lib: fix benchmark record acknowledgement race

lib/internal/bench_runner/cli.js

@@ -954,10 +954,11 @@ async function runChild(path, options, scope, onRecord) {
         }
         const pending = handleRecord(record);
         if (protocolError === undefined) {
-          const acknowledged = PromisePrototypeThen(
-            PromiseResolve(pending), () => sendAck(child, message.id));
-          trackPending(PromisePrototypeThen(acknowledged, () => {
+          trackPending(PromisePrototypeThen(PromiseResolve(pending), () => {
+            // The child can receive the acknowledgement and send its next
+            // record before the send callback runs.
             recordPending = false;
+            return sendAck(child, message.id);
           }));
         }
       } catch (error) {

test/fixtures/bench-runner/delayed-ack.cjs

@@ -0,0 +1,31 @@
+'use strict';
+
+const common = require('../../common');
+const childProcess = require('child_process');
+const { spawn } = childProcess;
+
+childProcess.spawn = (...args) => {
+  const child = spawn(...args);
+  const { send } = child;
+  child.send = function(message, handle, options, callback) {
+    if (message?.type === 'node:bench:ack') {
+      // Report send completion only after the child has responded to the ack.
+      const onComplete = common.mustCall(() => {
+        child.removeListener('message', onComplete);
+        child.removeListener('close', onComplete);
+        callback(null);
+      });
+      child.once('message', onComplete);
+      child.once('close', onComplete);
+      return send.call(this, message, handle, options, (error) => {
+        if (error) {
+          child.removeListener('message', onComplete);
+          child.removeListener('close', onComplete);
+          callback(error);
+        }
+      });
+    }
+    return send.call(this, message, handle, options, callback);
+  };
+  return child;
+};

test/parallel/test-bench-cli-ack.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { spawnSync } = require('child_process');
+const fixtures = require('../common/fixtures');
+
+const result = spawnSync(process.execPath, [
+  '--no-warnings', '--experimental-bench', '--bench',
+  '--require', fixtures.path('bench-runner/delayed-ack.cjs'),
+  '--bench-reporter=json',
+  fixtures.path('bench-runner/acknowledged-records.mjs'),
+], { encoding: 'utf8', timeout: common.platformTimeout(30_000) });
+
+assert.ifError(result.error);
+assert.strictEqual(result.signal, null);
+assert.strictEqual(result.status, 0, result.stdout + result.stderr);
+assert.strictEqual(result.stderr, '');
+const records = result.stdout.trim().split('\n').map((line) => JSON.parse(line));
+const diagnostics = records.filter(({ type }) => type === 'bench:diagnostic');
+assert.strictEqual(diagnostics.length, 32);
+assert(diagnostics.every(({ data }) => /^acknowledged 10\d{3}$/.test(data.message)));
+assert.strictEqual(records.at(-1).data.success, true);