Wednesday, 17 July 2019

Trimming the Phat

Fig 1. A very fancy Tomb

We all think we know how dead code elimination works, we can just reference code coverage, or run static analysis, or rely on our own internal model of the code, which is always absolutely perfect ...

Dead can mean multiple things when we're talking about code, at least:
  • Compiler context - dead code is "unreachable", it can never be executed
  • Execution context - dead code is "unused", it has not been, or is not going to be called
The distinction between compile and execute in PHP is somewhat blurred, as a result, some dead code detection that should be part of the compiler have traditionally been part of code coverage. In the latest versions of PHP, opcache eliminates unreachable code.

Static analysis and coverage reports can tell you about dead code in those narrow scopes defined, but there is another sense in which code might be considered dead:
  • Code that is in fact unused in production
My manager recently asked me to come up with something so that we can detect dead code in this production sense.

I'm quite adept at bending PHP to my will, however, this task presents some not insignificant challenges. Normally, when we want to abuse PHP in some strange way, we're doing so in the name of testing. 

Testing is a nice quiet place, where there's only one process to care about, not much can go wrong. If you are careful, you can write some really nice tooling, the overhead is very acceptable, and people rave about it on twitter (pcov).

Production on the other hand is a scary place, where mistakes may cost a lot of money, where there are in the order of hundreds of processes to care about: Extracting statistical information from hundreds of processes without adversely affecting their performance is not a simple task.

Tombs

Tombs is my solution to the problem of detecting code that is unused in production. Code that even though may be reported as reachable, covered, or used, is in fact never called in production.

There's something quite pleasing about the requirements for a task translating almost perfectly into the solution. The requirements for Tombs were:
  • Must not use more CPU time in PHP processes than is absolutely necessary (i.e. be production ready)
  • Must report statistics centrally for every process in a pool
The first requirement, aside from the obvious, means that Tombs needs to have an API that faces the system rather than user land PHP, we can't inject code into production so the processes that gather statistics must be separate and might be on different machines entirely.

The second requirement means that Tombs needs to use shared mapped memory, like O+ or APC(u).

O+ and APC(u) both achieve safety in their use of shared memory by multiple processes using mutual exclusion - implemented either as file locks, pthread mutex, or the windows equivalent - this makes perfect sense for them. It means that even though many processes may compile the about to be cached file, or execute the function that returns the about to be cached variable, only one process can insert the file or variable into shared memory.

Reporting live statistics about a system is similar to trying to count the number of birds in flight over the earth - it will change while reporting. In this environment, mutex makes very little sense, what we need here is a lock free implementation of the structure that stores information we need to share. We need to know that no matter how large the set of data being returned, we don't have to exclude other processes from continuing to manipulate that data concurrently.

Using Tombs

Simply load Tombs in a production environment and without modifying any code, allow normal execution to take place over the course of hours, days, or weeks. 

Now, when you open the Tombs socket the data returned represents the functions and methods that have not been executed by any process in the pool since Tombs was started.

Using this data, you can now make decisions about the removal or refactoring of code to reduce or hopefully eliminate dead code.

If you use Tombs, reach out to me and tell me how it worked out for you ....

Saturday, 30 March 2019

PHP GR8

Fig 1. A chap performing the Detroit JIT
Unless you have been living under a rock, or are from the past (in which case, welcome), you will be aware that a JIT is coming to PHP 8: The vote ended, quietly, today, with a vast majority in favour of merging into PHP 8, so, it's official.

Throw some crazy shapes in celebration, suggestion given in Fig 1, and it's even called "The (Detroit) JIT" ...

Now sit down and read the following myth busting article, we're going to clear up some confusion around what the JIT is, what it will benefit, and delve into how it works (but only a little, because I don't want you to be bored).

Since I don't know who I'm talking to, I'm going to start at the beginning with the simple questions and work up to the complex ones, if you already are sure you know the answer to the question in a heading, you can skip that part ...

What is JIT ?

PHP implements a virtual machine, a kind of virtual processor - we call it Zend VM. PHP compiles your human readable script into instructions that the virtual machine understands (we call them opcodes), this stage of execution is what we refer to as "Compile Time". At the "Runtime" stage of execution the virtual machine (Zend VM) executes your code's instructions (opcodes).

