[player] Fix for volumes of unselected devices being reduced

Closes #1077

src/outputs.c

@@ -532,9 +532,6 @@ vol_adjust(void)
 
   for (device = outputs_device_list; device; device = device->next)
     {
-      if (!OUTPUTS_DEVICE_DISPLAY_SELECTED(device) && (device->volume > outputs_master_volume))
-	device->volume = outputs_master_volume;
-
       device->relvol = vol_to_rel(device->volume, outputs_master_volume);
     }
 
@@ -815,12 +812,23 @@ outputs_device_remove(struct output_device *remove)
 }
 
 void
-outputs_device_select(struct output_device *device)
+outputs_device_select(struct output_device *device, int max_volume)
 {
   device->selected = 1;
   device->prevent_playback = 0;
   device->busy = 0;
 
+  // The purpose of this is to cap the volume for a newly selected device. It is
+  // used by the player to avoid this scenario:
+  // 1 Play on two speakers, say Kitchen (100) and Office (75), master is 100
+  // 2 Disable Office, reduce master to 25, Kitchen is now 25, Office is still 75
+  // 3 Turn on Office, it now blasts at 75
+  // We could avoid this by reducing the unselected Office in step 2, but that
+  // leads to issue #1077, where volumes of unselected devices go to 0 (e.g. by
+  // reducing master to 0 and then increasing again -> unselected stays at 0).
+  if (max_volume >= 0 && device->volume > max_volume)
+    device->volume = max_volume;
+
   vol_adjust();
 }
 

src/outputs.h

@@ -291,7 +291,7 @@ void
 outputs_device_remove(struct output_device *remove);
 
 void
-outputs_device_select(struct output_device *device);
+outputs_device_select(struct output_device *device, int max_volume);
 
 void
 outputs_device_deselect(struct output_device *device);

src/player.c

@@ -2091,7 +2091,7 @@ playback_start_item(void *arg, int *retval)
       if (*retval >= 0)
 	{
 	  DPRINTF(E_LOG, L_PLAYER, "Autoselected %s device '%s'\n", device->type_name, device->name);
-	  outputs_device_select(device);
+	  outputs_device_select(device, -1);
 	}
     }
 
@@ -2561,6 +2561,7 @@ speaker_set(void *arg, int *retval)
   struct speaker_set_param *speaker_set_param = arg;
   struct output_device *device;
   uint64_t *ids;
+  int max_volume;
   int nspk;
   int i;
 
@@ -2575,6 +2576,11 @@ speaker_set(void *arg, int *retval)
 
   DPRINTF(E_DBG, L_PLAYER, "Speaker set: %d speakers\n", nspk);
 
+  // Save the current master volume before we start selecting/unselecting, as
+  // that will affect master volume. See comment in outputs_device_select() for
+  // why we want to provide a max_volume.
+  max_volume = (player_state != PLAY_STOPPED) ? outputs_volume_get() : -1;
+
   for (device = outputs_list(); device; device = device->next)
     {
       for (i = 1; i <= nspk; i++)
@@ -2584,7 +2590,7 @@ speaker_set(void *arg, int *retval)
 	}
 
       if (i <= nspk)
-	outputs_device_select(device);
+	outputs_device_select(device, max_volume);
       else
 	outputs_device_deselect(device);
     }
@@ -2602,6 +2608,7 @@ speaker_enable(void *arg, int *retval)
 {
   uint64_t *id = arg;
   struct output_device *device;
+  int max_volume;
 
   *retval = -1;
 
@@ -2611,7 +2618,9 @@ speaker_enable(void *arg, int *retval)
 
   DPRINTF(E_DBG, L_PLAYER, "Speaker enable: '%s' (id=%" PRIu64 ")\n", device->name, *id);
 
-  outputs_device_select(device);
+  max_volume = (player_state != PLAY_STOPPED) ? outputs_volume_get() : -1;
+
+  outputs_device_select(device, max_volume);
 
   *retval = outputs_device_start(device, device_activate_cb, PLAYER_ONLY_PROBE);