bridge: a discovery request that arrives twice is answered once

A broker that mirrors another delivers every message twice, and the bridge
answered both copies of a discovery request. A request is now remembered
by its messageId for 60 s, as a directive is, and its copy is dropped with
a log line. The messageId is read from the header of the Discover
directive or from the request itself when that is the header alone.
A request without a messageId is answered every time, as before.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

dist/cjs/Alex2Node.js

@@ -53,6 +53,20 @@ const DESCRIBED = [
     "description", "manufacturerName", "manufacturer", "model", "serialNumber", "firmwareVersion", "softwareVersion",
     "customIdentifier", "cookie",
 ];
+// The messageId of a discovery request: in the header of the Discover directive, or beside the namespace and the
+// name when the request is the header alone. undefined for a request without one, which is answered every time.
+function requestId(text) {
+    let request;
+    try {
+        request = JSON.parse(text);
+    }
+    catch {
+        return undefined;
+    }
+    const header = request?.directive?.header ?? request?.header ?? request;
+    const { messageId } = (typeof header === "object" && header !== null ? header : {});
+    return typeof messageId === "string" ? messageId : undefined;
+}
 /**
  * The Alexa-to-MQTT bridge: one broker connection for a user's root topic, a set of registered devices, discovery and
  * directive dispatch.
@@ -157,6 +171,11 @@ class Alex2MQTT extends events_1.EventEmitter {
         this.log(`MQTT Message Received`, { topic, payload: text });
         try {
             if (topic === topics.discover(this.rootTopic)) {
+                const messageId = requestId(text);
+                if (messageId !== undefined && this.dispatcher.arrivedBefore("", messageId)) {
+                    this.log(`the discovery request ${messageId} is dropped: it arrived before`);
+                    return;
+                }
                 await this.answerDiscovery();
                 return;
             }

dist/cjs/dispatcher.js

@@ -163,6 +163,18 @@ class Dispatcher {
         this.deferralNoted.add(namespace);
         this.options.log(`warning: Amazon documents no DeferredResponse for ${namespace}, Alexa may not wait for the answer`);
     }
+    /**
+     * true for a message that came before, within REMEMBERED_MS: a broker that mirrors another delivers each message
+     * twice. endpointId is "" for a message to the root, a discovery request.
+     */
+    arrivedBefore(endpointId, messageId, now = Date.now()) {
+        this.forget(now);
+        const id = key(endpointId, messageId);
+        if (this.arrived.has(id))
+            return true;
+        this.arrived.set(id, now);
+        return false;
+    }
     /** The text of a message on the directive topic of endpointId. Resolves when the handlers are done. */
     async dispatch(endpointId, text) {
         const { log, emit } = this.options;
@@ -182,13 +194,9 @@ class Dispatcher {
         const { namespace, name, messageId } = raw.header;
         const correlationToken = typeof raw.header.correlationToken === "string" ? raw.header.correlationToken : "";
         const receivedAt = Date.now();
-        this.forget(receivedAt);
-        if (typeof messageId === "string") {
-            if (this.arrived.has(key(endpointId, messageId))) {
-                log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
-                return;
-            }
-            this.arrived.set(key(endpointId, messageId), receivedAt);
+        if (typeof messageId === "string" && this.arrivedBefore(endpointId, messageId, receivedAt)) {
+            log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
+            return;
         }
         const device = this.options.getDevice(endpointId);
         if (!device) {

dist/esm/Alex2Node.js

@@ -14,6 +14,20 @@ const DESCRIBED = [
     "description", "manufacturerName", "manufacturer", "model", "serialNumber", "firmwareVersion", "softwareVersion",
     "customIdentifier", "cookie",
 ];
+// The messageId of a discovery request: in the header of the Discover directive, or beside the namespace and the
+// name when the request is the header alone. undefined for a request without one, which is answered every time.
+function requestId(text) {
+    let request;
+    try {
+        request = JSON.parse(text);
+    }
+    catch {
+        return undefined;
+    }
+    const header = request?.directive?.header ?? request?.header ?? request;
+    const { messageId } = (typeof header === "object" && header !== null ? header : {});
+    return typeof messageId === "string" ? messageId : undefined;
+}
 /**
  * The Alexa-to-MQTT bridge: one broker connection for a user's root topic, a set of registered devices, discovery and
  * directive dispatch.
@@ -118,6 +132,11 @@ class Alex2MQTT extends EventEmitter {
         this.log(`MQTT Message Received`, { topic, payload: text });
         try {
             if (topic === topics.discover(this.rootTopic)) {
+                const messageId = requestId(text);
+                if (messageId !== undefined && this.dispatcher.arrivedBefore("", messageId)) {
+                    this.log(`the discovery request ${messageId} is dropped: it arrived before`);
+                    return;
+                }
                 await this.answerDiscovery();
                 return;
             }

