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

app/mix.exs

@@ -83,6 +83,7 @@ defmodule Firehose.MixProject do
        branch: "main"},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
       {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false},
+      {:muex, "~> 0.12", only: [:dev, :test], runtime: false},
       {:sobelow, "~> 0.16", only: [:dev, :test], runtime: false}
     ]
   end
@@ -93,6 +94,14 @@ defmodule Firehose.MixProject do
   #     $ mix setup
   #
   # See the documentation for `Mix` for more info on aliases.
+  #
+  # `mutate` / `mutate.fast` are a sensor, not a gate: they are deliberately
+  # absent from Makefile's VERIFY_SENSORS and always pass `--fail-at 0`, so a
+  # survivor is reported rather than fatal. The scope lives in the plain-text
+  # files beside this one; the aliases read them at run time, so the text file
+  # stays the single source of truth. Both are empty today because the app's
+  # candidate modules are only tested through the database-backed DataCase;
+  # an empty scope raises instead of silently running over nothing.
   defp aliases do
     [
       setup: ["deps.get", "ecto.setup", "assets.setup", "assets.build"],
@@ -106,10 +115,65 @@ defmodule Firehose.MixProject do
         "esbuild firehose --minify",
         "phx.digest"
       ],
-      precommit: ["compile --warning-as-errors", "deps.unlock --unused", "format", "test"]
+      precommit: ["compile --warning-as-errors", "deps.unlock --unused", "format", "test"],
+      mutate: &mutate/1,
+      "mutate.fast": &mutate_fast/1
     ]
   end
 
+  # No macro/`use` mutators: the web layer (where the DSLs live) is out of
+  # scope, and an empty scope never reaches muex at all. Spellings are muex
+  # 0.12's CLI names.
+  @mutation_mutators "arithmetic,boolean,comparison,conditional,function_call,literal,return_value"
+
+  defp mutate(args), do: run_mutation("mutation_scope.txt", args)
+
+  defp mutate_fast(args), do: run_mutation("mutation_scope_fast.txt", args)
+
+  defp run_mutation(scope_file, args) do
+    Mix.Task.run("muex", mutation_args(scope_file, args))
+  end
+
+  @doc false
+  def mutation_args(scope_file, extra_args \\ []) do
+    files = read_mutation_scope(scope_file)
+
+    # `--fail-at 0` comes first so an ad-hoc override cannot turn the sensor
+    # into a gate. Extra args come before our defaults, and OptionParser keeps
+    # the first value, so `mix mutate --files lib/foo.ex` targets one file.
+    ["--fail-at", "0"] ++
+      extra_args ++
+      ["--files", Enum.join(files, ","), "--mutators", @mutation_mutators]
+  end
+
+  @doc false
+  def read_mutation_scope(scope_file) do
+    path = Path.expand(scope_file, Path.dirname(Mix.Project.project_file()))
+
+    files =
+      case File.read(path) do
+        {:ok, contents} ->
+          contents
+          |> String.split("\n", trim: true)
+          |> Enum.map(&String.trim/1)
+          |> Enum.reject(&(&1 == ""))
+
+        {:error, reason} ->
+          Mix.raise(
+            "mutation scope #{scope_file} is unreadable (#{:file.format_error(reason)}); " <>
+              "refusing to run the mutation sensor over an unknown scope"
+          )
+      end
+
+    if files == [] do
+      Mix.raise(
+        "mutation scope #{scope_file} is empty; refusing to run the mutation sensor over nothing"
+      )
+    end
+
+    files
+  end
+
   # The coverage floor the Lens pass enforces (`make verify` -> `make coverage`
   # in app/). Rule: the threshold is a FLOOR. Raise it as behavior-test yaks
   # land; never lower it. A drop means the change (or an exclusion) is wrong,

