Even though using Linux has never been easier, newcomers often hit a confusing roadblock: the overwhelming choice of a distribution. Someone coming from Windows or MacOS will find the very concept of a ‘distribution’ to be alien, and the endless listicles and user arguments about which one is ‘best’ only make the confusion worse.

This article aims to clear up that confusion by providing a better way to think about Linux choices, so you won’t be overwhelmed by all the different options if you’re thinking of switching.

I’m delighted to share my paper I’ve presented at the IEEE/IFIP International Conference on Dependable Systems: ‘Be My Guest: Welcoming Interoperability into IBC-Incompatible Block­chains’.

It introduces the concept of a guest block­chain which runs on top of a block­chain and provides features necessary to support the Inter-Blockchain Communication (IBC) protocol. This enables trustless cross-chain interoperability between blockchains which would otherwise not support IBC-based communication. We demonstrate our approach by deploying the guest blockchain on Solana connecting it to the Cosmos ecosystem with performance comparable to native IBC implementations.

In the previous post, I criticised Rust’s contribution process, where a simple patch languished due to communication hurdles. Rust isn’t unique in struggling with its process. This time, the story is about Python.

Parsing HTML in Python

As its name implies, the html.parser module provides interfaces for parsing HTML documents. It offers an HTMLParser base class users can extend to implement their own handling of HTML markup. Of our interest is the unknown_decl method, which ‘is called when an unrecognised declaration is read by the parser.’ It’s called with an argument containing ‘the entire contents of the declaration inside the <![...]> markup.’ For example:

from html.parser import HTMLParser

class MyParser(HTMLParser):
    def unknown_decl(self, data: str) -> None:
        print(data)

parser = MyParser()
parser.feed('<![if test]>')
    # Prints out: if test
    # (unless Python 3.13.4+, see below)
parser.feed('<![CDATA[test]]>')
    # Prints out: CDATA[test

In November 2015, rmcgibbo opened Twine Issue #153. Less than two months later, he closed it with no explanation. The motive behind this baffling move might have remained an unsolved Internet mystery if not for one crucial fact: someone asked and rmcgibbo was willing to talk:

thedrow on Dec 31, 2015

Contributor

Were you able to resolve the issue?

---

rmcgibbo on Dec 31, 2015

Author

No. I decided I don’t care.

We all had such moments, and this humorous exchange serves as a reminder that certain matters are not worth stressing about. Like Marcus Aurelius once said, ‘choose not to be harmed — and you won’t feel harmed.’ However, instead of discussing philosophy, I want to bring up some of my experiences to make a point about contributions to free software projects.

The two pull requests

Rather than London and Paris, this tale takes place on GitHub and linux-kernel mailing list. The two titular pull requests (PRs) are of my own making and contrast between them help discuss and critique Rust’s development process.

In my previous post, I mentioned an ancient C++ technique of using ((X*)0)->f() syntax to simulate static methods. It ‘works’ by considering things from a machine code point of view, where a non-virtual method call is the same as a function call with an additional this argument. In general, a well-behaving obj->method() call is compiled into method(obj). With the assumption this is true, one might construct the following code:

struct Value {
    int safe_get() {
        return this ? value : -1;
    }
    int value;
};

void print(Value *val) {
    printf("value = %d", val->safe_get());
    if (val == nullptr) puts("val is null");
}

Will it work as expected though?

Draw an arbitrary triangle with corners A, B and C. (Bear with me; I promise this is a post about undefined behaviour). Draw a line parallel to line BC that goes through point A. On each side of point A, mark points B′ and C′ on the new line such that ∠B′AB, ∠BAC and ∠CAC′ form a straight angle, i.e., ∠B′AB + ∠BAC + ∠CAC′ = 180°.

Observe that line AB intersects two parallel lines: BC and B′C′. Via proposition 29, ∠B′AB = ∠ABC. Similarly, line AC intersects those lines, hence ∠C′AC = ∠ACB. We now get ∠BAC + ∠ABC + ∠ACB = ∠BAC + ∠B′AB + ∠C′AC = 180°. This proves that the sum of interior angles in a triangle is 180°.

