* Make resharding state transitions idempotent on replay

Why: consensus entries may be re-applied after a crash (partial state
on disk) or during raft recovery. The unchecked state-transition
helpers used `debug_assert!` to require a specific starting state, so
a replay would panic in debug or silently overwrite in release.

How to apply: use write_optional so the state file is only touched
when the in-memory state needs to change. This also avoids unnecessary
fsyncs when a replay is a no-op.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Skip replica set creation on resharding start replay

Why: on replay, `create_replica_set` would call `create_shard_dir`,
which removes and recreates the existing shard directory -- wiping
any shard contents that had already been migrated or written since
the first apply.

How to apply: check `contains_shard(shard_id)` before creating the
replica set and pass `None` to `start_resharding_unchecked` when a
replica set with the target shard id is already present. Also
relaxes `check_start_resharding` to return `Ok` (instead of a
swallowed `bad_request`) when a matching resharding state is
persisted, so the caller can fall through each idempotent step.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Derive resharding shard count from shard id for idempotency

Why: the start/finish/abort paths used ++/-- on the persisted shard
number with a `debug_assert` pinning the expected starting value. On
replay (e.g. after a crash between the shard holder mutation and the
config save) this either panics or produces a wrong count.

Since resharding always targets the last shard (auto sharding assigns
contiguous ids from zero), the target count is a pure function of the
shard id: `shard_id + 1` for start up, `shard_id` for finish down and
abort up. Set it directly and skip the save when it already matches,
so replay converges to the same value without touching the file.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Fall through resharding finish/abort steps on replay

Why: the early return on `resharding_state.is_none()` assumed that a
missing state means the operation was fully applied. But a crash can
leave the state cleared while the shard drop, key mapping removal, or
shard count update are still pending. Short-circuiting then skips the
reconciling work those replays need to do.

How to apply: drop the early return so every step runs; each step is
already individually idempotent (check_*, drop_and_remove_shard,
remove_shard_from_key_mapping, the set-based shard count update).
The abort_resharding down-invalidation path now reads nodes from the
router regardless of its variant, so a replay over an already-rolled-
back ring doesn't trip the removed debug_assert!s.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Skip shard key mapping write when nothing to remove

Why: `write_optional` returns `Some` whenever the shard key is
present, even if the shard id we're removing is already gone. That
still triggers a JSON rewrite and a cache notification on every
replay of a finished finish/abort.

How to apply: check that the shard id is actually in the set before
returning `Some`. If the set is missing or already doesn't contain
the id, return `None` so the on-disk file and in-memory data are
left untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Add tests for resharding replay idempotency

Covers the paths that must not error or panic on replay:
- `check_start_resharding` when matching state is already present
- `start_resharding_unchecked` preserves matching state verbatim
- `finish_resharding_unchecked` when state is already cleared

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Simplify finish_resharding_unchecked closure

Replace a manual `match` on `Option` with `as_ref().map(...)` to
satisfy clippy::manual_map.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* test: add a test for resharding replay (#8839)

* [ai] add a debug_assert on shard_number (#8846)

* Update lib/collection/src/shards/shard_holder/resharding.rs

Co-authored-by: Roman Titov <[email protected]>

* Reformat

---------

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>
Co-authored-by: tellet-q <[email protected]>
Co-authored-by: Roman Titov <[email protected]>

* Make resharding state transitions idempotent on replay

Why: consensus entries may be re-applied after a crash (partial state
on disk) or during raft recovery. The unchecked state-transition
helpers used `debug_assert!` to require a specific starting state, so
a replay would panic in debug or silently overwrite in release.

How to apply: use write_optional so the state file is only touched
when the in-memory state needs to change. This also avoids unnecessary
fsyncs when a replay is a no-op.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Skip replica set creation on resharding start replay

Why: on replay, `create_replica_set` would call `create_shard_dir`,
which removes and recreates the existing shard directory -- wiping
any shard contents that had already been migrated or written since
the first apply.

How to apply: check `contains_shard(shard_id)` before creating the
replica set and pass `None` to `start_resharding_unchecked` when a
replica set with the target shard id is already present. Also
relaxes `check_start_resharding` to return `Ok` (instead of a
swallowed `bad_request`) when a matching resharding state is
persisted, so the caller can fall through each idempotent step.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Derive resharding shard count from shard id for idempotency

Why: the start/finish/abort paths used ++/-- on the persisted shard
number with a `debug_assert` pinning the expected starting value. On
replay (e.g. after a crash between the shard holder mutation and the
config save) this either panics or produces a wrong count.

Since resharding always targets the last shard (auto sharding assigns
contiguous ids from zero), the target count is a pure function of the
shard id: `shard_id + 1` for start up, `shard_id` for finish down and
abort up. Set it directly and skip the save when it already matches,
so replay converges to the same value without touching the file.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Fall through resharding finish/abort steps on replay

Why: the early return on `resharding_state.is_none()` assumed that a
missing state means the operation was fully applied. But a crash can
leave the state cleared while the shard drop, key mapping removal, or
shard count update are still pending. Short-circuiting then skips the
reconciling work those replays need to do.

How to apply: drop the early return so every step runs; each step is
already individually idempotent (check_*, drop_and_remove_shard,
remove_shard_from_key_mapping, the set-based shard count update).
The abort_resharding down-invalidation path now reads nodes from the
router regardless of its variant, so a replay over an already-rolled-
back ring doesn't trip the removed debug_assert!s.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Skip shard key mapping write when nothing to remove

Why: `write_optional` returns `Some` whenever the shard key is
present, even if the shard id we're removing is already gone. That
still triggers a JSON rewrite and a cache notification on every
replay of a finished finish/abort.

How to apply: check that the shard id is actually in the set before
returning `Some`. If the set is missing or already doesn't contain
the id, return `None` so the on-disk file and in-memory data are
left untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Add tests for resharding replay idempotency

Covers the paths that must not error or panic on replay:
- `check_start_resharding` when matching state is already present
- `start_resharding_unchecked` preserves matching state verbatim
- `finish_resharding_unchecked` when state is already cleared

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* Simplify finish_resharding_unchecked closure

Replace a manual `match` on `Option` with `as_ref().map(...)` to
satisfy clippy::manual_map.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

* test: add a test for resharding replay (#8839)

* [ai] add a debug_assert on shard_number (#8846)

* Update lib/collection/src/shards/shard_holder/resharding.rs

Co-authored-by: Roman Titov <[email protected]>

* Reformat

---------

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>
Co-authored-by: tellet-q <[email protected]>
Co-authored-by: Roman Titov <[email protected]>