From: Justin Tobler Date: Wed, 18 Feb 2026 19:23:47 GMT Subject: Re: [PATCH 2/3] commit: make `repo_parse_commit_no_graph()` more robust Message-ID: In-Reply-To: <20260216-b4-pks-receive-pack-optimize-shallow-v1-2-e98886daff2b@pks.im> On 26/02/16 04:38PM, Patrick Steinhardt wrote: > In the next commit we will start to parse more commits via the > commit-graph. This change will lead to a segfault though because we try > to access the tree of a commit via `repo_get_commit_tree()`, but: > > - The commit has been parsed via the commit-graph, and thus its > `maybe_tree` field is not yet populated. > > - We cannot use the commit-graph to populate the commit's tree because > we're in the process of writing the commit-graph. > > The consequence is that we'll get a `NULL` pointer for the tree in > `write_graph_chunk_data()`. IIUC, when a commit has been parsed via the commit graph, if the commit graph is closed, there is no longer a way for commit object tree to be read. This results `repo_get_commit_tree()` always returning NULL in such scenarios. > In theory we are already mindful of this situation, as we explicitly use > `repo_parse_commit_no_graph()` to parse the commit without the help of > the commit-graph. But that doesn't do the trick as the commit is already > marked as parsed, so the function will not re-populate it. And as the > commit-graph has been closed, neither will `get_commit_tree_oid()` be > able to load the tree for us. And `repo_parse_commit_no_graph()` doesn't work because the commit has already been marked as parsed. > It seems like this issue can only be hit under artificial circumstances: > the error was hit via `git_test_write_commit_graph_or_die()`, which is > run by git-commit(1) and git-merge(1) in case `GIT_TEST_COMMIT_GRAPH=1`: > > $ GIT_TEST_COMMIT_GRAPH=1 meson test t7507-commit-verbose \ > --test-args=-ix -i > ... > ++ git -c commit.verbose=true commit --amend > hint: Waiting for your editor to close the file... > ./test-lib.sh: line 1012: 55895 Segmentation fault (core dumped) git -c commit.verbose=true commit --amend > > To the best of my knowledge, this is the only case where we end up > writing a commit-graph in the same process that might have already > consulted the commit-graph to look up arbitrary objects. But regardless > of that, this feels like a bigger accident that is just waiting to > happen. So I assume we end up closing the commit-graph when writing a new one. If we need to read the trees of commits parsed via commit-graph, this will trigger a segfault since the commit tree will always be NULL. > Make the code more robust by extending `repo_parse_commit_no_graph()` to > unparse a commit first in case we detect it's coming from a graph. This > ensures that we will re-read the object without it, and thus we will > populate `maybe_tree` properly. Hmm, I wonder if this is conceptually the correct place to address this problem. Naively, I would expect `repo_get_commit_tree()` to always be capable of providing the commit tree. I guess the problem though is that this would require `repo_get_commit_tree()` to detect this scenario and reparse the object itself. Maybe we could at least have `repo_get_commit_tree()` BUG() in this scenario though? > This fix shouldn't have any performance consequences: the function is > only ever called in the "commit-graph.c" code, and we'll only re-parse > the commit at most once. > > Add an exclusion to our Coccinelle rules so that it doesn't complain > about us accessing `maybe_tree` directly. > > Signed-off-by: Patrick Steinhardt > --- > commit.h | 14 ++++++++++++-- > contrib/coccinelle/commit.cocci | 2 +- > 2 files changed, 13 insertions(+), 3 deletions(-) > > diff --git a/commit.h b/commit.h > index 1635de418b..f2f39e1a89 100644 > --- a/commit.h > +++ b/commit.h > @@ -103,16 +103,26 @@ static inline int repo_parse_commit(struct repository *r, struct commit *item) > return repo_parse_commit_gently(r, item, 0); > } > > +void unparse_commit(struct repository *r, const struct object_id *oid); > + > static inline int repo_parse_commit_no_graph(struct repository *r, > struct commit *commit) > { > + /* > + * When the commit has been parsed but its tree wasn't populated then > + * this is an indicator that it has been parsed via the commit-graph. > + * We cannot read the tree via the commit-graph, as we're explicitly > + * told not to use it. We thus have to first un-parse the object so > + * that we can re-parse it without the graph. > + */ > + if (commit->object.parsed && !commit->maybe_tree) > + unparse_commit(r, &commit->object.oid); This unparses commits that were read via commit-graph so they can be reparsed normally. Looks good. -Justin