diagnostics_channel: lazily create tracing context

Default parameters are evaluated before the function body, so the
tracing context is allocated before the no-subscriber fast path.

Initialize it only for subscribed traces. Preserve method arity with an
undefined default and preserve explicit null behavior.

Add focused regression coverage and reusable TracingChannel benchmarks.

Signed-off-by: Romain Lanz <romain.lanz@pm.me>
PR-URL: https://github.com/nodejs/node/pull/65513
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>

benchmark/diagnostics_channel/tracing-channel-promise.js

@@ -0,0 +1,53 @@
+'use strict';
+
+const common = require('../common.js');
+const dc = require('node:diagnostics_channel');
+
+const bench = common.createBenchmark(main, {
+  n: [1e7],
+  context: ['omitted', 'undefined', 'provided'],
+  subscribers: [0, 1],
+});
+
+function noop() {}
+
+const thenable = {
+  then(onResolve) {
+    onResolve(undefined);
+  },
+};
+
+function returnThenable() {
+  return thenable;
+}
+
+function main({ n, context, subscribers }) {
+  const channel = dc.tracingChannel('test');
+  const providedContext = { __proto__: null };
+
+  if (subscribers) {
+    channel.subscribe({ start: noop });
+  }
+
+  bench.start();
+  switch (context) {
+    case 'omitted':
+      for (let i = 0; i < n; i++) {
+        channel.tracePromise(returnThenable);
+      }
+      break;
+    case 'undefined':
+      for (let i = 0; i < n; i++) {
+        channel.tracePromise(returnThenable, undefined);
+      }
+      break;
+    case 'provided':
+      for (let i = 0; i < n; i++) {
+        channel.tracePromise(returnThenable, providedContext);
+      }
+      break;
+    default:
+      throw new Error(`Unsupported context value: ${context}`);
+  }
+  bench.end(n);
+}

benchmark/diagnostics_channel/tracing-channel-sync.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const common = require('../common.js');
+const dc = require('node:diagnostics_channel');
+
+const bench = common.createBenchmark(main, {
+  n: [1e7],
+  context: ['omitted', 'undefined', 'provided'],
+  subscribers: [0, 1],
+});
+
+function noop() {}
+
+function main({ n, context, subscribers }) {
+  const channel = dc.tracingChannel('test');
+  const providedContext = { __proto__: null };
+
+  if (subscribers) {
+    channel.subscribe({ start: noop });
+  }
+
+  bench.start();
+  switch (context) {
+    case 'omitted':
+      for (let i = 0; i < n; i++) {
+        channel.traceSync(noop);
+      }
+      break;
+    case 'undefined':
+      for (let i = 0; i < n; i++) {
+        channel.traceSync(noop, undefined);
+      }
+      break;
+    case 'provided':
+      for (let i = 0; i < n; i++) {
+        channel.traceSync(noop, providedContext);
+      }
+      break;
+    default:
+      throw new Error(`Unsupported context value: ${context}`);
+  }
+  bench.end(n);
+}

lib/diagnostics_channel.js

@@ -522,11 +522,15 @@ class TracingChannel {
     return done;
   }
 
-  traceSync(fn, context = { __proto__: null }, thisArg, ...args) {
+  traceSync(fn, context = undefined, thisArg, ...args) {
     if (!this.hasSubscribers) {
       return ReflectApply(fn, thisArg, args);
     }
 
+    if (context === undefined) {
+      context = { __proto__: null };
+    }
+
     const { error } = this;
 
     // eslint-disable-next-line no-unused-vars
@@ -542,7 +546,7 @@ class TracingChannel {
     }
   }
 
-  tracePromise(fn, context = { __proto__: null }, thisArg, ...args) {
+  tracePromise(fn, context = undefined, thisArg, ...args) {
     if (!this.hasSubscribers) {
       const result = ReflectApply(fn, thisArg, args);
       if (typeof result?.then !== 'function') {
@@ -551,6 +555,10 @@ class TracingChannel {
       return result;
     }
 
