test_runner, cli: add --test-concurrency flag

doc/api/cli.md

@@ -1654,6 +1654,15 @@ Starts the Node.js command line test runner. This flag cannot be combined with
 See the documentation on [running tests from the command line][]
 for more details.
 
+### `--test-concurrency`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+The maximum number of test files that the test runner CLI will execute
+concurrently. The default value is `os.availableParallelism() - 1`.
+
 ### `--test-name-pattern`
 
 <!-- YAML

doc/api/test.md

@@ -350,11 +350,12 @@ in the [test runner execution model][] section.
 
 ### Test runner execution model
 
-Each matching test file is executed in a separate child process. If the child
-process finishes with an exit code of 0, the test is considered passing.
-Otherwise, the test is considered to be a failure. Test files must be
-executable by Node.js, but are not required to use the `node:test` module
-internally.
+Each matching test file is executed in a separate child process. The maximum
+number of child processes running at any time is controlled by the
+[`--test-concurrency`][] flag. If the child process finishes with an exit code
+of 0, the test is considered passing. Otherwise, the test is considered to be a
+failure. Test files must be executable by Node.js, but are not required to use
+the `node:test` module internally.
 
 Each test file is executed as if it was a regular script. That is, if the test
 file itself uses `node:test` to define tests, all of those tests will be
@@ -2551,6 +2552,7 @@ added:
 [TTY]: tty.md
 [`--experimental-test-coverage`]: cli.md#--experimental-test-coverage
 [`--import`]: cli.md#--importmodule
+[`--test-concurrency`]: cli.md#--test-concurrency
 [`--test-name-pattern`]: cli.md#--test-name-pattern
 [`--test-only`]: cli.md#--test-only
 [`--test-reporter-destination`]: cli.md#--test-reporter-destination

doc/node.1

@@ -418,6 +418,10 @@ Specify the minimum allocation from the OpenSSL secure heap. The default is 2. T
 .It Fl -test
 Starts the Node.js command line test runner.
 .
+.It Fl -test-concurrency
+The maximum number of test files that the test runner CLI will execute
+concurrently.
+.
 .It Fl -test-name-pattern
 A regular expression that configures the test runner to only execute tests
 whose name matches the provided pattern.

lib/internal/main/test_runner.js

@@ -22,7 +22,7 @@ const {
 prepareMainThreadExecution(false);
 markBootstrapComplete();
 
-let concurrency = true;
+let concurrency = getOptionValue('--test-concurrency') || true;
 let inspectPort;
 
 if (isUsingInspector()) {

src/node_options.cc

@@ -595,6 +595,9 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
   AddOption("--test",
             "launch test runner on startup",
             &EnvironmentOptions::test_runner);
+  AddOption("--test-concurrency",
+            "specify test runner concurrency",
+            &EnvironmentOptions::test_runner_concurrency);
   AddOption("--experimental-test-coverage",
             "enable code coverage in the test runner",
             &EnvironmentOptions::test_runner_coverage);

src/node_options.h

@@ -159,6 +159,7 @@ class EnvironmentOptions : public Options {
   std::string env_file;
   bool has_env_file_string = false;
   bool test_runner = false;
+  uint64_t test_runner_concurrency = 0;
   bool test_runner_coverage = false;
   std::vector<std::string> test_name_pattern;
   std::vector<std::string> test_reporter;

test/parallel/test-runner-cli-concurrency.js

@@ -0,0 +1,45 @@
+'use strict';
+const common = require('../common');
+const tmpdir = require('../common/tmpdir');
+const { deepStrictEqual, strictEqual } = require('node:assert');
+const { spawnSync } = require('node:child_process');
+const { readdirSync, writeFileSync } = require('node:fs');
+const { join } = require('node:path');
+const { beforeEach, test } = require('node:test');
+
+function createTestFile(name) {
+  writeFileSync(join(tmpdir.path, name), `
+    const fs = require('node:fs');
+
+    fs.unlinkSync(__filename);
+    setTimeout(() => {}, 1_000_000_000);
+  `);
+}
+
+beforeEach(() => {
+  tmpdir.refresh();
+  createTestFile('test-1.js');
+  createTestFile('test-2.js');
+});
+
+test('concurrency of one', () => {
+  const cp = spawnSync(process.execPath, ['--test', '--test-concurrency=1'], {
+    cwd: tmpdir.path,
+    timeout: common.platformTimeout(1000),
+  });
+
+  strictEqual(cp.stderr.toString(), '');
+  strictEqual(cp.error.code, 'ETIMEDOUT');
+  deepStrictEqual(readdirSync(tmpdir.path), ['test-2.js']);
+});
+
+test('concurrency of two', () => {
+  const cp = spawnSync(process.execPath, ['--test', '--test-concurrency=2'], {
+    cwd: tmpdir.path,
+    timeout: common.platformTimeout(1000),
+  });
+
+  strictEqual(cp.stderr.toString(), '');
+  strictEqual(cp.error.code, 'ETIMEDOUT');
+  deepStrictEqual(readdirSync(tmpdir.path), []);
+});