Rust 2018: maybe don't be too stable

I initially did not want to write a post with what I want and foresee for Rust in 2018, because I'm already very happy with it! I have spent more than 4 years tinkering with the language, experimenting, and I love the freedom I get when playing with low level stuff. In those 4 years, I discovered a wonderful, welcoming community and made some awesome friends. So, yes, I'm happy with Rust as it is :)

But some of the recent #Rust2018 posts made me react a bit. I'm interested in learning what other people see in Rust, so I read almost all of them, and there's an easy trend to follow. Rust should be stabilized. Rust should be boring and safe. Crates should be stabilized. We should have definitive crates for some purposes like HTTP clients or async programming.
This is not surprising, since there's already been a lot of focus on stability in 2017, with the impl period, the merge of the Rust epochs RFC, and the fact that more and more companies start relying on Rust.
We want Rust to be appealing to (big(ger)) companies, and to that end we need good compatibility between Rust versions, a high quality ecosystem of crates that work on stable Rust versions. We want newcomers to have a well prepared toolbox for their first projects.

Before that stabilization goal appeared, Rust looked a bit chaotic, with new features coming every 6 weeks, new crates popping up here and there, people hacking something quickly and publishing it the next minute. And this is something I love about this language.
People try stuff, cargo lets them publish it easily, Rust makes sure it's running smoothly. Sure, there's a lot of redundant crates, most of them are far from the big "1.0 stable" target, but it's fine.
This language and its community are full of that unabashed optimism that makes newcomers go "hey, should I really try to write my own kernel? OF COURSE I SHOULD". Should I try to make cool stuff with Web Assembly while it barely landed in nightly? YESSSSSS
I have seen over and over shitposting on twitter that ends up with people hacking on a cool new project. I have seen people publish a crate competing with another well known one, that will then send a PR for their idea to the bigger crate the next day.
I am overly enthusiastic about this, to the point that opening /r/rust often feels like Christmas: what new toys will we get today?

So, to be clear, I am all for getting more stuff stable. We need a stable, asynchronous hyper. We need futures to work. We need impl trait and various other Rust features that will appear in the following months or years. What we do not need is the attitude that wants everything to crystallize.
How many times have I seen people criticising the "yet another" asynchronous IO/command line argument system/web framework/parser, with the usual arguments that this is lost focus, redundant, that why didn't they try to do that in $BIG_PROJECT. This is fine.
Go on, make other parser libraries to compete with nom, keep me on my toes. Try other approaches than tokio. Test different approaches to writing web applications.

The underlying idea for me is that Rust is still incredibly young, extremely enthusiastic, and we still don't fully know how to write Rust. So, yes, we need some parts of Rust to stabilize, but we must balance that with its movement. What is stable and "the way we do things" now might not be the way to go in a year or so.

Let people experiment and lose focus. Keep hacking on cool stuff.

Adventures in logging

After working on the Sōzu HTTP reverse proxy for a while, I came up with an interesting approach to logging. Now why would I come up with my own logger, when there are existing solutions in Rust? Mainly, log and slog. That logging library grew up from testing things out with log, and changing requirements along the way.

Beginning with log and env_logger

Like a lot of other Rust developers, I started out with log and env_logger:


#[macro_use]
extern crate log;
extern crate env_logger;

fn main() {
env_logger::init().unwrap();

info!("starting up");
}

It's nice and easy: every library that depends on log will use that same set of logging macros (error, info, warn, debug, trace) that will use whatever global logger was defined. Here we use env_logger to define one.

env_logger is useful because it can apply a filter to the log, from an
environment variable:

[code lang=text]
# will show the logs above info level
RUST_LOG=info ./main

# will show the logs above info level, but also logs above debug level
# for the dependency 'dep1'
RUST_LOG=info,dep1=debug ./main
[/code]

You can also define the filter by module or apply a regular expression.

Custom formatter

env_logger allows you to build your own log formatter. This feature is especially important for me, as I like to add metadata to my logs.

