This tutorial describes how to make client calls and write servers in Rust using the FIDL InterProcess Communication (IPC) system in Fuchsia.
Refer to the main FIDL page for details on the design and implementation of FIDL, as well as the instructions for getting and building Fuchsia.
We'll use the echo.fidl
sample that we discussed in the FIDL Tutorial introduction section, by opening //garnet/examples/fidl/services/echo.fidl.
library fidl.examples.echo; [Discoverable] protocol Echo { EchoString(string? value) -> (string? response); };
@@@ What are the specific instructions for Rust?
Echo
serverThe echo server implementation can be found at: //garnet/examples/fidl/echo_server_rust/src/main.rs.
This file has two functions: main()
, and spawn_echo_server
:
main()
function creates an asynchronous task executor and a ServicesServer
and runs the ServicesServer
to completion on the executor.spawn_echo_server
spawns a new asynchronous task which will handle incoming echo service requests.To understand how the code works, here‘s a summary of what happens in the server to execute an IPC call. We will dig into what each of these lines means, so it’s not necessary to understand all of this before you move on.
ServicesServer
is the main top-level future being run on the executor. It binds itself to the startup handle of the current process and listens for incoming service requests.ServicesServer
containing the name of the service to connect to (“Echo”) and a channel to connect.async::Executor
executor and tells it that the ServicesServer
task can now make progress and should be run. The ServicesServer
wakes up, sees the request available on the startup handle of the process, and looks up the name of the requested service in the list of (service_name, service_startup_func)
provided through calls to add_service
. If a matching service_name
exists, it calls service_startup_func
with the channel to connect to the new service.|chan| spawn_echo_server(chan)
is called with the channel that wants to be connected to an Echo
service. spawn_echo_server
creates a new future which loops over each value in the incoming stream of requests. It spawns that future to be run on the thread-local async::Executor
.echo_string
request is sent on the channel. This makes the channel the Echo
service is running on readable, which wakes up the asynchronous task spawned in spawn_echo_server
. The task reads the request off of the channel and yields a value from the try_next()
future.responder.send
.Now let's go through the code and see how this works.
Here are the import declarations in the Rust server implementation:
use failure::{Error, ResultExt}; use fidl::endpoints2::{ServiceMarker, RequestStream}; use fidl_fidl_examples_echo::{EchoMarker, EchoRequest, EchoRequestStream}; use fuchsia_app::server::ServicesServer; use fuchsia_async as fasync; use futures::prelude::*;
failure
provides conveniences for error handling, including a standard dynamically-dispatched Error
type as well as a extension trait that adds the context
method to Result
for providing extra information about where the error occurred.fidl::endpoints2::ServiceMarker
is the trait implemented by XXXMarker
types. It provides the associated string NAME
.fidl_fidl_examples_echo
contains bindings for the Echo
protocol. This file is generated from the protocol defined in echo.fidl
. These bindings include:EchoMarker
type, a zero-sized type used to hold compile-time metadata about the Echo
service (such as NAME
)EchoRequest
type, an enum over all of the different request types that can be received.EchoRequestStream
type, a Stream
of incoming requests for the server to handle.ServicesServer
links service requests to service launcher functions.fuchsia_async
, often aliased to the abbreviated fasync
, is the runtime library for running asynchronous tasks on Fuchsia. It also provides asynchronous bindings to a number of Fuchsia primitives, such as channels, sockets, and TCP/UDP.futures
is a crate for working with asynchronous tasks. These tasks are composed of asynchronous units of work that may produce a single value (a Future
) or many values (a Stream
). Futures can be await!
ed inside an async
function or block, which will cause the current task to be suspended until the future is able to make more progress. For more about futures, see the crate's documentation. To understand more about how futures are structured internally, see this post on how futures connect to system waiting primitives like epoll
and Fuchsia's ports. Note that Fuchsia does not use Tokio, but employs a very similar strategy for managing asynchronous tasks.fn main
Everything starts with main():
fn main() -> Result<(), Error> { let mut executor = fasync::Executor::new().context("Error creating executor")?; let quiet = env::args().any(|arg| arg == "-q"); let fut = ServicesServer::new() .add_service((EchoMarker::NAME, move |chan| spawn_echo_server(chan, quiet))) .start() .context("Error starting echo services server")?; executor.run_singlethreaded(fut).context("failed to execute echo future")?; Ok(()) }
main
creates an asynchronous task executor and a ServicesServer
and runs the ServicesServer
to completion on the executor. You may notice that main
returns a Result
type: if an Error
is returned from main
as a result of one of the ?
lines, the error will be Debug
printed and the program will return with a status code indicating failure. Functions that return Result
, such as async::Executor::new()
, can have extra information appended to their error message via the context
function provided by failure::ResultExt
.
