Add support alsa source

pkg/alsa/open_linux.go

@@ -0,0 +1,39 @@
+package alsa
+
+import (
+	"fmt"
+	"net/url"
+
+	"github.com/AlexxIT/go2rtc/pkg/alsa/device"
+	"github.com/AlexxIT/go2rtc/pkg/core"
+)
+
+func Open(rawURL string) (core.Producer, error) {
+	// Example (ffmpeg source compatible):
+	// alsa:device?audio=/dev/snd/pcmC0D0p
+	// TODO: ?audio=default
+	// TODO: ?audio=hw:0,0
+	// TODO: &sample_rate=48000&channels=2
+	// TODO: &backchannel=1
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		return nil, err
+	}
+
+	path := u.Query().Get("audio")
+	dev, err := device.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	switch path[len(path)-1] {
+	case 'p': // playback
+		return newPlayback(dev)
+	case 'c': // capture
+		return newCapture(dev)
+	}
+
+	_ = dev.Close()
+
+	return nil, fmt.Errorf("alsa: unknown path: %s", path)
+}