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Timers - #7
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Timers#7
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Protocol-agnostic hierarchical timer wheel to replace O(n) TCP timer scanning. Designed for 10k+ connections with extensibility for DNS and QUIC timer consumers.
Introduces TimerWheel with three-tier hierarchical layout (1ms/256ms/65.5s resolution), TimerId/TimerHandle types, and the arm()/is_armed() operations backed by a Slab and per-slot doubly-linked lists.
Implements cancel() on TimerWheel with full doubly-linked list unlinking (head, middle, tail cases) and silent no-op on already-cancelled handles. Five TDD tests cover all unlink positions and the idempotent cancel case.
Implements advance(now_ms) which drains inner wheel slots tick by tick, cascades middle-tier entries to the inner wheel on inner-wrap, and cascades outer-tier entries to the middle wheel on middle-wrap. cascade() collects all entries before re-arming to avoid slab aliasing.
Adds four cascade tests: middle-to-inner placement after inner-wrap, correct firing tick after cascade, outer-to-middle cascade at the 65536 boundary, and simultaneous firing of multiple cascaded entries.
Adds entry_count() -> usize (delegates to slab len). Adds seven edge case tests: re-arm after cancel, arm at current tick, arm in past (slot wrap), entry_count tracking, advance noop when time unchanged, burst catch-up across multiple slots, and no-fire when no timers due.
Introduces timer_kinds.rs with the TcpTimerKind enum (6 variants, repr(u8)), free functions for packing/unpacking TimerId with a connection key and timer kind, and TcpTimerHandles for per-connection handle storage. Adds TimerHandle::from_raw(crate-visible) constructor to support tests.
Add individual fire_delayed_ack, fire_retransmit, fire_keep_alive, fire_persist, fire_linger, fire_time_wait, and handle_timer methods alongside existing poll_timers()/evict_stale(). These methods operate on a single connection by slab key, enabling O(1) timer-wheel dispatch instead of O(n) scanning.
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