This all works very well, and tools like APC (in the past) and OPCache (today) cache your code's instructions (opcodes) so that "Compile Time" only happens when it must.

First, one line to explain what JIT is in general: Just-in-time is a compiler strategy that takes an intermediate representation of code and turns it into architecture dependent machine code at runtime - just-in-time for execution.

In PHP, this means that the JIT treats the instructions generated for the Zend VM as the intermediate representation and emits architecture dependent machine code, so that the host of your code is no longer the Zend VM, but your CPU directly.

Why does PHP need a JIT ?

The focus of the PHP internals community since slightly before PHP 7.0 has been performance, brought about by healthy competition from Facebook's HHVM project. The majority of the core changes in PHP 7.0 were contained in the PHPNG patch, which improved significantly the way in which PHP utilizes memory and CPU at its core, since then every one of us has been forced to keep one eye on performance.

Since PHP 7.0 some performance improvements have been made, optimizations for the HashTable (a core data structure for PHP), specializations in the Zend VM for certain opcodes, specializations in the compiler for certain sequences, and a constant stream of improvements to the Optimizer component of OPCache ... and many others besides, too boring to list.

It's a brute fact that these optimizations can only take us so far and we are rapidly approaching, or maybe have already met, a brick wall in our ability to improve it any further.

Caveat: When we say things like "we can't improve it any further", what we really mean is, "the trade-offs we would have to make to improve it any further no longer look appealing" ... whenever we talk about performance optimizations, we're talking about trade-offs. Often, trade-offs in simplicity for performance. We would all like to think that the simplest code is the fastest code, but that simply is not the case in the modern world of C programming. The fastest code is often that code which is prepared to take advantage of architecture dependent intrinsics or platform (compiler) dependent builtins. Simplicity just is not a guarantee of the best performance ...

At this time, the ability for PHP to JIT would appear to be the best way to squeeze more performance from PHP.

Will the JIT make my website faster ?

In all probability, not significantly.

Maybe not the answer you were expecting: In the general case, applications written in PHP are I/O bound, and JIT works best on CPU bound code.

What on earth does "I/O and CPU bound" mean ?

When we want to describe the general performance characteristics of a piece of code, or an application, we use the terms I/O bound and CPU bound.

In the simplest possible terms:
  • An I/O bound piece of code would go faster if we could improve (reduce, optimize) the I/O it is doing.
  • A CPU bound piece of code would go faster if we could improve (reduce, optimize) the instructions the CPU is executing - or (magically) increase the clock speed of the CPU :)
A piece of code, or an application, may be I/O bound, CPU bound, or bound equally to CPU and I/O.

In general, PHP applications tend to be I/O bound - the thing that is slowing them down is the I/O which they are performing - connecting, reading, and writing to databases, caches, files, sockets and so on.

What does CPU bound PHP look like ?

CPU bound code is not something a lot of PHP programmers will be familiar with, because of the nature of most PHP applications - their job tends to be connect to some database, and or possibly a cache, do some light lifting and spit out an html/json/xml response.

You may look around your codebase and find lots of code that has nothing whatever to do with I/O, code that is calling functions completely disconnected from I/O even, and be confused that I seem to be implying that this doesn't make your application CPU bound, even though there may be many more lines of code that deal with non I/O than I/O.

PHP is actually quite fast, it's one of the fastest interpreted languages in the world. There is no remarkable difference between the Zend VM calling a function that has nothing to do with I/O, and making the same call in machine code. There is clearly a difference, but the fact is that machine code has a calling convention, and Zend VM has a calling convention, machine code has a prologue and Zend VM has a prologue: Whether you call some_c_level_function() in Zend Opcodes or machine code doesn't make a significant difference to the performance of the application making the call - although it may seem to make a significant difference to that call.

Note: A calling convention is (roughly) a sequence of instructions executed *before* entering into another function, a prologue is a sequence of instructions executed *at entry* into another function: The calling convention in both cases pushes arguments onto the stack, and the prologue pops them off the stack.

What about loops, and tail calls and X I hear you ask: PHP is actually quite smart, with the Optimizer component of OPCache enabled your code is transformed as if by magic into the most efficient form you could have written.

