Sometimes as developers we don’t always catch everything in our test, bugs being the tricky things they are will always slip through the net. When they do we need to fix them, but where do we begin?

The answer, PHPUnit + git bisect, using git bisect and PHPUnit we can quickly narrow down what commit caused the bug, which should allow us to find what caused and therefore solve the bug really quickly.

To see this in action let’s look at a very simple example. We have a FakeClass which contains a single method who’s job is to return three things. The class looks something like this.

                    class FakeClass
{
	/**
	 * Returns three things
	 *
	 * @return array
	 */
	public function getThreeThings()
	{
		$things = ['one', 'two', 'three'];

		return $things;
	}
}
                    
                    

The class does a very simple thing, and we’re convinced it’s so simple we’ve not written a unit test. Unfortunately this FakeClass doesn’t stay simple forever and over time we’ve ended up with a lot of changes to this file.

                    ad1a436 Add get four things method
708f57d Add return value PHPDoc
d4a81a4 Add phpdoc
33f9b00 Add get two things method
61deef0 Add returns phpdoc comment
85730ab Add get three things to FakeClass
                        

Somewhere in these commits, someone has broken the getThreeThings method, the customer is not happy! First let’s create the unit test we left out in our haste to get the code shipped.

                    class FakeClassTest extends PHPUnit_Framework_TestCase
{
        private $fakeClass;

        public function setUp()
        {
                $this->fakeClass = new FakeClass();
        }

        public function testGetThreeThings()
        {
                $things = $this->fakeClass->getThreeThings();
                $this->assertEquals(3, sizeof($things));
        }
}
PHPUnit 3.7.28 by Sebastian Bergmann.

Configuration read from /home/alan/workspaces/bisect-log/phpunit.xml

F

Time: 44 ms, Memory: 3.25Mb

There was 1 failure:

1) FakeClassTest::testGetThreeThings
Failed asserting that 4 matches expected 3.

/home/alan/workspaces/bisect-log/tests/FakeClassTest.php:14

FAILURES!
Tests: 1, Assertions: 1, Failures: 1.
                        
                        

Great now we have a test, and we know it’s failing. How can we find out which of the above commits caused the error?

First we need to enter git bisect mode.

git bisect start

We know that this functionality worked at our first commit so let’s mark that commit as good.

git bisect good 85730ab

And we know that the current commit is bad, so let’s mark that as bad.

git bisect bad fbe6fb8

Now we’ve done everything we need to find the broken commit. All that’s left is to run our unit test with git bisect.

git bisect run phpunit tests/FakeClassTest.php

Git moves through the commits using a binary search algorithm to find the first broken commit as quickly as possible.

After git bisect has zoomed through testing the commits it tells us the exact commit which caused the problem.

ad1a436f0c15676cd5251e1d73c3af667e739a72 is the first bad commit

If we do a git diff we see:

                    git diff ad1a436 HEAD

diff --git a/src/FakeClass.php b/src/FakeClass.php
index bd8a9a3..bbdf3a0 100644
--- a/src/FakeClass.php
+++ b/src/FakeClass.php
@@ -2,22 +2,13 @@
 class FakeClass
 {
        /**
-        */
-       public function getFourThings()
-       {
-               $things = ['one', 'two', 'three'];
-
-               return $things;
-       }
-
-       /**
         * Returns three things
         *
         * @return array
         */
        public function getThreeThings()
        {
-               $things = ['one', 'two', 'three', 'four'];
+               $things = ['one', 'two', 'three'];

                return $things;
        }
}
                        

                    

The problem is pretty obvious now. I added a ‘four’ to the array in the getThreeThingsMethod, how stupid.

I hope this blog post has shown how powerful git bisect is, and although this example was really simple, you can imagine how useful this is.

The code behind this blog post can be found at https://github.com/Alan01252/git-phpunit-bisect-test