[BUG] setUp() never restores TYPO3_PATH_ROOT, breaking every following forked test

[dkd-kaehm]

What are you trying to achieve?

Run functional test cases without processIsolation, where one of them is marked
#[RunInSeparateProcess] and loads a test extension by a relative path.

What do you get instead?

The forked test case resolves its paths against the instance directory of the previous test
case, so linking the test extension fails:

TYPO3\TestingFramework\Core\Exception: Test extension path
/var/www/html/public/typo3temp/var/tests/functional-<hashOfATest>/../vendor/acme/foo/Tests/Fixtures/Extensions/fake_extension
not found

The prefix is ATest's instance path instead of the document root.

FunctionalTestCase::setUp() points TYPO3_PATH_ROOT at the instance of the running test and
never restores it:

// Classes/Core/Functional/FunctionalTestCase.php:289
putenv('TYPO3_PATH_ROOT=' . $this->instancePath);
putenv('TYPO3_PATH_APP=' . $this->instancePath);

putenv() is process global, so the value outlives the test case. A forked test inherits it,
runs the bootstrap again and derives ORIGINAL_ROOT from the inherited value:

// Classes/Core/Testbase.php:91
define('ORIGINAL_ROOT', $this->getWebRoot()); // getWebRoot() reads TYPO3_PATH_ROOT

From there every path built on ORIGINAL_ROOT points one instance deeper, which
linkTestExtensionsToInstance() hits first:

// Classes/Core/Testbase.php:342
$absoluteExtensionPath = ORIGINAL_ROOT . $extensionPath;

Observed values:

process	ORIGINAL_ROOT	instancePath of BTest
parent, ATest	/var/www/html/public/	…/tests/functional-<hashOfATest>
fork, BTest	/var/www/html/public/typo3temp/var/tests/functional-<hashOfATest>/	…functional-<hashOfATest>/typo3temp/var/tests/functional-<hashOfBTest>

How to reproduce the issue?

1. Remove processIsolation="true" from the phpunit configuration.
2. Add two functional test cases, where the second one is forked and loads a test extension by
   a relative path:

final class ATest extends FunctionalTestCase
{
    #[Test]
    public function pollutesTheEnvironment(): void
    {
        self::assertTrue(true); // setUp() alone is enough
    }
}

final class BTest extends FunctionalTestCase
{
    protected array $testExtensionsToLoad = ['../vendor/acme/foo/Tests/Fixtures/Extensions/fake_extension'];

    #[Test]
    #[RunInSeparateProcess]
    public function fails(): void
    {
        self::assertTrue(true);
    }
}

3. Run both in one process, ATest first:

phpunit -c Build/Test/FunctionalTests.xml --filter='ATest|BTest'

BTest fails with the exception above. It passes when run on its own, with
processIsolation="true", or without #[RunInSeparateProcess].

Additional information you would like to provide?

processIsolation="true" hides the problem, because then every test case starts from the
pristine environment of the parent process.

A test case can work around it by unsetting the variable in its own tearDown()
(putenv('TYPO3_PATH_ROOT')), which is what led us to the cause.

Possible fix: remember TYPO3_PATH_ROOT and TYPO3_PATH_APP before overwriting them in
setUp() and restore them in tearDown().

Specify some data of the environment

• TYPO3 testing framework version: 9.6.1
• TYPO3 version: 14.3.6
• TYPO3 installation type: composer
• PHP version: 8.2.31
• Web server used + version: nginx-fpm (DDEV), tests run on CLI
• Operating system: DDEV environmen and/or Github Actions