doesn't match my recollection. See https://github.com/git/git/pull/1215, or more precisely https://github.com/git/git/blob/3b2c5ddb28fa42a8c944373bea2ca756b1723908/refs/reftable-backend.c#L1582 Removing the entire reflog means removing all the individual entries using tombstones. > Consequentially, if > we wrote the null OID, we'd only be able to see the last log entry here. > > It may totally be that I'm missing something obvious here. But if not, > it leaves us with a couple of options for how to fix it: > > 1. Disregard this edge case and accept that the reftable backend > does not do exactly the same thing as the files backend in very > specific situations like this. I remember discussing with Jun that it would be acceptable to have slight semantic differences if unavoidable for the reflogs, and there should be a record of this in the list. I think there will always be some differences: for example, dropping reflogs because of a dir/file conflict seems like a bug rather than a feature. > 2. Change the reftable format so that it can discern these cases, > e.g. by indicating deletion via a new log type. This will be a bit messy, because it means that every read of the reflog has to special case the "deletion marker" to make sure it is absent, and on every reflog write, you have to create a tombstone for the deletion marker, to make sure the reflog exists. Also, what if you have something that looks like: refs/main@1 : 0000... => 345.... refs/main@0xfffffff: DELETION the reflog doesn't exists ("DELETION"), and now it is recreated, ie. the following is added: refs/main@0xfffffff: TOMBSTONE DELETION (make sure reflog exists) refs/main@2 : 0000... => abc.... (the entry we want to write) the result is that the reflog would also surface the first entry (@1 , 000... => 345... ) again. To prevent that from happening, you have to write tombstones for all entries if you "delete the reflog", but then do you really need a separate existence marker? > None of these alternatives feel particularly great to me. In my opinion > (2) is likely the best option, but would require us to change the format > format that's already in use by Google in the context of multiple > projects. So I'm not quite sure how thrilled you folks at Google and > other users of the reftable library are with this prospect. Google uses reftable for datacenter-local serving. The reflog is stored in a database, because it's never involved in serving traffic. When I implemented reftable-on-filesystem, I found a few bugs in the reflog code because it was never exercised at Google. Terry/Jonathan may correct me if my knowledge is outdated. Luca manages large Gerrit installations. He might know if anyone has been using reftable in the wild. HTH. -- Han-Wen Nienhuys - hanwenn@xxxxxxxxx - http://www.xs4all.nl/~hanwen
