Runtime: fix lost wakeup from late-arriving atomicWaitBroken wait notify#107
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andreaTP merged 2 commits intoJul 20, 2026
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| for (Thread t : threads) { | ||
| final long remaining = Long.max(10, joinDeadline - System.currentTimeMillis()); | ||
| t.join(remaining); | ||
| } |
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There's actually no point in waiting for these threads to join. I could replace this loop with just a sleep until deadline + some leeway. We still expect no thread to be alive.
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Great PR as usual @andreas-karlsson , thanks for the help!
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Fixes #106
A pending wakeup left behind by atomicNotify could be stolen by an atomicWait call that arrives after the notify, letting it return without ever waiting — while the thread the wakeup was actually meant for gets no signal and can sleep forever (lost wakeup / deadlock).
Fixes ByteArrayMemory/ByteBufferMemory by tagging each WaitState with a generation counter, bumped on every notify. A waiter may only consume a pending wakeup minted since it registered, so late arrivals wait for the next notify instead of stealing the current one.
Split into two commits for review: a regression test that reproduces the steal (fails on main), then the fix. Also adds a stress test that drives a rw-lock built on atomicWait/atomicNotify under contention, to catch this class of bug more broadly.