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

[Patrick Steinhardt <ps@pks.im>]

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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.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 commit.c                |  7 ++++++-
 t/t5318-commit-graph.sh | 23 +++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/commit.c b/commit.c
index b3223478bc..109e9217e3 100644
--- a/commit.c
+++ b/commit.c
@@ -572,8 +572,13 @@ int repo_parse_commit_internal(struct repository *r,
 		return -1;
 	if (item->object.parsed)
 		return 0;
-	if (use_commit_graph && parse_commit_in_graph(r, item))
+	if (use_commit_graph && parse_commit_in_graph(r, item)) {
+		if (!has_object(r, &item->object.oid, 0))
+			return quiet_on_missing ? -1 :
+				error(_("commit %s exists in commit-graph but not in the object database"),
+				      oid_to_hex(&item->object.oid));
 		return 0;
+	}
 
 	if (oid_object_info_extended(r, &item->object.oid, &oi, flags) < 0)
 		return quiet_on_missing ? -1 :
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index ba65f17dd9..25f8e9e2d3 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -821,4 +821,27 @@ test_expect_success 'overflow during generation version upgrade' '
 	)
 '
 
+test_expect_success 'commit exists in commit-graph but not in object database' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit A &&
+		test_commit B &&
+		test_commit C &&
+		git commit-graph write --reachable &&
+
+		# Corrupt the repository by deleting the intermittent commit
+		# object. Commands should notice that this object is absent and
+		# thus that the repository is corrupt even if the commit graph
+		# exists.
+		oid=$(git rev-parse B) &&
+		rm .git/objects/"$(test_oid_to_path "$oid")" &&
+
+		test_must_fail git rev-parse HEAD~2 2>error &&
+		grep "error: commit $oid exists in commit-graph but not in the object database" error
+	)
+'
+
 test_done
-- 
2.42.0


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