scripts: Add script to find a suitable volume control

When using role based priorities a volume slider presented to the
user should adjust the volume of the currently playing role.

E.g. when a phone has an incoming call and is ringing the default volume
slider should adjust the ringing volume and once the call has been
picked up it should adjust the call volume and once the call ended it
should adjust the media volume again.

It's currently hard for e.g. desktop shells to find out what a suitable
sink for volume control is so add a script to find a suitable target for
volume control (based on media role priority) by storing it's name in
the metadata.

Signed-off-by: Guido Günther <agx@sigxcpu.org>

src/scripts/node/find-media-role-default-volume.lua

@@ -0,0 +1,94 @@
+-- WirePlumber
+--
+-- Copyright © 2025 Phosh.mobi e.V.
+--    @author Guido Günther <agx@sigxcpu.org>
+--
+-- SPDX-License-Identifier: MIT
+--
+-- Select the media role default volume
+
+log = Log.open_topic("s-node")
+
+local cutils = require ("common-utils")
+
+function findHighestPriorityRoleNode (node_om)
+  local best_role = nil
+  local best_prio = 0
+
+  local default_role = Settings.get ("node.stream.default-media-role")
+  if default_role then
+    default_role = default_role:parse()
+  end
+
+  for ni in node_om:iterate {
+     type = "node",
+     Constraint { "media.class", "=", "Audio/Sink" },
+     Constraint { "node.name", "#", "input.loopback.sink.role.*" },
+  } do
+     local ni_props = ni.properties
+     local roles = ni_props["device.intended-roles"]
+     local node_name = ni_props ["node.name"]
+     local prio = tonumber(ni_props ["policy.role-based.priority"])
+
+     -- Use the node that handles the default_role as fallback
+     -- when no node is in running state
+     if best_role == nil and roles and default_role then
+       local roles_table = Json.Raw(roles):parse()
+       for i, v in ipairs (roles_table) do
+         if default_role == v then
+           best_role = node_name
+           best_prio = prio
+           break
+         end
+       end
+     end
+
+     if ni.state == "running" then
+       if prio > best_prio then
+         best_role = node_name
+         best_prio = prio
+       end
+     end
+  end
+
+  log:info (string.format ("Volume control is on : '%s', prio %d", best_role, best_prio))
+  local metadata = cutils.get_default_metadata_object ()
+  metadata:set (0, "current.role-based.volume.control", "Spa:String:JSON",
+                Json.Object { ["name"] = best_role }:to_string ())
+end
+
+SimpleEventHook {
+  name = "node/rescan-for-media-role-volume",
+  interests = {
+    EventInterest {
+      Constraint  { "event.type", "=", "rescan-for-media-role-volume" }
+    },
+  },
+  execute = function (event)
+    local source = event:get_source ()
+    local node_om = source:call ("get-object-manager", "node")
+    findHighestPriorityRoleNode (node_om)
+  end
+}:register ()
+
+-- Track best volume control for media role based priorities
+SimpleEventHook {
+  name = "node/find-media-role-default-volume",
+  interests = {
+    EventInterest {
+      Constraint { "event.type", "=", "node-added" },
+      Constraint { "media.class", "=", "Audio/Sink" },
+      Constraint { "node.name", "#", "input.loopback.sink.role.*" }
+    },
+    EventInterest {
+      Constraint { "event.type", "=", "node-state-changed" },
+      Constraint { "media.class", "=", "Audio/Sink" },
+      Constraint { "node.name", "#", "input.loopback.sink.role.*" }
+    },
+  },
+  execute = function (event)
+    local source = event:get_source ()
+    local node_om = source:call ("get-object-manager", "node")
+    source:call ("schedule-rescan", "media-role-volume")
+  end
+}:register ()