[PATCH 0/4] Fix a few split-index bugs

I received an internal bug report that after upgrading from v2.39.2 to v2.40.0, some users ran into the following error message:

BUG: fsmonitor.c:21: fsmonitor_dirty has more entries than the index (57 > 0)

It sounds very much like the report we received in https://lore.kernel.org/git/CAC7ZvybvykKQyMWcZoKXxFDu_amnkxZCDq2C6KHoyhmHN2tcKw@mail.gmail.com/, but sadly that thread petered out when the reporter stopped being able to reproduce the problem.

After a few days of investigating, I am convinced that this is due to some old bugs, and not actually a regression in v2.40.0 (although I can believe that some improvements in v2.40.0 might make it easier to run into these bugs).

This patch series addresses those bugs.

Note: While the Git maintainer has stated a strong preference to introduce
regression tests in the same patch that fixes the corresponding regression,
this patch series starts with a stand-alone patch that demonstrates a
problematic scenario via a new test_expect_failure test case. The reason why
I specifically split out the test into its own commit is that there is a lot
of information to unpack in the commit message that is larger than any of
the subsequent bug fixes. Besides, it motivates not only the second patch
(which marks the test case as test_expect_success) but paints the larger
picture necessary to understand also the need for the remaining two patches.

This patch series is based on maint-2.37, the oldest maintenance branch it applies without merge conflicts. When merging with next, there are only trivial conflicts in unpack-trees.c due to en/dir-api-cleanup where o->result is now o->internal.result.

Johannes Schindelin (4):
  split-index & fsmonitor: demonstrate a bug
  split-index; stop abusing the `base_oid` to strip the "link" extension
  fsmonitor: avoid overriding `cache_changed` bits
  unpack-trees: take care to propagate the split-index flag

 fsmonitor.h                  |  2 +-
 read-cache.c                 | 37 ++++++++++++++++++++++--------------
 t/t7527-builtin-fsmonitor.sh | 37 ++++++++++++++++++++++++++++++++++++
 unpack-trees.c               |  2 ++
 4 files changed, 63 insertions(+), 15 deletions(-)

base-commit: eb88fe1ff5ceb34845f0919b8bdc60d8a1703cf6
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1497%2Fdscho%2Ffix-split-index-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1497/dscho/fix-split-index-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1497