audio: Bug fix: Fill empty data even when audio is suspended

When writing is stopped on Oto's players, unexpected delaying
happens.

The ideal solution is to have APIs to suspend and resume Oto's
player, but this is not easy. For a temporary solution, write
zero values on the players when audio is suspended.

Fixes #975

audio/audio.go

@@ -63,11 +63,11 @@ type Context struct {
 	sampleRate int
 	err        error
 	ready      bool
+	suspended  bool
 
 	players map[*playerImpl]struct{}
 
-	m         sync.Mutex
-	semaphore chan struct{}
+	m sync.Mutex
 }
 
 var (
@@ -75,6 +75,8 @@ var (
 	theContextLock sync.Mutex
 )
 
+var emptyBytes = make([]byte, 256)
+
 // NewContext creates a new audio context with the given sample rate.
 //
 // The sample rate is also used for decoding MP3 with audio/mp3 package
@@ -99,16 +101,19 @@ func NewContext(sampleRate int) (*Context, error) {
 		c:          newContext(sampleRate),
 		players:    map[*playerImpl]struct{}{},
 		inited:     make(chan struct{}),
-		semaphore:  make(chan struct{}, 1),
 	}
 	theContext = c
 
 	h := getHook()
 	h.OnSuspendAudio(func() {
-		c.semaphore <- struct{}{}
+		c.m.Lock()
+		c.suspended = true
+		c.m.Unlock()
 	})
 	h.OnResumeAudio(func() {
-		<-c.semaphore
+		c.m.Lock()
+		c.suspended = false
+		c.m.Unlock()
 	})
 
 	h.AppendHookOnBeforeUpdate(func() error {
@@ -455,10 +460,14 @@ func (p *playerImpl) read() ([]byte, bool) {
 
 	const bufSize = 2048
 
-	p.context.semaphore <- struct{}{}
-	defer func() {
-		<-p.context.semaphore
-	}()
+	// If audio is suspended, fill zero values not to cause delay (#975).
+	// TODO: Oto's players should be able to be suspended and resumed.
+	p.context.m.Lock()
+	s := p.context.suspended
+	p.context.m.Unlock()
+	if s {
+		return emptyBytes, true
+	}
 
 	newBuf := make([]byte, bufSize-len(p.buf))
 	n, err := p.src.Read(newBuf)