It's important to note now that JIT doesn't change the calling convention of Zend Functions from the convention established by the VM - Zend must be able to switch between JIT and VM modes at any time and so the decision was taken to retain the calling convention established by the VM. As a result those calls that you see everywhere aren't remarkably faster when JIT'd.

If you want to see what CPU bound PHP code looks like, look in Zend/bench.php ... This is obviously an extreme example of CPU bound code, but it should drive home the point that where the JIT really shines is in the area of mathematics.

Did PHP make the ultimate trade-off to make math faster ?

No. We did it to widen the scope of PHP, and considerably so. 

Without wanting to toot our own horn, we have the web covered - If you are a web programmer in 2019 and you haven't considered using PHP for your next project, then you are doing the web wrong - in this very biased PHP developer's opinion.

To improve the ability to execute math faster in PHP seems, at a glance, to be a very narrow scope. 

However, this in fact opens the door on things such as machine learning, 3d rendering, 2d (gui) rendering, and data analysis, to name just a few.

Why can't we have this in PHP 7.4 ?

I just called the JIT "the ultimate trade-off", and I think it is: It's arguably one of the most complex compiler strategies ever invented, maybe the most complex. To introduce a JIT is to introduce considerable complexity.

If you ask Dmitry (the author of the JIT) if he made PHP complex, he would say "No, I hate complexity" (that's a direct quote).

At bottom, complex is anything we do not understand, and at the moment, there are very few internals developers (less than a handful) that truly understand the implementation of JIT that we have.

PHP 7.4 is coming up fast, merging into PHP 7.4 would leave us with a version of PHP that less than a handful of people could debug, fix, or improve (in any real sense). This is just not an acceptable situation for those people that voted no on merging into PHP 7.4.

In the time between now and PHP 8, many of us will be working in our spare time to understand the JIT: We still have features we want to implement and tools we need to rewrite for PHP 8, and first we must understand the JIT. We need this time, and are very grateful that a majority of voters saw fit to give it to us.

Complex is not synonymous with horrible: Complex can be beautiful, like a nebula, and the JIT is that kind of complex. You can, in principle, fully understand something complex and only make a marginal reduction in the apparent complexity of that thing. In other words, even when there are 20 internals developers who are as familiar with the JIT as Dmitry is, it doesn't really change the complex nature of JIT.

Will development of PHP slow down ?

There's no reason to think that it will. We have enough time that we can say with confidence that by the time PHP 8 is generally available, there will be enough of us familiar with the JIT to function at least as well as we do today when it comes to fixing bugs and pushing PHP forward.

When trying to square this with the view that a JIT is inherently complex, consider that the majority of our time spent on introducing new features is actually spent discussing that feature. For the majority of features, and even fixes, code may take in the order of minutes or hours to write, and discussions take in the order of weeks or months. In rare cases, the code for a feature may take in the order of hours or days to write, but the discussion will always take longer in those rare cases.

That's all I have to say about that ... 

Enjoy your weekend.

Wednesday, 13 February 2019

Parallel PHP: The Next Chapter


Some years ago, to prove some people on the internet wrong, and because I had a break from normal work - the first such break in years - I decided to write pthreads. My memory fails me a little, but from what I can recall, nobody actually saw that first version, I developed the idea over the following weeks and months and was allowed to publish this work to PECL. It was my introduction to serious internals programming.

The thing I was proving wrong is that PHP is not designed to be used in threads: This is flat out wrong and has been since the 22nd of May in the year 2000, when TSRM was merged into PHP. TSRM - Thread Safe Resource Manager allows you to build PHP such that it can be embedded in a threaded server, like Apache. These builds are colloquially referred to as ZTS - Zend Thread Safe. PHP is very much designed to be used in threads. You may hear people say that TSRM is unstable, or that it's not safe ... there were mistakes in the original implementation, like any software. But it is safe, and it is theoretically sound, and it has been in use on Windows as the primary mode of executing PHP since shortly after it was merged, necessarily so because of a lack of support for proper forking.

