Fix a race in spawn_future() #520
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There's a race condition in `spawn_future()` between dropping a `future` and the completion of the spawn operation. It's possible for the spawned operation to record the "state" of the future as complete, have the dropping future observe that, which leads to deleting the shared state, whereupon the completing operation touches a deleted object when trying to set the event. This diff adds a unit test that repros the race (it crashed without the fix), and fixes the problem with a spin-wait. I don't *really* like this solution because the spin-wait seems bad, but we're seeing production crashes so I need *something*. We can reconsider the overall synchronization design once we've got the crashes fixed. Maybe we need a refcount on top of the state machine.
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There's a race condition in
spawn_future()between dropping afutureand the completion of the spawned operation. It's possible for the spawned operation to record the "state" of the future as complete, have the dropping future observe that, which leads to deleting the shared state, whereupon the completing operation touches a deleted object when trying to set the event.This diff adds a unit test that repros the race (it crashed without the fix), and fixes the problem with a spin-wait.
I don't really like this solution because the spin-wait seems bad, but we're seeing production crashes so I need something. We can reconsider the overall synchronization design once we've got the crashes fixed. Maybe we need a refcount on top of the state machine.