Split metadata tables into separate modules

[rshkv]

Split metadata tables into separate modules.

Context for this is to address #863 (comment) where the point was made that metadata_scan.rs will grow unwieldy if we shove all metadata table implementations in there. Especially as we're going to add extra utilities for those metadata tables.

The structure in this PR is:

inspect/
  metadata_table.rs: contains TableMetadata
  snapshots.rs: contains "snapshots" table
  manifests.rs: contains "manifests" table

In the future this can expand as described in #863 (comment).

[rshkv]

cc @liurenjie1024 @Xuanwo

[liurenjie1024]

Thanks @rshkv for this pr, LGTM! Some issues are tracked in others like #870 , just left one question to discuss.

[liurenjie1024]

I don't quite understand why we need this data struct here. It seems just a wrapper to provide more api, while just like this:

impl Table {
 pub fn snapshots(&self) -> SnapshotsTable {
  ...
}
 pub fn manifests(&self) -> ManifestsTable {
 ...
}
}

[liurenjie1024]

cc @Xuanwo @sdd @Fokko What do you think?

[Xuanwo]

I don't quite understand why we need this data struct here. It seems just a wrapper to provide more api, while just like this:

Yes, I intend to make the API exposed at Table more organized. For example, users will have:

table.metadata_table().snapshots();
table.metadata_table().manifests();

instead of:

// Could be confused with `table.metadata().snapshots()` which returns `Snapshot`.
table.snapshots();

// Verbose and long
table.metadata_snapshots_table();
table.metadata_manifests_table();

While I believe we could use better API names, such as table.inspect().snapshots(), the overall structure looks good to me and aligns better with other implementations.

[liurenjie1024]

Sounds reasonable to me.

[rshkv]

The metadata_table to inspect rename is here: #881

[liurenjie1024]

Same question as #868 . But we could defer this to later issue.

[liurenjie1024]

Same as #870

[liurenjie1024]

Thanks @rshkv for this pr, LGTM!

[liurenjie1024]

I'll merge this first as it's a pure refactoring.