A great example unit test for a PHP process that needs to fork to try concurrency locking

example-unit-test-locakable.php

<?php
/**
 * When lockMutex() happens, we aquire a semaphore
 *   no other processes have access while this is locked
 * 
 * This unit test forks into 2 processes and verifies locking
 */
 
	// Test that locking actually makes me wait if someone else has the lock.
	public function testForkLockMutex() {
		// tempFile used for communication between parent and child.
		$tempFile = tmpfile();

		// Split into two..
		$pid = pcntl_fork();
		if ($pid == -1) {
			$this->assertEqual('shit went down. [failed to fork]', 'no shit went down');
			return;
		}
		if ($pid) {
			// I am parent.

			// Wait 5000 microseconds / 5 milliseconds.  Give child time to grab lock.
			usleep(5000);

			$timeBeforeLock = microtime(true);
			// Get lock.
			$this->assertTrue($this->Model->lockMutex(1));
			$timeAfterLock = microtime(true);
			$this->assertTrue($this->Model->unlockMutex(1));

			// How much time elapsed?
			$timeElapsed = $timeAfterLock - $timeBeforeLock;
			// Amount of time elapsed should be (150ms - 5ms + x + y +/- z), where
			// x is the fork time, y is the overhead time from unlockMutex/assert, and z is
			// the "fudge factor" from OS scheduling...

			// Allow range [140, 200] ms
			$this->assertGreaterThanOrEqual(0.140, $timeElapsed);
			$this->assertLessThanOrEqual(   0.200, $timeElapsed);

			// Make sure Child passed tests and delete tempFile
			fseek($tempFile, 0);
			$messageFromChild = fread($tempFile, 1024);
			$this->assertEqual($messageFromChild, 'pass');
			fclose($tempFile);
		} else {
			// I am child.

			// Get lock.
			$lockStatus = $this->Model->lockMutex(1);
			// Hold lock for 150 milliseconds / 150000 microseconds, then unlock
			usleep(150000);
			$unlockStatus = $this->Model->unlockMutex(1);

			// Write test status to temp file, so parent can read it.
			if ($lockStatus === true && $unlockStatus === true) {
				fwrite($tempFile, "pass");
			} else {
				fwrite($tempFile, "fail");
			}

			// Kill self with a "kill" command.
			// If we used exit or die, then CakeTestCase
			// would take over and start to tear down all of the
			// fixtures!  Our parent still needs it.
			posix_kill(posix_getpid(), SIGTERM);
		}
	}