Implement mechanism to refresh Firestore access tokens before expiry

lib/firestore.ex

@@ -3,17 +3,32 @@ defmodule Firestore do
   This is the main entry point for the Firestore application.
   """
 
+  defmodule State do
+    defstruct [
+      :config,
+      :client
+    ]
+
+    @type t :: %__MODULE__{
+            config: map(),
+            client: Connection.t()
+          }
+  end
+
   use GenServer
 
-  def init(%{otp_app: app} = config) do
-    credentials = Enum.into(config, %{}, fn {k, v} -> {to_string(k), v} end)
+  alias Firestore.Connection
+  alias Goth.Token
 
-    with {:ok, %{token: token}} <- Goth.Token.fetch(source: {:service_account, credentials}) do
-      client = Firestore.Connection.init(token, config)
+  @ets_table :firestore_table
+  @refresh_token_interval_ms 1 * 60 * 1000
 
-      :ets.new(:firestore_conn_table, [:set, :public, :named_table])
-      :ets.insert(:firestore_conn_table, {:"#{app}_firestore_client", client})
-      {:ok, client}
+  @impl true
+  def init(config) do
+    :ets.new(@ets_table, [:set, :public, :named_table])
+
+    with {:ok, client} <- init_client(config) do
+      {:ok, %State{config: config, client: client}}
     end
   end
 
@@ -24,4 +39,28 @@ defmodule Firestore do
   def child_spec(opts) do
     %{id: __MODULE__, start: {__MODULE__, :start_link, [opts]}}
   end
+
+  @impl true
+  def handle_info(:refresh_token, %State{config: config} = state) do
+    case init_client(config) do
+      {:ok, client} ->
+        {:noreply, %State{state | client: client}}
+
+      {:error, reason} ->
+        {:stop, reason}
+    end
+  end
+
+  defp init_client(%{otp_app: app} = config) do
+    credentials = Enum.into(config, %{}, fn {k, v} -> {to_string(k), v} end)
+
+    with {:ok, %{token: token}} <- Token.fetch(source: {:service_account, credentials}) do
+      client = Connection.init(token, config)
+
+      :ets.insert(@ets_table, {:"#{app}_firestore_client", client})
+      Process.send_after(self(), :refresh_token, @refresh_token_interval_ms)
+
+      {:ok, client}
+    end
+  end
 end

lib/firestore/repo.ex

@@ -127,7 +127,7 @@ defmodule Firestore.Repo do
       end
 
       defp get_client() do
-        case :ets.lookup(:firestore_conn_table, :"#{@otp_app}_firestore_client") do
+        case :ets.lookup(:firestore_table, :"#{@otp_app}_firestore_client") do
           [{_, %Tesla.Client{} = client}] ->
             {:ok, client}