[WIP] diagnostics_channel: add meta buil-in channels

doc/api/diagnostics_channel.md

@@ -1153,6 +1153,20 @@ passed to `console.warn()`.
 Emitted when `console.error()` is called. Receives and array of the arguments
 passed to `console.error()`.
 
+#### Diagnotics Channel
+
+> Stability: 1 - Experimental
+
+<!-- YAML
+added: REPLACEME
+-->
+
+##### Event: `'diagnostics_channel.subscribe'`
+
+* `args` {any\[]}
+
+Emitted when `diagnostics_channel.subcribe()` or `channel.subscribe()` is called. Receives an object with the channel that was subscribed to, and the subscription function.
+
 #### HTTP
 
 > Stability: 1 - Experimental

lib/diagnostics_channel.js

@@ -33,6 +33,10 @@ const { triggerUncaughtException } = internalBinding('errors');
 
 const { WeakReference } = require('internal/util');
 
+const dcSubscribeChannel = dc.channel('diagnostics_channel.subscribe');
+const dcUnsubscribeChannel = dc.channel('diagnostics_channel.unsubscribe');
+const dcPublishChannel = dc.channel('diagnostics_channel.publish');
+
 // Can't delete when weakref count reaches 0 as it could increment again.
 // Only GC can be used as a valid time to clean up the channels map.
 class WeakRefMap extends SafeMap {
@@ -105,12 +109,21 @@ function wrapStoreRun(store, data, next, transform = defaultTransform) {
 class ActiveChannel {
   subscribe(subscription) {
     validateFunction(subscription, 'subscription');
+
+    if (dcSubscribeChannel.hasSubscribers) {
+      dcSubscribeChannel.publish({ channel: this, subscription });
+    }
+
     this._subscribers = ArrayPrototypeSlice(this._subscribers);
     ArrayPrototypePush(this._subscribers, subscription);
     channels.incRef(this.name);
   }
 
   unsubscribe(subscription) {
+    if (dcUnsubscribeChannel.hasSubscribers) {
+      dcUnsubscribeChannel.publish({ channel: this, subscription });
+    }
+
     const index = ArrayPrototypeIndexOf(this._subscribers, subscription);
     if (index === -1) return false;
 
@@ -148,7 +161,7 @@ class ActiveChannel {
     return true;
   }
 
-  publish(data) {
+  _publish(data) {
     const subscribers = this._subscribers;
     for (let i = 0; i < (subscribers?.length || 0); i++) {
       try {
@@ -162,6 +175,14 @@ class ActiveChannel {
     }
   }
 
+  publish(data) {
+    if (dcPublishChannel.hasSubscribers) {
+      dcPublishChannel._publish({ channel: this, data });
+    }
+
+    this._publish(data);
+  }
+
   runStores(data, fn, thisArg, ...args) {
     let run = () => {
       this.publish(data);
@@ -199,6 +220,9 @@ class Channel {
   }
 
   unsubscribe() {
+    if (dcUnsubscribeChannel.hasSubscribers) {
+      dcUnsubscribeChannel.publish({ channel: this, subscription });
+    }
     return false;
   }
 
@@ -215,7 +239,11 @@ class Channel {
     return false;
   }
 
-  publish() {}
+  publish(data) {
+    if (dcUnsubscribeChannel.hasSubscribers) {
+      dcUnsubscribeChannel.publish({ channel: this, subscription });
+    }
+  }
 
   runStores(data, fn, thisArg, ...args) {
     return ReflectApply(fn, thisArg, args);

test/parallel/test-diagnostics-channel-meta-channels.js

@@ -0,0 +1,35 @@
+'use strict';
+
+const common = require('../common');
+const dc = require('diagnostics_channel');
+const assert = require('assert');
+
+const testedChannel = dc.channel('test');
+const testedSubscription = () => {};
+const testedData = { foo: 'bar' };
+
+// should publish on meta channel for subscribe() on both inactive and active prototype
+dc.subscribe('diagnostics_channel.subscribe', common.mustCall(({ channel, subscription }) => {
+  assert.strictEqual(channel, testedChannel);
+  assert.strictEqual(subscription, testedSubscription);
+}, 2)); // called twice
+testedChannel.subscribe(testedSubscription); // inactive prototype
+testedChannel.subscribe(testedSubscription); // active prototype
+
+// should publish on meta channel for publish()
+dc.subscribe('diagnostics_channel.publish', common.mustCall(({ channel, data }) => {
+  assert.strictEqual(channel, testedChannel);
+  assert.strictEqual(data, testedData);
+}));
+testedChannel.publish(testedData);
+
+// should publish on meta channel for unsubscribe() on both inactive and active prototype
+dc.subscribe('diagnostics_channel.unsubscribe', common.mustCall(({ channel, subscription }) => {
+  assert.strictEqual(channel, testedChannel);
+  assert.strictEqual(subscription, testedSubscription);
+}, 2)); // called twice
+testedChannel.unsubscribe(testedSubscription); // active prototype
+testedChannel.unsubscribe(testedSubscription); // inactive prototype
+
+
+// TODO: should it publish on inactive channels ?