[PATCH 0/2] Sparse index: delete ignored files outside sparse cone

My commits are technically based on ds/add-with-sparse-index, but could apply earlier, I believe.

We launched an experimental release [1] of the sparse-index feature to our internal users. We immediately discovered a problem due to the isolated way in which we tested the sparse index: we were never building the project and changing our sparse-checkout definition.

[1] https://github.com/microsoft/git/releases/tag/v2.32.0.vfs.0.102.exp

Users who ran a build in one project and then moved to another still had build artifacts in their worktree that lived inside the old directories. Since the files are marked by the .gitignore patterns, these files were not removed by the 'git sparse-checkout set' command. However, they make the sparse-index unusable because every 'git status' command needs to expand the sparse-directory entries in order to see if the files are tracked or not. This made the first experimental release actually slower for all users because of this cost.

The solution we shipped to these customers was to change the way our fork handles these ignored files. Specifically, instead of Git completely ignoring the files, we changed Git to understand that with cone-mode sparse-checkout patterns, the users is asking for entire directories to be removed from the worktree. The link [1] included earlier has this change.

I believe that this is a reasonable expectation, though I recognize that it might look like breaking the expectations of how .gitignore files work.

To limit the scope of this change, the focus is limited to behave this way only when the sparse-index is enabled, because otherwise the sparse index does not behave as intended. With that restriction in mind, the implementation can use the sparse directory entries to assist.

I imagine that when the sparse index is mature enough, we will enable it by default when cone mode sparse-checkout is enabled, in which case this behavior would expand to all users with cone mode sparse-checkout.

I'm interested in the community's thoughts about this change, as it seems like one that we should make carefully and intentionally.

While the rewrite of the t7519 test seems unrelated, it is required to avoid a test failure with this change that deletes files outside of the cone. By moving the test into repositories not at $TRASH_DIRECTORY, we gain more control over the repository structure.

Thanks, -Stolee

Derrick Stolee (2):
  t7519: rewrite sparse index test
  sparse-checkout: clear tracked sparse dirs

 Documentation/git-sparse-checkout.txt |  6 +++
 builtin/sparse-checkout.c             | 73 +++++++++++++++++++++++++++
 t/t1091-sparse-checkout-builtin.sh    | 42 +++++++++++++++
 t/t7519-status-fsmonitor.sh           | 38 +++++++-------
 4 files changed, 142 insertions(+), 17 deletions(-)

base-commit: adf5b15ac3d44d92e0438451ef36631ed3ee2a63
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1009%2Fderrickstolee%2Fsparse-index%2Fignored-files-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1009/derrickstolee/sparse-index/ignored-files-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1009