app/mix.lock

@@ -34,6 +34,7 @@
   "microprints": {:git, "https://gitea.apps.sustainabledelivery.com/QWAN/microprints-phoenix.git", "f2a361555310518870ad3c093050169f30ff870c", [branch: "main"]},
   "mime": {:hex, :mime, "2.0.7", "b8d739037be7cd402aee1ba0306edfdef982687ee7e9859bee6198c1e7e2f128", [:mix], [], "hexpm", "6171188e399ee16023ffc5b76ce445eb6d9672e2e241d2df6050f3c771e80ccd"},
   "mint": {:hex, :mint, "1.11.0", "a713551624815c0435237b93d90ea8b9b14254690c66f732d0ef8930f76ff1d9", [:mix], [{:castore, "~> 0.1.0 or ~> 1.0", [hex: :castore, repo: "hexpm", optional: true]}, {:hpax, "~> 1.1", [hex: :hpax, repo: "hexpm", optional: false]}], "hexpm", "c6279ba2d6aa3a383a1d4cfbe7b59f42e6efd400f58d8e2acfeac48a438693ab"},
+  "muex": {:hex, :muex, "0.12.0", "80d8f1b2f7bfcba8ecb46b7b43dce55b555d33da54b84e74feedc4079fa2bd36", [:mix], [{:jason, "~> 1.4", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "a4b395b87d23b8d34f4508d0c13494b734f1fff8156eca2360fd279a0799d52e"},
   "nimble_options": {:hex, :nimble_options, "1.1.1", "e3a492d54d85fc3fd7c5baf411d9d2852922f66e69476317787a7b2bb000a61b", [:mix], [], "hexpm", "821b2470ca9442c4b6984882fe9bb0389371b8ddec4d45a9504f00a66f650b44"},
   "nimble_parsec": {:hex, :nimble_parsec, "1.4.2", "8efba0122db06df95bfaa78f791344a89352ba04baedd3849593bfce4d0dc1c6", [:mix], [], "hexpm", "4b21398942dda052b403bbe1da991ccd03a053668d147d53fb8c4e0efe09c973"},
   "nimble_pool": {:hex, :nimble_pool, "1.1.0", "bf9c29fbdcba3564a8b800d1eeb5a3c58f36e1e11d7b7fb2e084a643f645f06b", [:mix], [], "hexpm", "af2e4e6b34197db81f7aad230c1118eac993acc0dae6bc83bac0126d4ae0813a"},

app/test/mutation_config_test.exs

@@ -0,0 +1,49 @@
+defmodule Firehose.MutationConfigTest do
+  # Same wiring pins as blogex, plus the app-specific one: because the app
+  # scope is deliberately empty (Q45), the aliases must fail loudly. A sensor
+  # that reports a score for a run that tested nothing is worse than no sensor.
+  use ExUnit.Case
+
+  test "muex is a dev/test-only, non-runtime dependency" do
+    deps = Mix.Project.config()[:deps]
+
+    assert {:muex, requirement, opts} = dep!(deps, :muex)
+    assert requirement =~ "0.12"
+    assert opts[:only] == [:dev, :test]
+    assert opts[:runtime] == false
+  end
+
+  test "mutate and mutate.fast aliases are defined" do
+    aliases = Mix.Project.config()[:aliases]
+
+    assert is_function(aliases[:mutate], 1)
+    assert is_function(aliases[:"mutate.fast"], 1)
+  end
+
+  test "an empty app scope fails loudly instead of running over nothing" do
+    assert_raise Mix.Error, ~r/empty/, fn ->
+      Firehose.MixProject.mutation_args("mutation_scope.txt", [])
+    end
+  end
+
+  test "ad-hoc args do not bypass the empty-scope failure" do
+    assert_raise Mix.Error, ~r/empty/, fn ->
+      Firehose.MixProject.mutation_args("mutation_scope.txt", [
+        "--files",
+        "lib/firehose/accounts/user.ex"
+      ])
+    end
+  end
+
+  defp dep!(deps, name) do
+    case Enum.find(deps, fn
+           {^name, _requirement} -> true
+           {^name, _requirement, _opts} -> true
+           _ -> false
+         end) do
+      {^name, requirement} -> {name, requirement, []}
+      {^name, requirement, opts} -> {name, requirement, opts}
+      nil -> flunk("expected #{inspect(name)} in deps/0")
+    end
+  end
+end

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

blogex/mix.exs

@@ -48,7 +48,8 @@ defmodule Blogex.MixProject do
       {:plug, "~> 1.15"},
       {:ex_doc, "~> 0.34", only: :dev, runtime: false},
       {:rustler, "~> 0.38.0", optional: true},
-      {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false}
+      {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false},
+      {:muex, "~> 0.12", only: [:dev, :test], runtime: false}
     ]
   end
 
@@ -91,12 +92,74 @@ defmodule Blogex.MixProject do
   # Same `precommit` alias name and shape as app/mix.exs, so both apps share
   # one loop: compile (warnings fatal), format, test. Heavy sensors belong in
   # the Lens pass (`make verify`), not here.
+  #
+  # `mutate` / `mutate.fast` are a sensor, not a gate: they are deliberately
+  # absent from Makefile's VERIFY_SENSORS and always pass `--fail-at 0`, so a
+  # survivor is reported rather than fatal. The scope lives in the plain-text
+  # files beside this one; the aliases read them at run time, so the text file
+  # stays the single source of truth.
   defp aliases do
     [
-      precommit: ["compile --warning-as-errors", "format", "test"]
+      precommit: ["compile --warning-as-errors", "format", "test"],
+      mutate: &mutate/1,
+      "mutate.fast": &mutate_fast/1
     ]
   end
 