Defining a custom formatter with env_logger is quite straightforward:

[code lang=text]
let format = |record: &LogRecord| {
format!("{} - {}", record.level(), record.args())
};

let mut builder = LogBuilder::new();
builder.format(format).filter(None, LogLevelFilter::Info);

if env::var("RUST_LOG").is_ok() {
builder.parse(&env::var("RUST_LOG").unwrap());
}

builder.init().unwrap();
[/code]

It is easily combined with the filtering and usage of the RUST_LOG environment variable.

Where things get annoying: reducing allocations

If you take a look at env_logger, you'll realize that it will allocate a String for every log line that will be written, using a formatting closure.

Let's get one thing out of the way first: I completely agree with the idea you should not try to optimize stuff too much. But I'm in the case of a networking component that will handle a lot of traffic. I had debugging sessions where I generated tens of gigabytes of logs in a few seconds, and needed almost all of them, to debug async IO issues. In those cases, the time spent allocating and deallocating log lines becomes relevant.

So, how would I get a custom log formatter that does not allocate much? As it turns out, when you tell log to use your logger with log::set_logger, it requires something that implements Log. The logger's log method receives a LogRecord, a structure that's created on the fly from LogLocation, LogMetadata and Arguments.
The first two are internal to log, I can't create them myself. The last one is interesting.

Arguments can be created from the format_args macro. That structure will roughly contain the format string split in the various substrings that appear between arguments. if you do println!("hello {}!", name), you would get a structure that contains "hello ", the content of name and "!". println! and other macros use this.

You can then use that Arguments with io::Write::write_fmt to write it directly to, say, a file or a socket. And it is implemented so that the individual parts are written one after another instead of allocating one big string.

So, how do I use that?

Well, it turns out that, basically, I can't. If I implement Log, I can get a Logrecord which gives me a &Arguments, while write requires a Arguments. So now I have to clone it, which defeats a bit the purpose.

So let's write our own then

There was another reason for the custom logging library: using a custom logging backend. Having the option between stdout and stderr is fine, but I might want to send them to a file or a socket.

So I started writing a specific logging library for sozu. First, copying the log filtering from env_logger. That part is mostly straightforward, but that's still a lot of code to copy around.

The logging macros specify a logging level then they all call the same common macro.

[code lang=text]
#[macro_export]
macro_rules! error {
($format:expr, $($arg:tt)*) => {
log!($crate::logging::LogLevel::Error, $format, "ERROR", $($arg)*);
};
($format:expr) => {
log!($crate::logging::LogLevel::Error, $format, "ERROR");
};
}
[/code]

The main logging macro has two interesting parts. First, we define some static metadata (that's coming from the log crate):

[code lang=text]
static _META: $crate::logging::LogMetadata = $crate::logging::LogMetadata {
level: $lvl,
target: module_path!(),
};
[/code]

That object will not be allocated over and over, all the data in there will be defined at compile time.

Then we call the logger itself (ignore the line with try_lock for now):

[code lang=text]
if let Ok(mut logger) = LOGGER.try_lock() {
logger.log(
&_META,
format_args!(
concat!("{}\t{}\t{}\t{}\t{}\t", $format, '\n'),
::time::now_utc().rfc3339(), ::time::precise_time_ns(), *$crate::logging::PID,
$level_tag, *$crate::logging::TAG));
}
[/code]

So we give this metadata structure to our logger, then we make an Arguments structure with format_args!. The concat! macro is there to concatenate the formatting string with the custom prefix. That way, I could write debug!("hello {}", name) and have the resulting format string be "{}\t{}\t{}\t{}\t{}\thello {}\n", generated at compile time and transformed through the format_args call.

I added the date in ISO format, along with a monotonic timestamp (that becomes handy when multiple workers might write logs concurrently), the process identifier, the log level and a process wide logging tag (to better identify workers).

