commit 7a5d604443ffc7afcd3788818f8fe00fc68c054d
author Patrick Steinhardt <ps@pks.im> Tue Oct 31 08:16:18 2023 +0100
committer Junio C Hamano <gitster@pobox.com> Wed Nov 01 12:04:06 2023 +0900
tree 2fa0f7fc834c17b43c06c6b642d363c677cb6e2a
parent e04838ea828651cc122de505320e5ea85b43f1b1

commit: detect commits that exist in commit-graph but not in the ODB

Commit graphs can become stale and contain references to commits that do
not exist in the object database anymore. Theoretically, this can lead
to a scenario where we are able to successfully look up any such commit
via the commit graph even though such a lookup would fail if done via
the object database directly.

As the commit graph is mostly intended as a sort of cache to speed up
parsing of commits we do not want to have diverging behaviour in a
repository with and a repository without commit graphs, no matter
whether they are stale or not. As commits are otherwise immutable, the
only thing that we really need to care about is thus the presence or
absence of a commit.

To address potentially stale commit data that may exist in the graph,
our `lookup_commit_in_graph()` function will check for the commit's
existence in both the commit graph, but also in the object database. So
even if we were able to look up the commit's data in the graph, we would
still pretend as if the commit didn't exist if it is missing in the
object database.

We don't have the same safety net in `parse_commit_in_graph_one()`
though. This function is mostly used internally in "commit-graph.c"
itself to validate the commit graph, and this usage is fine. We do
expose its functionality via `parse_commit_in_graph()` though, which
gets called by `repo_parse_commit_internal()`, and that function is in
turn used in many places in our codebase.

For all I can see this function is never used to directly turn an object
ID into a commit object without additional safety checks before or after
this lookup. What it is being used for though is to walk history via the
parent chain of commits. So when commits in the parent chain of a graph
walk are missing it is possible that we wouldn't notice if that missing
commit was part of the commit graph. Thus, a query like `git rev-parse
HEAD~2` can succeed even if the intermittent commit is missing.

It's unclear whether there are additional ways in which such stale
commit graphs can lead to problems. In any case, it feels like this is a
bigger bug waiting to happen when we gain additional direct or indirect
callers of `repo_parse_commit_internal()`. So let's fix the inconsistent
behaviour by checking for object existence via the object database, as
well.

This check of course comes with a performance penalty. The following
benchmarks have been executed in a clone of linux.git with stable tags
added:

    Benchmark 1: git -c core.commitGraph=true rev-list --topo-order --all (git = master)
      Time (mean ± σ):      2.913 s ±  0.018 s    [User: 2.363 s, System: 0.548 s]
      Range (min … max):    2.894 s …  2.950 s    10 runs

    Benchmark 2: git -c core.commitGraph=true rev-list --topo-order --all (git = pks-commit-graph-inconsistency)
      Time (mean ± σ):      3.834 s ±  0.052 s    [User: 3.276 s, System: 0.556 s]
      Range (min … max):    3.780 s …  3.961 s    10 runs

    Benchmark 3: git -c core.commitGraph=false rev-list --topo-order --all (git = master)
      Time (mean ± σ):     13.841 s ±  0.084 s    [User: 13.152 s, System: 0.687 s]
      Range (min … max):   13.714 s … 13.995 s    10 runs

    Benchmark 4: git -c core.commitGraph=false rev-list --topo-order --all (git = pks-commit-graph-inconsistency)
      Time (mean ± σ):     13.762 s ±  0.116 s    [User: 13.094 s, System: 0.667 s]
      Range (min … max):   13.645 s … 14.038 s    10 runs

    Summary
      git -c core.commitGraph=true rev-list --topo-order --all (git = master) ran
        1.32 ± 0.02 times faster than git -c core.commitGraph=true rev-list --topo-order --all (git = pks-commit-graph-inconsistency)
        4.72 ± 0.05 times faster than git -c core.commitGraph=false rev-list --topo-order --all (git = pks-commit-graph-inconsistency)
        4.75 ± 0.04 times faster than git -c core.commitGraph=false rev-list --topo-order --all (git = master)

We look at a ~30% regression in general, but in general we're still a
whole lot faster than without the commit graph. To counteract this, the
new check can be turned off with the `GIT_COMMIT_GRAPH_PARANOIA` envvar.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>