Re: [PATCH v2] ls-tree: add --sparse-filter-oid argument

[William Sprent <williams@unity3d.com>]

On 24/01/2023 21.11, Victoria Dye wrote:
> William Sprent via GitGitGadget wrote:
>> From: William Sprent <williams@unity3d.com>
>>
>> There is currently no way to ask git the question "which files would be
>> part of a sparse checkout of commit X with sparse checkout patterns Y".
>> One use-case would be that tooling may want know whether sparse checkouts
>> of two commits contain the same content even if the full trees differ.
>> Another interesting use-case would be for tooling to use in conjunction
>> with 'git update-index --index-info'.
> 
> These types of use cases align nicely with "Behavior A" as described in
> 'Documentation/technical/sparse-checkout.txt' [1]. I think adding a
> '--scope=(sparse|all)' option to 'ls-trees' would be a good way to make
> progress on the goals of that design document as well as serve the needs
> described above.
> 
> [1] https://lore.kernel.org/git/pull.1367.v4.git.1667714666810.gitgitgadget@gmail.com/
> >>
>> 'rev-list --objects --filter=sparse:oid' comes close, but as rev-list is
>> concerned with objects rather than directory trees, it leaves files out
>> when the same blob occurs in at two different paths.
>>
>> It is possible to ask git about the sparse status of files currently in
>> the index with 'ls-files -t'. However, this does not work well when the
>> caller is interested in another commit, intererested in sparsity
>> patterns that aren't currently in '.git/info/sparse-checkout', or when
>> working in with bare repo.
> 
> I'm a bit confused by your described use cases here, though. Right now,
> 'sparse-checkout' is a local repo-only optimization (for performance and/or
> scoping user workspaces), but you seem to be implying a need for
> "sparse-checkout patterns" as a general-purpose data format (like a config
> file or 'rebase-todo') that can be used to manually configure command
> behavior.
> 
> If that is what you're getting at, it seems like a pretty substantial
> expansion to the scope of what we consider "sparse checkout". That's not to
> say it isn't a good idea - I can definitely imagine tooling where this type
> of functionality is useful - just that it warrants careful consideration so
> we don't over-complicate the typical sparse-checkout user experience.
> 

I guess it is sort of what I'm getting at. I want to enable tooling (in 
particular) to be able to interrogate git about sparse checkouts without 
having to checkout a commit and load a set of patterns. There's already 
a deterministic relationship between a (commit * sparse checkout 
patterns) and a list of included files, so I think it makes sense to be 
able to ask git about it.

I agree with your concerns about not over complicating things for the 
average user. I think the cone/non-cone distinctions are already a bit 
hard to grasp (at least it was for me). FWIW, it isn't my intention that 
an average user should use the "explicitly pass patterns to ls-tree" 
functionality. But of course we should still get it right.


>>
>> To fill this gap, add a new argument to ls-tree '--sparse-filter-oid'
>> which takes the object id of a blob containing sparse checkout patterns
>> that filters the output of 'ls-tree'. When filtering with given sparsity
>> patterns, 'ls-tree' only outputs blobs and commit objects that
>> match the given patterns.
> 
> I don't think a blob OID is the best way to specify an arbitrary pattern set
> in this case. OIDs will only be created for blobs that are tracked in Git;
> it's possible that your project tracks potential sparse-checkout patterns in
> Git, but it seems overly restrictive. You could instead specify the file
> with a path on-disk (like the '--file' options in various commands, e.g.
> 'git commit'); if you did need to get that file from the object store, you
> could first get its contents with 'git cat-file'.
> 

I'm fine with changing it to be a file. I picked it to align with

  'git rev-list --filter=sparse:<blob-oid>'

which I think is the only other place in the code base where you can 
query about arbitrary filters. But I agree, it is a bit awkward to use.

>>
>> While it may be valid in some situations to output a tree object -- e.g.
>> when a cone pattern matches all blobs recursively contained in a tree --
>> it is less unclear what should be output if a sparse pattern matches
>> parts of a tree.
>>
>> To allow for reusing the pattern matching logic found in
>> 'path_in_sparse_checkout_1()' in 'dir.c' with arbitrary patterns,
>> extract the pattern matching part of the function into its own new
>> function 'recursively_match_path_with_sparse_patterns()'.
>>
>> Signed-off-by: William Sprent <williams@unity3d.com>
>> ---
>>      ls-tree: add --sparse-filter-oid argument
>>      
>>      I'm resubmitting this change as rebased on top of 'master', as it
>>      conflicted with the topic 'ls-tree.c: clean-up works' 1
>>      [https://public-inbox.org/git/20230112091135.20050-1-tenglong.tl@alibaba-inc.com],
>>      which was merged to 'master' recently.
>>      
>>      This versions also incorporates changes based on the comments made in 2
>>      [https://public-inbox.org/git/CAPig+cRgZ0CrkqY7mufuWmhf6BC8yXjXXuOTEQjuz+Y0NA+N7Q@mail.gmail.com/].
>>      
>>      I'm also looping in contributors that have touched ls-tree and/or
>>      sparse-checkouts recently. I hope that's okay.
> 
> Definitely okay, thanks for CC-ing!
> 
> Overall, this is an interesting extension to 'sparse-checkout', and opens up
> some opportunities for expanded tooling. However, at an implementation
> level, I think it's addressing two separate needs ("constrain 'ls-files' to
> display files matching patterns" and "specify sparse patterns not in
> '.git/info/sparse-checkout'") in one option, which makes for a somewhat
> confusing and inflexible user experience. What about instead breaking this
> into two options:
> 
> * --scope=(sparse|all): limits the scope of the files output by ("Behavior
>    A" vs. "Behavior B" from the document linked earlier).
> * --sparse-patterns=<file>: specifies "override" patterns to use instead of
>    those in '.git/info/sparse-checkout' (whether 'sparse-checkout' is enabled
>    for the repository or not).
> 

That makes sense to me. I'll look into making those changes.

> I haven't looked at your implementation in detail yet, but I did want to
> offer a recommendation in case you hadn't considered it: if you want to
> allow the use of patterns from a user-specified specific file, it would be
> nice to do it in a way that fully replaces the "default" sparse-checkout
> settings at the lowest level (i.e., override the values of
> 'core_apply_sparse_checkout', 'core_sparse_checkout_cone', and
> 'get_sparse_checkout_filename()'). Doing it that way would both make it
> easier for other commands to add a '--sparse-patterns' option, and avoid the
> separate code path ('path_in_sparse_checkout_1()' vs.
> 'recursively_match_path_with_sparse_patterns()', for example) when dealing
> with '.git/info/sparse-checkout' patterns vs. manually-specified patterns.
> 

Thanks for the pointers. I'll see what I can do. Do you mean something 
along the line of the following?

    set_sparse_checkout_file(filename);
    init_sparse_checkout_patterns(istate);
    _ = path_in_sparse_checkout_1(some_path, istate, ...);


I do I think I like the explicitness of first loading the pattern_list 
and then passing it to the matching function, though. But maybe it is 
too cumbersome.

>>      
>>      William
>>
>> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1459%2Fwilliams-unity%2Fls-tree-sparse-v2
>> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1459/williams-unity/ls-tree-sparse-v2
>> Pull-Request: https://github.com/gitgitgadget/git/pull/1459
>