So this starts looking good, right? Now how do we write this to configurable backends? Some backends already implement io::Write, others will need an intermediary buffer:

[code lang=text]
pub fn log<'a>(&mut self, meta: &LogMetadata, args: Arguments) {
if self.enabled(meta) {
match self.backend {
LoggerBackend::Stdout(ref mut stdout) => {
stdout.write_fmt(args);
},
LoggerBackend::Unix(ref mut socket) => {
socket.send(format(args).as_bytes());
},
LoggerBackend::Udp(ref mut socket, ref address) => {
socket.send_to(format(args).as_bytes(), address);
}
LoggerBackend::Tcp(ref mut socket) => {
socket.write_fmt(args);
},
}
}
}
[/code]

For Unix sockets and UDP, instead of allocating on the fly, it should probably use a buffer (hey, anyone wants to implement that?). Stdout and a TcpStream can be written to directly. Adding buffers might still be a good idea here, depending on what you want, because that write could fail. Would you like a logger that will send a partial log if it can't write on the socket, or one using a buffer that can be filled up?

So, now, what's next? Originally, sozu worked as one process with multiple threads, but evolved as a bunch of single threaded processes. But that raises an interesting question. How do you write logs concurrently?

Highly concurrent logs

It turns out that problem is not really easy. Most solutions end up in this list:

  • every thread or process writes to stdout or a file at the same time
  • synchronized access to the logging output
  • one common logger everybody sends to
  • every thread or process has its own connection to syslog (or even its own file to write to)

The first solution is easy, but has a few issues. First, writing to stdout is slow, and it can quickly overwhelm your terminal (yes, I know you can redirect to a file). Second, it's not really synchronized, so you might end up with incoherently interleaved log lines.

So we often move to the second solution, where access to the logging stream is protected by a mutex. Now you get multiple threads or processes that might spend their time waiting on each other for serializing and writing logs. Having all threads sharing one lock can quickly affect your performance. It's generally a better idea to have every thread or process running independently from the others (it's one of the principles in sozu's architecture, you can learn more about it in this french talk).

Alright then, moving on to the third solution: let's have one of the threads or processes handle the logging, and send the logs to it via cross thread channels or IPC. That will surely be easier than having everybody share a lock, right? This is also intersting because you can offload serialization and filtering to the logging thread. Unfortunately, that means one thread will handle all the logs, and it can be overwhelming. That also means a lot of data moving between workers (if using processes).

The last solution relies on external services: use the syslog daemon on your system, or a log aggregator somewhere on another machine, that every worker will send the logs to. Let that service interleave logs, and maybe scale up if necessary. Since in a large architecture, you might have such an aggregator, talking to it directly might be a good idea (oh hey BTW I wrote a syslog crate if you need).

With sozu, I ended up with a mixed solution. You can send the logs to various backends. If you choose stdout, then all workers will write to it directly without synchronization which will be mostly fine if you don't have a large traffic. But if you want, each worker can open its own connection to the logging service.

Now that concurrency is taken care of, there's a last issue that has annoyed me for months: how to use the logger from everywhere in the code, when it's obviously one big global mutable object?

the dreaded logging singleton

One thing I like about the macros from the log crate: you can use them anywhere in your code, and it will work. The other approach, used in languages like Haskell, or proposed by slog, is to carry your logger around, in function arguments or structure members. I can understand the idea, but I don't like it much, because I'll often need to add a debug call anywhere in the code, and when it's deep in a serie of five method calls, with those methods coming from traits implemented here and there, updating the types quickly gets annoying.

