test_runner: support running tests in process

doc/api/cli.md

@@ -1091,6 +1091,20 @@ generated as part of the test runner output. If no tests are run, a coverage
 report is not generated. See the documentation on
 [collecting code coverage from tests][] for more details.
 
+### `--experimental-test-isolation=mode`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.0 - Early development
+
+Configures the type of test isolation used in the test runner. When `mode` is
+`'process'`, each test file is run in a separate child process. When `mode` is
+`'none'`, all test files run in the same process as the test runner. The default
+isolation mode is `'process'`. This flag is ignored if the `--test` flag is not
+present. See the [test runner execution model][] section for more information.
+
 ### `--experimental-test-module-mocks`
 
 <!-- YAML
@@ -2196,7 +2210,9 @@ added:
 -->
 
 The maximum number of test files that the test runner CLI will execute
-concurrently. The default value is `os.availableParallelism() - 1`.
+concurrently. If `--experimental-test-isolation` is set to `'none'`, this flag
+is ignored and concurrency is one. Otherwise, concurrency defaults to
+`os.availableParallelism() - 1`.
 
 ### `--test-coverage-exclude`
 
@@ -2361,7 +2377,7 @@ added: v22.3.0
 
 > Stability: 1.0 - Early development
 
-Regenerates the snapshot file used by the test runner for [snapshot testing][].
+Regenerates the snapshot files used by the test runner for [snapshot testing][].
 Node.js must be started with the `--experimental-test-snapshots` flag in order
 to use this functionality.
 
@@ -3534,6 +3550,7 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [snapshot testing]: test.md#snapshot-testing
 [syntax detection]: packages.md#syntax-detection
 [test reporters]: test.md#test-reporters
+[test runner execution model]: test.md#test-runner-execution-model
 [timezone IDs]: https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
 [tracking issue for user-land snapshots]: https://github.com/nodejs/node/issues/44014
 [ways that `TZ` is handled in other environments]: https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html

doc/api/test.md

@@ -445,18 +445,26 @@ in the [test runner execution model][] section.
 
 ### Test runner execution model
 
-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.
+When process-level test isolation is enabled, 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
 executed within a single application thread, regardless of the value of the
 `concurrency` option of [`test()`][].
 
+When process-level test isolation is disabled, each matching test file is
+imported into the test runner process. Once all test files have been loaded, the
+top level tests are executed with a concurrency of one. Because the test files
+are all run within the same context, it is possible for tests to interact with
+each other in ways that are not possible when isolation is enabled. For example,
+if a test relies on global state, it is possible for that state to be modified
+by a test originating from another file.
+
 ## Collecting code coverage
 
 > Stability: 1 - Experimental
@@ -933,7 +941,7 @@ the [`--experimental-test-snapshots`][] command-line flag.
 Snapshot files are generated by starting Node.js with the
 [`--test-update-snapshots`][] command-line flag. A separate snapshot file is
 generated for each test file. By default, the snapshot file has the same name
-as `process.argv[1]` with a `.snapshot` file extension. This behavior can be
+as the test file with a `.snapshot` file extension. This behavior can be
 configured using the `snapshot.setResolveSnapshotPath()` function. Each
 snapshot assertion corresponds to an export in the snapshot file.
 
@@ -1239,6 +1247,9 @@ added:
   - v18.9.0
   - v16.19.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/53927
+    description: Added the `isolation` option.
   - version: v22.6.0
     pr-url: https://github.com/nodejs/node/pull/53866
     description: Added the `globPatterns` option.
@@ -1274,8 +1285,13 @@ changes:
   * `inspectPort` {number|Function} Sets inspector port of test child process.
     This can be a number, or a function that takes no arguments and returns a
     number. If a nullish value is provided, each process gets its own port,
-    incremented from the primary's `process.debugPort`.
-    **Default:** `undefined`.
+    incremented from the primary's `process.debugPort`. This option is ignored
+    if the `isolation` option is set to `'none'` as no child processes are
+    spawned. **Default:** `undefined`.
+  * `isolation` {string} Configures the type of test isolation. If set to
+    `'process'`, each test file is run in a separate child process. If set to
+    `'none'`, all test files run in the current process. **Default:**
+    `'process'`.
   * `only`: {boolean} If truthy, the test context will only run tests that
     have the `only` option set
   * `setup` {Function} A function that accepts the `TestsStream` instance
@@ -1727,9 +1743,9 @@ added: v22.3.0
 
 * `fn` {Function} A function used to compute the location of the snapshot file.
   The function receives the path of the test file as its only argument. If the
-  `process.argv[1]` is not associated with a file (for example in the REPL),
-  the input is undefined. `fn()` must return a string specifying the location of
-  the snapshot file.
+  test is not associated with a file (for example in the REPL), the input is
+  undefined. `fn()` must return a string specifying the location of the snapshot
+  snapshot file.
 
 This function is used to customize the location of the snapshot file used for
 snapshot testing. By default, the snapshot filename is the same as the entry

doc/node.1

@@ -185,6 +185,9 @@ Enable the experimental node:sqlite module.
 .It Fl -experimental-test-coverage
 Enable code coverage in the test runner.
 .
+.It Fl -experimental-test-isolation Ns = Ns Ar mode
+Configures the type of test isolation used in the test runner.
+.
 .It Fl -experimental-test-module-mocks
 Enable module mocking in the test runner.
 .

lib/internal/main/test_runner.js

@@ -21,7 +21,7 @@ markBootstrapComplete();
 
 const options = parseCommandLine();
 
-if (isUsingInspector()) {
+if (isUsingInspector() && options.isolation === 'process') {
   process.emitWarning('Using the inspector with --test forces running at a concurrency of 1. ' +
   'Use the inspectPort option to run with concurrency');
   options.concurrency = 1;

lib/internal/test_runner/harness.js

@@ -334,4 +334,5 @@ module.exports = {
   after: hook('after'),
   beforeEach: hook('beforeEach'),
   afterEach: hook('afterEach'),
+  startSubtestAfterBootstrap,
 };