Add convenience for dynamic changes to routing (#2704)

tests/test_late_adds.py

@@ -0,0 +1,54 @@
+import pytest
+
+from sanic import Sanic, text
+
+
+@pytest.fixture
+def late_app(app: Sanic):
+    app.config.TOUCHUP = False
+    app.get("/")(lambda _: text(""))
+    return app
+
+
+def test_late_route(late_app: Sanic):
+    @late_app.before_server_start
+    async def late(app: Sanic):
+        @app.get("/late")
+        def handler(_):
+            return text("late")
+
+    _, response = late_app.test_client.get("/late")
+    assert response.status_code == 200
+    assert response.text == "late"
+
+
+def test_late_middleware(late_app: Sanic):
+    @late_app.get("/late")
+    def handler(request):
+        return text(request.ctx.late)
+
+    @late_app.before_server_start
+    async def late(app: Sanic):
+        @app.on_request
+        def handler(request):
+            request.ctx.late = "late"
+
+    _, response = late_app.test_client.get("/late")
+    assert response.status_code == 200
+    assert response.text == "late"
+
+
+def test_late_signal(late_app: Sanic):
+    @late_app.get("/late")
+    def handler(request):
+        return text(request.ctx.late)
+
+    @late_app.before_server_start
+    async def late(app: Sanic):
+        @app.signal("http.lifecycle.request")
+        def handler(request):
+            request.ctx.late = "late"
+
+    _, response = late_app.test_client.get("/late")
+    assert response.status_code == 200
+    assert response.text == "late"