So, even if the idea of that global mutable logger singleton usually looks like a bad pattern, it can still be useful. In log, the log macro calls the log::Log::log method, getting the logger instance from the logger method. That method gets the logger instance from a global pointer to the logger, with an atomic integer used to indicate if the logger was initialized:

[code lang=text]
static mut LOGGER: *const Log = &NopLogger;
static STATE: AtomicUsize = ATOMIC_USIZE_INIT;

const UNINITIALIZED: usize = 0;
const INITIALIZING: usize = 1;
const INITIALIZED: usize = 2;

[...]

pub fn logger() -> &'static Log {
unsafe {
if STATE.load(Ordering::SeqCst) != INITIALIZED {
static NOP: NopLogger = NopLogger;
&NOP
} else {
&*LOGGER
}
}
}
[/code]

So how can that global mutable pointer be used from any thread? That's because the Log trait requires Send and Sync. As a reminder, Send means it can be sent to another thread, Sync means it can be shared between threads.

That's a cool trick, but usually we employ another pattern:

[code lang=text]
lazy_static! {
pub static ref LOGGER: Mutex<Logger> = Mutex::new(Logger::new());
pub static ref PID: i32 = unsafe { libc::getpid() };
pub static ref TAG: String = LOGGER.lock().unwrap().tag.clone();
}
[/code]

lazy_static allows you to define static variables that will be initialized at runtime, at first access (using the same pattern as log with std::sync::Once. Since our logger's log method needs an &amp;mut, it's wrapped in a Mutex. That's where the call to try_lock comes from.

We might think the mutex is costly compared to log's solution, but remember that the logger instance has to be Sync, so depending on your implementation, there might be some synchronization somewhere. Except that for sozu, it's not the case! Each worker is single threaded, and has its own instance of the logger (possibly with each of them a connection to the logging service). Can't we have a logging system that does not require that mutex used everywhere?

Removing the last Mutex

This is a problem that annoyed me for months. It's not that I really mind the cost of that mutex (since no other thread ever touches it). It's just that I'd feel better not using one when I don't need it :)

And the solution to that problem got quite interesting. To mess around a bit, here's a playground of the logging solution based on lazy_static. You'll see why the code in Foo::modify is important.

There's a feature you can use to have a global variable available anywhere in a thread: thread_local. It uses thread local storage with an UnsafeCell to initialize and provide a variable specific to each thread:

[code lang=text]
thread_local!(static FOO: RefCell<u32> = RefCell::new(1));

FOO.with(|f| {
assert_eq!(*f.borrow(), 1);
*f.borrow_mut() = 2;
});
[/code]

So I tried to use this with my logger, but encountered an interesting bug, as you can see in another playground. I replaced my logging macro with this:

[code lang=text]
#[macro_export]
macro_rules! log {
($format:expr, $($arg:tt)+) => ({
{
LOGGER.with(|l| {
l.borrow_mut().log(
format_args!(
concat!("\t{}\t", $format, '\n'),
"a", $($arg)+));
});
}
});
}
[/code]

And got this error:

[code lang=text]
error[E0502]: cannot borrow `self` as immutable because `self.opt.0` is also borrowed as mutable
--> src/main.rs:36:21
|
36 | LOGGER.with(|l| {
| ^^^ immutable borrow occurs here
...
54 | if let Some(ref mut s) = self.opt {
| --------- mutable borrow occurs here
55 | log!("changing {} to {}", self.bar, new);
| -----------------------------------------
| | |
| | borrow occurs due to use of `self` in closure
| in this macro invocation
56 | }
| - mutable borrow ends here
[/code]

When implementing this inside sozu's source, I got about 330 errors like this one... So what happened? That with method requires a closure. Since we use a macro, if we use self.bar as argument, it will appear inside the closure. That becomes an issue with anything that has been already mutably borrowed somewhere.

I tried a few things, like calling format_args outside the closure, but I get the error error[E0597]: borrowed value does not live long enough. This is apparently a common problem with format_args.

