role-based-policy: Allow to set target sink for media role loopbacks

There are streams that should go to a speaker rather than a headphone
or earpiece by default. Examples are alarms and emergency alerts on
phones. Allow to set a preference via
`policy.role-based.preferred-target` which then looks up the target
via `node.name` and `node.nick`.

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

src/config/wireplumber.conf

@@ -727,11 +727,16 @@ wireplumber.components = [
     provides = hooks.node.role-based.default-volume
     requires = [ hooks.linking.role-based.rescan ]
   }
+  {
+    name = linking/find-media-role-sink-target.lua, type = script/lua
+    provides = hooks.linking.target.find-media-role-sink
+  }
   {
     type = virtual, provides = policy.linking.role-based
     requires = [ policy.linking.standard,
                  hooks.linking.role-based.rescan,
-                 hooks.node.role-based.default-volume ]
+                 hooks.node.role-based.default-volume,
+                 hooks.linking.target.find-media-role-sink ]
   }
 
   ## Standard policy definition

src/config/wireplumber.conf.d.examples/media-role-nodes.conf

@@ -150,6 +150,7 @@ wireplumber.components = [
         policy.role-based.priority = 100
         policy.role-based.action.same-priority = "mix"
         policy.role-based.action.lower-priority = "cork"
+        policy.role-based.preferred-target = "Speaker"
       }
     }
     provides = loopback.sink.role.alert

src/scripts/linking/find-default-target.lua

@@ -13,7 +13,8 @@ SimpleEventHook {
   name = "linking/find-default-target",
   after = { "linking/find-defined-target",
             "linking/find-filter-target",
-            "linking/find-media-role-target" },
+            "linking/find-media-role-target",
+            "linking/find-media-role-sink-target" },
   before = "linking/prepare-link",
   interests = {
     EventInterest {

src/scripts/linking/find-media-role-sink-target.lua

@@ -0,0 +1,81 @@
+-- WirePlumber
+--
+-- Copyright © 2025 Phosh.mobi e.V.
+--
+-- SPDX-License-Identifier: MIT
+--
+-- Pick up a preferred target node for the output stream of role-based loopbacks
+
+lutils = require ("linking-utils")
+cutils = require ("common-utils")
+
+log = Log.open_topic ("s-linking")
+
+SimpleEventHook {
+  name = "linking/find-media-role-sink-target",
+  after = { "linking/find-defined-target",
+	    "linking/find-media-role-target" },
+  before = "linking/prepare-link",
+  interests = {
+    EventInterest {
+      Constraint { "event.type", "=", "select-target" },
+    },
+  },
+  execute = function (event)
+    local _, om, si, si_props, _, target =
+       lutils:unwrap_select_target_event (event)
+
+    local node_name = si_props["node.name"]
+    local target_direction = cutils.getTargetDirection (si_props)
+    local media_class = si_props["media.class"]
+    local link_group = si_props["node.link-group"]
+    local is_virtual = si_props["node.virtual"]
+
+    log:info (si, string.format ("Lookup for '%s' (%s) / '%s' / '%s'",
+				 node_name, tostring (si_props ["node.id"]), media_class, link_group))
+
+    --- bypass the hook if the target is already set or there's no link group
+    if target or media_class ~= "Stream/Output/Audio" or not is_virtual or link_group == nil then
+      return
+    end
+
+    --- We link the output node but the relevant properties are on the input node
+    --- of the link group
+    local input_node = om:lookup {
+      type = "SiLinkable",
+      Constraint { "media.class", "=", "Audio/Sink" },
+      Constraint { "node.link-group", "=", link_group },
+    }
+
+    if input_node == nil then
+       log:warning (si, string.format("No input node for %s found", link_group))
+       return
+    end
+
+    local target_name = input_node.properties["policy.role-based.preferred-target"]
+    --- no preferred target
+    if target_name == nil then
+       return
+    end
+
+    local si_target = om:lookup {
+      type = "SiLinkable",
+      Constraint { "item.factory.name", "c", "si-audio-adapter", "si-node" },
+      Constraint { "node.name", "=", target_name },
+    }
+    if si_target == nil then
+       si_target = om:lookup {
+	  type = "SiLinkable",
+	  Constraint { "item.factory.name", "c", "si-audio-adapter", "si-node" },
+	  Constraint { "node.nick", "=", target_name },
+       }
+    end
+    if si_target then
+       log:info (si,
+        string.format ("... role based sink target picked: %s (%s)",
+          tostring (si_target.properties ["node.name"]),
+          tostring (si_target.properties ["node.id"])))
+       event:set_data ("target", si_target)
+    end
+  end
+}:register ()