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Writing a "Hello World" session

Sessions are regular components that the session_manager can launch at startup. This means that creating a session component follows all of the same steps as creating any other component. This document discusses creating an example session that launches at startup and prints "Hello World!" to the system log.

Create the directory structure

Components require a specific directory structure. The fx tool provides a generator that creates this structure for you. It takes the name of the component and the language you want to use as arguments. For example, this component is called hello-world-session and is written in Rust.

Run the following command to create the directory structure for this example:

fx create component-v2 hello-world-session --lang rust

This command creates the following directory structure with a template for a component offering a service:

hello-world-session
  |- meta
  |   |- hello-world-session-unittests.cml
  |   |- hello-world-session.cml
  |
  |- src
  |   |- main.rs
  |
  |- BUILD.gn

Create a component manifest

The component manifest file (hello-world-session.cml in this case) gives Fuchsia information about our component. The component manifest file has to have the same name as the component that it refers to. This session doesn't do much so you don't need to add anything to the one that was generated.

The following lines of code are from hello-world-session.cml:

  1. The file starts by including other cml files if needed.

    include: [ "sdk/lib/diagnostics/syslog/client.shard.cml" ],
    

    This include key lets the session component use the fuchsia.logger.LogSink capability so that it can print to the system log.

  2. Next is the program block.

    program: {
        // Use the built-in ELF runner to run native binaries.
        runner: "elf",
        // The binary to run for this component.
        binary: "bin/hello_world_session",
    },
    

    The program block tells the component_manager where the binary for the session component can be found. The runner key tells the component_manager that is should run the component binary using the ELF runner.

  3. Finally the component manifest describes capabilities that the component can use, offer, or expose.

    use: [
        // List your component's dependencies here, ex:
        // { protocol: "fuchsia.net.NameLookup" }
    ],
    

Create a session config

session_manager needs to know to which session component to launch at startup. To do this create a session config JSON file in the meta directory that contains the URL of the session component.

Component URLs follow the format:

fuchsia-pkg://fuchsia.com/package_name#meta/your_session.cm

Notice that the path points to a .cm file. .cm files are compiled versions of .cml files that are generated when fx build is run. So, in this case, the component URL is:

fuchsia-pkg://fuchsia.com/hello-world-session#meta/hello-world-session.cm

The whole session config file looks like this:

{
    "session_url": "fuchsia-pkg://fuchsia.com/hello-world-session#meta/hello-world-session.cm"
}

Writing a session in Rust

Now you can write the actual code for the session component. Inside the src/main.rs file that was generated there is a lot of code that isn't needed for this example. Replace the contents of src/main.rs with the following code:

use {
    anyhow::{Context, Error},
    fuchsia_async as fasync,
    fuchsia_syslog::fx_log_info,
};

/// Creates a simple session that just prints "Hello World" to the syslog.
#[fasync::run_singlethreaded]
async fn main() -> Result<(), Error> {
    fuchsia_syslog::init_with_tags(&["hello_world_session"])
        .context("Failed to initialize logger.")?;

    fx_log_info!("Hello World!");

    Ok(())
}

#[cfg(test)]
mod tests {
    #[fuchsia::test]
    async fn smoke_test() {
        assert!(true);
    }
}

This code initializes the system log and then prints "Hello World!". fuchsia_syslog::init_with_tags can cause an error so the main function returns Result<(), Error> as well. fx_log_info! is a macro that prints to the system log with a level of info. There are similar marcros for error and warn.

Writing the BUILD.gn

The last file to modify is the BUILD.gn. This tells the compiler how to build the the session component.

Imports

The file starts by importing GN templates that are used in this BUILD.gn. To build a session component you must import the session_config.gni.

import("//src/session/build/session_config.gni")

Session config

The added import statement gives the BUILD.gn access to the session_config command. This command tells the build where to find the session_config.json for this component. Add the session_config to the BUILD.gn just below the import statements:

session_config("hello-world-session-config.json") {
  config = "meta/hello-world-session-config.json"
}

Rust binary

The next section describes the actual Rust binary. It tells the compiler what the name of the binary should be, that it includes unit tests, what dependencies it has, and where it's source is located. The only thing that needs to be changed in this section are the dependencies. Since this session component doesn't have much functionality there are things in the list that aren't needed. But, also, syslog is missing so the session wouldn't have access to do any logging. Replace the dependencies with the following code:

deps = [
  "//src/lib/fuchsia",
  "//src/lib/fuchsia-async",
  "//src/lib/syslog/rust:syslog",
  "//third_party/rust_crates:anyhow",
]

Everything else can stay as it is in the generated BUILD.gn. The fuchsia_component and fuchsia_package commands tell Fuchsia more about the component including what it is called, where to find the manifest, and what dependencies the package and component have.

Building the session

To build the session fx set must first be used to configure the build so that session_manager, your session component, and the session config are included in the base image. This is done with the --with-base command.

fx set product.board --with-base=//src/session,//path/to/your/session,//path/to/your/session:your_session_config

This project is in the session/examples directory so the fx set for it would be:

fx set core.x64 --with-base=//src/session,//src/session/examples/hello-world-session,//src/session/examples/hello-world-session:hello-world-session-config.json

Once that's done and built session_manager should automatically start your session on boot.