fix deadlock in SoundSDL::deinit() #139 #130

On SoundSDL destructor and `reset()`, there was a deadlock with Linux
pthreads, it happened sometimes that the reader thread would call
`SDL_SemWait(data_available)` while `deinit()` would destroy the
semaphore and the `SDL_SemWait()` would end up waiting forever on a null
semaphore.

Change the sequencing of deinit() to prevent this from happening, set
`initialized = false` at the beginning of the sequence to prevent
subsequent entries into the reader callback, and add an SDL_Delay(100)
between the final `SDL_SemPost()` and `SDL_UnlockMutex()`s and the
`SDL_DestroySemaphore()` and `SDL_DestroyMutex()`s to allow a running
reader to complete with a valid mutex and semaphores before they are
destroyed and the thread is joined.

Resetting the sound system also sometimes triggers a memory corruption
bug, but that's a separate issue.

src/common/SoundSDL.cpp

@@ -51,12 +51,13 @@ std::size_t SoundSDL::buffer_size() {
 }
 
 void SoundSDL::read(uint16_t* stream, int length) {
-    if (!initialized || length <= 0)
+    if (length <= 0)
         return;
 
     SDL_memset(stream, audio_spec.silence, length);
 
-    if (!emulating)
+    // if not initialzed, paused or shutting down, do nothing
+    if (!initialized || !emulating)
         return;
 
     if (!buffer_size())
@@ -148,6 +149,8 @@ void SoundSDL::deinit() {
     if (!initialized)
         return;
 
+    initialized = false;
+
     SDL_LockMutex(mutex);
     int is_emulating = emulating;
     emulating = 0;
@@ -155,6 +158,8 @@ void SoundSDL::deinit() {
     SDL_SemPost(data_read);
     SDL_UnlockMutex(mutex);
 
+    SDL_Delay(100);
+
     SDL_DestroySemaphore(data_available);
     data_available = nullptr;
     SDL_DestroySemaphore(data_read);
@@ -166,8 +171,6 @@ void SoundSDL::deinit() {
     SDL_CloseAudioDevice(sound_device);
 
     emulating = is_emulating;
-
-    initialized = false;
 }
 
 SoundSDL::~SoundSDL() {