index: convert ReadonlyIndex and MutableIndex methods to be fallible

[bts]

Context

This is part of a series of changes to make most methods on index traits (i.e. ChangeIdIndex, MutableIndex, ReadonlyIndex, Index) fallible. The changes will enable networked implementations of these traits, which can produce I/O errors during operation. See #7825 for more information.

This is a follow-up to #7799 and #7823.

Changes

These two changes convert the necessary methods on ReadonlyIndex and MutableIndex to be fallible:

• ReadonlyIndex::change_id_index
• MutableIndex::change_id_index
• MutableIndex::merge_in

Two things to note:

• I did not also convert ReadonlyIndex::start_modification to return a Result for the reasons described in FR: Support distributed implementations of index traits #7825.
• I changed MutableRepo::merge to return a RepoLoaderError instead of a BackendError, but we should verify that we think this makes sense. My line of thinking was as follows:
  • Without a larger change to error organization, the only other error type that seems to make sense in this context would be to smuggle the new IndexError from merge_in into BackendError::Other.
  • MutableRepo::merge is only used in two contexts (cmd_op_revert and Transaction::merge_operation). In both situations we are loading two repos and then merging them, and Transaction::merge_operation is a caller that already returns a RepoLoaderError.
  • ReadonlyRepo has a few methods that are already returning RepoLoaderError.

EDIT: ended up dropping conversion of change_id_index -- for any implementers looking to perform effects in this method, we can recommend they use interior mutation like they'll have to with start_modification.

Thanks!
Brian

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes

[thoughtpolice]

lgtm, but someone else will have to give the real green light

[bts]

Thanks for taking a look! Going to re-request a review so folks see the PR is not approved to land.