The ServicesServer
represents a collection of services that can be provided. add_service
takes a tuple of service_name
and service_start_fn
. We pass it the name of our Echo
service, EchoMarker::NAME
, and a function which takes a channel and spawns the echo server onto that channel. We then attempt to start
the ServicesServer
, which binds it to the startup handle of the current component. If that binding fails, the “Error starting echo services server” occurs. Otherwise, we get back a Future
which, when run on the executor, will process and delegate incoming service requests until a protocol error occurs or the startup handle is closed.
fn spawn_echo_server
fn spawn_echo_server(chan: fasync::Channel, quiet: bool) { fasync::spawn(async move { let mut stream = EchoRequestStream::from_channel(chan); while let Some(EchoRequest::EchoString { value, responder }) = await!(stream.try_next()).context("error running echo server")? { if !quiet { println!("Received echo request for string {:?}", value); } responder.send(value.as_ref().map(|s| &**s)).context("error sending response")?; if !quiet { println!("echo response sent successfully"); } } Ok(()) }.unwrap_or_else(|e: failure::Error| eprintln!("{:?}", e))); }
When a request for an echo service is received, spawn_echo_server
is called with the channel to host the Echo
service on. The channel that will contain incoming requests is turned into an EchoRequestStream
, an asynchronous stream of EchoRequest
s.
We use async move { ... }
to create an asynchronous block, and spawn that asynchronous task onto the local executor using fasync::spawn
.
The .try_next()
function will return a future which yields a value of type Result<Option<EchoRequest>, fidl::Error>
. We await!
the future, causing the current task to yield if no request is yet available. When a value becomes available, await!
returns the result. We apply a context("...")
to give some information about the error that may have occurred, and use ?
to return early in the error case. If no request is available, this expression will result in None
, the while
loop will exit, and we return Ok
.
When a request is received, we use pattern-matching to extract the contents of the EchoString
variant of the EchoRequest
enum. For a protocol with more than one type of request, we would instead write |x| match x { MyServiceRequest::Req1 { ... } => ... }
. In our case, we receive value
, an optional string, and responder
, a control handle with a send
method for sending a response. We log the request using println!
, and then do a bit of complicated-looking nonsense. :) The as_ref().map(|s| &**s)
trick isn‘t related to FIDL, but is a specific issue with converting Option<String>
into Option<&str>
. If you’re not interested in the details of this conversion, feel free to skip the following paragraph.
s
is an Option<String>
, but our send
method takes back an Option<&str>
to allow sending back non-heap-allocated strings. To convert between the two, we use .as_ref()
to go from Option<String>
to Option<&String>
, and then .map(|s| &**s)
to get Option<&str>
using the Deref<Target=str>
implementation for String
. The first *
goes from &String
to String
, the next goes from String
to str
, and the last goes from str
to &str
. You might well ask why we used as_ref
at all, since we immediately dereference the resulting &String
. This necessary in order to make sure that we're still borrowing from the initial Option<String>
value. Option::map
takes self
by value and so consumes its input, but we want to instead create a reference to its input.
Once we've done the conversion from Option<String>
to Option<&str>
, we call send
, which returns a Result<(), Error>
which we use ?
on to return an error on failure.
Finally, we call .unwrap_or_else(|e| ...)
on our async move { ... }
block to handle the case in which an error occurred.
Echo
clientThe echo client implementation can be found at:
//garnet/examples/fidl/echo_client_rust/src/main.rs
Our simple client does everything in main()
.
Note: a component can be a client, a service, or both, or many. The distinction in this example between Client and Server is purely for demonstration purposes.
Here is the summary of how the client makes a connection to the echo service.
connect_to_service
on the launched server component and get back a proxy with methods for making IPC calls to the remote server.echo_string
method with the desired value to echo, get back a Future
of the response, and map
the future so that the response will be logged once it is received.#[fasync::run_singlethreaded] async fn main() -> Result<(), Error> { #[derive(StructOpt, Debug)] #[structopt(name = "echo_client_rust")] struct Opt { #[structopt(long = "server", help = "URL of echo server", default_value = "fuchsia-pkg://fuchsia.com/echo_server_rust#meta/echo_server_rust.cmx")] server_url: String, } // Launch the server and connect to the echo service. let Opt { server_url } = Opt::from_args(); let launcher = Launcher::new().context("Failed to open launcher service")?; let app = launcher.launch(server_url, None) .context("Failed to launch echo service")?; let echo = app.connect_to_service(EchoMarker) .context("Failed to connect to echo service")?; let res = await!(echo.echo_string(Some("hello world!")))?; println!("response: {:?}", res); Ok(()) }
You can run the echo example like this:
$ run fuchsia-pkg://fuchsia.com/echo_client_rust#meta/echo_client_rust.cmx