Add success return value in diagnostics_channel unsubscribe

doc/api/diagnostics_channel.md

@@ -264,9 +264,14 @@ channel.subscribe((message, name) => {
 added:
  - v15.1.0
  - v14.17.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/40433
+    description: Added return value. Added to channels without subscribers.
 -->
 
 * `onMessage` {Function} The previous subscribed handler to remove
+* Returns: {boolean} `true` if the handler was found, `false` otherwise.
 
 Remove a message handler previously registered to this channel with
 [`channel.subscribe(onMessage)`][].

lib/diagnostics_channel.js

@@ -32,15 +32,17 @@ class ActiveChannel {
 
   unsubscribe(subscription) {
     const index = ArrayPrototypeIndexOf(this._subscribers, subscription);
-    if (index >= 0) {
-      ArrayPrototypeSplice(this._subscribers, index, 1);
+    if (index === -1) return false;
 
-      // When there are no more active subscribers, restore to fast prototype.
-      if (!this._subscribers.length) {
-        // eslint-disable-next-line no-use-before-define
-        ObjectSetPrototypeOf(this, Channel.prototype);
-      }
+    ArrayPrototypeSplice(this._subscribers, index, 1);
+
+    // When there are no more active subscribers, restore to fast prototype.
+    if (!this._subscribers.length) {
+      // eslint-disable-next-line no-use-before-define
+      ObjectSetPrototypeOf(this, Channel.prototype);
     }
+
+    return true;
   }
 
   get hasSubscribers() {
@@ -79,6 +81,10 @@ class Channel {
     this.subscribe(subscription);
   }
 
+  unsubscribe() {
+    return false;
+  }
+
   get hasSubscribers() {
     return false;
   }

test/parallel/test-diagnostics-channel-object-channel-pub-sub.js

@@ -35,9 +35,12 @@ assert.ok(channel instanceof Channel);
 channel.publish(input);
 
 // Should not publish after subscriber is unsubscribed
-channel.unsubscribe(subscriber);
+assert.ok(channel.unsubscribe(subscriber));
 assert.ok(!channel.hasSubscribers);
 
+// unsubscribe() should return false when subscriber is not found
+assert.ok(!channel.unsubscribe(subscriber));
+
 assert.throws(() => {
   channel.subscribe(null);
 }, { code: 'ERR_INVALID_ARG_TYPE' });