Running for Coverage

Today we're going to look at the history and the future of coverage collection in PHP.

History is the easy bit: For most of the history of PHP, Xdebug has provided the only implementation to php-code-coverage. Simple.

Then in 2015, just after phpdbg was merged into PHP, some clever sausages extended the instruction logging facility that I wrote into phpdbg for internals developers in order to provide another implementation to php-code-coverage. To paraphrase a popular book "and they saw that it was good" ...

But was it good !? It was fast, it didn't add any complication to phpdbg and for a lot of people, they didn't notice (or didn't care about) the mistakes phpdbg was making.

That's right, it was merged with mistakes ...

You might think the job of a coverage collector is just to hook into Zend and find out what lines have been executed any way it can, and on the face of it, that's true (and difficult to get wrong, you might think).

But, think more carefully about it, and you'll realise that actually a coverage collector must know what instructions are executable (and important to the user), if for no other reason than Zend will insert an implicit return statement into all functions (or top level code, file) even if you have an explicit one. It does this so that all functions certainly end with return, this is important for boring internal reasons as well as obvious ones.

A graphic example of why it's important to know which instructions are executable is this:

/* 1 */ function foo($bar) {
/* 2 */    if ($bar) {
/* 3 */        return true;
/* 4 */    }
/* 5 */ }

At the end of this function, on line 5, Zend inserts that implicit return, a collector that doesn't know if that return is an executable instruction must ignore it, and so will report inaccurate coverage of the function if the first control path is taken.

Quite early on in the life of Xdebug, Derick developed branch analysis. At the time, it was the only implementation of branch analysis for PHP code, so a very valuable thing. Branch analysis allows Xdebug to determine that the implicit return is important, and so mark it as coverable/executable code to be included in any trace.

In addition the branch analysis in Xdebug is the basis for its support for branch or path coverage, which is in my opinion, and Derick's, the most valuable feature of Xdebug's coverage, but unfortunately unusable in its current state. Although there are plans to improve that, and I may even help. There's no real bias here.

phpdbg has no such analysis, and used a fast, but inaccurate method that results in ignoring all implicit returns, executable or not. This makes the reports phpdbg generates less than accurate, and less than trustworthy ... but it does do it fast ... ish.

So now it's 2015, we have two debuggers, both with support for coverage, one of them obviously superior to the other, and one of them fast, but inferior.

Now I want you to question whether it makes sense for a debugger to have support for code coverage at all; gdb has no such thing, I haven't used any Java since 2015, but I don't remember seeing code coverage collection as a feature of any debugger I ever used for it, quite late on in the game Visual Studio did get support for coverage, but not as an extension of debugging, but a feature of the IDE itself ...

The answer is no, it doesn't really make sense, the two things interfere with each other. A debugger must gain such a degree of control over the execution environment it is debugging that they come with unavoidable overhead, they may even need to change (or have the ability to change) the path that is taken through code which is antithetical to collecting coverage. A coverage collection tool needs to do the opposite of a debugger: Change as little as possible, try not to slow down the execution of code more than is absolutely necessary to do your job - Not a concern for a debugger, nobody really cares if a debugger is slow - it will spend a lot of its time paused, waiting for you to figure out what to do next !

I should say now that Derick flat disagrees me with here, and his reasoning is not wrong: Xdebug started as a debugging aid, the more fully blown features such as step debugging and profiling were added later. So adding coverage to the set of tools wasn't crazy, it makes sense.

But we have two debuggers that support coverage, we're so spoiled, or unlucky ... or foolhardy ... or doomed ...

The fact is that while Xdebug has superior collection to phpdbg, most people disable Xdebug in their CI, and in their development environments for performance reasons. If they run coverage in CI, it's for a small project, or they use phpdbg and put up with the mistakes (or don't know about  them).

When Sebastian Bergmann was conversing with the clever sausages that made phpdbg support coverage collection, he even warned them they were following a doomed path, and suggested it might be better if code coverage was a standalone extension, nevertheless he merged their work and we all moved forward.

Doing something about this situation has been on my todo list since shortly after the driver for phpdbg was merged into php-code-coverage, not very high up on my todo list, but present.

Quite recently I saw a blog post from Sebastian about making Xdebug collect coverage faster, he failed to mention phpdbg in the whole article which was questioned by reddit users and twitterers alike. He didn't mention it because he knows very well what its limitations are, and didn't want to encourage people to use something that makes mistakes. Totally fair.

But, someone on the internet said a thing and my mind became occupied, completely, with fixing this problem. There's no good reason that in 2019, we can't collect accurate coverage, and fast.

