Set the source gain to 0 before stopping it

Stopping a source in the middle of playback can causing undesirable "popping"
from a sudden change in amplitude. Setting the source gain to 0 gives OpenAL
the opportunity to ramp the source's output to 0, where it can be safely
stopped without causing a sudden change in amplitude.

src/sound/oalsound.cpp

@@ -230,7 +230,7 @@ class OpenALSoundStream : public SoundStream
 		if(Renderer->FreeSfx.Size() == 0)
 		{
 			FSoundChan *lowest = Renderer->FindLowestChannel();
-			if(lowest) Renderer->StopChannel(lowest);
+			if(lowest) Renderer->ForceStopChannel(lowest);
 
 			if(Renderer->FreeSfx.Size() == 0)
 				return false;
@@ -810,6 +810,14 @@ OpenALSoundRenderer::OpenALSoundRenderer()
 		return;
 	}
 
+	ALCint refresh=0;
+	alcGetIntegerv(Device, ALC_REFRESH, 1, &refresh);
+	if(refresh > 0)
+	{
+		// Round up instead of down
+		UpdateTimeMS = (1000+refresh-1) / refresh;
+	}
+
 	ALCint numMono=0, numStereo=0;
 	alcGetIntegerv(Device, ALC_MONO_SOURCES, 1, &numMono);
 	alcGetIntegerv(Device, ALC_STEREO_SOURCES, 1, &numStereo);
@@ -1315,14 +1323,28 @@ void OpenALSoundRenderer::UnloadSound(SoundHandle sfx)
 			if((ALuint)bufID == buffer)
 			{
 				FSoundChan *next = schan->NextChan;
-				StopChannel(schan);
+				ForceStopChannel(schan);
 				schan = next;
 				continue;
 			}
 		}
 		schan = schan->NextChan;
 	}
 
+	// Make sure to kill any currently fading sounds too
+	for(auto iter = FadingSources.begin();iter != FadingSources.end();)
+	{
+		ALint bufID = 0;
+		alGetSourcei(iter->first, AL_BUFFER, &bufID);
+		if(static_cast<ALuint>(bufID) == buffer)
+		{
+			FreeSource(iter->first);
+			iter = FadingSources.erase(iter);
+		}
+		else
+			++iter;
+	}
+
 	alDeleteBuffers(1, &buffer);
 	getALError();
 }
@@ -1359,7 +1381,7 @@ FISoundChannel *OpenALSoundRenderer::StartSound(SoundHandle sfx, float vol, int
 	if(FreeSfx.Size() == 0)
 	{
 		FSoundChan *lowest = FindLowestChannel();
-		if(lowest) StopChannel(lowest);
+		if(lowest) ForceStopChannel(lowest);
 
 		if(FreeSfx.Size() == 0)
 			return NULL;
@@ -1462,7 +1484,7 @@ FISoundChannel *OpenALSoundRenderer::StartSound3D(SoundHandle sfx, SoundListener
 		{
 			if(lowest->Priority < priority || (lowest->Priority == priority &&
 			                                   lowest->DistanceSqr > dist_sqr))
-				StopChannel(lowest);
+				ForceStopChannel(lowest);
 		}
 		if(FreeSfx.Size() == 0)
 			return NULL;
@@ -1666,16 +1688,8 @@ void OpenALSoundRenderer::ChannelVolume(FISoundChannel *chan, float volume)
 	alSourcef(source, AL_GAIN, SfxVolume * volume);
 }
 
-void OpenALSoundRenderer::StopChannel(FISoundChannel *chan)
+void OpenALSoundRenderer::FreeSource(ALuint source)
 {
-	if(chan == NULL || chan->SysChannel == NULL)
-		return;
-
-	ALuint source = GET_PTRID(chan->SysChannel);
-	// Release first, so it can be properly marked as evicted if it's being
-	// forcefully killed
-	S_ChannelEnded(chan);
-
 	alSourceRewind(source);
 	alSourcei(source, AL_BUFFER, 0);
 	getALError();
@@ -1685,11 +1699,48 @@ void OpenALSoundRenderer::StopChannel(FISoundChannel *chan)
 		PausableSfx.Delete(i);
 	if((i=ReverbSfx.Find(source)) < ReverbSfx.Size())
 		ReverbSfx.Delete(i);
+	if((i=SfxGroup.Find(source)) < SfxGroup.Size())
+		SfxGroup.Delete(i);
 
