feat: adds support for detached commits

[westonpace]

A detached commit is a commit that is not part of the regular dataset lineage. It will never show up as the latest commit and is completely separate from the linear history of the dataset.

This can be useful for:

• Making temporary changes to a dataset, like adding a column of values while computing an index without ever truly materializing the column.
• Situations where the dataset's lineage is managed remotely and all commits are detached commits without any linear history (blob storage will use this)

Closes #2889

[westonpace]

My original approach was much more complicated and used UUIDs as the version. However, if we keep the version as a u64 but borrow the most significant bit to flag detached vs. normal then we end up with much fewer changes and less overall complexity.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.97826% with 127 lines in your changes missing coverage. Please review.

Project coverage is 78.23%. Comparing base (f9024ce) to head (bb46d90).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance/src/io/commit.rs	2.98%	65 Missing ⚠️
rust/lance/src/dataset.rs	44.82%	44 Missing and 4 partials ⚠️
rust/lance-table/src/io/commit.rs	48.14%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3028      +/-   ##
==========================================
+ Coverage   78.19%   78.23%   +0.03%     
==========================================
  Files         239      240       +1     
  Lines       76782    77227     +445     
  Branches    76782    77227     +445     
==========================================
+ Hits        60043    60419     +376     
- Misses      13669    13705      +36     
- Partials     3070     3103      +33     

Flag	Coverage Δ
unittests	78.23% <30.97%> (+0.03%)	⬆️

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[wjones127]

This seems reasonable. Is there any special considerations we need to make for clean up?

[wjones127]

👍

[westonpace]

This seems reasonable. Is there any special considerations we need to make for clean up?

@wjones127

I'm planning on tackling cleanup later. My thinking is that cleanup will be something along the lines of:

cleanup_old_versions(
  since: ...,
  delete_unverified: bool,
  versions_to_retain: &[u64],
)

For the "detached versions are just temporary versions" case then versions_to_retain will be empty and the temporary versions will only be deleted if they are older than 7 days or delete_unverified (assuming the caller setting delete_unverified is confirming there are no ongoing temporary operations)

For the "this is a secondary database and everything is detached" case then cleanup will be triggered by a cleanup of the primary database. After the cleanup of the primary database we will scan all remaining versions (in the primary database) and collect which secondary versions are still referenced. These will be passed in as versions_to_retain.