Deadlock when reloading cache

[ksuprynowicz]

Deadlock happens in:
_nonPersistentEntitiesScriptManager->unloadAllEntityScripts(true);

[ksuprynowicz]

It deadlocks with entity script thread:

QtLinuxFutex::_q_futex qfutex_p.h:116
QtLinuxFutex::futexWait<…> qfutex_p.h:135
futexSemaphoreTryAcquire_loop<false> 0x00007ffff5acdd9b
futexSemaphoreTryAcquire<false> 0x00007ffff5acdd9b
QSemaphore::acquire 0x00007ffff5acdd9b
QMetaMethod::invoke 0x00007ffff5cbc78f
QMetaObject::invokeMethod 0x00007ffff5cc1726
hifi::qt::blockingInvokeMethod QtHelpers.cpp:86
ResourceCache::prefetch ResourceCache.cpp:222
ScriptableResourceCache::prefetch ResourceCache.cpp:165
ScriptableResourceCache::prefetch ResourceCache.h:382
ScriptableResourceCache::qt_static_metacall moc_ResourceCache.cpp:582
QMetaMethod::invoke 0x00007ffff5cbc88e
ScriptMethodV8Proxy::call ScriptObjectV8Proxy.cpp:1098
ScriptMethodV8Proxy::callback ScriptObjectV8Proxy.cpp:949
v8::internal::FunctionCallbackArguments::Call 0x00007fffe25c6c6a
v8::internal::(anonymous namespace)::HandleApiCallHelper<false> 0x00007fffe25c7632
v8::internal::Builtin_Impl_HandleApiCall 0x00007fffe25c7b1f
v8::internal::Builtin_HandleApiCall 0x00007fffe25c7b1f
Builtins_CEntry_Return1_DontSaveFPRegs_ArgvOnStack_BuiltinExit 0x00007fffe2479c79
Builtins_InterpreterEntryTrampoline 0x00007fffe23fdbd0

It's caused by another blocking invoke call:

    return QMetaObject::invokeMethod(obj, member,
            Qt::BlockingQueuedConnection, ret, val0, val1, val2, val3, val4, val5, val6, val7, val8, val9);
}

[ksuprynowicz]

It happens between

QMetaObject::invokeMethod(this, "unloadAllEntityScripts",
            blockingCall ? Qt::BlockingQueuedConnection : Qt::QueuedConnection);

in ScriptManager::unloadAllEntityScripts
and

        BLOCKING_INVOKE_METHOD(this, "prefetch",
            Q_RETURN_ARG(ScriptableResource*, result),
            Q_ARG(QUrl, url), Q_ARG(void*, extra), Q_ARG(size_t, extraHash));
        return result;
    }

in ScriptableResource* ResourceCache::prefetch

[ksuprynowicz]

This only affects domains that have a lot of entity scripts