In May 2026, the Shopify/rails repo shipped two high-impact fixes to strengthen concurrency safety and reload reliability under fiber-based execution and long-lived connections. The work improves developer confidence in hot-reload during active development and under websockets/WebSocket upgrades while maintaining compatibility with existing rack-based workflows. Key actions: - Replaced thread-based lock ownership with fiber-aware ownership for ShareLock, keyed by ActiveSupport::IsolatedExecutionState.context. This prevents deadlocks when fibers share a thread under :fiber isolation and clarifies ownership semantics. Included renaming of internal state fields (@exclusive_thread -> @exclusive_owner) and updates to the raw_state hash and ActionDispatch::DebugLocks to handle Fiber status gracefully. - Ensured eager release of reloader state on rack hijack (e.g., HTTP 101 upgrades and rack.hijack_io) in ActionDispatch::Executor. By detecting hijacks in Executor#call and applying an idempotent finalize closure, the system releases autoloaded code resolution early, preventing blocking of subsequent reloads and avoiding stale state during long-lived connections. Impact: - Reduced deadlock risk and NoMethodError surface during file reloads. - Faster, more reliable reloads in fiber-scheduled environments and under long-lived streaming connections. - Improved maintainability and alignment with Rails framework conventions. Technologies/skills demonstrated: - ActiveSupport concurrency primitives, fiber-based scheduling, and context-aware ownership models. - Rack hijack handling, ActionDispatch::Executor lifecycle, and reload lifecycle management. - Code refactoring for clearer ownership semantics and updated debugging tooling.