PHPUnit: How do I mock this file system?

Question

Consider the following scenario (this is not production code):

 class MyClass {
    public function myMethod() {
        // create a directory
        $path = sys_get_temp_dir() . '/' . md5(rand());
        if(!mkdir($path)) {
            throw new Exception("mkdir() failed.");
        }

        // create a file in that folder
        $myFile = fopen("$path/myFile.txt", "w");
        if(!$myFile) {
            throw new Exception("Cannot open file handle.");
        }
    }
}

Right, so what's the problem? Code coverage reports that this line is not covered:

throw new Exception("Cannot open file handle.");

Which is correct, but since I'm creating the folder above logically it would seem impossible for the fopen() to fail (except maybe in extreme circumstances, like disk at 100 %).

I could ignore the code from code coverage but thats kind of cheating. Is there any way I can mock the file system so that it can recognise myFile.txt and mock the file system unable to create the file?

Answer 1

vfsStream is a stream wrapper for a virtual filesystem that is useful in unit tests to mock the real filesystem. You can install it from composer.

More Info at:

https://github.com/mikey179/vfsStream

https://phpunit.de/manual/current/en/test-doubles.html

Answer 2

Yes!

You should inject the full path somehow, and don't call sys_get_temp_dir() right in that method.

Any non-existant path should trigger a problem. You don't need VFS for that.

BUT you will get a E_NOTICE (or warning perhaps?) before the exception is triggered. So you should probably first check is_writable, and throw the exception if it returns false.

Answer 3

You could also break the function into 2 methods, one to create the path, and the other to use it. Then, individual tests could be done to ensure the path is created. A second set of tests could check and capture the exception when you try to use a bad path.