Don't busy wait in the audio thread (ALSA)

When the emulation is paused and the ALSA backend is used, make the audio
thread wait on a condition variable instead of busy-waiting. This commit
fixes bug #7729

Since the ALSA API is not thread-safe, calls to snd_pcm_drop() and snd_pcm_prepare()
in AlsaSound::Clear() are protected by the same mutex as the condition variable in AlsaSound::SoundLoop()
to make sure that we do not call these functions while a call to
snd_pcm_writei() is ongoing.

Source/Core/AudioCommon/AlsaSoundStream.cpp

@@ -2,6 +2,8 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#include <mutex>
+
 #include "AudioCommon/AlsaSoundStream.h"
 #include "Common/CommonTypes.h"
 #include "Common/Thread.h"
@@ -39,6 +41,10 @@ bool AlsaSound::Start()
 void AlsaSound::Stop()
 {
 	m_thread_status.store(ALSAThreadStatus::STOPPING);
+
+	//Give the opportunity to the audio thread
+	//to realize we are stopping the emulation
+	cv.notify_one();
 	thread.join();
 }
 
@@ -53,8 +59,11 @@ void AlsaSound::SoundLoop()
 	Common::SetCurrentThreadName("Audio thread - alsa");
 	while (m_thread_status.load() == ALSAThreadStatus::RUNNING)
 	{
+		std::unique_lock<std::mutex> lock(cv_m);
+		cv.wait(lock, [this]{return !m_muted || m_thread_status.load() != ALSAThreadStatus::RUNNING;});
+
 		m_mixer->Mix(reinterpret_cast<short *>(mix_buffer), frames_to_deliver);
-		int rc = m_muted ? 1337 : snd_pcm_writei(handle, mix_buffer, frames_to_deliver);
+		int rc = snd_pcm_writei(handle, mix_buffer, frames_to_deliver);
 		if (rc == -EPIPE)
 		{
 			// Underrun
@@ -69,6 +78,24 @@ void AlsaSound::SoundLoop()
 	m_thread_status.store(ALSAThreadStatus::STOPPED);
 }
 
+
+void AlsaSound::Clear(bool muted)
+{
+	m_muted = muted;
+	if (m_muted)
+	{
+		std::lock_guard<std::mutex> lock(cv_m);
+		snd_pcm_drop(handle);
+	}
+	else
+	{
+		std::unique_lock<std::mutex> lock(cv_m);
+		snd_pcm_prepare(handle);
+		lock.unlock();
+		cv.notify_one();
+	}
+}
+
 bool AlsaSound::AlsaInit()
 {
 	unsigned int sample_rate = m_mixer->GetSampleRate();

Source/Core/AudioCommon/AlsaSoundStream.h

@@ -5,6 +5,8 @@
 #pragma once
 
 #include <atomic>
+#include <condition_variable>
+#include <mutex>
 #include <thread>
 
 #if defined(HAVE_ALSA) && HAVE_ALSA
@@ -25,6 +27,7 @@ public:
 	void SoundLoop() override;
 	void Stop() override;
 	void Update() override;
+	void Clear(bool) override;
 
 	static bool isValid()
 	{
@@ -45,6 +48,8 @@ private:
 	u8 *mix_buffer;
 	std::thread thread;
 	std::atomic<ALSAThreadStatus> m_thread_status;
+	std::condition_variable cv;
+	std::mutex cv_m;
 
 	snd_pcm_t *handle;
 	int frames_to_deliver;