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.test.fidl
sample that we discussed in the FIDL concepts doc, by opening //garnet/examples/fidl/services/echo.test.fidl..
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/services/echo.test.fidl" adjust_indentation="auto" %}
--with //garnet/examples/fidl/echo_server_rust
to your fx set
invocation.--with //garnet/examples/fidl/echo_client_rust
to your fx set
invocation.Echo
serverThe echo server implementation can be found at: //garnet/examples/fidl/echo_server_rust/src/main.rs.
This file has two functions:
main()
: An async task executor starts this function through the #[fasync::run_singlethreaded]
annotation. This function starts an instance of ServiceFs
and runs it to completion by await
ing on its future.run_echo_server()
: This is an async function that handles incoming service requests. It returns a future that completes once the client channel is closed.To understand how the code works, here‘s a summary of what happens in the server to execute an IPC call. This section explains what each of these lines means, so it’s not necessary to understand all of this before you move on.
NOTE: Rust uses a polling asynchronous execution model. Futures do not make progress unless they are polled through the poll
method or by await
ing them.
ServiceFs
is the main top-level future being run on the executor. It binds itself to the startup handle of the current process through ServiceFs::take_and_serve_directory_handle
and listens for incoming service requests.ServiceFs
containing the name of the service to connect to (“Echo”) and a channel to connect.async::Executor
executor and tells it that the ServiceFs
task can now make progress and should be run. The ServiceFs
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
, add_fidl_service
, etc. If a matching service_name
exists, it calls service_startup_func
with the channel to connect to the new service.IncomingService::Echo
is called with a RequestStream
(typed-channel) of the Echo
FIDL protocol that is registered with add_fidl_service
. The incoming request channel is stored in IncomingService::Echo
and is added to the stream of incoming requests. for_each_concurrent
consumes the ServiceFs
into a Stream
of type IncomingService
. A handler is run for each entry in the stream, which matches over the incoming requests and dispatches to the run_echo_server
. The resulting futures from each call to run_echo_server
are run concurrently when the ServiceFs
stream is await
ed.echo_string
request is sent on the channel. This makes the channel the Echo
service is running on readable, which wakes up the asynchronous code in the body of run_echo_server
. The request is read from the channel and yielded by 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:
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/echo_server_rust/src/main.rs" region_tag="import_declarations" adjust_indentation="auto" %}
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_fidl_examples_echo
contains bindings for the Echo
protocol. This file is generated from the protocol defined in echo.test.fidl
. These bindings include: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.ServiceFs
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():
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/echo_server_rust/src/main.rs" region_tag="main" adjust_indentation="auto" %}
main
creates a ServiceFs
and asynchronously runs it to completion. You may notice that main
is async
. The run_singlethreaded
, run
, and run_until_stalled
macros from the fuchsia_async
crate can be used to run asynchronous main
or test functions to completion using the fuchsia_async::Executor
.
main
also returns Result<(), Error>
. 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.
The ServiceFs
represents a filesystem containing various services. Services exposed inside the "svc"
directory will be offered to other components. The add_fidl_service
function can be used to offer a \[Discoverable\]
FIDL service inside the file system.
The add_fidl_service
function accepts any closure with a RequestStream
argument type. This closure can return a value of any type, but the return type of all closures passed to add_fidl_service
must match. The return values of all add_fidl_service
closures will become the elements in the ServiceFs
stream.
In this case, the argument to add_fidl_service
is an IncomingService
enum variant constructor which accepts a value of type EchoRequestStream
and returns a value of type IncomingService
. In this simple example, the IncomingService
enum is redundant and could be replaced with a simple function |stream| stream
that directly passed-through the EchoRequestStream
(causing the ServiceFs
stream to yield values of type EchoRequestStream
rather than values of type IncomingService
). However, more complex servers may offer multiple services, in which case the various types of incoming RequestStream
s will need to be returned from the stream as a single enum
type.
In order to offer services to the outside world, we need to call the take_and_serve_directory_handle
function. This function removes the current process‘s directory handle and connects it to ServiceFs
. Note that, since this removes the handle from the process’s handle table, this function can only be called once per process. If you wish to provide a ServiceFs
to a different channel, you can use the serve_connection
function.
To actually run our filesystem, we'll need to handle the incoming stream of request streams (one request stream per client connection). We use for_each_concurrent
to loop over the IncomingService
s and run_echo_server
for each of them. Note that we use for_each_concurrent
rather than for_each
or a manual while let
loop in order to serve multiple client connections concurrently.
fn run_echo_server
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/echo_server_rust/src/main.rs" region_tag="run_echo_server" adjust_indentation="auto" %}
In run_echo_server
, we serve all requests for a particular client connection (one EchoRequestStream
). Because we don‘t need to do any asynchronous work when processing a request, there’s no value in processing requests concurrently, so we use a simple while let
loop to iterate over and respond to each request.
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 convert Option<String>
into Option<&str>
. This is necessary because 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.as_str())
to go from Option<&String>
to Option<&str>
.
You might well ask why we used as_ref
at all, since we immediately dereference the resulting &String
(this happens implicitly, when we call the .as_str()
method). This is 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 the future returned from run_echo_server
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.{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/echo_client_rust/src/main.rs" region_tag="main" adjust_indentation="auto" %}
You can run the echo example like this:
$ run fuchsia-pkg://fuchsia.com/echo_client_rust#meta/echo_client_rust.cmx