Add distribution example

Author: ConnorRigby

README.md

@@ -25,6 +25,7 @@ application's root directory.
 
 * [`blinky`](https://github.com/nerves-project/nerves-examples/blob/main/blinky/README.md)
 * [`hello_erlang`](https://github.com/nerves-project/nerves-examples/blob/main/hello_erlang/README.md)
+* [`hello_distribution`](https://github.com/nerves-project/nerves-examples/blob/main/hello_distribution/README.md)
 * [`hello_gpio`](https://github.com/nerves-project/nerves-examples/blob/main/hello_gpio/README.md)
 * [`hello_leds`](https://github.com/nerves-project/nerves-examples/blob/main/hello_leds/README.md)
 * [`hello_lfe`](https://github.com/nerves-project/nerves-examples/blob/main/hello_lfe/README.md)

hello_distribution/.formatter.exs

@@ -0,0 +1,8 @@
+# Used by "mix format"
+[
+  inputs: [
+    "{mix,.formatter}.exs",
+    "{config,lib,test}/**/*.{ex,exs}",
+    "rootfs_overlay/etc/iex.exs"
+  ]
+]

hello_distribution/.gitignore

@@ -0,0 +1,17 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump

hello_distribution/README.md

@@ -0,0 +1,54 @@
+# Hello Distribution
+
+This example builds a Nerves firmware image for supported Nerves devices that demonstrates using Mdns Lite, Erlang Distribution and Phoenix PubSub
+to build a communication mechanism between two or more Nerves devices.
+
+This example has all the same configuration as the [https://github.com/nerves-project/nerves_examples/tree/main/hello_wifi](Hello WiFi Example).
+
+The first step will be to build firmware for your boards:
+
+```bash
+cd hello_distribution
+
+# Set the target to rpi0, rpi3, or rpi4 depending on what you have
+export MIX_TARGET=rpi0
+mix deps.get
+mix firmware
+
+# Insert a MicroSD card or whatever media your board takes
+mix burn
+```
+
+Next configure the board so it connects to you WiFi network.
+
+Finally, open two ssh sessions - one to each board and use `Node.connect/1` to connect them via Erlang Distribution.
+For example (you will need to replace `nerves-bea0.local` with your devices hostname.)
+
+```elixir
+iex(hello@nerves-080c.local)4> Node.connect(:"[email protected]")
+true
+```
+
+Once connected, you can use Phoenix PubSub to send messages back and forth on the network:
+
+on one device:
+
+```elixir
+iex(hello@nerves-bea0.local)3> Phoenix.PubSub.subscribe(HelloDistribution.PubSub, "test-event") 
+```
+
+and the other:
+
+```elixir
+iex(hello@nerves-080c.local)5> Phoenix.PubSub.broadcast(HelloDistribution.PubSub, "test-event", {:hello, :world})
+```
+
+Now back on the first device, you should be able to see the message:
+
+```elixir
+iex(hello@nerves-bea0.local)4> receive do
+...(hello@nerves-bea0.local)4> event -> IO.inspect(event)
+...(hello@nerves-bea0.local)4> end
+...(hello@nerves-bea0.local)5> flush()
+{:hello, :world}
+```

hello_distribution/config/config.exs

@@ -0,0 +1,33 @@
+# This file is responsible for configuring your application
+# and its dependencies with the aid of the Mix.Config module.
+#
+# This configuration file is loaded before any dependency and
+# is restricted to this project.
+import Config
+
+# Enable the Nerves integration with Mix
+Application.start(:nerves_bootstrap)
+
+config :hello_distribution, target: Mix.target()
+
+# Customize non-Elixir parts of the firmware. See
+# https://hexdocs.pm/nerves/advanced-configuration.html for details.
+
+config :nerves, :firmware, rootfs_overlay: "rootfs_overlay"
+
+# Set the SOURCE_DATE_EPOCH date for reproducible builds.
+# See https://reproducible-builds.org/docs/source-date-epoch/ for more information
+
+config :nerves, source_date_epoch: "1630590634"
+
+# Use Ringlogger as the logger backend and remove :console.
+# See https://hexdocs.pm/ring_logger/readme.html for more information on
+# configuring ring_logger.
+
+config :logger, backends: [RingLogger]
+
+if Mix.target() == :host or Mix.target() == :"" do
+  import_config "host.exs"
+else
+  import_config "target.exs"
+end

hello_distribution/config/host.exs

@@ -0,0 +1,3 @@
+import Config
+
+# Add configuration that is only needed when running on the host here.

