feat(engine): defer RouterContext.attach to run after registry bootstrap

runtime/engine/src/main/java/io/aklivity/zilla/runtime/engine/Engine.java

@@ -44,6 +44,7 @@
 import java.util.Objects;
 import java.util.ServiceLoader;
 import java.util.ServiceLoader.Provider;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
@@ -372,6 +373,12 @@ public void start() throws Exception
         if (!readonly)
         {
             manager.start();
+
+            workers.stream()
+                .map(w -> w.attachRouter(routerConfig))
+                .reduce(CompletableFuture::allOf)
+                .ifPresent(CompletableFuture::join);
+
             Ready.markReady(config.directory());
         }
     }

runtime/engine/src/main/java/io/aklivity/zilla/runtime/engine/internal/registry/EngineRouterContext.java

@@ -33,12 +33,17 @@ final class EngineRouterContext implements RouterContext
     }
 
     @Override
-    public BindingHandler attach(
-        RouterConfig config)
+    public BindingHandler streamFactory()
     {
         return streamFactory;
     }
 
+    @Override
+    public void attach(
+        RouterConfig config)
+    {
+    }
+
     @Override
     public void detach(
         long routerId)

runtime/engine/src/main/java/io/aklivity/zilla/runtime/engine/internal/registry/EngineWorker.java

@@ -430,6 +430,7 @@ public EngineWorker(
         EngineRouteable routeable = new EngineRouteable(config, this::newStream,
             this::attachComposite, this::detachComposite, this::supplyStore);
         this.router = router.supply(routeable);
+        this.streamFactory = this.router.streamFactory();
 
         this.bindings = bindings;
         this.exporters = exporters;
@@ -993,8 +994,6 @@ public void onStart()
 
     private void doInit()
     {
-        this.streamFactory = router.attach(routerConfig);
-
         Map<String, BindingContext> bindingsByType = bindings.stream()
             .collect(toMap(Binding::name, b -> b.supply(this), (a, b) -> a, LinkedHashMap::new));
 
@@ -1194,6 +1193,28 @@ public CompletableFuture<Void> detach(
         return detachTask.future();
     }
 
+    public CompletableFuture<Void> attachRouter(
+        RouterConfig config)
+    {
+        assert thread != Thread.currentThread();
+
+        CompletableFuture<Void> future = new CompletableFuture<>();
+        dispatch(() ->
+        {
+            try
+            {
+                router.attach(config);
+                future.complete(null);
+            }
+            catch (Throwable ex)
+            {
+                future.completeExceptionally(ex);
+            }
+        });
+
+        return future;
+    }
+
     public AgentRunner runner()
     {
         return runner;

runtime/engine/src/main/java/io/aklivity/zilla/runtime/engine/router/RouterContext.java

@@ -31,13 +31,27 @@
 public interface RouterContext
 {
     /**
-     * Attaches a router configuration to this thread's context, returning the composed
-     * {@link BindingHandler} that will be installed as the engine's stream factory.
+     * Returns this thread's composed {@link BindingHandler}, installed as the engine's
+     * stream factory.
+     * <p>
+     * Available immediately once this context is constructed, independent of {@link #attach},
+     * so the engine can wire it into bindings before any registry-dependent setup has run.
+     * </p>
+     *
+     * @return the composed stream factory
+     */
+    BindingHandler streamFactory();
+
+    /**
+     * Attaches a router configuration to this thread's context.
+     * <p>
+     * Called once this thread's namespace registry reflects the engine's bootstrap
+     * configuration, so implementations may safely attach synthesized namespaces here.
+     * </p>
      *
      * @param config  the router configuration to activate
-     * @return a {@link BindingHandler} that the engine installs as its stream factory
      */
-    BindingHandler attach(
+    void attach(
         RouterConfig config);
 
     /**

runtime/engine/src/test/java/io/aklivity/zilla/runtime/engine/test/internal/router/TestRouterContext.java

@@ -34,12 +34,17 @@ public TestRouterContext(
     }
 
     @Override
-    public BindingHandler attach(
-        RouterConfig config)
+    public BindingHandler streamFactory()
     {
         return streamFactory;
     }
 
+    @Override
+    public void attach(
+        RouterConfig config)
+    {
+    }
+
     @Override
     public void detach(
         long routerId)