+    if (context === undefined) {
+      context = { __proto__: null };
+    }
+
     const { error } = this;
     const continuationWindow = this.#continuationWindow;
 

test/parallel/test-diagnostics-channel-tracing-channel-context.js

@@ -0,0 +1,85 @@
+'use strict';
+
+const common = require('../common');
+
+// This test ensures that tracing channels create an independent mutable
+// context for each call while preserving explicitly provided contexts.
+
+const assert = require('node:assert');
+const dc = require('node:diagnostics_channel');
+
+const lengthChannel = dc.tracingChannel('test:length');
+assert.strictEqual(lengthChannel.traceSync.length, 1);
+assert.strictEqual(lengthChannel.tracePromise.length, 1);
+
+function subscribe(channel, contexts, isPromise = false) {
+  let endCalls = 0;
+  let asyncStartCalls = 0;
+  let asyncEndCalls = 0;
+
+  channel.subscribe({
+    start: common.mustCall((context) => {
+      context.invocation = contexts.length;
+      contexts.push(context);
+    }, 4),
+    end: common.mustCall((context) => {
+      assert.strictEqual(context.invocation, endCalls++);
+    }, 4),
+    asyncStart: common.mustCall((context) => {
+      assert.strictEqual(context.invocation, asyncStartCalls++);
+    }, isPromise ? 4 : 0),
+    asyncEnd: common.mustCall((context) => {
+      assert.strictEqual(context.invocation, asyncEndCalls++);
+    }, isPromise ? 4 : 0),
+  });
+}
+
+const syncChannel = dc.tracingChannel('test:sync-context');
+const syncContexts = [];
+subscribe(syncChannel, syncContexts);
+
+const syncProvided = { provided: true };
+assert.strictEqual(syncChannel.traceSync(() => 'omitted'), 'omitted');
+assert.strictEqual(syncChannel.traceSync(() => 'undefined', undefined),
+                   'undefined');
+assert.strictEqual(syncChannel.traceSync(() => 'provided', syncProvided),
+                   'provided');
+
+assert.strictEqual(Object.getPrototypeOf(syncContexts[0]), null);
+assert.strictEqual(Object.getPrototypeOf(syncContexts[1]), null);
+assert.notStrictEqual(syncContexts[0], syncContexts[1]);
+assert.strictEqual(syncContexts[2], syncProvided);
+assert.deepStrictEqual(syncContexts.slice(0, 3).map(({ result }) => result),
+                       ['omitted', 'undefined', 'provided']);
+
+const syncError = new Error('sync');
+assert.throws(() => syncChannel.traceSync(() => {
+  throw syncError;
+}), (error) => error === syncError);
+assert.strictEqual(syncContexts[3].error, syncError);
+
+const promiseChannel = dc.tracingChannel('test:promise-context');
+const promiseContexts = [];
+subscribe(promiseChannel, promiseContexts, true);
+
+const promiseProvided = { provided: true };
+const promiseError = new Error('promise');
+Promise.resolve()
+  .then(() => promiseChannel.tracePromise(() => Promise.resolve('omitted')))
+  .then(() => promiseChannel.tracePromise(
+    () => Promise.resolve('undefined'), undefined))
+  .then(() => promiseChannel.tracePromise(
+    () => Promise.resolve('provided'), promiseProvided))
+  .then(() => assert.rejects(
+    promiseChannel.tracePromise(() => Promise.reject(promiseError)),
+    promiseError))
+  .then(common.mustCall(() => {
+    assert.strictEqual(Object.getPrototypeOf(promiseContexts[0]), null);
+    assert.strictEqual(Object.getPrototypeOf(promiseContexts[1]), null);
+    assert.notStrictEqual(promiseContexts[0], promiseContexts[1]);
+    assert.strictEqual(promiseContexts[2], promiseProvided);
+    assert.deepStrictEqual(
+      promiseContexts.slice(0, 3).map(({ result }) => result),
+      ['omitted', 'undefined', 'provided']);
+    assert.strictEqual(promiseContexts[3].error, promiseError);
+  }));