But the real solution came from tomaka, with some macro trickery, as seen in one last playground:

[code lang=text]
#[macro_export]
macro_rules! log {
($format:expr $(, $args:expr)*) => ({
log!(__inner__ $format, [], [a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v]
$(, $args)*)
});

(__inner__ $format:expr, [$($transformed_args:ident),*], [$first_ident:ident $(, $other_idents:ident)*], $first_arg:expr $(, $other_args:expr)*) => ({
let $first_ident = &$first_arg;
log!(__inner__ $format, [$($transformed_args,)* $first_ident], [$($other_idents),*] $(, $other_args)*);
});

(__inner__ $format:expr, [$($final_args:ident),*], [$($idents:ident),*]) => ({
LOGGER.with(move |l| {
//let mut logger = *l.borrow_mut();
l.borrow_mut().log(
format_args!(
concat!("\t{}\t", $format, '\n'),
"a" $(, $final_args)*));
});
});
}
[/code]

The basic idea is that we could avoid the borrowing issue by doing an additional borrow. But since some of the arguments might by expressions (like 1+1 or self.size), we will store the reference to it in a local variable, with let $first_ident = &amp;$first_arg;.

We cannot create variable or function names in macros out of thin air (sadly, because that would be extremely useful), so we instead do recursive macros calls, consuming arguments one after another.
In [$($transformed_args:ident),*], [$first_ident:ident $(, $other_idents:ident)*], $first_arg:expr $(, $other_args:expr)*,
transformed_args accumulates the idents (variable names) in which we stored the data.

[$first_ident:ident $(, $other_idents:ident)*] is matching on the list that started as [a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v], to get the first one in that list (storing it in $first_ident), and using it as variable names. As you might have guessed, that means I won't be able to use a log line with more than 21 arguments. That's a limitation I can live with.

The $first_arg:expr $(, $other_args:expr)* part matches on the log call's arguments, and gets the first in the list as $first_arg. We then use those in the line let $first_ident = &amp;$first_arg; and recursively call log!, adding the variable name $first_ident to the list of transformed arguments, and the rest of the variable names list and the log call's arguments:

[code lang=text]
log!(__inner__ $format, [$($transformed_args,)* $first_ident], [$($other_idents),*] $(, $other_args)*);
[/code]

Once all of the logger's arguments are consumed, we can call format_args on the list of $transformed_args:

[code lang=text]
(__inner__ $format:expr, [$($transformed_args:ident),*], [$($idents:ident),*]) => ({
LOGGER.with(move |l| {
//let mut logger = *l.borrow_mut();
l.borrow_mut().log(
format_args!(
concat!("\t{}\t", $format, '\n'),
"a" $(, $transformed_args)*));
});
});
});
[/code]

and it works!

So, that last part may not be completely relevant to your logger implementation, but I thought
it was quite cool :)

Despite my issues with the log crate, it's quite useful and supported by a lot of libraries. It is currently getting a lot better as part of the libz blitz. I'd also encourage you to check out slog. I haven't felt the need to integrate it in sozu yet, but it can be interesting for new projects, as it comes with an ecosystem of composable libraries to extend it.

How to rewrite your project in Rust

In a previous post, I explained why rewriting existing software in Rust could be a good idea. The main point being that you should not rewrite the whole application, but replace the weaker parts without disturbing most of the code, to strengthen the codebase without disruption.

I also provided pointers to projects where other people and I did it succesfully, but without giving too many details. So let's get a real introduction to Rust rewrites now. This article requires a little bit of knowledge about Rust, but you should be able to follow it even as a
beginner.

As a reminder, here are the benefits Rust bring into a rewrite:

  • it can easily call C code
  • it can easily be called by C code (it can export C compatible functions and structures)
  • it does not need a garbage collector
  • if you want, it does not even need to handle allocations
  • the Rust compiler can produce static and dynamic libraries, and even object files
  • the Rust compiler avoids most of the memory vulnerabilities you get in C (yes, I had to mention it)
  • Rust is easier to maintain than C (this is discutable, but not the point of this article)

As it turns out, this is more or less the plan to replace C code with Rust:

  • import C structures and functions in Rust
  • import Rust structures and functions from C
  • reuse the host application's memory allocations whenever possible
  • write code (yes, we have to do it at some point)
  • produce artefacts that can be linked with the host application
  • integrate with the build system

We'll see how to apply this with examples from the Rust VLC plugin.

