[PATCH v2 1/4] split-index & fsmonitor: demonstrate a bug

From: Johannes Schindelin <johannes.schindelin@gmx.de>

This commit adds a new test case that demonstrates a bug in the split-index code that is triggered under certain circumstances when the FSMonitor is enabled, and its symptom manifests in the form of one of the following error messages:

    BUG: fsmonitor.c:20: fsmonitor_dirty has more entries than the index (2 > 1)

    BUG: unpack-trees.c:776: pos <n> doesn't point to the first entry of <dir>/ in index

    error: invalid path ''
    error: The following untracked working tree files would be overwritten by reset:
            initial.t

Which of these error messages appears depends on timing-dependent conditions.

Technically the root cause lies with a bug in the split-index code that has nothing to do with FSMonitor, but for the sake of this new test case it was the easiest way to trigger the bug.

The bug is this: Under specific conditions, Git needs to skip writing the "link" extension (which is the index extension containing the information pertaining to the split-index). To do that, the `base_oid` attribute of the `split_index` structure in the in-memory index is zeroed out, and `do_write_index()` specifically checks for a "null" `base_oid` to understand that the "link" extension should not be written. However, this violates the consistency of the in-memory index structure, but that does not cause problems in most cases because the process exits without using the in-memory index structure anymore, anyway.

But: _When_ the in-memory index is still used (which is the case e.g. in
`git rebase`), subsequent writes of `the_index` are at risk of writing
out a bogus index file, one that _should_ have a "link" extension but
does not. In many cases, the `SPLIT_INDEX_ORDERED` flag _happens_ to be
set for subsequent writes, forcing the shared index to be written, which
re-initializes `base_oid` to a non-bogus state, and all is good.

When it is _not_ set, however, all kinds of mayhem ensue, resulting in above-mentioned error messages, and often enough putting worktrees in a totally broken state where the only recourse is to manually delete the `index` and the `index.lock` files and then call `git reset` manually. Not something to ask users to do.

The reason why it is comparatively easy to trigger the bug with FSMonitor is that there is _another_ bug in the FSMonitor code: `mark_fsmonitor_valid()` sets `cache_changed` to 1, i.e. treating that variable as a Boolean. But it is a bit field, and 1 happens to be the `SOMETHING_CHANGED` bit that forces the "link" extension to be skipped when writing the index, among other things.

"Comparatively easy" is a relative term in this context, for sure. The essence of how the new test case triggers the bug is as following:

1. The `git rebase` invocation will first reset the worktree to
   a commit that contains only the `one.t` file, and then execute a
   rebase script that starts with the following commands (commit hashes
   skipped):

   label onto

   reset initial
   pick two
   label two

   reset two
   pick three
   [...]

2. Before executing the `label` command, a split index is written, as
   well as the shared index.

3. The `reset initial` command in the rebase script writes out a new
   split index but skips writing the shared index, as intended.

4. The `pick two` command updates the worktree and refreshes the index,
   marking the `two.t` entry as valid via the FSMonitor, which sets the
   `SOMETHING_CHANGED` bit in `cache_changed`, which in turn causes the
   `base_oid` attribute to be zeroed out and a full (non-split) index
   to be written (making sure _not_ to write the "link" extension).

5. Now, the `reset two` command will leave the worktree alone, but
   still write out a new split index, not writing the shared index
   (because `base_oid` is still zeroed out, and there is no index entry
   update requiring it to be written, either).

6. When it is turn to run `pick three`, the index is read, but it is
   too short: It only contains a single entry when there should be two,
   because the "link" extension is missing from the written-out index
   file.

There are three bugs at play, actually, which will be fixed over the course of the next commits:

- The `base_oid` attribute should not be zeroed out to indicate when
  the "link" extension should not be written, as it puts the in-memory
  index structure into an inconsistent state.

- The FSMonitor should not overwrite bits in `cache_changed`.

- The `unpack_trees()` function tries to reuse the `split_index`
  structure from the source index, if any, but does not propagate the
  `SPLIT_INDEX_ORDERED` flag.

While a fix for the second bug would let this test case pass, there are other conditions where the `SOMETHING_CHANGED` bit is set. Therefore, the bug that most crucially needs to be fixed is the first one.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/t7527-builtin-fsmonitor.sh | 37 ++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)