Reflecting on ppbert

I had an itch: I was pretty-printing the BERT-encoded terms that we use in a production system at work and it was very slow. The Erlang shell took more than two minutes to dump the largest file. (It took about 0.1 second to read and parse the file; the rest was spent in io:format.) I decided to scratch that itch: I wrote ppbert, a command-line utility that reads BERT-encoded values and pretty-prints them. I’ve worked sporadically on ppbert for almost a year now, I use it daily at work, I’m happy with it, and I want to write about some of the things I learned during that journey.

I wanted ppbert to be a good Unix tool. It was essential that it can read from stdin, write to stdout, and thus can be inserted in a Unix pipeline. (I’m astounded—and annoyed—when programs can’t be used in a pipe.) The interface of ppbert should be familiar to Unix users: it reads from stdin, from files, or from a mix of both with - acting as the filename for stdin; normal output goes to stdout, diagnostics and errors go to stderr; the format of warnings and errors follows the typical Unix style (ppbert: reason for error); the return code is 0 on success and 1 on failure; ppbert has a manpage. The main “failure” of ppbert as a Unix tool is that it consumes a whole input file before it starts pretty-printing it (a bit like sort(1)).

I wrote ppbert in Rust because I like the language and because I wanted to use it. But if you’d like to hear technical reasons why choosing Rust made sense, here are a few. First, I wanted something that would be faster than Erlang and it would’ve been counter-productive to reach for Python or Ruby; Rust compiles to native code, uses LLVM to apply thousands of optimizations, and many benchmarks shows that the performance of Rust programs is right up there with C and C++. Rust’s type system and ownership/borrowing systems are also right up my alley: I suck at programming, I make mistakes with strings and memory in C all the time, and the Rust bondage helps me write better software. Rust has a very good story for third-party libraries: cargo is an absolute joy to use, and the selection of quality libraries on crates.io is growing quickly (more about good libraries later). I tried to limit the number of dependencies in ppbert—for example, I don’t use serde to pretty-print JSON—, but I like that complex functionality is a single line in Cargo.toml away. Finally, the Rust compiler produces statically-linked executable, which makes it easier to use ppbert on remote servers that don’t have development tools installed: I just transfer the executable with scp.

I’m happy that I chose Rust: I had a faster pretty-printer the first time that I compiled an executable and with some further tweaks, I was able to improve the speed to a level that I’m happy with. Today, I can pretty-print the large BERT file in 3 seconds rather than 2 minutes. I got some of the extra performance gains by applying lessons that one can learn from any language: buffering your output—the File struct in Rust doesn’t buffer by default; finding ways to do less work in the common cases—in ppbert I got a satisfying speed boost by avoiding UTF-8 validation when all the bytes of an atom’s name where in the ASCII range. I also got some speed boosts by reaching for some of Rust’s unsafe functions, for example by using the .get_unchecked() Vec method when the bound checks had already been performed. I found that using perf with Rust is underwhelming; it could tell me in which functions ppbert spent most of its time, but I had to divine the reasons.

I’m quite happy with the third-party crates that I use. I started using nom, a parser combinator library, but I found that using its macros was unnatural and injecting my own errors was awkward, so I stopped using it and wrote my own parser instead. (This was with nom 2; I have not tried the newer nom 3.) For command-line arguments parsing, I like clap: it’s easy to add new options, and easy to consume them. Rayon blew me away! I used it in ppbert’s sibling program, bert-convert, and simply by using .par_iter() rather than .iter(), I made the conversion of all our production .bert files to .bert2 twice as fast. The crates num and encoding, which I use for bigints and latin-1 support respectively, were easy to use and because they do their jobs without problems, I haven’t needed to look at that code in a long time.

Some people lament the state of Rust’s tooling, particularly IDE support. I’m a simple man: I like some syntax highlighting, automatic indentation, jump-to-definition, and auto-completion. Emacs, rust-mode, and racer-mode gave me all that.

One issue I ran into with Rust itself was how the println!() macro behaves in a pipeline: if a pipe is closed early while printing a long string, println panics. The code below exhibits the behaviour in a simple program.

/tmp$ cat sample.rs
fn main() {
    let mut buf = String::new();
    for _ in 0 .. 65536 {
        buf.push_str("foo\n")
    }
    println!("{}", buf);
}

/tmp$ rustc -O sample.rs

/tmp$ ./sample | head -n 1
foo
thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', /checkout/src/libstd/io/stdio.rs:690:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.

There is an easy fix, and that’s to use the writeln!() macro and to ignore errors.

/tmp$ cat sample.rs
use std::io::{self, Write};

fn main() {
    let mut buf = String::new();
    for _ in 0 .. 65536 {
        buf.push_str("foo\n")
    }
    let _ = writeln!(io::stdout(), "{}", buf);
}

/tmp$ rustc -O sample.rs

/tmp$ ./sample | head -n 1
foo

It’s something to be wary of if you write Unix utilities in Rust.

Ppbert is not a popular program—I’m probably its only user—because BERT is not a very popular format. Still, I’m happy with the work I did, I’m happy that I scratched that itch, and I’m happy that ppbert has been a good vehicle for learning about Rust, improving performance, and creating good Unix programs.


Posted in on 2018-02-09.
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