add support for TracingChannel#hasSubscribers

Author: tlhunter

.github/workflows/pull-request.yml

@@ -79,9 +79,13 @@ jobs:
           - 20.0.0 # first 20
           - 20.5.1 # sync unsubscribe bug
           - 20.6.0 # sync unsubscribe bug fixed
-          - 20.x # nothing special, full support dc, latest 20
-          - 21.0.0 # nothing special, full support dc, first 21
-          - 21.x # nothing special, full support dc, latest 21
+          - 20.x # nothing special, latest 20
+          - 20.12.x # last version without TC early exit and TC#hasSubscribers()
+          - 20.13.0 # introduces TC early exit and TC#hasSubscribers()
+          - 21.0.0 # nothing special, first 21
+          - 21.x # nothing special, latest 21
+          - 22.0.0 # introduces TC early exit and TC#hasSubscribers()
+          - 22.x # nothing special, full support DC
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-node@v1

README.md

@@ -13,6 +13,8 @@ Since this package recreates a Node.js API, read the [Node.js `diagnostics_chann
 | Oldest Supported Node.js Version | 12.17.0 |
 | Target Node.js DC API Version    | 20.6.0  |
 
+> Note that `dc-polyfill` currently has the `TracingChannel#hasSubscribers` getter backported from Node.js v22 however it doesn't yet support the tracing channel early exit feature. Once that's been added we'll delete this clause and update the above table.
+
 Whenever the currently running version of Node.js ships with `diagnostics_channel` (i.e. v16+, v15.14+, v14.17+), **dc-polyfill** will make sure to use the global registry of channels provided by the core module. For older versions of Node.js **dc-polyfill** instead uses its own global collection of channels. This global collection remains in the same location and is shared across all instances of **dc-polyfill**. This avoids the issue wherein multiple versions of an npm library installed in a module dependency hierarchy would otherwise provide different singleton instances.
 
 Ideally, this package will forever remain backwards compatible, there will never be a v2.x release, and there will never be an additional global channel collection.

checks.js

@@ -56,3 +56,12 @@ function hasSyncUnsubscribeBug() {
   return MAJOR === 20 && MINOR <= 5;
 }
 module.exports.hasSyncUnsubscribeBug = hasSyncUnsubscribeBug;
+
+// if there is a TracingChannel#hasSubscribers() method
+// @see https://github.com/nodejs/node/pull/51915
+// TODO: note that we still need to add the TC early exit from this same version
+function hasTracingChannelHasSubscribersMethod() {
+  return MAJOR >= 22
+    || (MAJOR == 20 && MINOR >= 13);
+};
+module.exports.hasTracingChannelHasSubscribersMethod = hasTracingChannelHasSubscribersMethod;

dc-polyfill.js

@@ -30,5 +30,9 @@ if (checks.hasSyncUnsubscribeBug()) {
   dc = require('./patch-sync-unsubscribe-bug.js')(dc);
 }
 
+if (!checks.hasTracingChannelHasSubscribersMethod()) {
+  dc = require('./patch-tracing-channel-has-subscribers.js')(dc);
+}
+
 module.exports = dc;
 

patch-tracing-channel-has-subscribers.js

@@ -0,0 +1,44 @@
+const {
+  ReflectApply,
+  PromiseReject,
+  PromiseResolve,
+  PromisePrototypeThen,
+  ArrayPrototypeSplice,
+  ArrayPrototypeAt,
+} = require('./primordials.js');
+
+const { ERR_INVALID_ARG_TYPE } = require('./errors.js');
+
+const traceEvents = [
+  'start',
+  'end',
+  'asyncStart',
+  'asyncEnd',
+  'error',
+];
+
+module.exports = function (unpatched) {
+  const { channel } = unpatched;
+
+  const dc = { ...unpatched };
+
+  const tracingChannel = dc.tracingChannel;
+
+  dc.tracingChannel = function() {
+    const tc = tracingChannel(...arguments);
+
+    Object.defineProperty(tc, 'hasSubscribers', { // TODO: on prototype?
+      get: function () {
+        return this.start.hasSubscribers
+          || this.end.hasSubscribers
+          || this.asyncStart.hasSubscribers
+          || this.asyncEnd.hasSubscribers
+          || this.error.hasSubscribers;
+      }
+    });
+
+    return tc;
+  };
+
+  return dc;
+};

test/test-diagnostics-channel-tracing-channel-has-subscribers.spec.js

@@ -0,0 +1,55 @@
+'use strict';
+
+const test = require('tape');
+const common = require('./common.js');
+const dc = require('../dc-polyfill.js');
+
+test('test-diagnostics-channel-tracing-has-subscribers', (t) => {
+  t.plan(10);
+
+  const handler = common.mustNotCall();
+
+  {
+    const handlers = {
+      start: common.mustNotCall()
+    };
+
+    const channel = dc.tracingChannel('test');
+
+    t.strictEqual(channel.hasSubscribers, false);
+
+    channel.subscribe(handlers);
+    t.strictEqual(channel.hasSubscribers, true);
+
+    channel.unsubscribe(handlers);
+    t.strictEqual(channel.hasSubscribers, false);
+
+    channel.start.subscribe(handler);
+    t.strictEqual(channel.hasSubscribers, true);
+
+    channel.start.unsubscribe(handler);
+    t.strictEqual(channel.hasSubscribers, false);
+  }
+
+  {
+    const handlers = {
+      asyncEnd: common.mustNotCall()
+    };
+
+    const channel = dc.tracingChannel('test');
+
+    t.strictEqual(channel.hasSubscribers, false);
+
+    channel.subscribe(handlers);
+    t.strictEqual(channel.hasSubscribers, true);
+
+    channel.unsubscribe(handlers);
+    t.strictEqual(channel.hasSubscribers, false);
+
+    channel.asyncEnd.subscribe(handler);
+    t.strictEqual(channel.hasSubscribers, true);
+
+    channel.asyncEnd.unsubscribe(handler);
+    t.strictEqual(channel.hasSubscribers, false);
+  }
+});