Disconnected/broken/lost audio devices now continue to fire their callback.

The data produced by the callback is just thrown away and the audio thread
delays as if it's waiting for the hardware to drain.

This lets apps that rely on their audio callback firing regularly continue
to make progress to function as properly as possible in the face of disaster.
Apps that want to know that the device is really gone and deal with that
scenario can use the new hotplug functionality.

src/audio/SDL_audio.c

@@ -727,7 +727,7 @@ SDL_RunAudio(void *devicep)
            resampling process can be any number. We will have to see what a good size for the
            stream's maximum length is, but I suspect 2*max(len_cvt, stream_len) is a good figure.
          */
-        while (device->enabled) {
+        while (!device->shutdown) {
 
             if (device->paused) {
                 SDL_Delay(delay);
@@ -810,31 +810,27 @@ SDL_RunAudio(void *devicep)
         const int silence = (int) device->spec.silence;
 
         /* Loop, filling the audio buffers */
-        while (device->enabled) {
-
+        while (!device->shutdown) {
             /* Fill the current buffer with sound */
             if (device->convert.needed) {
-                if (device->convert.buf) {
-                    stream = device->convert.buf;
-                } else {
-                    continue;
-                }
-            } else {
+                stream = device->convert.buf;
+            } else if (device->enabled) {
                 stream = current_audio.impl.GetDeviceBuf(device);
-                if (stream == NULL) {
-                    stream = device->fake_stream;
-                }
+            } else {
+                /* if the device isn't enabled, we still write to the
+                    fake_stream, so the app's callback will fire with
+                    a regular frequency, in case they depend on that
+                    for timing or progress. They can use hotplug
+                    now to know if the device failed. */
+                stream = NULL;
             }
 
-            /* !!! FIXME: this should be LockDevice. */
-            SDL_LockMutex(device->mixer_lock);
-
-            /* Check again, in case device was removed while a lock was held. */
-            if (!device->enabled) {
-                SDL_UnlockMutex(device->mixer_lock);
-                break;
+            if (stream == NULL) {
+                stream = device->fake_stream;
             }
 
+            /* !!! FIXME: this should be LockDevice. */
+            SDL_LockMutex(device->mixer_lock);
             if (device->paused) {
                 SDL_memset(stream, silence, stream_len);
             } else {
@@ -843,14 +839,15 @@ SDL_RunAudio(void *devicep)
             SDL_UnlockMutex(device->mixer_lock);
 
             /* Convert the audio if necessary */
-            if (device->convert.needed) {
+            if (device->enabled && device->convert.needed) {
                 SDL_ConvertAudio(&device->convert);
                 stream = current_audio.impl.GetDeviceBuf(device);
                 if (stream == NULL) {
                     stream = device->fake_stream;
+                } else {
+                    SDL_memcpy(stream, device->convert.buf,
+                               device->convert.len_cvt);
                 }
-                SDL_memcpy(stream, device->convert.buf,
-                           device->convert.len_cvt);
             }
 
             /* Ready current buffer for play and change current buffer */
@@ -1084,6 +1081,7 @@ static void
 close_audio_device(SDL_AudioDevice * device)
 {
     device->enabled = 0;
+    device->shutdown = 1;
     if (device->thread != NULL) {
         SDL_WaitThread(device->thread, NULL);
     }
@@ -1178,6 +1176,7 @@ open_audio_device(const char *devname, int iscapture,
     SDL_AudioDevice *device;
     SDL_bool build_cvt;
     void *handle = NULL;
+    int stream_len;
     int i = 0;
 
     if (!SDL_WasInit(SDL_INIT_AUDIO)) {
@@ -1304,14 +1303,6 @@ open_audio_device(const char *devname, int iscapture,
     }
     device->opened = 1;
 
-    /* Allocate a fake audio memory buffer */
-    device->fake_stream = (Uint8 *)SDL_AllocAudioMem(device->spec.size);
-    if (device->fake_stream == NULL) {
-        close_audio_device(device);
-        SDL_OutOfMemory();
-        return 0;
-    }
-
     /* See if we need to do any conversion */
     build_cvt = SDL_FALSE;
     if (obtained->freq != device->spec.freq) {
@@ -1370,6 +1361,18 @@ open_audio_device(const char *devname, int iscapture,
         }
     }
 
+    /* Allocate a fake audio memory buffer */
+    stream_len = (device->convert.needed) ? device->convert.len_cvt : 0;
+    if (device->spec.size > stream_len) {
+        stream_len = device->spec.size;
+    }
+    device->fake_stream = (Uint8 *)SDL_AllocAudioMem(stream_len);
+    if (device->fake_stream == NULL) {
+        close_audio_device(device);
+        SDL_OutOfMemory();
+        return 0;
+    }
+
     if (device->spec.callback == NULL) {  /* use buffer queueing? */
         /* pool a few packets to start. Enough for two callbacks. */
         const int packetlen = SDL_AUDIOBUFFERQUEUE_PACKETLEN;

src/audio/SDL_sysaudio.h

@@ -155,7 +155,8 @@ struct SDL_AudioDevice
     /* Current state flags */
     /* !!! FIXME: should be SDL_bool */
     int iscapture;
-    int enabled;
+    int enabled;  /* true if device is functioning and connected. */
+    int shutdown; /* true if we are signaling the play thread to end. */
     int paused;
     int opened;