@matheusbsbs / matheus-bagliano / commits / 2dd804cd12

t5318: demonstrate commit-graph generation v2 corruption

When upgrading a commit-graph using generation v1 to one using generation v2, it is possible to force Git into a corrupt state where it (incorrectly) believes that a GDO2 chunk is necessary, *after* deciding not to write one. This makes subsequent reads using the commit-graph produce the following error message: fatal: commit-graph requires overflow generation data but has none Demonstrate this bug by increasing our test coverage to include a minimal example of upgrading a commit-graph from generation v1 to v2. The only notable components of this test are: - The committer date of the commit is chosen carefully so that the offset underflows when computed using a v1 generation number, but would not overflow when using v2 generation numbers. - The upgrade to generation number v2 must read in the v1 generation numbers, which we can do by passing `--changed-paths`, which will force the commit-graph internals to call `fill_commit_graph_info()`. A future patch will squash this bug. Reported-by: Jeff King <peff@peff.net> Reproduced-by: Will Chandler <wfc@wfchandler.org> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Taylor Blau committed Jul 12, 2022 at 19:10 UTC 2dd804cd12143741ea4188346fba250e821609b5
1 file changed +27
t/t5318-commit-graph.sh
+27
@@ -811,4 +811,31 @@ test_expect_success 'set up and verify repo with generation data overflow chunk'
811
812 graph_git_behavior 'generation data overflow chunk repo' repo left right
813
814 +test_expect_failure 'overflow during generation version upgrade' '
815 + git init overflow-v2-upgrade &&
816 + (
817 + cd overflow-v2-upgrade &&
818 +
819 + # This commit will have a date at two seconds past the Epoch,
820 + # and a (v1) generation number of 1, since it is a root commit.
821 + #
822 + # The offset will then be computed as 1-2, which will underflow
823 + # to 2^31, which is greater than the v2 offset small limit of
824 + # 2^31-1.
825 + #
826 + # This is sufficient to need a large offset table for the v2
827 + # generation numbers.
828 + test_commit --date "@2 +0000" base &&
829 + git repack -d &&
830 +
831 + # Test that upgrading from generation v1 to v2 correctly
832 + # produces the overflow table.
833 + git -c commitGraph.generationVersion=1 commit-graph write &&
834 + git -c commitGraph.generationVersion=2 commit-graph write \
835 + --changed-paths &&
836 +
837 + git rev-list --all
838 + )
839 +'
840 +
841 test_done