hello_distribution/config/target.exs

@@ -0,0 +1,108 @@
+import Config
+
+# Use shoehorn to start the main application. See the shoehorn
+# docs for separating out critical OTP applications such as those
+# involved with firmware updates.
+
+config :shoehorn,
+  init: [:nerves_runtime, :nerves_pack],
+  app: Mix.Project.config()[:app]
+
+# Nerves Runtime can enumerate hardware devices and send notifications via
+# SystemRegistry. This slows down startup and not many programs make use of
+# this feature.
+
+config :nerves_runtime, :kernel, use_system_registry: false
+
+# Erlinit can be configured without a rootfs_overlay. See
+# https://github.com/nerves-project/erlinit/ for more information on
+# configuring erlinit.
+
+config :nerves,
+  erlinit: [
+    hostname_pattern: "nerves-%s"
+  ]
+
+# Configure the device for SSH IEx prompt access and firmware updates
+#
+# * See https://hexdocs.pm/nerves_ssh/readme.html for general SSH configuration
+# * See https://hexdocs.pm/ssh_subsystem_fwup/readme.html for firmware updates
+
+keys =
+  [
+    Path.join([System.user_home!(), ".ssh", "id_rsa.pub"]),
+    Path.join([System.user_home!(), ".ssh", "id_ecdsa.pub"]),
+    Path.join([System.user_home!(), ".ssh", "id_ed25519.pub"])
+  ]
+  |> Enum.filter(&File.exists?/1)
+
+if keys == [],
+  do:
+    Mix.raise("""
+    No SSH public keys found in ~/.ssh. An ssh authorized key is needed to
+    log into the Nerves device and update firmware on it using ssh.
+    See your project's config.exs for this error message.
+    """)
+
+config :nerves_ssh,
+  authorized_keys: Enum.map(keys, &File.read!/1)
+
+# Configure the network using vintage_net
+# See https://github.com/nerves-networking/vintage_net for more information
+config :vintage_net,
+  regulatory_domain: "US",
+  additional_name_servers: [{127, 0, 0, 53}],
+  config: [
+    {"usb0", %{type: VintageNetDirect}},
+    {"eth0",
+     %{
+       type: VintageNetEthernet,
+       ipv4: %{method: :dhcp}
+     }},
+    {"wlan0", %{type: VintageNetWiFi}}
+  ]
+
+# Set the SSID for the network to join and the DNS name to use
+# in the browser.
+# see https://github.com/nerves-networking/vintage_net_wizard
+config :vintage_net_wizard,
+  dns_name: "hello_wifi.config"
+
+config :mdns_lite,
+  dns_bridge_enabled: true,
+  dns_bridge_ip: {127, 0, 0, 53},
+  dns_bridge_port: 53,
+  dns_bridge_recursive: true,
+  # The `host` key specifies what hostnames mdns_lite advertises.  `:hostname`
+  # advertises the device's hostname.local. For the official Nerves systems, this
+  # is "nerves-<4 digit serial#>.local".  mdns_lite also advertises
+  # "nerves.local" for convenience. If more than one Nerves device is on the
+  # network, delete "nerves" from the list.
+
+  host: [:hostname, "nerves"],
+  ttl: 120,
+
+  # Advertise the following services over mDNS.
+  services: [
+    %{
+      protocol: "ssh",
+      transport: "tcp",
+      port: 22
+    },
+    %{
+      protocol: "sftp-ssh",
+      transport: "tcp",
+      port: 22
+    },
+    %{
+      protocol: "epmd",
+      transport: "tcp",
+      port: 4369
+    }
+  ]
+
+# Import target specific config. This must remain at the bottom
+# of this file so it overrides the configuration defined above.
+# Uncomment to use target specific configurations
+
+# import_config "#{Mix.target()}.exs"

hello_distribution/lib/hello_distribution.ex

@@ -0,0 +1,18 @@
+defmodule HelloDistribution do
+  @moduledoc """
+  Documentation for HelloDistribution.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> HelloDistribution.hello
+      :world
+
+  """
+  def hello do
+    {:ok, :world}
+  end
+end

hello_distribution/lib/hello_distribution/application.ex

@@ -0,0 +1,96 @@
+defmodule HelloDistribution.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  use Application
+  require Logger
+
+  @ifname "wlan0"
+
+  def start(_type, _args) do
+    maybe_start_wifi_wizard()
+    maybe_start_distribution()
+    opts = [strategy: :one_for_one, name: HelloDistribution.Supervisor]
+    gpio_pin = Application.get_env(:hello_distribution, :button_pin, 17)
+
+    children = [
+      {HelloDistribution.Button, gpio_pin},
+      {Phoenix.PubSub, name: HelloDistribution.PubSub}
+    ]
+
+    Supervisor.start_link(children, opts)
+  end
+
+  @doc false
+  def on_wizard_exit() do
+    # This function is used as a callback when the WiFi Wizard
+    # exits which is useful if you need to do work after
+    # configuration is done, like restart web servers that might
+    # share a port with the wizard, etc etc
+    Logger.info("[#{inspect(__MODULE__)}] - WiFi Wizard stopped")
+  end
+
+  def maybe_start_distribution() do
+    _ = :os.cmd('epmd -daemon')
+    {:ok, hostname} = :inet.gethostname()
+
+    case Node.start(:"hello@#{hostname}.local") do
+      {:ok, _pid} -> Logger.info("Distribution started at hello@#{hostname}.local")
+      _error -> Logger.error("Failed to start distribution")
+    end
+  end
+
+  def maybe_start_wifi_wizard() do
+    with true <- has_wifi?() || :no_wifi,
+         true <- wifi_configured?() || :not_configured,
+         true <- has_networks?() || :no_networks do
+      # By this point we know there is a wlan interface available
+      # and already configured with networks. This would normally
+      # mean that you should then skip starting the WiFi wizard
+      # here so that the device doesn't start the WiFi wizard after
+      # every reboot.
+      #
+      # However, for the example we want to always run the
+      # WiFi wizard on startup. Comment/remove the function below
+      # if you want a more typical experience skipping the wizard
+      # after it has been configured once.
+      VintageNetWizard.run_wizard(on_exit: {__MODULE__, :on_wizard_exit, []})
+    else
+      :no_wifi ->
+        Logger.error(
+          "[#{inspect(__MODULE__)}] Device does not support WiFi - Skipping wizard start"
+        )
+
+      status ->
+        info_message(status)
+        VintageNetWizard.run_wizard(on_exit: {__MODULE__, :on_wizard_exit, []})
+    end
+  end
+
+  def has_wifi?() do
+    @ifname in VintageNet.all_interfaces()
+  end
+
+  def wifi_configured?() do
+    @ifname in VintageNet.configured_interfaces()
+  end
+
+  def has_networks?() do
+    VintageNet.get_configuration(@ifname)[:vintage_net_wifi][:networks] != []
+  end
+
+  def info_message(status) do
+    msg =
+      case status do
+        :not_configured -> "WiFi has not been configured"
+        :no_networks -> "WiFi was configured without any networks"
+      end
+
+    Logger.info("[#{inspect(__MODULE__)}] #{msg} - Starting WiFi Wizard")
+  end
+
+  def target() do
+    Application.get_env(:hello_distribution, :target)
+  end
+end

hello_distribution/lib/hello_distribution/button.ex

@@ -0,0 +1,44 @@
+defmodule HelloDistribution.Button do
+  use GenServer
+
+  @moduledoc """
+  This GenServer starts the wizard if a button is depressed for long enough.
+  """
+
+  alias Circuits.GPIO
+
+  @doc """
+  Start the button monitor
+
+  Pass an index to the GPIO that's connected to the button.
+  """
+  @spec start_link(non_neg_integer()) :: GenServer.on_start()
+  def start_link(gpio_pin) do
+    GenServer.start_link(__MODULE__, gpio_pin)
+  end
+
+  @impl true
+  def init(gpio_pin) do
+    {:ok, gpio} = GPIO.open(gpio_pin, :input)
+    :ok = GPIO.set_interrupts(gpio, :both)
+    {:ok, %{pin: gpio_pin, gpio: gpio}}
+  end
+
+  @impl true
+  def handle_info({:circuits_gpio, gpio_pin, _timestamp, 1}, %{pin: gpio_pin} = state) do
+    # Button pressed. Start a timer to launch the wizard when it's long enough
+    {:noreply, state, 5_000}
+  end
+
+  @impl true
+  def handle_info({:circuits_gpio, gpio_pin, _timestamp, 0}, %{pin: gpio_pin} = state) do
+    # Button released. The GenServer timer is implicitly cancelled by receiving this message.
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_info(:timeout, state) do
+    :ok = VintageNetWizard.run_wizard(on_exit: {HelloDistribution, :on_wizard_exit, []})
+    {:noreply, state}
+  end
+end