Make NULL always fail.

Change of behaviour with the NULL device. It will now simply fail when trying to open it. The consequence of this is that audio will be disabled which was ultimately why the NULL device exists to start with.
Change the NULL device to report that it supports all audio types, channels and format, so audio processing will never occur

Fixes #8816

mythtv/libs/libmyth/audio/audiooutputnull.cpp

@@ -20,13 +20,13 @@ using namespace std;
 #include "mythverbose.h"
 #include "audiooutputnull.h"
 
+#define CHANNELS_MIN 1
+#define CHANNELS_MAX 8
+
 AudioOutputNULL::AudioOutputNULL(const AudioSettings &settings) :
     AudioOutputBase(settings),
     pcm_output_buffer_mutex(QMutex::NonRecursive),
-    current_buffer_size(0),
-    locked_channels(settings.channels),
-    locked_format(settings.format),
-    locked_samplerate(settings.samplerate)
+    current_buffer_size(0)
 {
     memset(pcm_output_buffer, 0, sizeof(char) * NULLAUDIO_OUTPUT_BUFFER_SIZE);
     InitSettings(settings);
@@ -41,22 +41,45 @@ AudioOutputNULL::~AudioOutputNULL()
 
 bool AudioOutputNULL::OpenDevice()
 {
-    VERBOSE(VB_GENERAL, "Opening NULL audio device.");
+    VERBOSE(VB_GENERAL, "Opening NULL audio device, will fail.");
 
     fragment_size = NULLAUDIO_OUTPUT_BUFFER_SIZE / 2;
     soundcard_buffer_size = NULLAUDIO_OUTPUT_BUFFER_SIZE;
 
-    format = locked_format;
-    channels = locked_channels;
-    samplerate = locked_samplerate;
-
-    return true;
+    return false;
 }
 
 void AudioOutputNULL::CloseDevice()
 {
 }
 
+AudioOutputSettings* AudioOutputNULL::GetOutputSettings(bool /*digital*/)
+{
+    // Pretend that we support everything
+    AudioFormat fmt;
+    int rate;
+    AudioOutputSettings *settings = new AudioOutputSettings();
+
+    while ((rate = settings->GetNextRate()))
+    {
+        settings->AddSupportedRate(rate);
+    }
+
+    while ((fmt = settings->GetNextFormat()))
+    {
+        settings->AddSupportedFormat(fmt);
+    }
+
+    for (uint channels = CHANNELS_MIN; channels <= CHANNELS_MAX; channels++)
+    {
+        settings->AddSupportedChannels(channels);
+    }
+
+    settings->setPassthrough(-1);   // no passthrough
+
+    return settings;
+}
+
 
 void AudioOutputNULL::WriteAudio(unsigned char* aubuf, int size)
 {

mythtv/libs/libmyth/audio/audiooutputnull.h

@@ -40,15 +40,12 @@ class AudioOutputNULL : public AudioOutputBase
     virtual void CloseDevice(void);
     virtual void WriteAudio(unsigned char *aubuf, int size);
     virtual int  GetBufferedOnSoundcard(void) const;
+    virtual AudioOutputSettings* GetOutputSettings(bool digital);
 
   private:
     QMutex        pcm_output_buffer_mutex;
     unsigned char pcm_output_buffer[NULLAUDIO_OUTPUT_BUFFER_SIZE];
     int           current_buffer_size;
-
-    int           locked_channels;
-    AudioFormat   locked_format;
-    int           locked_samplerate;
 };
 
 #endif