ML-21: Classify API errors as transient vs permanent

test/music_brainz/api_test.exs

@@ -0,0 +1,85 @@
+defmodule MusicBrainz.APITest do
+  use ExUnit.Case, async: true
+
+  alias MusicBrainz.API
+
+  @config %MusicBrainz.Config{
+    user_agent: "test_agent",
+    req_options: [plug: {Req.Test, __MODULE__}, max_retries: 0],
+    api_cooldown: 0
+  }
+
+  describe "error classification" do
+    @tag :capture_log
+    test "503 is treated as :rate_limit (MusicBrainz's rate-limit signal, not 429)" do
+      Req.Test.stub(__MODULE__, fn conn ->
+        conn
+        |> Plug.Conn.put_status(503)
+        |> Req.Test.json(%{"error" => "Your requests are exceeding the allowable rate limit."})
+      end)
+
+      assert {:error, %API.ErrorResponse{} = err} =
+               API.get_artist("mbid", @config)
+
+      assert err.status == 503
+      assert err.kind == :rate_limit
+      assert API.ErrorResponse.retryable?(err)
+    end
+
+    @tag :capture_log
+    test "429 falls under :rate_limit via default mapping (even though MusicBrainz doesn't use it)" do
+      Req.Test.stub(__MODULE__, fn conn ->
+        conn
+        |> Plug.Conn.put_status(429)
+        |> Req.Test.json(%{"error" => "Too many requests"})
+      end)
+
+      assert {:error, %API.ErrorResponse{kind: :rate_limit} = err} =
+               API.get_artist("mbid", @config)
+
+      assert API.ErrorResponse.retryable?(err)
+    end
+
+    @tag :capture_log
+    test "404 is treated as :not_found and is not retryable" do
+      Req.Test.stub(__MODULE__, fn conn ->
+        conn
+        |> Plug.Conn.put_status(404)
+        |> Req.Test.json(%{"error" => "Not Found"})
+      end)
+
+      assert {:error, %API.ErrorResponse{kind: :not_found} = err} =
+               API.get_artist("mbid", @config)
+
+      refute API.ErrorResponse.retryable?(err)
+    end
+
+    @tag :capture_log
+    test "400 is treated as :client_error and is not retryable" do
+      Req.Test.stub(__MODULE__, fn conn ->
+        conn
+        |> Plug.Conn.put_status(400)
+        |> Req.Test.json(%{"error" => "Invalid MBID"})
+      end)
+
+      assert {:error, %API.ErrorResponse{kind: :client_error} = err} =
+               API.get_artist("mbid", @config)
+
+      refute API.ErrorResponse.retryable?(err)
+    end
+
+    @tag :capture_log
+    test "500 is treated as :server_error and is retryable" do
+      Req.Test.stub(__MODULE__, fn conn ->
+        conn
+        |> Plug.Conn.put_status(500)
+        |> Req.Test.json(%{"error" => "Internal Server Error"})
+      end)
+
+      assert {:error, %API.ErrorResponse{kind: :server_error} = err} =
+               API.get_artist("mbid", @config)
+
+      assert API.ErrorResponse.retryable?(err)
+    end
+  end
+end