Cycle 8: Dead Actor Cleanup (Memory Leak Fix) — Development History

Commit: 0213938 · 4 files · 120 insertions, 14 deletions

Motivation

After Cycles 7 (recovery) and the pre-existing poison-on-panic behavior, dead actors accumulated in both ActorPool and AddressMap forever. Their slots were never reclaimed, their addresses remained registered, and the system gradually leaked memory. This is a known bug class in actor frameworks.

Competitor Analysis

Framework	Dead Actor Handling	Known Bugs
Akka	Automatic cleanup via DeathWatch	#22990 — ActorRef leak in certain paths
CAF	Manual cleanup expected	#420 — actor leak in specific failure modes
Erlang/OTP	Automatic — process exits free all resources	N/A (VM handles cleanup)
Ractor	Supervisor-driven cleanup	Memory bloat per actor at scale
Swactor (before)	None — permanent leak	Both ActorPool and AddressMap leak

Implementation

Added AddressMap::remove(addr) to delivery.rs — O(1) removal from address map
Added ActorPool::cleanup_dead() to worker.rs — collects and removes poisoned actors, returns their addresses
Added Phase 7 to tick_once: cleanup_dead → remove from address_map → re-publish num_actors stat
Stats immediately reflect removal (no stale counts)

Key files modified: src/delivery.rs, src/worker.rs, tests/runtime_api.rs

Design Decisions

Automatic cleanup in tick_once — no manual API needed. Dead actors are cleaned up every tick, preventing accumulation.
Phase 7 (after all message processing) — cleanup happens after tick_all and pending_local, so any final messages to dead actors correctly fail. No risk of cleaning up an actor that's about to receive a message.
Re-publish num_actors after cleanup — ensures stats are immediately consistent. Without this, stats would show stale actor counts until the next tick.

Behavior Change

Before: Sending to a poisoned actor silently discarded the message (address still in map, delivery succeeded, but processing was skipped)
After: Sending to a cleaned-up actor returns Err (address removed from map, send fails)
This is better — callers learn the actor is gone instead of silently losing messages.

Tests Added

2 new tests + 2 existing tests updated (68 → 70 total):

dead_actor_cleaned_up_from_stats_and_address_map — good actor persists, bad actor's address is removed
bulk_dead_actor_cleanup — 20 panicked actors all cleaned up in one tick
Updated send_to_poisoned_actor_is_a_silent_black_hole → now asserts send returns Err (behavior change)
Updated poisoned_actor_messages_not_counted_as_processed → sends fail to cleaned-up actor

Result

70 tests pass
All workspace crates compile
Memory leak closed: dead actors no longer accumulate in ActorPool or AddressMap

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