Refactor HAProxy interface

lib/ha_handler/ha_proxy.ex

@@ -1,23 +1,108 @@
 defmodule HAHandler.HAProxy do
+  @moduledoc """
+  Interface to HAProxy's runtime API:
+    https://www.haproxy.com/documentation/hapee/latest/api/runtime-api/
+  """
+
+  # TODO: can we keep the socket open by keeping the port state in a GenServer
+  # and abusing the prompt mode of HAProxy's socket somehow? The later is
+  # somewhat clunky, but I'm not sure if there is a way around it since HAProxy
+  # seems to return EOF once a command is executed (unless prompt mode is
+  # enabled).
+
+  import HAHandler, only: [haproxy_socket: 0]
+
   alias :procket, as: Socket
 
-  @haproxy_socket "/run/haproxy.sock"
+  # How long do we wait for an answer - in milliseconds.
+  @socket_read_timeout 5_000
 
-  def open_socket() do
-    pad = 8 * (Socket.unix_path_max() - byte_size(@haproxy_socket))
+  @doc """
+  Sends a command to HAProxy's socket - please refer to HAProxy's runtime API
+  documentation for available commands (see moduledoc).
 
-    sockaddr =
-      Socket.sockaddr_common(1, byte_size(@haproxy_socket)) <> @haproxy_socket <> <<0::size(pad)>>
+  This method will either return an answer with `{:ok, answer}` or error with
+  `{:error, err}`.
+  """
+  def execute(command) when is_binary(command) do
+    case open_socket(haproxy_socket()) do
+      {:ok, socket} ->
+        send(socket, {self(), {:command, command <> "\n"}})
+        read_from_socket(socket)
+      {:error, err} ->
+        {:error, err}
+    end
+  end
 
-    family = 1
+  @doc """
+  Executes and parse the output of the `show stats` HAProxy command, returning
+  a list of Maps.
 
-    {:ok, socket} = Socket.socket(family, 1, 0)
+  TODO: extract more informations from stats output.
+  """
+  def get_stats() do
+    case execute("show stat json") do
+      {:ok, raw} ->
+        case Poison.decode(raw) do
+          {:ok, data} ->
+            for entry <- data do
+              attrs =
+                entry
+                |> Enum.filter(fn m -> get_in(m, ["field", "name"]) == "pxname" end)
+                |> Enum.at(0)
+
+              %{
+                type: attrs |> Map.get("objType"),
+                name: get_in(attrs, ["value", "value"])
+              }
+            end
+
+          {:error, err} ->
+            {:error, err}
+        end
+
+      {:error, err} ->
+        {:error, err}
+    end
+  end
+
+  # Opens an UNIX socket - there is no built-in support in Elixir/Erlang so we
+  # use low-level C bindings provided by :procket
+  # (https://github.com/msantos/procket).
+  #
+  # Heavily inspired by the `NVim.Link` module:
+  # https://github.com/kbrw/neovim-elixir/blob/master/lib/link.ex
+  defp open_socket(path) do
+    family = Socket.family(:unix)
+    type = :stream
+    # 0 means use the default protocol in the family.
+    protocol = 0
+
+    pad = 8 * (Socket.unix_path_max() - byte_size(path))
+    sockaddr = Socket.sockaddr_common(family, byte_size(path)) <> path <> <<0::size(pad)>>
+
+    {:ok, socket} = Socket.socket(family, type, protocol)
 
     case Socket.connect(socket, sockaddr) do
       :ok ->
-        {stdin, stdout} = {socket, socket}
-        port = Port.open({:fd, stdin, stdout}, [{:line, 10_000}, :binary])
-        Process.link(port)
+        # We connect our UNIX socket to an erlang Port:
+        #
+        # > Ports provide a mechanism to start operating system processes
+        # > external to the Erlang VM and communicate with them via message
+        # > passing.
+        # See https://hexdocs.pm/elixir/Port.html for details.
+        #
+        # See https://www.erlang.org/doc/man/erlang.html#open_port-2 for more
+        # port options.
+        #
+        # * :binary: all I/O from the port is binary data objects as opposed to
+        #   lists of bytes.
+        # * Messages are delivered on a per line basis. Each line (delimited by
+        #   the OS-dependent newline sequence) is delivered in a single message.
+        #   The message data format is {Flag, Line}, where Flag is eol or noeol,
+        #   and Line is the data delivered (without the newline sequence).
+        {fdin, fdout} = {socket, socket}
+        port = Port.open({:fd, fdin, fdout}, [{:line, 10_000}, :binary])
 
         {:ok, port}
 
@@ -26,37 +111,21 @@ defmodule HAHandler.HAProxy do
     end
   end
 
+  # Messages may be split due to the `{:line, L}` option specific in
+  # `open_socket/1`.
   defp read_from_socket(socket, acc \\ "") do
     receive do
       {_port, {:data, {:noeol, data}}} ->
         read_from_socket(socket, acc <> data)
+
       {_port, {:data, {:eol, data}}} ->
-        Poison.decode(acc <> data)
-      _ ->
-        {:error, :unexpected}
-    after 5000 ->
+        {:ok, acc <> data}
+
+      msg ->
+        {:error, {:unexpected_message, msg}}
+    after
+      @socket_read_timeout ->
         {:error, :timeout}
     end
   end
-
-  def get_stats() do
-    case open_socket() do
-      {:ok, socket} ->
-        send socket, {self(), {:command, "show stat json\n"}}
-        case read_from_socket(socket) do
-          {:ok, raw} ->
-            for entry <- raw do
-              attrs = entry
-                          |> Enum.filter(fn m -> get_in(m, ["field", "name"]) == "pxname" end)
-                          |> Enum.at(0)
-
-              %{type: attrs |> Map.get("objType"), name: get_in(attrs, ["value", "value"])}
-            end
-
-          {:error, err} ->
-            {:error, err}
-        end
-      {:error, err} -> {:error, err}
-    end
-  end
 end