Testing PHP Network Code

I work on this Open Source webmail client. I don’t think I have ever written about it here before. It’s called Cypht. It connects to services, like an IMAP, SMTP, or POP3 server. It uses the PHP function stream_socket_client to create a connection to these services, then it sends commands and reads responses with standard read/write functions like fgets and fwrite.

Recently I decided I hate myself, so I tried to build a way to unit-test this. Turns out it’s possible, and not nearly as hard as I deserve. I did bang my head around the desk area for a few days figuring it out, so not a total loss. Here is how I did it.

Step 1: Abstract low-down-no-good functions

No matter how amazingly awesome your PHP code base is, if your code actually does anything and you want comprehensive unit test coverage, you have no choice but to abstract a few built-in PHP functions that simply don’t play nice (sessions, cookies, header, curl, streams, you get the picture). I use the following pattern for this:

  • Create a class of all static methods that “wrap” the naughty functions
  • Only define that class at run time if it does not already exist
  • Change your code to call the naughty_class::function version
  • Create the same class in your unit test bootstrap, that has friendly versions of these functions (like doing nothing, or returning true or whatever)
  • Include your unit test version before the run time version when running tests.
  • Realize your wildest dreams of success and good fortune.

An example:

class NaughtyFunctions {
    /**
     * @param string $server host to connect to
     * @param integer $port port to connect to
     * @param integer $errno error number
     * @param string $errstr error string
     * @param integer $mode connection mode
     * @param object $ctx context
    */
    public static function stream_socket_client($server, $port,
        &$errno, &$errstr, $timeout, $mode, $ctx) {
        return stream_socket_client($server.':'.$port, $errno,
            $errstr, $timeout, $mode, $ctx);
    }
}

Instead of calling stream_socket_client in code, we call NaughtyFunctions::stream_socket_client with the same (similar) arguments. This pattern (or something like it) is required to make this work, so no skipping step 1. It’s also a great way to deal with PHP functions that disagree with PHPUnit, and as a way to fool tests into taking a different code path they would not normally take, like by overriding function_exists for example. Here is what Cypht uses at runtime:

https://github.com/jasonmunro/cypht/blob/master/lib/framework.php#L59-L202

Step 2: Build a stream wrapper to fake out your code

In PHP you can fake a “stream” AKA a file handle or network connection, by creating and registering a “stream wrapper“. For file operations and stateless protocols like HTTP, this is pretty simple – read until the “file” ends. But for persistent network protocols, this takes a bit of cleverness.

You need the ability to read from the stream until you reach “End Of File” (EOF). But then you need to reset the EOF status the next time you issue a command, so you can read from the stream again. There is no way (I know of) to do this from within the stream wrapper prototype, and we don’t want to alter the network code we are testing.

Thus the cleverness. Using the abstract in step 1, we can save a reference to the stream resource, and rewind it every time we send a new command, effectively resetting the EOF. Seems less clever now that I write this, but it was the most difficult part.

Here is an example of of both the NaughtyFunctions class and a stream wrapper in action:


/**
 * Generic stream wrapper. This will be extended for protocol
 * specific commands and responses.
 */
class Fake_Server {

    /* position within the response string */
    protected $position;

    /* current response string */
    protected $response = '';

    /* list of commands to responses, varies per protocol */
    public $command_responses = array();

    /* open */
    function stream_open($path, $mode, $options, &$opened) {
        $this->position = 0;
        return true;
    }

    /* read */
    function stream_read($count) {
        $this->position += strlen($this->response);
        return $this->response;
    }

    /* write */
    function stream_write($data) {
        $data = trim($data);

        /* look for and set the correct response */
        if (array_key_exists($data, $this->command_responses)) {
            $this->response =  $this->command_responses[$data];
        }

        /* request not found, so set an error value */
        else {
            $this->response = $this->error_resp($data);
        }
        /* CLEVERNESS: here we rewind the stream so we
           can read from it again */
        rewind(NaughtyFunctions::$resource);
        return (strlen($data)+2);
    }

    /* tell */
    function stream_tell() {
        return $this->position;
    }

    /* seek */
    function stream_seek($pos, $whence) {
        $this->position = 0;
        return true;
    }

    /* end of file */
    function stream_eof() {
        return $this->position >= strlen($this->response);
    }

    /* generic error */
    function error_resp($data) {
        return "ERROR\r\n";
    }
}

/**
 * IMAP specific fake server that extends the generic one
 */
class Fake_IMAP_Server extends Fake_Server {

    /* array of commands and their corresponding responses */
    public $command_responses = array(
        'A1 CAPABILITY' => "* CAPABILITY IMAP4rev1 LITERAL+ ".
            "LOGIN-REFERRALS ID ENABLE AUTH=PLAIN AUTH=CRAM-MD5\r\n",
        /* other commands and responses go here */
    );

    /* IMAP friendly error */
    function error_resp($data) {
        $bits = explode(' ', $data);
        $pre = $bits[0];
        return $pre." BAD Error in IMAP command\r\n";
    }
}

/**
 * Naughty functions wrapper to be used in unit tests. Unlike the
 * run time version, this one returns a "connection" to our fake
 * server.
 */
class NaughtyFunctions {

    /* this will hold a reference to our fake network connection */
    public static $resource = false;

    /* we can toggle this to simulate a bad connection */
    public static $no_stream = false;

    /* fake out stream_socket_client and start the wrapper */
    public static function stream_socket_client($server, $port,
        &$errno, &$errstr, $timeout, $mode, $ctx) {

        /* bad connection */
        if (self::$no_stream) {
            return false;
        }
        /* don't call twice from the same test */
        if (!in_array('foo', stream_get_wrappers(), true)) {
            stream_wrapper_register('foo', 'Fake_IMAP_Server');
        }

        /* open, save a reference to, and return the connection
           to our fake server */
        $res = fopen('foo://', 'w+');
        self::$resource = $res;
        return $res;
    }
}

Step 3: Correlate requests and responses for your protocol

Now all you have to do is map requests to the server with appropriate (or inappropriate) responses to exercise your network code from a unit test. In this case that would be adding to the $command_responses array in Fake_IMAP_Server. This is where we cross over from “cool problem solving” to “incredibly tedious unit test production”. looks like I will be receiving extra punishment after all.

Step 4. See a doctor about your wrist pain from writing all the unit tests

Cypht has about 14,000 lines of code I need to test this way. I’m about 1% through the process. I love that it can be done without standing up an IMAP/POP3/SMTP server, but my fingers hurt just thinking about it.