+  # No macro/`use` mutators: the scoped modules are plain functions, and the
+  # macro-heavy modules (blog.ex, router.ex, components.ex, layout.ex) are out
+  # of scope entirely. Spellings are muex 0.12's CLI names.
+  @mutation_mutators "arithmetic,boolean,comparison,conditional,function_call,literal,return_value"
+
+  defp mutate(args), do: run_mutation("mutation_scope.txt", args)
+
+  defp mutate_fast(args), do: run_mutation("mutation_scope_fast.txt", args)
+
+  defp run_mutation(scope_file, args) do
+    Mix.Task.run("muex", mutation_args(scope_file, args))
+  end
+
+  @doc false
+  def mutation_args(scope_file, extra_args \\ []) do
+    files = read_mutation_scope(scope_file)
+
+    # `--fail-at 0` comes first so an ad-hoc override cannot turn the sensor
+    # into a gate. Extra args come before our defaults, and OptionParser keeps
+    # the first value, so `mix mutate --files lib/blogex/seo.ex` targets one
+    # file while `mix mutate` covers the whole scope file.
+    ["--fail-at", "0"] ++
+      extra_args ++
+      ["--files", Enum.join(files, ","), "--mutators", @mutation_mutators]
+  end
+
+  @doc false
+  def read_mutation_scope(scope_file) do
+    path = Path.expand(scope_file, Path.dirname(Mix.Project.project_file()))
+
+    files =
+      case File.read(path) do
+        {:ok, contents} ->
+          contents
+          |> String.split("\n", trim: true)
+          |> Enum.map(&String.trim/1)
+          |> Enum.reject(&(&1 == ""))
+
+        {:error, reason} ->
+          Mix.raise(
+            "mutation scope #{scope_file} is unreadable (#{:file.format_error(reason)}); " <>
+              "refusing to run the mutation sensor over an unknown scope"
+          )
+      end
+
+    if files == [] do
+      Mix.raise(
+        "mutation scope #{scope_file} is empty; refusing to run the mutation sensor over nothing"
+      )
+    end
+
+    files
+  end
+
   defp package do
     [
       licenses: ["MIT"],

blogex/mix.lock

@@ -11,6 +11,7 @@
   "makeup_erlang": {:hex, :makeup_erlang, "1.0.3", "4252d5d4098da7415c390e847c814bad3764c94a814a0b4245176215615e1035", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm", "953297c02582a33411ac6208f2c6e55f0e870df7f80da724ed613f10e6706afd"},
   "mdex_native": {:hex, :mdex_native, "0.2.4", "a74b9d799d3fc487b1f97b70ee9b04108c7af0514920d56828e173d19312c2a9", [:mix], [{:rustler, "~> 0.32", [hex: :rustler, repo: "hexpm", optional: true]}, {:rustler_precompiled, "~> 0.7", [hex: :rustler_precompiled, repo: "hexpm", optional: false]}], "hexpm", "d71185c43c5209c6939f4b65f903f71c892512c93ea6843171227855d2d976f5"},
   "mime": {:hex, :mime, "2.0.7", "b8d739037be7cd402aee1ba0306edfdef982687ee7e9859bee6198c1e7e2f128", [:mix], [], "hexpm", "6171188e399ee16023ffc5b76ce445eb6d9672e2e241d2df6050f3c771e80ccd"},
+  "muex": {:hex, :muex, "0.12.0", "80d8f1b2f7bfcba8ecb46b7b43dce55b555d33da54b84e74feedc4079fa2bd36", [:mix], [{:jason, "~> 1.4", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "a4b395b87d23b8d34f4508d0c13494b734f1fff8156eca2360fd279a0799d52e"},
   "nimble_parsec": {:hex, :nimble_parsec, "1.4.2", "8efba0122db06df95bfaa78f791344a89352ba04baedd3849593bfce4d0dc1c6", [:mix], [], "hexpm", "4b21398942dda052b403bbe1da991ccd03a053668d147d53fb8c4e0efe09c973"},
   "nimble_publisher": {:hex, :nimble_publisher, "2.0.0", "f4fc100949ccdf1831094ff303b79f27c6f04b2ead829409426ee0d9ed3aa55b", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: true]}, {:mdex_native, "~> 0.1", [hex: :mdex_native, repo: "hexpm", optional: false]}], "hexpm", "88997f74b763c24a15d20a90d7cd4ef1fd01172a2b4651e54d8021a44659f2c8"},
   "phoenix": {:hex, :phoenix, "1.8.15", "dcdb304113660ec97e1579458824d3c34b5d2eed07a0fdf25a836a9515b63146", [:mix], [{:bandit, "~> 1.0", [hex: :bandit, repo: "hexpm", optional: true]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:phoenix_pubsub, "~> 2.1", [hex: :phoenix_pubsub, repo: "hexpm", optional: false]}, {:phoenix_template, "~> 1.0", [hex: :phoenix_template, repo: "hexpm", optional: false]}, {:phoenix_view, "~> 2.0", [hex: :phoenix_view, repo: "hexpm", optional: true]}, {:plug, "~> 1.14", [hex: :plug, repo: "hexpm", optional: false]}, {:plug_cowboy, "~> 2.7", [hex: :plug_cowboy, repo: "hexpm", optional: true]}, {:plug_crypto, "~> 2.2", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}, {:websock_adapter, "~> 0.5", [hex: :websock_adapter, repo: "hexpm", optional: false]}], "hexpm", "7b83ed6b3d544f24a29277eab7f051be38b76f390bb511bb6ddb7ec6e8e05b95"},

