On Tue, Jun 02, 2020 at 10:52:48PM -0600, Taylor Blau wrote: > Hi Jonathan, > > On Tue, Jun 02, 2020 at 08:42:13PM -0700, Jonathan Nieder wrote: > > Hi, > > > > Taylor Blau wrote: > > > > > Signed-off-by: Taylor Blau <me@xxxxxxxxxxxx> > > > --- > > > builtin/receive-pack.c | 4 ++-- > > > commit.h | 2 ++ > > > fetch-pack.c | 10 +++++----- > > > shallow.c | 30 +++++++++++++++++++++--------- > > > t/t5537-fetch-shallow.sh | 29 +++++++++++++++++++++++++++++ > > > 5 files changed, 59 insertions(+), 16 deletions(-) > > > > I haven't investigated the cause yet, but I've run into an interesting > > bug that bisects to this commit. Jay Conrod (cc-ed) reports: > > > > | I believe this is also the cause of Go toolchain test failures we've > > | been seeing. Go uses git to fetch dependencies. > > | > > | The problem we're seeing can be reproduced with the script below. It > > | should print "success". Instead, the git merge-base command fails > > | because the commit 7303f77963648d5f1ec5e55eccfad8e14035866c > > | (origin/master) has no history. > > > > -- 8< -- > > #!/bin/bash > > > > set -euxo pipefail > > if [ -d legacytest ]; then > > echo "legacytest directory already exists" >&2 > > exit 1 > > fi > > mkdir legacytest > > cd legacytest > > git init --bare > > git config protocol.version 2 > > git config fetch.writeCommitGraph true > > git remote add origin -- https://github.com/rsc/legacytest > > git fetch -f --depth=1 origin refs/heads/master:refs/heads/master > > git fetch -f origin 'refs/heads/*:refs/heads/*' 'refs/tags/*:refs/tags/*' > > git fetch --unshallow -f origin > > git merge-base --is-ancestor -- v2.0.0 7303f77963648d5f1ec5e55eccfad8e14035866c > > echo success > > -- >8 -- > > Thanks to you and Jay for the report and reproduction script. Indeed, I > can reproduce this on the tip of master (which is equivalent to v2.27.0 > at the time of writing). > > > The fetch.writeCommitGraph part is interesting. When does a commit > > graph file get written in this sequence of operations? In an > > unshallow operation, does the usual guard against writing a commit > > graph in a shallow repo get missed? > > The last 'git fetch' is the one that writes the commit-graph. You can > verify this by sticking a 'ls objects/info' after each 'git' invocation > in your script. > > Here's where things get weird, though. Prior to this patch, Git would > pick up that the repository is shallow before unshallowing, but never > invalidate this fact after unshallowing. That means that once we got to > 'write_commit_graph', we'd exit immediately since it appears as if the > repository is shallow. > > In this patch, we don't do that anymore, since we rightly unset the fact > that we are (were) shallow. > > In a debugger, I ran your script and a 'git commit-graph write --split > --reachable' side-by-side, and found an interesting discrepancy: some > commits (loaded from 'copy_oids_to_commits') *don't* have their parents > set when invoked from 'git fetch', but *do* when invoked as 'git > commit-graph write ...'. > > I'm not an expert in the object cache, but my hunch is that when we > fetch these objects they're marked as parsed without having loaded their > parents. When we load them again via 'lookup_object', we get objects > that look parsed, but don't have parents where they otherwise should. Ah, this only sort of has to do with the object cache. In 'parse_commit_buffer()' we stop parsing parents in the case that the repository is shallow (this goes back to 7f3140cd23 (git repack: keep commits hidden by a graft, 2009-07-23)). That makes me somewhat nervous. We're going to keep any objects opened prior to unshallowing in the cache, along with their hidden parents. I suspect that this is why Git has kept the shallow bit as sticky for so long. I'm not quite sure what to do here. I think that any of the following would work: * Keep the shallow bit sticky, at least for fetch.writeCommitGraph (i.e., pretend as if fetch.writecommitgraph=0 in the case of '--unshallow'). * Dump the object cache upon un-shallowing, forcing us to re-discover the parents when they are no longer hidden behind a graft. The latter seems like the most complete feasible fix. The former should work fine to address this case, but I wonder if there are other call-sites that are affected by this behavior. My hunch is that this is a unique case, since it requires going from shallow to unshallow in the same process. I have yet to create a smaller test case, but the following should be sufficient to dump the cache of parsed objects upon shallowing or un-shallowing: diff --git a/shallow.c b/shallow.c index b826de9b67..06db857f53 100644 --- a/shallow.c +++ b/shallow.c @@ -90,6 +90,9 @@ static void reset_repository_shallow(struct repository *r) { r->parsed_objects->is_shallow = -1; stat_validity_clear(r->parsed_objects->shallow_stat); + + parsed_object_pool_clear(r->parsed_objects); + r->parsed_objects = parsed_object_pool_new(); } int commit_shallow_file(struct repository *r, struct shallow_lock *lk) Is this something we want to go forward with? Are there some far-reaching implications that I'm missing? > I'm going to CC Stolee to see if he has any thoughts on how to handle > this and/or if my idea is on the right track. > > > "rm -fr objects/info/commit-graphs" recovers the full history in the > > repo, so this is not a case of writing the wrong shallows --- it's > > only a commit graph issue. > > > > I'll take a closer look, but thought I'd give others a chance to look > > to in case there's something obvious. :) > > > > Thanks, > > Jonathan > > Thanks, > Taylor Thanks, Taylor