What it's not designed to do, or rather I should say what has been given no attention except by me, is exposing threads to userland. The reason for this is the architecture that PHP has, often referred to as "share nothing" seems to be antithetical to threads: Normally, when you start a thread, it operates in the same address space as the thread or process that created it, they share everything. I've been using threads in other languages for a very long time, and the very thing that other people think makes PHP an unsuitable candidate makes me think it more suitable than other languages. The thing that makes programming with threads hard is precisely that they share data, the cognitive overhead increases with each additional thread you create. The models you have to build in your head become unreasonable and prone to mistakes. That is why more modern languages like Go try to hide all of the complexities of threading behind a nice simple API, so that the programmer doesn't actually need to learn about the intricacies of how to use a condition variable, or a mutex, or when to synchronize, they only have to learn how the simple API works.

Having never written a threading API before pthreads, and being left entirely on my own to do it even when the code became public, maybe I made some questionable decisions. I couldn't accept that using parallelism could be easy, I would repeat like a mantra that threading is hard and API's can't solve that. I wanted pthreads to expose the same kind of API that Java has, and my focus could not be shifted by reason. I vaguely remember the first time I went into IRC on internals to talk about pthreads, and people, including Rasmus, tried to reason with me that I was maybe making a mistake, that threading at the frontend of a website doesn't make sense, others said they would have preferred a simpler API ... these pleas fell on deaf ears, and I regret it. I spent many hundreds, possibly thousands of hours writing and rewriting pthreads until it is what you see today, a kind of monster that about 4 people really understand excluding myself, that only the same number of projects have really managed to deploy with any success.

A slight tangent: Threading at the frontend of a website, on the face of it, doesn't make sense: If you have a web request that creates a "reasonable" number of threads, let's say 8, and 1000 clients come along at once, you are asking your hardware to execute 8*1000 threads not including the threads or processes that done the creation, this is unreasonable and cannot possibly scale (with a 1:1 threading model, which is what pthreads has). That said, other languages do manage to integrate parallelism into the web response, but it's tricky, and takes a lot of thought and expertise. I've never suggested that you should build software in this way, and would never suggest it, eventually I prohibited the use of pthreads in anything but the CLI in an attempt to force users to be reasonable.

Tangent over: The great thing about being wrong, and acknowledging that you are wrong, is that you have the opportunity to learn, and do better. If we were never wrong, programming would be boring, we would just chug out code for our entire lives, and miss out on the feeling of truly understanding something for the first time, a feeling I love and cherish, and that keeps me at my keyboard.

I've been aware of my mistakes for some time, but pthreads does have some large projects relying on it, and mostly it's development is controlled by other people now. Occasionally I will give advice or commit something, but I can't, in good conscience, tell anyone to use it, it's simply too hard.

While aware, there was nothing driving me to write another API, until recently when Zend made their intention to merge the JIT  they have been working on for years into PHP. Just think for a moment what it would mean to be able to execute machine code in parallel in user land PHP ... this is not a thing I could have ever imagined happening all those years ago, but it would seem a possibility in the not too distant future.

pthreads has to jump through so many hoops to make threads work and provide the API that it does, that it's not reasonable to talk about it being able to execute machine code.

Recently, I set to work on a new threading API, named Parallel, it is not an exact clone of any existing threading API, it is an API focused on being simple and hiding the complexity inherent in utilising parallelism in your application, it is also focused on being forward compatible with the JIT, for that day when we can actually execute machine code in userland and in parallel.

I should mention now that the current implementation of the JIT, which is not finished, doesn't actually have proper support for TSRM, a fact that only became evident in the past few days. However, the conversation some of us had with Dmitry (the author of the JIT) about the lack of support for ZTS in JIT has inspired him to look at ways to make ZTS with and without a JIT much more efficient. So, it's highly likely that this support will come, although I can't say when.

While the JIT is an exciting prospect for parallelism, I'm also excited to be able to provide a really nice, simple API, that any PHP programmer can understand and use. For the next year at the very least, the JIT doesn't exist for most PHP developers, and you can get to work getting to know how to use parallel ...

Parallel is not complete, but is stable: More features are planned, something like Go's channels would be a nice addition, and I've already started to think about and discuss the implementation of this with other internals developers.

I wish you all the best of luck ... and for those people thinking about using pthreads for new code: don't be silly.