Import C structures and functions in Rust

Rust can easily use C code directly, by writing functions and structures definitions. A lot of the techniques you would use for this come from the "unsafe Rust" chapter of "The Rust Programming Language" book. For the following C code:

[code lang=C]
struct vlc_object_t {
const char *object_type;
char *header;
int flags;
bool force;
libvlc_int_t *libvlc;
vlc_object_t *parent;
};
[/code]

You would get the following Rust structure:

[code lang=C]
extern crate libc;
use libc::c_char;

#[repr(C)]
pub struct vlc_object_t {
pub psz_object_type: *const c_char,
pub psz_header: *mut c_char,
pub i_flags: c_int,
pub b_force: bool,
pub p_libvlc: *mut libvlc_int_t,
pub p_parent: *mut vlc_object_t,
}
[/code]

the #[repr(C)] tag indicates to the compiler that the structure should have a memory layout similar to the one generated by a C
compiler. We import types from the libc crate, like c_char. Those types are platform dependent (with their different form already handled in libc). Here, we use a lot of raw pointers (indicated by *), which means by using this structure directly, we're basically writing C, which is no good! A good approach, as we'll see later, is to write safer wrappers above those C bindings.

Importing C functions is quite straightforward too:

[code lang=C]
ssize_t vlc_stream_Peek(stream_t *, const uint8_t **, size_t);
ssize_t vlc_stream_Read(stream_t *, void *buf, size_t len);
uint64_t vlc_stream_Tell(const stream_t *);
[/code]

These C function declarations would get translated to:

[code lang=C]
#[link(name = "vlccore")]
extern {
pub fn vlc_stream_Peek(stream: *mut stream_t, buf: *mut *const uint8_t, size: size_t) -> ssize_t;
pub fn vlc_stream_Read(stream: *mut stream_t, buf: *const c_void, size: size_t) -> ssize_t;
pub fn vlc_stream_Tell(stream: *const stream_t) -> uint64_t;
}
[/code]

The #[link(name = "vlccore")] tag indicates to which library we are linking. It is equivalent to passing a -lvlccore argument to the linker. Libvlccore is a library all VLC plugins must link to. Those functions are declared like regular Rust functions, but like the previous structures, will mainly work on raw pointers.

bindgen

You can always write all your bindings manually like this, but when the amount of code to import is a bit large, it can be a good idea to employ the awesome bindgen tool, that will generate Rust code from C headers.

It can work as a command line tool, but can also work at compile time from a build script. First, add the dependency to your Cargo.toml file:

[code lang=toml]
[build-dependencies.bindgen]
version = "^0.25"
[/code]

You can then write your build script like this:

[code lang=C]
extern crate bindgen;
use std::fs::File;
use std::io::Write;
use std::path::Path;

fn main() {
let include_arg = concat!("-I", env!("INCLUDE_DIR"));
let vlc_common_path = concat!(env!("INCLUDE_DIR"), "/vlc_common.h");

let _ = bindgen::builder()
.clang_arg(include_arg)
.clang_arg("-include")
.clang_arg(vlc_common_path)
.header(concat!(env!("INCLUDE_DIR"), "/vlc_block.h"))
.hide_type("vlc_object_t")
.whitelist_recursively(true)
.whitelisted_type("block_t")
.whitelisted_function("block_Init")
.raw_line("use ffi::common::vlc_object_t;")
.use_core()
.generate().unwrap()
.write_to_file("src/ffi/block.rs");
}
[/code]

So there's a lot to unpack here, because bindgen is very flexible:

  • we use clang_arg to pass the include folder path and pre include a header everywhere (vlc_common.h is included pretty puch everywhere in VLC)
  • the header method specifies the header from which we will import definitions
  • hide_type prevents redefinition of elements we already defined (liek the ones from the common header)
  • whitelisted_type and whitelisted_function specify types and functions for which bindgen will create definitions
  • raw_line writes its argument at the top of the file. I apply it to reuse definitions from other files
  • write_to_file writes the whole definition to the specified path

You can apply that process to any C header you must import. With the build script, it can run every time the library is compiled, but be careful, generating a lot of headers can take some time. It might be a good idea to pregenerate them and commit the generated files, and update them from time to time.

