By the time we decide we need to do a partial clone fetch, we already know the object is missing, even if the_repository->parsed_objects thinks it exists. But --refetch bypasses the local object check, so we can guarantee that a JIT fetch will fix incorrect local caching. This manifested at $DAYJOB in a repo with the following features: * blob-filtered partial clone enabled * commit graph enabled * ref Foo pointing to commit object 6aaaca * object 6aaaca missing[a] With these prerequisites, we noticed that `git fetch` in the repo produced an infinite loop: 1. `git fetch` tries to fetch, but thinks it has all objects, so it noops. 2. At the end of cmd_fetch(), we try to write_commit_graph_reachable(). 3. write_commit_graph_reachable() does a reachability walk, including starting from Foo 4. The reachability walk tries to peel Foo, and notices it's missing 6aaaca. 5. The partial clone machinery asks for a per-object JIT fetch of 6aaaca. 6. `git fetch` (child process) is asked to fetch 6aaaca. 7. We put together the_repository->parsed_objects, adding all commit IDs reachable from local refs to it (fetch-pack.c:mark_complete_and_common_refs(), trace region mark_complete_local_refs). We see Foo, so we add 6aaaca to the_repository->parsed_objects and mark it as COMPLETE. 8. cmd_fetch notices that the object ID it was asked for is already known, so does not fetch anything new. 9. GOTO 2. The culprit is that we're assuming all local refs already must have objects in place. Using --refetch means we ignore that assumption during JIT fetch. Signed-off-by: Emily Shaffer <emilyshaffer@xxxxxxxxxx> --- There are a few alternative approaches for this issue that I talked about with some folks at $DAYJOB: i. Just disabling the commit graph rewrite allows this to fall eventually into a path where the fetch actually succeeds. I didn't like this solution - it's just whack-a-mole - so I didn't look too hard into why it succeeds that way. It *could* make sense to disable commit graph rewrite when we do a JIT fetch with blob or tree filter provided - but if later we want to implement commit filter (something we've talked about at Google) then I'd worry about this situation coming up again. ii. We could decide not to mark local refs (and commits reachable from them) as COMPLETE in the_repository->parsed_objects. I didn't try this solution out, and I'm not sure what the performance implications are, but Jonathan Tan likes this solution, so I may try it out and see what breaks shortly. iii. We could do all the JIT fetches with --refetch. In my opinion, this is the safest/most self-healing solution; the JIT fetch only happens when we really know we're missing the object, so it doesn't make sense for that fetch to be canceled by any cache. It doesn't have performance implications as far as I can guess (except that I think we still build the parsed_objects hash even though we are going to ignore it, but we already were doing that anyway). Of course, that's what this patch does. iv. We could do nothing; when cmd_fetch gets a fetch-by-object-id but decides there is nothing more to do, it could terminate with an error. That should stop the infinite recursion, and the error could suggest the user to run `git fsck` and discover what the problem is. Depending on the remediation we suggest, though, I think a direct fetch to fix this particular loop would not work. I'm curious to hear thoughts from people who are more expert than me on partial clone and fetching in general, though. This change is also still in RFC, for two reasons: First, it's intermittently failing tests for me locally, in weirdly flaky ways: - t0410-partial-clone.sh fails when I run it from prove, but passes when I run it manually, every time. - t5601-clone.sh and t5505-remote.sh fail nonsensically on `rm -rf` that should succeed (and does succeed if I stop the test with test_pause), which makes me think there's something else borked in my setup, but I'm not sure what. - t5616-partial-clone.sh actually does fail in a way that I could see having to do with this change (since I guess we might download more packs than usual), but I was so confused by the other two errors I haven't looked closely yet. And secondly, I didn't write tests verifying the breakage and that this change fixes it yet, either. I'm going to work on both those things in the background, but I wanted to get the description and RFC out early so that folks could take a look and we could decide which approach is best. Thanks, - Emily a: That commit object went missing as a byproduct of this partial clone gc issue that Calvin, Jonathan, Han Young, and others have been investigating: https://lore.kernel.org/git/20241001191811.1934900-1-calvinwan@xxxxxxxxxx/ --- promisor-remote.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/promisor-remote.c b/promisor-remote.c index 9345ae3db2..cf00e31d3b 100644 --- a/promisor-remote.c +++ b/promisor-remote.c @@ -43,7 +43,7 @@ static int fetch_objects(struct repository *repo, strvec_pushl(&child.args, "-c", "fetch.negotiationAlgorithm=noop", "fetch", remote_name, "--no-tags", "--no-write-fetch-head", "--recurse-submodules=no", - "--filter=blob:none", "--stdin", NULL); + "--filter=blob:none", "--refetch", "--stdin", NULL); if (!git_config_get_bool("promisor.quiet", &quiet) && quiet) strvec_push(&child.args, "--quiet"); if (start_command(&child)) -- 2.47.0.rc0.187.ge670bccf7e-goog
