Type safety error in fiber serialisation/TLS swapping code can cause weird crashes

[mikehearn]

Fiber defines the following fields:

private Object fiberLocals; private Object inheritableFiberLocals;

It is unclear to me why type safety is disabled for these fields, but it appears that normally they are meant to contain a ThreadLocalMap object, judging from Fiber.switchFiberAndThreadLocals()

However FiberSerializer changes the type of these fields with no explanation:

f.fiberLocals = f.fiberLocals != null ? filterThreadLocalMap(ThreadAccess.toMap(f.fiberLocals)).keySet().toArray() : null;
f.inheritableFiberLocals = f.inheritableFiberLocals != null ? filterThreadLocalMap(ThreadAccess.toMap(f.inheritableFiberLocals)).keySet().toArray() : null;

After this code runs f.fiberLocals is now an array, not a ThreadLocalMap. No explanation is provided in the code for the type switch. This causes a crash when debugging is enabled on the next run through switchFiberAndThreadLocals when it tries to print out the contents to the debug log.

[pron]

It is unclear to me why type safety is disabled for these fields

Because ThreadLocalMap is package-private.

However FiberSerializer changes the type of these fields with no explanation

That's because ThreadLocalMap is not serializable.

on the next run through switchFiberAndThreadLocals

Ah, but in the original use-case for serialization, there would be no next run. Once a fiber is serialized, the original fiber was presumed to be discarded until a new one is recreated from the serialized representation.

It seems like fiber serialization needs to be retrofitted to allow your use-case. I don't think it's a big change by any means, but the current assumptions are wrong.