55 lines
2.9 KiB
Markdown
55 lines
2.9 KiB
Markdown
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# Cycle 8: Dead Actor Cleanup (Memory Leak Fix) — Development History
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> Commit: `0213938` · 4 files · 120 insertions, 14 deletions
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---
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## Motivation
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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.
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## Competitor Analysis
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| Framework | Dead Actor Handling | Known Bugs |
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|-----------|-------------------|------------|
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| Akka | Automatic cleanup via DeathWatch | #22990 — ActorRef leak in certain paths |
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| CAF | Manual cleanup expected | #420 — actor leak in specific failure modes |
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| Erlang/OTP | Automatic — process exits free all resources | N/A (VM handles cleanup) |
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| Ractor | Supervisor-driven cleanup | Memory bloat per actor at scale |
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| **Swactor (before)** | **None — permanent leak** | **Both ActorPool and AddressMap leak** |
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## Implementation
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- Added `AddressMap::remove(addr)` to `delivery.rs` — O(1) removal from address map
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- Added `ActorPool::cleanup_dead()` to `worker.rs` — collects and removes poisoned actors, returns their addresses
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- Added Phase 7 to `tick_once`: `cleanup_dead` → remove from address_map → re-publish `num_actors` stat
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- Stats immediately reflect removal (no stale counts)
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**Key files modified:** `src/delivery.rs`, `src/worker.rs`, `tests/runtime_api.rs`
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## Design Decisions
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- **Automatic cleanup in tick_once** — no manual API needed. Dead actors are cleaned up every tick, preventing accumulation.
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- **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.
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- **Re-publish `num_actors` after cleanup** — ensures stats are immediately consistent. Without this, stats would show stale actor counts until the next tick.
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### Behavior Change
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- **Before**: Sending to a poisoned actor silently discarded the message (address still in map, delivery succeeded, but processing was skipped)
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- **After**: Sending to a cleaned-up actor returns `Err` (address removed from map, send fails)
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- This is **better** — callers learn the actor is gone instead of silently losing messages.
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## Tests Added
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2 new tests + 2 existing tests updated (68 → 70 total):
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- `dead_actor_cleaned_up_from_stats_and_address_map` — good actor persists, bad actor's address is removed
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- `bulk_dead_actor_cleanup` — 20 panicked actors all cleaned up in one tick
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- Updated `send_to_poisoned_actor_is_a_silent_black_hole` → now asserts send returns `Err` (behavior change)
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- Updated `poisoned_actor_messages_not_counted_as_processed` → sends fail to cleaned-up actor
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## Result
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- 70 tests pass
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- All workspace crates compile
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- Memory leak closed: dead actors no longer accumulate in ActorPool or AddressMap
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