Brian Schroeder 7810926c35 index: make Index::is_ancestor fallible ...

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

- Introduced a few more instances of the existing anti-pattern, `TODO:
  indexing error shouldn't be a "BackendError"`. We're tracking this
  known issue in #7849.
- Converted `MutableRepo::merge_view` to return a `RepoLoaderError`
  instead of a `BackendError`. The only caller, `MutableRepo::merge`,
  already returns a `RepoLoaderError`.
- Added three "`fallible_`" iterator helpers to reduce the amount of
  noise at `is_ancestor` call sites due to the method now being
  fallible. Using these helpers seem to produce code that's a little
  more readable than using `process_results` from itertools. One
  consideration in this trade-off is that these helpers do not
  themselves return iterators: if we find that we need more support for
  fallible combinators mid-"chain" of iterator combinators, we might
  either want to use `process_results` only in those cases, or switch to
  use of `process_results` across the board (in lieu of these new
  helpers).

2025-10-28 13:47:44 +00:00

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