Re: [PATCH v2 2/2] shallow.c: use '{commit,rollback}_shallow_file'

[Taylor Blau <me@ttaylorr.com>]

Hi Stolee,

On Wed, Jun 03, 2020 at 09:08:26AM -0400, Derrick Stolee wrote:
> On 6/3/2020 1:16 AM, Taylor Blau wrote:
> > 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@ttaylorr.com>
> >>>> ---
> >>>>  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').
>
> I'm in favor of this option, if possible. Anything that alters the
> commit history in-memory at any point in the Git process is unsafe to
> combine with a commit-graph read _or_ write. I'm sorry that the guards
> in commit_graph_compatible() are not enough here.
>
> >   * 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.
>
> The latter would solve issues that could arise outside of the commit-graph
> space. But it also presents an opportunity for another gap if someone edits
> the shallow logic without putting in the proper guards.
>
> To be extra safe, I'd be in favor of adding an "if (grafts_ever_existed)"
> condition in commit_graph_compatible() based on a global that is assigned
> a non-zero value whenever grafts are loaded at any point in the process,
> mostly because it would be easy to guarantee that it is safe. It could
> even be localized to the repository struct.
>
> > 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'd like to see the extra-careful check, in addition to this one. This
> is such a rarely-used and narrowly-tested case that we need to be really
> really careful to avoid regressions.

I'm a little confused at which suggestion you're in favor of ;-). For
clarity, are you suggesting that we add a new 'r->grafts_ever_existed'
bit in addition to doing a hard reset of the object pool?

> Thanks,
> -Stolee

Thanks,
Taylor