I set to work on PCOV, which is a standalone extension that implements the kind of interface that php-code-coverage needs, it does this with as little overhead as possible, as it should. At first, I copied the faulty method of ignoring executable returns from phpdbg, I done this to prototype it as fast as possible and see just what kind of performance we can get. The results were remarkable, the overhead was so very low that it managed to outpace phpdbg on every test suite I ran, by a considerable margin.

Even though flawed, I thought this is worth sharing, so I made it nice and made a readme and opened a pull request to have the PHP part of the driver merged into php-code-coverage. But I didn't stop thinking ...

I then read from a post on Dericks blog that mentioned he was looking for ways to improve the performance of coverage collection in Xdebug. In the blog post is a one liner about preferring correctness over speed.

I absolutely agree with preferring correctness over speed, and I couldn't sleep knowing that I had just introduced a known flaw in brand new code, sure it was faster than phpdbg, but objectively not better at the job of collecting accurate coverage.

It so happens that Xdebug is not the only software in the ecosystem that performs analysis of code, in fact Optimizer, part of PHP for many years now, also performs analysis and is the "source of truth" for what is an executable Zend instruction, since non-executable ones are destined to be removed automatically during one of its many optimization passes.

You will notice that nobody ever complains that Optimizer is slow to analyze code, the reason for this, is that it's not slow at all, it has a very succinct implementation of a control flow graph ... not much of PHP is succinct, but so well abstracted is this feature of Optimizer that you can lift it from opcache and drop it into whatever you like with very little work.

That is what I did next ... So now PCOV and Zend agree absolutely, and always will, about what is executable code.

It may seem presumptious of me to talk about the future of coverage being PCOV, but humbly, I'd like to suggest that it should be, and like you to consider that I'm talking about the distant future, not tomorrow: I think phpdbg and maybe Xdebug should drop that feature altogether and maybe we can team up and add some really cool but usable and fast features to PCOV that php-code-coverage always wanted, such as branch or path coverage, a much superior criteria than line coverage.

I should make clear at this point that Derick is not so keen on that idea currently, and would like to pursue his own path for Xdebug, with plans to refactor and improve upcoming Xdebug releases, possibly containing the many features within Xdebug - so it is able to behave only as a profiler, or a debugger, or a collection tool. Honestly I would be surprised if he wanted to drop anything from such mature and widely deployed software, but let's see where we are in 5 years, perhaps ...

At this moment, you will find it hard to use PCOV in your projects as I'm waiting for Sebastian to review the pull request and make his decision, presumably about the version of php-code-coverage that PCOV will first be included in.

It's Monday morning, and I've got nothing better to do than write a blog post ... When you can use it easily, another post will follow.

That's all for now, enjoy your week :)

Faking It

Fig 1. A Mockingbird

As well as mentoring and code review one of my main tasks at work is to improve the test suites and improve the testing and development methodologies we use. This is no small task and has resulted in the publication of a few extensions, one of them is uopz.

Before we continue; I work in the real world, where not all code was written yesterday using the best standards and best methods, and it's fine to say "just fix your code", but totally unrealistic, we have to deal with reality.

You may not hear people raving about using uopz, because for most of us, if you are doing the sort of things that uopz makes possible, there is something wrong with the way you are writing code. This is technically true, of course, nevertheless, we have work to do.

When I first started my current job, our test suite was bound tightly to runkit, and failed all the time. When I wrote uopz and we adopted it, it stabilised and developers could once again get on with their work. All the while we would repeat to each other that we would move away from uopz by improving our tests and code. This hasn't really happened, instead, because uopz allows certain crazy things, they are the crazy things that were being done in tests.

It's five years later, and uopz takes up a fair amount of my time with each new PHP version, it's quite a headache, for a temporary solution. Realising that actually this is entirely my fault because I gave them the tools to work like this, I decided we needed new tools.

Some time ago, I wrote an extension called Componere (latin, composer ... I like latin names), the purpose of Componere is to allow the programmer to (re)define complex types at runtime. I showed this to a couple of colleagues, got some "wow, cool", but they never picked it up. I later realised that they didn't pick it up because it relies on the author having at least some knowledge of the way classes are built internally. So even though extremely powerful, it got ignored.

We required a higher level API, and so I've written a mocking framework in PHP, built on top of Componere that is almost without limitation. It's name is "mimus" which is taken from Latin, "mimic" and shared with the first part of the Latin binomial name for the animals everyone knows as Mockingbirds.

