PHPUnit process isolation (@runInSeparateProcess) silently dies in the managed WordPress sandbox — child exits with no stderr and no result file

[chubes4]

Symptom

Every @runInSeparateProcess / @runTestsInSeparateProcesses test in Extra-Chill/extrachill-users fails in CI (managed sandbox, wp_codebox_phpunit_bootstrap_mode: managed) with:

Test was run in child process and ended unexpectedly

Reproduced with a minimal probe — a noop isolated test dies identically:

class Test_Isolation_Probe extends WP_UnitTestCase {
    /** @runInSeparateProcess @preserveGlobalState disabled */
    public function test_noop_in_separate_process(): void { $this->assertTrue( true ); }
}

Evidence: Extra-Chill/extrachill-users#391 (19 tests across 5 classes), probe at branch fix-391-test-baseline (commit 5d389b5), GitHub run 34170973133.

Diagnosis

Per PHPUnit 9's AbstractPhpProcess::processChildResult() (vendor/phpunit/phpunit/src/Util/PHP/AbstractPhpProcess.php:276-309), this failure means BOTH:

• child stderr was empty — no PHP fatal occurred (the child template sets display_errors=stderr before autoload, so any fatal would surface), and
• the child result file was never written — the child exited or was killed before file_put_contents(processResultFile) at the end of __phpunit_run_isolated_test().

The child runs php with the template job on STDIN (DefaultPhpProcess::runProcess → proc_open), bare ini, then requires the composer autoload plus $GLOBALS['__PHPUNIT_BOOTSTRAP'] (the managed bootstrap) before running the test. A silent death with no output points at an exit() in the child bootstrap path or a process kill — not a PHP error.

What it breaks

• 19 tests in extrachill-users cannot run in CI at all.
• Any future test using process isolation in any repo on this harness hits the same wall.

Ask

Investigate the child-process path in the sandbox: whether proc_open/STDIN execution of the PHPUnit template is permitted, whether the managed bootstrap exits early when re-run in a child context, and whether the result temp file (parent sys_get_temp_dir()) is writable from the child's mount namespace.

Repro: push a noop @runInSeparateProcess test on any repo configured with wp_codebox_phpunit_bootstrap_mode: managed.

Filed from Extra-Chill/extrachill-users#391 / PR Extra-Chill/extrachill-users#393.