policy: don't link endpoints on startup

Sometimes the default device node might not exist when reevaluating endpoints
for the first time on startup, so the policy would link endpoints to another
device node. Then, the default device node appears and the policy moves the
endpoints to the default device node while the previous link has not finish its
activation yet. This race condition can cause endpoint links to fail when being
activated. Delaying the reevaluation of endpoint links until the first client
link is created avoids this issue.

src/scripts/policy-endpoint-device.lua

@@ -176,15 +176,21 @@ end
 default_nodes = Plugin.find("default-nodes-api")
 siendpoints_om = ObjectManager { Interest { type = "SiEndpoint" }}
 silinkables_om = ObjectManager { Interest { type = "SiLinkable",
-  -- only handle si-audio-adapter and si-node
-  Constraint {
-    "si.factory.name", "c", "si-audio-adapter", "si-node", type = "pw-global" },
+  -- only handle device si-audio-adapter items
+  Constraint { "si.factory.name", "=", "si-audio-adapter", type = "pw-global" },
+  Constraint { "is.device", "=", true, type = "pw-global" },
   }
 }
 silinks_om = ObjectManager { Interest { type = "SiLink",
   -- only handle links created by this policy
   Constraint { "is.policy.endpoint.device.link", "=", true, type = "pw-global" },
 } }
+clientlinks_om = ObjectManager {
+  Interest {
+    type = "SiLink",
+    Constraint { "is.policy.endpoint.client.link", "=", true },
+  },
+}
 
 -- listen for default node changes if config.follow is enabled
 if config.follow then
@@ -193,10 +199,20 @@ if config.follow then
   end)
 end
 
-silinkables_om:connect("objects-changed", function (om)
+-- start reevaluating endpoints when the first client link is created
+clientlinks_om:connect("object-added", function (om, l)
   reevaluateLinks ()
+
+  -- stop listening to client links from now on
+  clientlinks_om = nil
+
+  -- only reevaluate endpoints when device nodes change from now on
+  silinkables_om:connect("objects-changed", function (om)
+    reevaluateLinks ()
+  end)
 end)
 
 siendpoints_om:activate()
 silinkables_om:activate()
 silinks_om:activate()
+clientlinks_om:activate()