Now, take a ball whose circumference is c. Start drawing a straight line of length c/4 on it. Turn 90° and draw another straight line of length c/4. Finally, make another 90° turn in the same direction and draw a straight line closing the loop. You’ve just drawn a triangle whose internal angles sum to over 180°. Something we’ve just proved is impossible‽

There is no secret. Everyone sees what is happening. The geometry of a sphere’s surface is non-Euclidean, so the proof doesn’t work on it. The real question is: what does this have to do with undefined behaviour?

‘Ctrl+D in terminal is like pressing Enter,’ Gynvael claims. A surprising proclamation, but pondering on it one realises that it cannot be discarded out of hand. Indeed, there is a degree of truth to it. However, the statement can create more confusion if it’s made without further explanations which I provide in this article.

To keep things focused, this post assumes terminal in canonical mode. This is what one gets when running bash --noediting or one of many non-interactive tools which can read data from standard input such as cat, sed, sort etc. Bash, other shells and TUI programs normally run in raw mode and provide their own line editing capabilities.

Mark Rober’s video where he fooled Tesla car into hitting fake road has been making rounds on the Internet recently. It questions Musk’s decision to abandon lidars and adds to the troubles Tesla has been facing. But it also hints at a more general issue with automation of complex systems and artificial intelligence.

A drawing of a Tesla car with self-driving technology engaged.

‘Klyne and I belong to two different generations,’ testifies Pirx in Lem’s 1971 short story Ananke. ‘When I was getting my wings, servo-mechanisms were more error-prone. Distrust becomes second nature. My guess is… he trusted in them to the end.’ The end being the Ariel spacecraft crashing killing everyone onboard. If Klyne put less trust in ship’s computer, could the catastrophe be averted? We shall never know, mostly because the crash is a fictional story, but with proliferation of so called artificial intelligence, failures directly attributed to it has happened in real world as well:

In 2004, Brady Haran published the infamous Astounding: \(1 + 2 + 3 + 4 + 5 + \cdots = -\frac{1}{12}\) video in which Dr Tony Padilla demonstrates how sum of all natural numbers equals minus a twelfth. The video prompted a flurry of objections from viewers rejecting the result. But riddle me this: Would you agree the following equation is true: \[1 + \frac{1}{2} + \frac{1}{4} + \cdots = 2\]

Obviously the equation does not hold. How could it? The thing on the left side of the equal sign is an infinite series. The thing on the right side is a real number. Those are completely different objects therefore they cannot be equal. Anyone saying the two are equal might just as well say \(\mathbb{N} = 🐘\) (and while I grant you that elephants are quite large, they’re not sets of natural numbers).

And yet, people usually agree the infinite sum equals 2. Why is that? They use ‘mathematical trickery’ and redefine the meaning of the equal sign. Indeed, \(1 + \frac{1}{2} + \frac{1}{4} + \cdots\) does not equal 2. Rather, given an infinite series \((a_n)\) where \(a_n = 2^{-n}\), we can define an infinite series \((S_n)\) where \(S_n = \sum_{0}^{n} a_n\) and only now we get: \[\lim_{n\to\infty} S_n = \lim_{n\to\infty} 2 - \frac{1}{2^n} = 2\]

But that’s a different equation than the one I’ve enquired about.

Even though Bitcoin is technically Turing complete, in practice implementing a non-trivial computation on Bitcoin is borderline impossible.¹ Only after Ethereum was introduced, smart contracts entered common parlance. But even recent block­chains offer execution environments much more constrained than those available on desktop computers, servers or even home appliances such as routers or NASes.

Things are no different on Solana. I’ve previously discussed its transaction size limit, but there’s more: Despite using the ELF file format, Solana Virtual Machine (SVM) does not support mutable global state.

Meanwhile, to connect Solana and Composable Foundation’s Picasso network, I was working on an on-chain light client based on the tendermint crate. The crate supports custom signature verification implementations via a Verifier trait, but it does not offer any way to pass state to such implementation. This became a problem since I needed to pass the address of the signatures account to the Ed25519 verification code.²

This article describes how I overcame this issue with a custom allocator and how the allocator can be used in other projects. The custom allocator implements other features which may be useful in Solana programs, so it may be useful even for projects that don’t need access to mutable global state.