test_runner: flush report stream before force exit

lib/internal/test_runner/test.js

@@ -85,6 +85,7 @@ const {
 } = require('internal/validators');
 const {
   clearTimeout,
+  setImmediate,
   setTimeout,
 } = require('timers');
 const { TIMEOUT_MAX } = require('internal/timers');
@@ -1476,14 +1477,31 @@ class Test extends AsyncResource {
             if (!destination.closed && typeof destination.close === 'function') {
               destination.close(resolve);
             } else {
-              resolve();
+              // The destination has no close() to await - for example a child
+              // process's stdout, which is a pipe to the test runner's parent
+              // process. Writes to a pipe are asynchronous, so the process.exit()
+              // below can truncate buffered output and silently drop test
+              // results (https://github.com/nodejs/node/issues/64833). Switch the
+              // underlying handle to blocking so any queued bytes are flushed to
+              // the OS, then end the stream and wait for it to finish before
+              // exiting.
+              if (destination._handle &&
+                  typeof destination._handle.setBlocking === 'function') {
+                destination._handle.setBlocking(true);
+              }
+              destination.end(resolve);
             }
           });
         }));
       }
 
       this.harness.teardown();
       await SafePromiseAllReturnVoid(promises);
+      // Yield once more so the event loop can drain any report events that are
+      // still in flight from child processes (their stdout is piped into this
+      // process) before we force the exit. The blocking flush above guarantees
+      // the writable side; this covers the readable side on the parent.
+      await new Promise((resolve) => setImmediate(resolve));
       process.exit();
     }
   }

test/parallel/test-runner-force-exit-no-verdict-loss.js

@@ -0,0 +1,71 @@
+'use strict';
+require('../common');
+const tmpdir = require('../common/tmpdir');
+const assert = require('node:assert');
+const { spawnSync } = require('node:child_process');
+const { writeFileSync, mkdirSync, readFileSync } = require('node:fs');
+const { join } = require('node:path');
+
+// Regression test for https://github.com/nodejs/node/issues/64833.
+//
+// When --test-force-exit is combined with process isolation and concurrency,
+// each test file runs in a child process that streams its results back to the
+// parent over a pipe. Writes to a pipe are asynchronous, so a forced
+// process.exit() could truncate the still-flushing report stream, silently
+// dropping test verdicts while the process still exited with code 0.
+//
+// Generate several test files, each of which keeps itself alive with a ref'd
+// handle (so --test-force-exit is required to finish), then assert that every
+// single verdict survives the forced exit.
+
+const FILES = 12;
+const TESTS_PER_FILE = 1000;
+const EXPECTED = FILES * TESTS_PER_FILE;
+
+tmpdir.refresh();
+const testsDir = tmpdir.resolve('tests');
+mkdirSync(testsDir);
+
+for (let f = 0; f < FILES; f++) {
+  let body = "'use strict';\n";
+  body += "const { test } = require('node:test');\n";
+  // A ref'd handle keeps the child alive so it never exits on its own; only
+  // --test-force-exit ends it, forcing a process.exit() while the child's
+  // stdout may still be flushing verdicts to the parent.
+  body += 'setInterval(() => {}, 1_000_000);\n';
+  for (let t = 0; t < TESTS_PER_FILE; t++) {
+    body += `test('f${f}_t${t}', () => {});\n`;
+  }
+  writeFileSync(join(testsDir, `f${f}.test.js`), body);
+}
+
+// Write the aggregated report to a file rather than piping it through
+// spawnSync (12k TAP lines would exceed the default maxBuffer). Running from
+// within testsDir lets the runner auto-discover the *.test.js files.
+const destination = tmpdir.resolve('out.tap');
+const args = [
+  '--test',
+  '--test-force-exit',
+  '--test-concurrency=16',
+  '--test-reporter=tap',
+  `--test-reporter-destination=${destination}`,
+];
+const child = spawnSync(process.execPath, args, { encoding: 'utf8', cwd: testsDir });
+
+assert.strictEqual(
+  child.status,
+  0,
+  `unexpected exit code ${child.status} (signal ${child.signal})\n${child.stderr}`,
+);
+
+const output = readFileSync(destination, 'utf8');
+const pass = output.match(/# pass (\d+)/);
+const fail = output.match(/# fail (\d+)/);
+assert.notStrictEqual(pass, null, `missing pass summary in output:\n${output}`);
+assert.strictEqual(
+  Number(pass[1]),
+  EXPECTED,
+  `expected ${EXPECTED} passing verdicts but only ${pass[1]} reached the reporter ` +
+  `(force-exit truncated the child -> parent report stream)`,
+);
+assert.strictEqual(Number(fail?.[1]), 0, `unexpected failures:\n${output}`);