# Rust language FIDL tutorial

[TOC]

## About this tutorial

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][fidl-entry] for details on the
design and implementation of FIDL, as well as the
[instructions for getting and building Fuchsia](/docs/getting_started.md).

## Getting started

We'll use the `echo.test.fidl` sample that we discussed in the
[FIDL concepts][concepts] doc, by opening
[//garnet/examples/fidl/services/echo.test.fidl](/garnet/examples/fidl/services/echo.test.fidl)..

<!-- NOTE: the code snippets here need to be kept up to date manually by
     copy-pasting from the actual source code. Please update a snippet
     if you notice it's out of date. -->


```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/services/echo.test.fidl" adjust_indentation="auto" %}
```

## Build

* To build the echo server, add `--with //garnet/examples/fidl/echo_server_rust` to your `fx set` invocation.
* To build the echo client, add `--with //garnet/examples/fidl/echo_client_rust` to your `fx set` invocation.

## `Echo` server

The echo server implementation can be found at:
[//garnet/examples/fidl/echo_server_rust/src/main.rs](/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.

1.  **ServiceFs:** The `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.
1.  **Service Request:** When another component needs to access an "Echo"
    server, it sends a request to the `ServiceFs` containing the name of
    the service to connect to ("Echo") and a channel to connect.
1.  **Service Lookup:** The incoming service request wakes up the
    `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.
1.  **Server Creation:**  At this point in our example,
    `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.
1.  **API Request:** An `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.
1.  **API Response:** Upon receiving a request, the task sends a response
    back to the client with `responder.send`.

Now let's go through the code and see how this works.

### File headers

Here are the import declarations in the Rust server implementation:

```rust
{% 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:
    -   The `EchoRequest` type, an enum over all of the different request types
        that can be received.
    -   The `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][docs].
    To understand more about how futures
    are structured internally, see [this post][Tokio internals] 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():

```rust
{% 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`

```rust
{% 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` client

The echo client implementation can be found at:

[//garnet/examples/fidl/echo_client_rust/src/main.rs](/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.

1.  **Launch:** The server component is specified, and we request for it to
    be launched if it wasn't already. Note that this step isn't included in
    most production FIDL-using components: generally you're connecting with
    an already-running server component.
1.  **Connect:** We call `connect_to_service` on the launched server
    component and get back a proxy with methods for making IPC calls to
    the remote server.
1.  **Call:** We call the `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.
1.  **Run:** We run the future to completion on an asynchronous task executor.

```rust
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="garnet/examples/fidl/echo_client_rust/src/main.rs" region_tag="main" adjust_indentation="auto" %}
```

### Run the sample

You can run the echo example like this:

```sh
$ run fuchsia-pkg://fuchsia.com/echo_client_rust#meta/echo_client_rust.cmx
```

<!-- xrefs -->
[concepts]: /docs/concepts/fidl/overview.md
[fidl-entry]: /docs/development/languages/fidl
[docs]: https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.5/futures/
[`Stream`]: https://docs.rs/futures/0.2.0/futures/stream/trait.Stream.html
[Tokio internals]: https://cafbit.com/post/tokio_internals/
[zero-sized type]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts