Running tests in separate processes

PHPUnit offers the possibility to run tests in a separate PHP process; Codeception does not officially support the option as of version 4.0.
The wp-browser project tries to fill that gap by supporting the @runInSeparateProcess annotation.
This support comes with some caveats, though:
1.The support is only for test cases extending the Codeception\TestCase\WPTestCase class (the base test case for integration or "WordPress unit" tests)
2.The support wp-browser provides only supports the @preserveGlobalState annotation with the disabled value; this means there is no support for preserving global state between tests.
Read more about what this means in PHPUnit documentation.
Why run tests in a separate PHP process?
One main reason: isolation.
What does "isolation" means?
Before answering that question, it's essential to understand, via an example, why a lack of isolation might be an issue.
I want to test the get_api function. The function will return the correct singleton instance of an API handling class: an instance of Api when the function is called in non-admin context, and an instance of AdminApi when the function is called in admin context. The get_api function is acting as a service locator.
<?php
function get_api(){
    static $api;
​
    if(null !== $api){
        return $api;
    }
​
    if( is_admin() ) {
        $api = new Admin_Api();
    } else {
        $api = new Api();
    }
​
    return $api;
}
There are two challenges to testing this function:
1.The is_admin function, defined by WordPress, looks up a WP_ADMIN constant to know if the context of the current request is an administration UI one or not.
2.The get_api function will check for the context and resolve and build the correct instance only once, the first time it's called in the context of a request. 
There are some possible solutions to this problem:
a. Refactor the get_api function into a method of an Api_Factory object taking the context as a dependency, thus allowing injection of the "context" (which implies the creation of a Context adapter that will proxy its is_admin method to the is_admin function). You can find the code for such refactoring in the OOP refactoring of get_api section. b. Refactor the get_api function to accept the current is_admin value as an input argument, get_api( $is_admin ), this refactoring moves part of the complexity of getting hold of the correct instance of the API handler on the client code. Adding more build condition and checks, e.g., if the current request is a REST request or not or some tests on the user authorizations, then, requires adding more input arguments to the get_api function: the knowledge of the implementation of the get_api method will "leak" to the client code having to replicate complexity throughout the system.
I want to layout possible solutions to the problem to show there is always a design alternative to make code testable that might or might not fit the current time or scope constraint.
In this example, I've inherited the get_api function from the existing code, and it cannot be changed, yet I want to test it dealing with the two problems outlined above.
Running tests in separate PHP processes
To test the get_api function shown above I've created a new wpunit type of test:
vendor/bin/codecept g:wpunit integration "api"
The command scaffolds a test/integration/apiTest.php file that I've modified to ensure full coverage of the get_api function:
<?php
​
class apiTest extends \Codeception\TestCase\WPTestCase
{
    public function test_get_api_exists()
    {
        $this->assertTrue(function_exists('get_api'));
    }
​
    public function test_get_api_will_cache()
    {
        $this->assertSame(get_api(), get_api());
    }
​
    /**
     * @runInSeparateProcess
     * @preserveGlobalState disabled
     */
    public function test_get_api_will_return_api_if_not_admin()
    {
        // Let's make sure we're NOT in admin context.
        define('WP_ADMIN', false);
​
        $api = get_api();
​
        $this->assertInstanceOf(Api::class, $api);
    }
​
    /**
     * @runInSeparateProcess
     * @preserveGlobalState disabled
     */
    public function test_get_api_will_cache_api_if_not_admin()
    {
        // Let's make sure we're NOT in admin context.
        define('WP_ADMIN', false);
​
        $api = get_api();
​
        $this->assertSame(get_api(), $api);
    }
​
    /**
     * @runInSeparateProcess
     * @preserveGlobalState disabled
     */
    public function test_get_api_will_return_api_if_is_admin()
    {
        // Let's make sure we're NOT in admin context.
        define('WP_ADMIN', true);
​
        $api = get_api();
​
        $this->assertInstanceOf(AdminApi::class, $api);
    }
​
    /**
     * @runInSeparateProcess
     * @preserveGlobalState disabled
     */
    public function test_get_api_will_cache_api_if_is_admin()
    {
        // Let's make sure we're NOT in admin context.
        define('WP_ADMIN', true);
​
        $api = get_api();
​
        $this->assertSame(get_api(), $api);
    }
}
Some pieces of this code are worth pointing out:
1.There are two test methods, test_get_api_exists and test_get_api_will_cache that are not running in a separate process. Running tests in a separate process provide isolation at the cost of speed, only tests that require isolation should run in a separate PHP process.  
2.I instruct the Codeception and PHPUnit test runner to run a test method in a different process by adding two annotations that are both required  precisely as shown:
 /**
  * @runInSeparateProcess
  * @preserveGlobalState disabled
  */
3.The isolation part of this testing approach shines through when I define, in the last four tests, the WP_ADMIN constant multiple times. If I try to do that in test code running in the same PHP process, then the second define call would cause a fatal error.
4.The isolation has also taken care of the second issue where the get_api function caches the $api instance after its first resolution in a static variable: since each test happens in a self-contained, dedicated PHP process, the static $api variable will be null at the start of each test.
Can I run some tests in the same process and some in a separate process?
Yes. In the example test code in the previous section, the test_get_api_exists and test_get_api_will_cache test methods are not running in separate processes.
In your test cases extending the Codeception\TestCase\WPTestCase, you can mix test methods running in the primary PHP process and those running in a separate PHP process without issues.
OOP refactoring of get_api
In the Why run tests in a separate PHP process? section I've outlined a possible refactoring of the get_api function to make it testable without requiring the use of separate PHP processes.
I'm providing this refactoring code below for the sake of completeness, the judgment of which approach is "better" is up to the reader.
<?php
​
class Context_Adapter{
​
    public function is_admin(){
        return \is_admin();
    }
​
}
​
class Api_Factory{
​
    private $api;
    private $context;
​
    public function __construct(Context_Adapter $context){
        $this->context = $context;
    }
​
    public function getApi(){
        if(null !== $this->api){
            return $this->api;    
        }
​
        if($this->context->is_admin()){
            $api = new Admin_Api;
        } else {
            $api = new Api;
        }
​
        return $api;
    }
}
Now the Api_Factory class can be injected by injecting a mocked Context_Adapter class, modifying the return value of the Context_Adapter::is_admin method.
Due to the supposed requirement of the API instance being a singleton, this solution will also require some container or service-locator to ensure at most only one instance of the Api_Factory exists at any given time in the context of a request.
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Why run tests in a separate PHP process?
Running tests in separate PHP processes
Can I run some tests in the same process and some in a separate process?
OOP refactoring of get_api