I am fully aware that the ecosystem contains within it many mocking frameworks, however they all have the same set of problems, limitations on what methods you can mock or how. This may be good enough for the vast majority of people, you can just write your code so that you can mock the parts you need at test time. However, if you have 3M LOC, it's not so simple; We need to do some of the things that can't be supported properly if you write the whole framework in PHP, such as stubbing privates, statics, finals. We also have 14 tonnes (number pulled from the air) of tests split across many suites and projects, this makes invoking PHP parsing in PHP several tens of thousands of times unrealistic.  Wipe that cringe off your face, real world, remember ...

While you don't hear people raving about uopz, I happen to know it's used in some very large scale deployments of PHP: I know that there's a number of people out there doing exactly the same sort of horrible things in tests that we were doing, and that mimus is freeing us from, slowly.

We're a month into the switch from uopz and hacking the engine apart to a more modern, more sensible world. The developers are really enjoying themselves using mimus too, which is a bonus, and probably born of the fact that they don't have to feel quite so "dirty" when writing tests.

I'm not going to repeat the readme for mimus here, or show any code, because it's Sunday and you probably don't want that. Tomorrow morning, before you write another test that uses uopz, check out mimus ...

Enjoy the rest of your Sunday ...

Boxes of Sand

Fig 1. A Sandbox

Sandboxing is a technique used in testing and security to execute unsafe, or untrusted code in a safe environment. There are different levels of sandboxing: In security a sandboxed environment may refer to a (virtual) machine dedicated to the execution of unsafe code. In testing, a sandbox may refer to a thread or process dedicated to the same purpose.

For most of the history of PHP, sandboxing has been provided by Runkit, and the first thing I'd like to do is clear up some confusion that is present in the manual:

Instantiating the Runkit_Sandbox class creates a new thread with its own scope and program stack.

This has never actually been true. Runkit never created a thread for the sandbox: What it created was a PHP thread context provided by a much misunderstood layer of software called TSRM. It is TSRM that provides the "share nothing" architecture that PHP requires when running in a threaded SAPI, such as Apache on windows. Builds of PHP that use TSRM are colloquially known as ZTS - Zend Thread Safe(ty).

Runkit "switched" between the parent and the sandbox context to execute code as if it were in another thread, but there was never actually another thread, so this extremely old documentation is very misleading. For the sake of nostalgia, I don't intend to fix the documentation, it has been that way for 13 years, and the feature is all but dead, so it's not affecting anyone anymore.

There is a version of runkit available for PHP7, however, sandboxing was removed because it looks like the new maintainer couldn't figure out how to make it work, maybe mislead by current documentation.

The only people who care about TSRM/ZTS in PHP are the windows people, and myself. The windows people need it for Apache, and I need it for pthreads, what's more I recognise the value in the abilities that TSRM provides. So when PHP 7 quietly improved the performance of ZTS with improvements to TSRM, nobody really noticed, I've even heard people say TSRM is going to be removed. It isn't.

Those improvements broke the ability to abuse TSRM as Runkit did, for many boring reasons, runkits sandbox cannot work as it did before, it's not possible.

I've bumped up against runkit before; uopz exists because 5 years ago (roughly) I started a new job and was presented with test suites that would crash all the time because runkit was doing bad things. This is the reason I didn't and won't work on runkit, but do think a sandbox is a useful thing.

So I wrote Sandbox, it's a PHP 7.1+ extension (because nobody is using 7.0, right !?) that requires a ZTS build of PHP and creates real sandboxes. A Sandbox is truly a separate thread, isolation is provided by TSRM as it always has been, this means a Sandbox thread is as isolated from the thread that created it as are two PHP threads inside apache, almost complete isolation.

While the Runkit implementation provided a lot of methods to affect the context it created, the new implementation provides just an entry point into the sandbox thread:

$sandbox = new \sandbox\Runtime();

$sandbox->enter(function(){
    /* I will execute in the sandbox thread */
});

This doesn't give you multi-threading, the sandbox and parent threads are synchronized so that no user code executes in parallel, this is the safest (only) thing to do for a sandbox.

The code executed in the sandbox may do anything up to but excluding making PHP segfault, and not affect the thread (process) that created it. It may exhaust memory limits and time constraints and not only will the parent thread remain stable, so will the child (you may re-enter after such failures).

By passing an array of INI options to the constructor of the runtime you can gain great control over the sandbox.

I'm afraid I haven't created a PHP manual for Sandbox, so that job is up for grabs ... I'm quite happy to leave it quietly in a corner and have the kind of people who read my blog know about it, but not everyone who reads the manunal, although I won't object to anyone commiting manual entries.

Thanks to my awesome QA/RM team, Anatol and Remi, who I adopted from PHP for my own purposes; Sandbox is available on PECL, windows builds are available, and it's available from Remi's repositories.

So, now we have a sandbox for PHP7.1+ ... have fun using it ...