Monday, 28 January 2019

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 :)

Sunday, 27 January 2019

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 ...

Sunday, 3 June 2018

Preface to idbg

Fig 1. A tweet from earlier this month
We already have several options for debugging code within the PHP ecosystem. XDebug is extremely mature software, and phpdbg has been slowly gaining traction also, if for no other reason than it's very fast to collect code coverage compared to XDebug.

Although phpdbg and XDebug are different from one another, they have some things in common: They are both complicated (to varying degrees), and they are both written in a language that 97% of the people reading this text do not understand (number pulled from air, based on nothing at all).

Why do we need a debugger at all?

Slightly tangential perhaps ... Debugging is a necessary part of writing code; If you disagree with this statement, then I don't know what you are talking about.

If debugging to you means sprinkling code with debugging statements like var_dump, or print_r, then I implore you to learn how to use a debugger; You are wasting a lot of time. I don't say this from a position of arrogance because I happen to be one of the authors of phpdbg, but from a position of experience; I remember trying to write code before I had a good handle on using a debugger. 

Sprinkling code with debugging statements is like crouching on a stool in the corner of a cockroach infested room and hoping that blowing upon the blanketed floor will destroy and eliminate the little beasts creating the blanket.

Using a debugger is like having an army of nano bots at your disposal, each one trained exquisitely in a top nano-bot-training-camp, they live to kill cockroaches, some of them also have mean looking tattoos, chew tobacco, and spit on the ground at the start of every sentence ...

I think we understand each other ...

Why do we need a debugger written in PHP ?

Here are some statistics (from github api):
  • XDebug has had 50 contributors in the 7 years it has been on github
  • phpdbg has had a handful of contributors (20-30) in the 4 and a half years it has been on github
  • PHPUnit has had 342 contributors in the 8 and a half years it has been on github
  • phpstan has had 70 contributors in the 2 and a half years it has been on github
XDebug predates it's github repository (by a very wide margin), still it doesn't matter for the point I'm trying to make here: In the PHP ecosystem, we have very many very talented programmers, with a whole host of knowledge about how the PHP engine works - they may have been using it for their entire professional career - they are able to write and contribute to arguably comparatively complicated software like PHPUnit or phpstan. Alas we have vanishingly few programmers in the ecosystem that are able to improve, fix, or develop in any way software like phpdbg or XDebug, and I think it's mostly because of the language they are written in.

You might also just like to scan the number of contributors to projects like Laravel and Symfony ... although I think these numbers less relevant, they are surprisingly high.

It's not all about the language, the domain specific knowledge required to implement a debugging engine might not be so disseminated. But maybe it doesn't need to be ...

You may not find these arguments convincing, you may not be convinced that we need another debugger written in any language, after all XDebug is extremely mature, and using phpdbg makes you at least 20% cooler (in the same way as go faster stripes make any vehicle 20% faster). That's a perfectly rationale position to take, and I can't think of another way to argue my case, you can probably stop reading ;)

Domain Specifics

I don't know how obvious it is that it's not reasonable to talk about implementing a debugger entirely in PHP; The kind of control you need over the engine just isn't reasonably attained in userland by default.

The debugger itself, the thing that interacts with a person or an ide can be written in PHP alone, and is much easier to write in PHP. But the core of the "debugging engine" (terminology borrowed from dbgp specification) should be written in C.

krakjoe/inspector is a disasembler and debugging kit for PHP, it exposes the necessary API for the development of a debugger in userland. It is an advanced extension of the existing Reflection API, giving it a shallow learning curve for anyone already familiar with Reflection.

While it's annoying that we still must have a binary dependency, I'm hoping that inspector becomes a defacto part of php installations in the not too distant future. Although I have no intention of making an RFC to merge inspector into core - it belongs outside of core, the release cycle in core does not lend itself to new software and there is nothing to be gained by merging. Being a defacto part of installations doesn't necessarily mean merged into core.

Code or STFU

Fig 1. idbg help
This isn't just pipe dreams, the PHP code exists, it's alpha quality and largely untested ...

There is much to do and you shouldn't design your workflow around this (or any alpha quality software) yet.

What you should do is start reading code, testing, and opening pull requests ... consider me waiting ...