-	SfxGroup.Delete(SfxGroup.Find(source));
 	FreeSfx.Push(source);
 }
 
+void OpenALSoundRenderer::StopChannel(FISoundChannel *chan)
+{
+	if(chan == NULL || chan->SysChannel == NULL)
+		return;
+
+	ALuint source = GET_PTRID(chan->SysChannel);
+	// Release first, so it can be properly marked as evicted if it's being killed
+	S_ChannelEnded(chan);
+
+	ALint state = AL_INITIAL;
+	alGetSourcei(source, AL_SOURCE_STATE, &state);
+	if(state != AL_PLAYING)
+		FreeSource(source);
+	else
+	{
+		// The sound is being killed while playing, so set its gain to 0 and track it
+		// as it fades.
+		alSourcef(source, AL_GAIN, 0.f);
+		getALError();
+
+		FadingSources.insert(std::make_pair(
+			source, std::chrono::steady_clock::now().time_since_epoch().count()
+		));
+	}
+}
+
+void OpenALSoundRenderer::ForceStopChannel(FISoundChannel *chan)
+{
+	ALuint source = GET_PTRID(chan->SysChannel);
+	if(!source) return;
+
+	S_ChannelEnded(chan);
+	FreeSource(source);
+}
+
+
 unsigned int OpenALSoundRenderer::GetPosition(FISoundChannel *chan)
 {
 	if(chan == NULL || chan->SysChannel == NULL)
@@ -1946,6 +1997,23 @@ void OpenALSoundRenderer::UpdateSounds()
 {
 	alProcessUpdatesSOFT();
 
+	if(!FadingSources.empty())
+	{
+		auto cur_time = std::chrono::steady_clock::now().time_since_epoch();
+		for(auto iter = FadingSources.begin();iter != FadingSources.end();)
+		{
+			auto time_diff = std::chrono::duration_cast<std::chrono::milliseconds>(cur_time -
+				std::chrono::steady_clock::time_point::duration(iter->second));
+			if(time_diff.count() >= UpdateTimeMS)
+			{
+				FreeSource(iter->first);
+				iter = FadingSources.erase(iter);
+			}
+			else
+				++iter;
+		}
+	}
+
 	if(ALC.EXT_disconnect)
 	{
 		ALCint connected = ALC_TRUE;
@@ -2031,17 +2099,18 @@ void OpenALSoundRenderer::PrintStatus()
 
 FString OpenALSoundRenderer::GatherStats()
 {
-	ALCint updates = 1;
-	alcGetIntegerv(Device, ALC_REFRESH, 1, &updates);
+	FString out;
+
+	ALCint refresh = 1;
+	alcGetIntegerv(Device, ALC_REFRESH, 1, &refresh);
 	getALCError(Device);
 
 	uint32_t total = Sources.Size();
 	uint32_t used = SfxGroup.Size()+Streams.Size();
 	uint32_t unused = FreeSfx.Size();
 
-	FString out;
-	out.Format("%u sources (" TEXTCOLOR_YELLOW"%u" TEXTCOLOR_NORMAL" active, " TEXTCOLOR_YELLOW"%u" TEXTCOLOR_NORMAL" free), Update interval: " TEXTCOLOR_YELLOW"%d" TEXTCOLOR_NORMAL"ms",
-			   total, used, unused, 1000/updates);
+	out.Format("%u sources (" TEXTCOLOR_YELLOW"%u" TEXTCOLOR_NORMAL" active, " TEXTCOLOR_YELLOW"%u" TEXTCOLOR_NORMAL" free), Update interval: " TEXTCOLOR_YELLOW"%.1f" TEXTCOLOR_NORMAL"ms",
+			   total, used, unused, 1000.f/static_cast<float>(refresh));
 	return out;
 }
 
@@ -2106,7 +2175,7 @@ void OpenALSoundRenderer::PurgeStoppedSources()
 		{
 			if(schan->SysChannel != NULL && src == GET_PTRID(schan->SysChannel))
 			{
-				StopChannel(schan);
+				ForceStopChannel(schan);
 				break;
 			}
 			schan = schan->NextChan;

src/sound/oalsound.h

@@ -5,6 +5,7 @@
 #include <mutex>
 #include <atomic>
 #include <condition_variable>
+#include <unordered_map>
 
 #include "i_sound.h"
 #include "s_sound.h"
@@ -200,8 +201,10 @@ class OpenALSoundRenderer : public SoundRenderer
     void RemoveStream(OpenALSoundStream *stream);
 
 	void LoadReverb(const ReverbContainer *env);
+	void FreeSource(ALuint source);
 	void PurgeStoppedSources();
 	static FSoundChan *FindLowestChannel();
+	void ForceStopChannel(FISoundChannel *chan);
 
     std::thread StreamThread;
     std::mutex StreamLock;
@@ -222,6 +225,10 @@ class OpenALSoundRenderer : public SoundRenderer
 	TArray<ALuint> ReverbSfx;
 	TArray<ALuint> SfxGroup;
 
+	int UpdateTimeMS;
+	using SourceTimeMap = std::unordered_map<ALuint,int64_t>;
+	SourceTimeMap FadingSources;
+
 	const ReverbContainer *PrevEnvironment;
 
     typedef TMap<uint16_t,ALuint> EffectMap;