Re: [PATCH 0/3] sparse-checkout: add 'clean' command

> When using cone-mode sparse-checkout, users specify which tracked
> directories they want (recursively) and any directory not part of the parent
> paths for those directories are considered "out of scope". When changing
> sparse-checkouts, there are a variety of reasons why these "out of scope"
> directories could remain, including:
> 
>  * The user has .gitignore or .git/info/exclude files that tell Git to not
>    remove files of a certain type.
>  * Some filesystem blocker prevented the removal of a tracked file. This is
>    usually more of an issue on Windows where a read handle will block file
>    deletion.
> 
> Typically, this would not mean too much for the user experience. A few extra
> filesystem checks might be required to satisfy git status commands, but the
> scope of the performance hit is relative to how many cruft files are left
> over in this situation.
> 
> However, when using the sparse index, these tracked sparse directories cause
> significant performance issues. When noticing that the index contains a
> sparse directory but that directory exists on disk, Git needs to expand that
> sparse directory to determine which files are tracked or untracked. The
> current mechanism expands the entire index to a full one, an expensive
> operation that scales with the total number of paths at HEAD and not just
> the number of cruft files left over.
> 
> Advice was added in 9479a31d603 (advice: warn when sparse index expands,
> 2024-07-08) to help users determine that they were in this state. However,
> the advice doesn't actually recommend helpful ways to get out of this state.
> Recommending "git clean" on its own is incomplete, as typically users
> actually need 'git clean -dfx' to clear out the ignored or excluded files.
> Even then, they may need 'git sparse-checkout reapply' afterwards to clear
> the sparse directories.
> 
> The advice was successful in helping to alert users to the problem, which is
> how I got wind of many of these cases for how users get into this state.
> It's now time to give them a tool that helps them out of this state.

> This series adds a new 'git sparse-checkout clean' command that currently
> only works for cone-mode sparse-checkouts. The only thing it does is
> collapse the index to a sparse index (as much as possible) and make sure
> that any sparse directories are removed. These directories are listed to
> stdout.
> 
> A --dry-run option is available to list the directories that would be
> removed without actually deleting the directories.
> 
> This option would be preferred to something like 'git clean -dfx' since it
> does not clear the excluded files that are still within the sparse-checkout.
> Instead, it performs the exact filesystem operations required to refresh the
> sparse index performance back to what is expected.
> 
> I spent a few weeks debating with myself about whether or not this was the
> right interface, so please suggest alternatives if you have better ideas.
> Among my rejected ideas include:
> 
>  * 'git sparse-checkout reapply -f -x' or similar augmentations of
>    'reapply'.
>  * 'git clean --sparse' to focus the clean operation on things outside of
>    the sparse-checkout.
> 
> The implementation is rather simple with the current CLI. Future
> augmentations could include a --quiet option to silence the output and a
> --verbose option to list the files that exist within each directory and
> would/will be removed.