Validate repo without behaviour check

Repo wrappers that don't implement all functions of the Ecto.Repo
behaviour are still viable and shouldn't be validated with a behaviour
check. This changes repo validation back to the way it was done in older
versions, by checking that it's a valid module that exports `config/0`.

All other `:behaviour` checks are for behavious defined by Oban, and
remain intact.

Closes #1014

lib/oban/config.ex

@@ -142,7 +142,7 @@ defmodule Oban.Config do
       plugins: {:custom, &validate_plugins/1},
       prefix: {:or, [:falsy, :string]},
       queues: {:custom, &validate_queues(opts, &1)},
-      repo: {:behaviour, Ecto.Repo},
+      repo: {:module, [config: 0]},
       shutdown_grace_period: :non_neg_integer,
       stage_interval: :timeout,
       testing: {:enum, @testing_modes}

lib/oban/validation.ex

@@ -153,6 +153,19 @@ defmodule Oban.Validation do
     {:error, "expected #{inspect(key)} to be between #{min}..#{max}, got: #{inspect(val)}"}
   end
 
+  defp validate_type({:module, funs}, key, val) do
+    cond do
+      not Code.ensure_loaded?(val) ->
+        {:error, "expected #{inspect(key)} to be a loaded module, got: #{inspect(val)}"}
+
+      not Enum.all?(funs, fn {fun, arity} -> function_exported?(val, fun, arity) end) ->
+        {:error, "expected #{inspect(key)} to implement #{inspect(funs)}, got: #{inspect(val)}"}
+
+      true ->
+        :ok
+    end
+  end
+
   defp validate_type(:schedule, key, val) do
     case Expression.parse(val) do
       {:ok, _cron} ->

test/oban/config_test.exs

@@ -107,6 +107,20 @@ defmodule Oban.ConfigTest do
       assert_valid(queues: [default: [limit: 1, dispatch_cooldown: 10]])
     end
 
+    test ":repo is validated as a repo-like module" do
+      defmodule NotRepo do
+      end
+
+      defmodule SomeRepo do
+        def config, do: []
+      end
+
+      refute_valid(repo: NotReal)
+      refute_valid(repo: NotRepo)
+
+      assert_valid(repo: SomeRepo)
+    end
+
     test ":shutdown_grace_period is validated as an integer" do
       refute_valid(shutdown_grace_period: -1)
       refute_valid(shutdown_grace_period: "5")