Limiter No.6 causes deadlock when changing latency mode while processing

[PhysSong]

To reproduce:

1. Load Limiter No.6 on master channel
2. Press "Fix. latency" while playing audio

Here's the relevant part of backtrace for RemoteVstPlugin.exe.so:

Thread 3 (Thread 0x2393b40 (LWP 14783)):
#0  0xf7fd5059 in __kernel_vsyscall ()
#1  0xf7dda5c2 in __lll_lock_wait () from /lib/i386-linux-gnu/libpthread.so.0
#2  0xf7dd2b7c in pthread_mutex_lock ()
   from /lib/i386-linux-gnu/libpthread.so.0
#3  0x7fb0dffd in RemoteVstPlugin::lockShm() ()
   from /home/hjsong/.wine/dosdevices/z:/usr/local/lib/lmms/RemoteVstPlugin.exe.so
#4  0x7fb0ba8c in RemoteVstPlugin::updateInOutCount() ()
   from /home/hjsong/.wine/dosdevices/z:/usr/local/lib/lmms/RemoteVstPlugin.exe.so
#5  0x7fb0c1b0 in RemoteVstPlugin::hostCallback(AEffect*, int, int, int, void*, float) ()
   from /home/hjsong/.wine/dosdevices/z:/usr/local/lib/lmms/RemoteVstPlugin.exe.so
#6  0x1001b6ab in ?? ()
#7  0x10017ee3 in ?? ()
#8  0x1001a63d in ?? ()
#9  0x7fb07946 in RemotePluginClient::doProcessing() ()

The problem is the callback tries to lock m_shmLock but it's already locked by doProcessing. I don't know which solution will be the best, but what I think is: make m_shmLock to a recursive mutex.

@DomClark Any ideas/suggestions?

[DomClark]

I don't think making m_shmLock recursive is a good solution in general - the purpose of the mutex is to prevent m_inputs, m_outputs and the shared memory from being deleted during a call to m_plugin->processReplacing. It might work in this particular case, but for other plugins it may well result in use-after-free crashes within the plugin.
In this case, the plugin is using audioMasterIOChanged to announce a latency change, not a change in the number of inputs and outputs. We can easily check if the IO count hasn't changed (inputCount() == RemotePluginClient::inputCount() && outputCount() == RemotePluginClient::outputCount()) and return immediately without taking the lock in this case. A more general solution might be trickier. I'll work on a fix.