Crash in Java/C# code: GADRegisterSignalHandlers: race condition in signal handler cause NULL handler to be installed once SIGBUS is handled.

[dkimitsa]

Hello,
GAD version v12.9.0.
Observer behaviour: when two thread has SIGBUS signal, first thread restores previous in chain, but second nulls it.
Reproducible: 100%

This causes C#/Java signals handlers to be removed from chain and they are not able to handle Null Pointer Exceptions in try/catch blocks.
Code to reproduce (RoboVM one):

Runnable NPE = () -> {
    try {
        ((String) null).equals("hello");
    } catch (NullPointerException ignored) {}
};
Thread t1 = new Thread(NPE);
Thread t2 = new Thread(NPE);
t1.start();
t2.start();

^^^ this code will crash.
It happens due possible scenarios:

• thread1 and thread2 have both NPE at same time;
• signals are raised in both of these;
• first thread restores valid previous handler, removes GAM from handling and calls it;
• second thread processing signal as it was raised before removed by thread 1;
• second thread restores NULL handlers (e.g. not RoboVM ones) and call it;
• crash

full investigation story