Cycle 4: Shutdown Fix + Bug-Inspired Tests — Development History

Commit: cf61619 · 3 files · 161 insertions, 12 deletions

Motivation

Cycle 3 introduced thread parking, but created a new problem: shutdown() didn't unpark workers. Parked workers wouldn't notice the shutdown signal until their park_timeout expired, causing delayed shutdown. Additionally, studying bug reports from competitor projects (Ractor, Kameo, Actix) revealed specific failure modes worth testing in swactor.

Competitor Bug Analysis

The 5 new tests were directly inspired by real bug reports from other actor frameworks:

Test	Inspired By	Bug
`stats_snapshot_is_read_only`	Ractor #310	`get_children()` was destructive — moved children out of supervisor
`stats_under_load_do_not_interfere_with_processing`	General	Stats collection shouldn't slow down message processing
`shutdown_wakes_parked_workers_immediately`	Cycle 3 regression	Parked workers must notice shutdown promptly
`mt_send_after_run_delivers_to_running_actors`	Kameo #185	Messages sent after `run()` weren't delivered during startup race
`budget_respected_even_with_self_sends`	Actix #515	Self-sends bypassed mailbox capacity, defeating backpressure

Implementation

Shutdown Fix

shutdown() now iterates all workers and calls unpark() on each thread handle
Parked workers wake immediately and check the shutdown flag
Workers that aren't parked are unaffected (unpark is a no-op)

Bug-Inspired Tests

Each test encodes a real bug class discovered in competitor frameworks, ensuring swactor doesn't have the same vulnerability.

Key files modified: src/runtime.rs, tests/runtime_api.rs

Design Decisions

Unpark-all on shutdown rather than a dedicated shutdown condvar — simpler, reuses existing parking infrastructure from Cycle 3
Bug-inspired testing methodology — studying competitor bug trackers yields high-value test cases that target real failure modes, not theoretical ones

Tests Added

5 new tests (52 → 57 total):

stats_snapshot_is_read_only — reading stats doesn't mutate runtime state (from Ractor #310)
stats_under_load_do_not_interfere_with_processing — stats don't affect message processing throughput
shutdown_wakes_parked_workers_immediately — validates fast shutdown with thread parking
mt_send_after_run_delivers_to_running_actors — messages sent after run() are delivered (from Kameo #185)
budget_respected_even_with_self_sends — self-sends don't bypass budget (from Actix #515)

Result

57 tests pass
All workspace crates compile
Shutdown latency with parked workers reduced from up to 1ms to near-zero

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