Tuesday, March 20, 2018

Behat AfterScenario, PHP Garbage Collection, and Singletons

In Behat, I added a singleton to our contexts to store things across scenarios, but I ran into trouble when trying to keep separation between my tests.  The storage object allowed me to be creative with builders, validators, and similar ways of reducing repetition and making the PHP code behind easier to read.  There was a problem though: it would randomly be cleared in the middle of a test.

The only thing I knew was the object would get cleared at relatively the same time.  I had a set of about 50 different tests in a single feature.  This would call an API multiple times, run validations on the responses, and then move on to the next test.  All the while, it would put information into the storage object.  The test would not just fail in the middle of a scenario, it would generally fail near the same part of a scenario every time.  it was timing, an async process, or something was clearing a logjam.

While designing the storage object, I had the bright idea to clear it with every scenario.  The singleton acts like a global variable, and a clear after each one would ensure data from one test didn't pop up in another.  To make sure i was running this at the last possible moment, I put the clear into the __destruct() method of my context class.  By putting the clear in the destructor, I gave PHP permission to handle it as it saw fit.  In reality, it sometimes left my scenario objects to linger while running the next (due to a memory leak or similar in Behat itself, or a problem in my code; I couldn't tell).

/**
 * Destructor
 */
public function __destruct()
{
 ApiContextStore::clear();
}


I first stopped clearing the store and the bugs went away.  Whew!  But how could I make sure I wasn't contaminating my tests with other data and sloppy design?  I tried two things:

1) gc_collect_cycles() forces the garbage collector to run.  This seems to have the same effect of stopping the crashes, but it was kind of a cryptic thing to do.  I had to put it in the constructor of the Context rather than something that made more sense.

/**
 * FeatureContext constructor.
 */
public function __construct()
{
 /**
 * Bootstrap The Store
 */
 gc_collect_cycles();
 ApiContextStore::create(); // Creates an instance if needed
}
2) Putting in an @AfterScenario test provided the same protection, but it ran, purposefully, after every test was complete.  I'm not freeing memory with my clear, so relying on garbage collection wasn't a priority.  I just needed it to run last.


/**
 * @AfterScenario
 *
 * Runs after every scenario */
public function cleanUpStore()
{
 ApiContextStore::clear();
}

http://php.net/manual/en/function.gc-collect-cycles.php
http://docs.behat.org/en/v2.5/guides/3.hooks.html
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