It is usually a good idea to separate the imported definitions in another crate with the -sys suffix, and write the safe code in the main crate.
As an example, see the crates openssl and openssl-sys.

Writing safe wrappers

Previously, we imported the C function ssize_t vlc_stream_Read(stream_t *, void *buf, size_t len) as the Rust version pub fn vlc_stream_Read(stream: *mut stream_t, buf: *const c_void, size: size_t) -&gt; ssize_t but kept an unsafe interface. Since we want to use those functions safely, we can now make a better wrapper:

[code lang=C]
use ffi;

pub fn stream_Read(stream: *mut stream_t, buf: &mut [u8]) -> ssize_t {
unsafe {
ffi::vlc_stream_Read(stream, buf.as_mut_ptr() as *mut c_void, buf.len())
}
}
[/code]

Here we replaced the raw pointer to memory and the length with a mutable slice. We still use a raw pointer to the stream_t instance, maybe we can do better:

[code lang=C]
use ffi;

pub struct Stream(*mut stream_t);

pub fn stream_Read(stream: Stream, buf: &mut [u8]) -> ssize_t {
unsafe {
ffi::vlc_stream_Read(stream.0, buf.as_mut_ptr() as *mut c_void, buf.len())
}
}
[/code]

Be careful if you plan to implement Drop for this type: is the Rust code supposed to free that object? Is there some reference counting involved? Here is an example of Drop implementation from the openssl crate:

[code lang=C]
pub struct SslContextBuilder(*mut ffi::SSL_CTX);

impl Drop for SslContextBuilder {
fn drop(&mut self) {
unsafe { ffi::SSL_CTX_free(self.as_ptr()) }
}
}
[/code]

Remember that it's likely the host application has a lot of infrastructure to keep track of memory, and as a rule, we should reuse the tools it offers for the code at the interface between Rust and C. See the Rust FFI omnibus for more examples of safe wrappers you can write.

Side note: as of now (2017/07/10) custom allocators are still not stable

Exporting Rust code to be called from C

Since the host application is written in C, it might need to call your code. This is quite easy in Rust: you need to write unsafe wrappers.

Here we will use as example the inverted index library for mobile apps I wrote for a conference. In this library, we have an Index type that we want to use from Java. Here is its definition:

[code lang=C]
#[repr(C)]
pub struct Index {
pub index: HashMap<String, HashSet<i32>>,
}
[/code]

This type has a few method we want to provide:

[code lang=C]
impl Index {
pub fn new() -> Index {
Index {
index: HashMap::new(),
}
}

pub fn insert(&mut self, id: i32, data: &str) {
[...]
}

pub fn search_word(&self, word: &str) -> Option<&HashSet<i32>> {
self.index.get(word)
}

pub fn search(&self, text: &str) -> HashSet<i32> {
[...]
}
}
[/code]

First, we need to write the functions to allocate and deallocate our index. Every use from C will be wrapped in a Box.

[code lang=C]
#[no_mangle]
pub extern "C" fn index_create() -> *mut Index {
Box::into_raw(Box::new(Index::new()))
}
[/code]

The Box type indicates and owns a heap allocation. When the box is dropped, the underlying data is dropped as well and the memory is freed. The following function takes ownership of its argument, so it is dropped at the end.

[code lang=C]
#[no_mangle]
pub extern "C" fn index_free(ptr: *mut Index) {
let _ = unsafe { Box::from_raw(ptr) };
}
[/code]

Now that allocation is handled, we can work on a real method. The following method takes an index, and id for a text, and the text itself, as a C string (ie, terminated by a null character).