dist/esm/dispatcher.d.ts

@@ -93,6 +93,11 @@ export declare class Dispatcher {
     send(topic: string, message: object): Promise<PublishResult>;
     /** defer() for an interface Amazon documents no DeferredResponse for: said once, the DeferredResponse is sent. */
     noteDeferral(namespace: string): void;
+    /**
+     * true for a message that came before, within REMEMBERED_MS: a broker that mirrors another delivers each message
+     * twice. endpointId is "" for a message to the root, a discovery request.
+     */
+    arrivedBefore(endpointId: string, messageId: string, now?: number): boolean;
     /** The text of a message on the directive topic of endpointId. Resolves when the handlers are done. */
     dispatch(endpointId: string, text: string): Promise<void>;
     private route;

dist/esm/dispatcher.js

@@ -127,6 +127,18 @@ export class Dispatcher {
         this.deferralNoted.add(namespace);
         this.options.log(`warning: Amazon documents no DeferredResponse for ${namespace}, Alexa may not wait for the answer`);
     }
+    /**
+     * true for a message that came before, within REMEMBERED_MS: a broker that mirrors another delivers each message
+     * twice. endpointId is "" for a message to the root, a discovery request.
+     */
+    arrivedBefore(endpointId, messageId, now = Date.now()) {
+        this.forget(now);
+        const id = key(endpointId, messageId);
+        if (this.arrived.has(id))
+            return true;
+        this.arrived.set(id, now);
+        return false;
+    }
     /** The text of a message on the directive topic of endpointId. Resolves when the handlers are done. */
     async dispatch(endpointId, text) {
         const { log, emit } = this.options;
@@ -146,13 +158,9 @@ export class Dispatcher {
         const { namespace, name, messageId } = raw.header;
         const correlationToken = typeof raw.header.correlationToken === "string" ? raw.header.correlationToken : "";
         const receivedAt = Date.now();
-        this.forget(receivedAt);
-        if (typeof messageId === "string") {
-            if (this.arrived.has(key(endpointId, messageId))) {
-                log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
-                return;
-            }
-            this.arrived.set(key(endpointId, messageId), receivedAt);
+        if (typeof messageId === "string" && this.arrivedBefore(endpointId, messageId, receivedAt)) {
+            log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
+            return;
         }
         const device = this.options.getDevice(endpointId);
         if (!device) {

dist/types/dispatcher.d.ts

@@ -93,6 +93,11 @@ export declare class Dispatcher {
     send(topic: string, message: object): Promise<PublishResult>;
     /** defer() for an interface Amazon documents no DeferredResponse for: said once, the DeferredResponse is sent. */
     noteDeferral(namespace: string): void;
+    /**
+     * true for a message that came before, within REMEMBERED_MS: a broker that mirrors another delivers each message
+     * twice. endpointId is "" for a message to the root, a discovery request.
+     */
+    arrivedBefore(endpointId: string, messageId: string, now?: number): boolean;
     /** The text of a message on the directive topic of endpointId. Resolves when the handlers are done. */
     dispatch(endpointId: string, text: string): Promise<void>;
     private route;

src/Alex2Node.ts

@@ -50,6 +50,20 @@ const DESCRIBED = [
   "customIdentifier", "cookie",
 ];
 
+// The messageId of a discovery request: in the header of the Discover directive, or beside the namespace and the
+// name when the request is the header alone. undefined for a request without one, which is answered every time.
+function requestId(text: string): string | undefined {
+  let request: { directive?: { header?: unknown }; header?: unknown } | null;
+  try {
+    request = JSON.parse(text);
+  } catch {
+    return undefined;
+  }
+  const header = request?.directive?.header ?? request?.header ?? request;
+  const { messageId } = (typeof header === "object" && header !== null ? header : {}) as { messageId?: unknown };
+  return typeof messageId === "string" ? messageId : undefined;
+}
+
 /**
  * The Alexa-to-MQTT bridge: one broker connection for a user's root topic, a set of registered devices, discovery and
  * directive dispatch.
@@ -160,6 +174,11 @@ class Alex2MQTT extends EventEmitter {
     this.log(`MQTT Message Received`, { topic, payload: text });
     try {
       if (topic === topics.discover(this.rootTopic)) {
+        const messageId = requestId(text);
+        if (messageId !== undefined && this.dispatcher.arrivedBefore("", messageId)) {
+          this.log(`the discovery request ${messageId} is dropped: it arrived before`);
+          return;
+        }
         await this.answerDiscovery();
         return;
       }

src/dispatcher.ts

@@ -240,6 +240,18 @@ export class Dispatcher {
     this.options.log(`warning: Amazon documents no DeferredResponse for ${namespace}, Alexa may not wait for the answer`);
   }
 
