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refactor(xpp): replace atomic waker slots with mutex in ScheduledIo - #201
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The previous implementation stored wakers as 4 atomic pointers (Schedule* + SpawnTaskBase* per direction) with manual release/acq_rel ordering. This was a lock-free reimplementation of a mutex but without the actual mutual exclusion — the Waker struct itself was subject to concurrent read/write data races. Replace with a mutex-protected Waiters struct (xpp::sys::Mutex<Waiters>), following Tokio's ScheduledIo design: - Readiness bitmask stays lock-free (atomic<uint8_t>) - Waker slots (reader/writer) behind a single mutex - Double-check-under-lock in poll_read/poll_write prevents lost wakes - wake_read/wake_write take the mutex before firing This eliminates the data race, reduces fields from 10 to 5, and removes 3 static helper functions (set_waker/clear_waker/fire_waker). Also add 15 unit tests covering all public methods: poll_read, poll_write, clear_readable, clear_writable, close_fd, is_closed, accessors, and concurrent poll+event. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Summary
xpp::sys::Mutex<Waiters>, following Tokio's ScheduledIo designpoll_read/poll_writeprevents lost wakesMotivation
The previous
set_waker/fire_wakerhelpers storedWakeras two separate atomic pointers (Schedule*+SpawnTaskBase*) with manual release/acq_rel ordering. This was a lock-free reimplementation of a mutex but without actual mutual exclusion — theWakerstruct was subject to concurrent read/write data races. Tokio uses aMutex<Waiters>for exactly this reason.Test plan
ScheduledIoTestcases: accessors, poll_read (not ready, ready, clear+re-poll, waker wake), poll_write (ready, clear+re-poll, not ready), close_fd (sets closed, idempotent, wakes read/write), concurrent poll+eventctest -R xppin build (C++14)ctest -R xppin build-cxx20 (C++20)🤖 Generated with Claude Code