Warn if no env set in config

We can't infer the env of the parent app, so without this explict config
value electric will always launch in prod mode with persistent shapes.

Author: magnetised

config/test.exs

@@ -1,6 +1,6 @@
 import Config
 
-config :logger, level: :critical
+config :logger, level: :warning
 
 config :phoenix_sync, mode: :disabled
 

lib/phoenix/sync/application.ex

@@ -9,14 +9,17 @@ defmodule Phoenix.Sync.Application do
 
   @impl true
   def start(_type, _args) do
-    case children() do
-      {:ok, children} ->
-        Supervisor.start_link(children, strategy: :one_for_one, name: Phoenix.Sync.Supervisor)
+    children =
+      case children() do
+        {:ok, children} ->
+          children
 
-      {:error, reason} ->
-        Logger.warning(reason)
-        Supervisor.start_link([], strategy: :one_for_one, name: Phoenix.Sync.Supervisor)
-    end
+        {:error, reason} ->
+          Logger.warning(reason)
+          []
+      end
+
+    Supervisor.start_link(children, strategy: :one_for_one, name: Phoenix.Sync.Supervisor)
   end
 
   @doc false
@@ -41,6 +44,8 @@ defmodule Phoenix.Sync.Application do
 
   @doc false
   def children(opts) when is_list(opts) do
+    warn_missing_env(opts)
+
     {adapter, env} = adapter_env(opts)
 
     apply(adapter, :children, [env, opts])
@@ -73,4 +78,23 @@ defmodule Phoenix.Sync.Application do
       :error -> {:error, "Missing required key #{inspect(key)}"}
     end
   end
+
+  defp warn_missing_env(config) do
+    if config[:mode] != :disabled && !config[:env] do
+      Logger.warning("""
+      No `env` specified for :phoenix_sync: defaulting to `:prod`.
+
+      Add the following to your config:
+
+      config :phoenix_sync,
+        env: config_env(),
+        # the rest of your config
+
+      In `:prod` mode, shapes are persisted between server restarts
+      which may cause problems in `:dev` or `:test` environments.
+      """)
+    end
+
+    config
+  end
 end

test/phoenix/sync/application_test.exs

@@ -3,6 +3,8 @@ defmodule Phoenix.Sync.ApplicationTest do
 
   alias Phoenix.Sync.Application, as: App
 
+  import ExUnit.CaptureLog
+
   Code.ensure_loaded!(Support.ConfigTestRepo)
 
   defp validate_repo_connection_opts!(opts, overrides \\ []) do
@@ -78,7 +80,20 @@ defmodule Phoenix.Sync.ApplicationTest do
     end
 
     test "disabled mode" do
-      assert {:ok, []} = App.children(mode: :disabled)
+      refute capture_log(fn ->
+               assert {:ok, []} = App.children(mode: :disabled)
+             end) =~ ~r/No `env` specified for :phoenix_sync: defaulting to `:prod`/
+    end
+
+    test "warns if env not set" do
+      config = [
+        mode: :embedded,
+        repo: Support.ConfigTestRepo
+      ]
+
+      assert capture_log(fn ->
+               assert {:ok, _} = App.children(config)
+             end) =~ ~r/No `env` specified for :phoenix_sync: defaulting to `:prod`/
     end
 
     test "embedded mode dev env" do