audio: Use a dummy driver for testing

Fixes #750

This also fixes an out-of-index bug when all players are skipped.

audio/audio.go

@@ -74,10 +74,13 @@ func (p *players) Read(b []byte) (int, error) {
 	l := len(b)
 	l &= mask
 
+	allSkipped := true
+
 	for player := range p.players {
 		if player.shouldSkip() {
 			continue
 		}
+		allSkipped = false
 		s := player.bufferSizeInBytes()
 		if l > s {
 			l = s
@@ -85,6 +88,10 @@ func (p *players) Read(b []byte) (int, error) {
 		}
 	}
 
+	if allSkipped {
+		l = 0
+	}
+
 	if l == 0 {
 		// If l is 0, all the players might reach EOF at the next update.
 		// However, this Read might block forever and never causes context switch
@@ -233,6 +240,15 @@ func CurrentContext() *Context {
 	return c
 }
 
+var driverForTesting io.WriteCloser
+
+func newDriver(sampleRate int) (io.WriteCloser, error) {
+	if driverForTesting != nil {
+		return driverForTesting, nil
+	}
+	return oto.NewPlayer(sampleRate, channelNum, bytesPerSample/channelNum, bufferSize())
+}
+
 func (c *Context) loop() {
 	initCh := make(chan struct{})
 
@@ -255,7 +271,7 @@ func (c *Context) loop() {
 	// e.g. a variable for JVM on Android might not be set.
 	<-initCh
 
-	p, err := oto.NewPlayer(c.sampleRate, channelNum, bytesPerSample/channelNum, bufferSize())
+	p, err := newDriver(c.sampleRate)
 	if err != nil {
 		c.err = err
 		return

audio/driver_test.go

@@ -0,0 +1,29 @@
+// Copyright 2018 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package audio
+
+type dummyDriver struct{}
+
+func (d *dummyDriver) Write(b []byte) (int, error) {
+	return len(b), nil
+}
+
+func (d *dummyDriver) Close() error {
+	return nil
+}
+
+func init() {
+	driverForTesting = &dummyDriver{}
+}