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 ...