+  /**
+   * true for a message that came before, within REMEMBERED_MS: a broker that mirrors another delivers each message
+   * twice. endpointId is "" for a message to the root, a discovery request.
+   */
+  arrivedBefore(endpointId: string, messageId: string, now = Date.now()): boolean {
+    this.forget(now);
+    const id = key(endpointId, messageId);
+    if (this.arrived.has(id)) return true;
+    this.arrived.set(id, now);
+    return false;
+  }
+
   /** The text of a message on the directive topic of endpointId. Resolves when the handlers are done. */
   async dispatch(endpointId: string, text: string): Promise<void> {
     const { log, emit } = this.options;
@@ -258,13 +270,9 @@ export class Dispatcher {
     const { namespace, name, messageId } = raw.header;
     const correlationToken = typeof raw.header.correlationToken === "string" ? raw.header.correlationToken : "";
     const receivedAt = Date.now();
-    this.forget(receivedAt);
-    if (typeof messageId === "string") {
-      if (this.arrived.has(key(endpointId, messageId))) {
-        log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
-        return;
-      }
-      this.arrived.set(key(endpointId, messageId), receivedAt);
+    if (typeof messageId === "string" && this.arrivedBefore(endpointId, messageId, receivedAt)) {
+      log(`the directive ${messageId} for ${endpointId} is dropped: it arrived before`);
+      return;
     }
 
     const device = this.options.getDevice(endpointId);

test/transport.test.js

@@ -89,3 +89,38 @@ test("a discovery answer that cannot be published: receive() resolves and the \"
     await bridge.receive("root/discover", "");
   }
 });
+
+test("a discovery request that arrives twice is answered once, by its messageId", async () => {
+  const sent = new MemoryPublisher();
+  const logged = [];
+  const bridge = new Alex2MQTT("user", "password", "root", false, { publisher: sent, log: (message) => logged.push(message) });
+  bridge.addDevice({ endpointId: "lamp-1", name: "Lamp", categories: ["LIGHT"] }).add(PowerController);
+  const header = (messageId) => ({ namespace: "Alexa.Discovery", name: "Discover", payloadVersion: "3", messageId });
+  const answers = () => sent.published.filter(({ topic }) => topic === "root/discover_r").length;
+
+  // The request as the Discover directive of Alexa, with its header alone, and the header by itself
+  const requests = [{ directive: { header: header("d-1"), payload: {} } }, { header: header("d-2") }, header("d-3")];
+  for (const [i, request] of requests.entries()) {
+    await bridge.receive("root/discover", JSON.stringify(request));
+    await bridge.receive("root/discover", JSON.stringify(request));
+    assert.equal(answers(), i + 1, `d-${i + 1}`);
+    assert.ok(logged.includes(`the discovery request d-${i + 1} is dropped: it arrived before`));
+  }
+
+  // A request without a messageId cannot be told from the next one: each is answered
+  for (const request of ["", "{}", "not JSON", JSON.stringify({ namespace: "Alexa.Discovery", name: "Discover" })]) {
+    await bridge.receive("root/discover", request);
+    await bridge.receive("root/discover", request);
+  }
+  assert.equal(answers(), 3 + 8);
+
+  // The messageId of a discovery request is not the one of a directive to an endpoint
+  const called = [];
+  bridge.getDevice("lamp-1").capability("Alexa.PowerController").on("TurnOn", (ctx) => { called.push(ctx.name); return ctx.respond(); });
+  await bridge.receive("root/lamp-1/alexaDirective", JSON.stringify({
+    header: { namespace: "Alexa.PowerController", name: "TurnOn", messageId: "d-1", correlationToken: "ct", payloadVersion: "3" },
+    endpoint: { endpointId: "lamp-1" },
+    payload: {},
+  }));
+  assert.deepEqual(called, ["TurnOn"]);
+});