Since we're kinda writing C in Rust here, we have to first check if the pointers are null. Then we can transform the C string in a slice. Then we check if it is correctly encoded as UTF-8 before inserting it into our index.

[code lang=C]
#[no_mangle]
pub extern "C" fn index_insert(index: *mut Index, id: i32, raw_text: *const c_char) {
unsafe { if index.is_null() || raw_text.is_null() { return } };
let slice = unsafe { CStr::from_ptr(raw_text).to_bytes() };
if let Ok(text) = str::from_utf8(slice) {
(*index).insert(id, text);
}
}
[/code]

Most of the code for those kinds of wrappers is just there to transform between C and Rust types and checking that the arguments coming from C code are correct. Even if we have to trust the host application, we should program defensively at the boundary.

There are other methods we could implement for the index, we'll leave those as exercise for the reader :)

Now, we need to write the C definitions to import those functions and types:

[code lang=C]
typedef struct Index Index;

Index* index_create();
void index_free(Index* index);
void index_insert(Index* index, int32_t id, char const* raw_text);
[/code]

We defined Index as an opaque type here. Since Rust structures can be compatible with C structures, we could export the real type, but since it only contains a Rust specific type, HashMap, it is better to hide it completely and write accessors and wrappers.

Generating bindings with rusty-cheddar

Writing function imports from C to Rust is tedious, so we have bindgen for this. We also have a great tool to go the other way: rusty-cheddar.

In the same way, it can be used from a build script:

[code lang=C]
extern crate cheddar;

fn main() {
cheddar::Cheddar::new().expect("could not read definitions")
.run_build("include/main.h");
cheddar::Cheddar::new().expect("could not read definitions")
.module("index").expect("malformed module path")
.insert_code("#include \"main.h\"")
.run_build("include/index.h");
}
[/code]

Here we run rusty-cheddar a first time without specifying the module: it will default to generate a header for the definitions in src/lib.rs.
The second run specifies a different module, and can insert a file inclusion at the top.

It can be a good idea to commit the generated headers, since you will see immediately if you changed the interface in a breaking way.

Integrating with the build system

As you might know, we can make dynamic libraries and executables with rustc and cargo. But often, the host application will have its own build system, and it might disagree with the way cargo builds its projects. So we have multiple strategies:

  • build Rust code separately, store libraries and headers in Maven or something (don't laugh, I've worked with such a system once, and it was actually great)
  • try to let rustc build dynamic libraries from inside the build system. We tried that for VLC and it was not great at all
  • build a static library from inside or outside the build system, include it in the libraries at link. This was done in Rusticata
  • build an object file and let the build system link it. This is what we ended up doing with VLC

Building a static library is as easy as specifying crate-type = ["staticlib"] in your Cargo.toml file. To build an object file, use the command cargo rustc --release -- --emit obj. You can see how we added it to the autotools usage in VLC.

Unfortunately, for this part we still do not have automated ways to fix the issues. Maybe with some time, people will write scripts for autotools,
CMake and others to handle Rust and Cargo.

Side note on reproducible builds: if you want to fix the set of Rust dependencies used in your project and make them always available, you can use cargo-vendor to store them in a specific folder

As you might have guessed, this is the most complex part, for which I have no good generic answer. I'd recommend that you spend the most time on this during the project's prototyping phase: import very little C code, export very little Rust code, try to make it build entirely from within the host application's build system. Once this is done, extending the project will get much easier. You really don't want to discover this task at the end of your project and try to retrofit your code in there.

Going further

While this article just explores the surface of Rust rewrites, I hope it provides a good starting point on the tools and techniques you can apply.
Any rewrite will be a large and complex project, but the result is worth the effort. The code you will write will be stronger, and Rust's type system will force you to review the assumptions made in the C version. You might even find better ways to write it once you start refactoring your code in a more Rusty way, safely hidden behind your wrappers.