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coverage: 86.679% (+0.05%) from 86.626%
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drmingdrmer 
fix: client_api: ensure_linearizable_read must respect transfer_to

# Summary

Replace `try_leader_handler` with `ensure_writable_leader_handler` at
the top of `handle_ensure_linearizable_read`. The read path now returns
`ForwardToLeader(target)` when `transfer_to` is set, mirroring the
write path. Fixes #1747.

# Details

Before this change, the only gate on the read path was "am I a
leader?". The write path had a stricter "am I a *writable* leader?"
gate that also checks `transfer_to`. The asymmetry caused two bugs:

- `ReadIndex` on the source emits AppendEntries that bump follower
  `last_update` regardless of lease state. Under sustained reads, the
  cluster could not recover from a transfer if the target was
  unreachable -- followers never election-time-out, so no new leader
  was established.
- `LeaseRead` short-circuits on `last_quorum_acked + leader_lease`,
  which `transfer_leader` does not clear. The source kept serving
  cached reads while a new leader could already be committing fresh
  entries -- a stale read.

Both paths now go through the same gate. `ensure_writable_leader_handler`
returns `ForwardToLeader(target)` when transfer is in progress; the
existing `tx.send(Err(forward.into()))` path propagates that to the
client unchanged.

The two reproducer tests added in the previous commit now pass:

- `transfer_leader_with_dead_target_does_not_block_election` --
  the cluster recovers (n2 elects) within `5 * election_timeout_max`,
  because n0 stops emitting AppendEntries for reads.
- `transfer_leader_blocks_lease_read` -- LeaseRead returns
  `ForwardToLeader(target)` immediately; no stale-read window.

- Fix: #1747

16332 of 18842 relevant lines covered (86.68%)

145287.39 hits per line

Coverage Regressions

Lines Coverage ∆ File
2
92.59
 3.7% openraft/src/core/sm/handle.rs
Jobs
ID Job ID Ran Files Coverage
1 25491647501.1 264
86.68
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Source Files on build 25491647501