yak(mutation-testing-with-muex-blogex-pure-app-code-1341): mutation testing with muex (blogex + pure app code)

app/test/mutation_scope_test.exs

@@ -0,0 +1,53 @@
+defmodule Firehose.MutationScopeTest do
+  # The app's candidate pure modules (Firehose.Accounts.User, Scope and
+  # UserToken) are only exercised through Firehose.DataCase: their tests use
+  # the SQL sandbox, fixtures and Repo. Per the yak's Q45 decision, an app
+  # mutation run would therefore need PostgreSQL, so the scope is deliberately
+  # empty and the alias fails loudly rather than silently running over nothing.
+  #
+  # These pins keep that decision visible: if the scope grows, this file must
+  # change with it, and it may never grow into the web layer.
+  use ExUnit.Case
+
+  @root Path.expand("..", __DIR__)
+
+  test "scope files exist" do
+    assert File.exists?(Path.join(@root, "mutation_scope.txt"))
+    assert File.exists?(Path.join(@root, "mutation_scope_fast.txt"))
+  end
+
+  test "app scope is empty because the candidate modules' tests need PostgreSQL" do
+    assert scope("mutation_scope.txt") == []
+  end
+
+  test "fast scope is a subset of full scope" do
+    out_of_scope =
+      MapSet.difference(
+        MapSet.new(scope("mutation_scope_fast.txt")),
+        MapSet.new(scope("mutation_scope.txt"))
+      )
+
+    assert MapSet.size(out_of_scope) == 0,
+           "mutate.fast names files outside the audited scope: #{inspect(MapSet.to_list(out_of_scope))}"
+  end
+
+  test "scope never includes web-layer code" do
+    web_entries =
+      Enum.filter(
+        scope("mutation_scope.txt") ++ scope("mutation_scope_fast.txt"),
+        &String.starts_with?(&1, "lib/firehose_web")
+      )
+
+    assert web_entries == [],
+           "the web layer (controllers, LiveViews, components) is out of scope: #{inspect(web_entries)}"
+  end
+
+  defp scope(file) do
+    @root
+    |> Path.join(file)
+    |> File.read!()
+    |> String.split("\n", trim: true)
+    |> Enum.map(&String.trim/1)
+    |> Enum.reject(&(&1 == ""))
+  end
+end