Queue Lua MESSAGEMAN:Broadcast to be ran on main thread #2
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Works around a potential deadlock: T1 = Thread 1 (main thread) T2 = Thread 2 (background thread) M = MessageManager mutex L = Lua mutex 1. MessageManager::Broadcast is looping subscribers on T1, while keeping M locked. Subscribers might lock L. 2. A network request completes on T2, obtains lock on L before M is released. Lua code wants to run MessageManager::Broadcast, which waits for M to be released. We are now in a deadlock as T1 is waiting for L while holding M and T2 is waiting for M while holding L. This commit changes the Lua API to never run MessageManager::Broadcast from a non-main thread. Instead, a Message is allocated and stored in a queue, which the main thread checks in the main GameLoop. Access to the queue is guarded with a separate mutex. No other mutexes are obtained while this new mutex is locked. With this change, it should no longer be possible for M to be waited on while L is locked from a non-main thread and when attempting to broadcast. (Potentially, it could be possible for MessageManager::Broadcast to lock M and L on the main thread and run Lua which attempts to Broadcast again, that would try to obtain M which is already locked... I did not investigate whether there are some safeguards preventing this.)
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Works around a potential deadlock:
T1 = Thread 1 (main thread)
T2 = Thread 2 (background thread)
M = MessageManager mutex
L = Lua mutex
We are now in a deadlock as T1 is waiting for L while holding M and T2 is waiting for M while holding L.
This commit changes the Lua API to never run MessageManager::Broadcast from a non-main thread. Instead, a Message is allocated and stored in a queue, which the main thread checks in the main GameLoop.
Access to the queue is guarded with a separate mutex. No other mutexes are obtained while this new mutex is locked.
With this change, it should no longer be possible for M to be waited on while L is locked from a non-main thread and when attempting to broadcast.
(Potentially, it could be possible for MessageManager::Broadcast to lock M and L on the main thread and run Lua which attempts to Broadcast again, that would try to obtain M which is already locked... I did not investigate whether there are some safeguards preventing this.)