[o11y] Avoid tail stream cancellation warning with empty handler

src/workerd/api/tests/BUILD.bazel

@@ -64,6 +64,7 @@ wd_test(
         "tail-worker-test-receiver.js",
         "tail-worker-test-dummy.js",
         "tail-worker-test-invalid.js",
+        "tail-worker-test-no-op.js",
         "tail-worker-test-jsrpc.js",
         "websocket-hibernation.js",
         "connect-handler-test.js",

src/workerd/api/tests/tail-worker-test-no-op.js

@@ -0,0 +1,14 @@
+// Copyright (c) 2017-2023 Cloudflare, Inc.
+// Licensed under the Apache 2.0 license found in the LICENSE file or at:
+//     https://opensource.org/licenses/Apache-2.0
+
+export default {
+  // https://developers.cloudflare.com/workers/observability/logs/tail-workers/
+  tailStream(event, env, ctx) {
+    // Return a handler object that does not include any handler functions for individual tail
+    // events – serves as a regression test for a bug where a handler that did not have handler
+    // functions for any events in the final tail stream RPC call caused stream cancellation
+    // warnings.
+    return {};
+  },
+};

src/workerd/api/tests/tail-worker-test.wd-test

@@ -13,7 +13,7 @@ const unitTests :Workerd.Config = (
           ( name = "CACHE_ENABLED", json = "false" ),
         ],
         compatibilityFlags = ["nodejs_compat", "service_binding_extra_handlers", "cache_option_disabled", "queues_json_messages", "url_standard", "workers_api_getters_setters_on_prototype", "fetch_legacy_url", "web_socket_auto_reply_to_close"],
-        streamingTails = ["log", "log-invalid"],
+        streamingTails = ["log", "log-invalid", "log-no-op"],
       ),
     ),
     ( name = "queue-test",
@@ -74,6 +74,15 @@ const unitTests :Workerd.Config = (
           (name = "worker", esModule = embed "tail-worker-test-invalid.js")
         ],
       ),
+    ),
+    # tail worker with a handler object that doesn't handle any of the individual tail event types,
+    # used to confirm that this does not cause warnings
+    ( name = "log-no-op",
+      worker = (
+        modules = [
+          (name = "worker", esModule = embed "tail-worker-test-no-op.js")
+        ],
+      ),
     )
   ],
   sockets = [

src/workerd/io/trace-stream.c++

@@ -934,6 +934,12 @@ class TailStreamTarget final: public rpc::TailStreamTarget::Server {
         });
       }
       return ioContext.awaitJs(js, kj::mv(p));
+    } else if (doFulfill) {
+      // If we have no promises, but doFulfill is true, then none of the events we have had a
+      // handler available, but we still need to indicate that we are done since we got the outcome
+      // event – do this by calling fulfill right away.
+      doneReceiving = true;
+      doneFulfiller->fulfill();
     }
     return kj::READY_NOW;
   }