blogex/mutation_scope.txt

@@ -0,0 +1,8 @@
+lib/blogex/link_validator.ex
+lib/blogex/seo.ex
+lib/blogex/feed.ex
+lib/blogex/post.ex
+lib/blogex/external_links.ex
+lib/blogex/registry.ex
+lib/blogex/not_found_error.ex
+lib/blogex/link_error.ex

blogex/mutation_scope_fast.txt

@@ -0,0 +1,3 @@
+lib/blogex/link_validator.ex
+lib/blogex/seo.ex
+lib/blogex/post.ex

blogex/test/blogex/link_validator_test.exs

@@ -245,6 +245,24 @@ defmodule Blogex.LinkValidatorTest do
   end
 
   describe "validate_body/2" do
+    # The guard is a conjunction, not a disjunction: both arguments must have
+    # the documented types. Mutating `and` to `or` would let a string blog id
+    # (or a non-binary body) through the guard to crash somewhere else, so pin
+    # the contract at the door.
+    test "rejects a non-atom blog id at the function clause guard" do
+      assert_raise FunctionClauseError, fn ->
+        # apply/3 keeps the compiler's type checker quiet: the point is the
+        # runtime guard, and the test deliberately passes an invalid type.
+        apply(LinkValidator, :validate_body, ["Just text, no links.", "engineering"])
+      end
+    end
+
+    test "rejects a non-binary body at the function clause guard" do
+      assert_raise FunctionClauseError, fn ->
+        apply(LinkValidator, :validate_body, [[], :engineering])
+      end
+    end
+
     test "returns :ok when body has no internal blog links" do
       body = "Just text, no links."
 

blogex/test/mutation_config_test.exs

@@ -0,0 +1,82 @@
+defmodule Blogex.MutationConfigTest do
+  # The mutation sensor is opt-in: a dependency and two aliases, deliberately
+  # absent from `make verify`'s VERIFY_SENSORS. These pins keep the wiring
+  # honest (dev/test only, never runtime) and keep the aliases reading the
+  # scope files rather than duplicating a list in mix.exs.
+  use ExUnit.Case
+
+  test "muex is a dev/test-only, non-runtime dependency" do
+    deps = Mix.Project.config()[:deps]
+
+    assert {:muex, requirement, opts} = dep!(deps, :muex)
+    assert requirement =~ "0.12"
+    assert opts[:only] == [:dev, :test]
+    assert opts[:runtime] == false
+  end
+
+  test "mutate and mutate.fast aliases are defined" do
+    aliases = Mix.Project.config()[:aliases]
+
+    assert is_function(aliases[:mutate], 1)
+    assert is_function(aliases[:"mutate.fast"], 1)
+  end
+
+  test "mutation args always include the scope, the curated mutators and --fail-at 0" do
+    args = Blogex.MixProject.mutation_args("mutation_scope_fast.txt", [])
+    scope = Blogex.MixProject.read_mutation_scope("mutation_scope_fast.txt")
+
+    assert "--fail-at" in args
+    assert Enum.at(args, Enum.find_index(args, &(&1 == "--fail-at")) + 1) == "0"
+    assert files_arg(args) == Enum.join(scope, ",")
+
+    mutators = Enum.at(args, Enum.find_index(args, &(&1 == "--mutators")) + 1)
+
+    assert mutators ==
+             "arithmetic,boolean,comparison,conditional,function_call,literal,return_value"
+  end
+
+  test "extra CLI args pass through so a single file can be targeted" do
+    args =
+      Blogex.MixProject.mutation_args("mutation_scope_fast.txt", ["--files", "lib/blogex/seo.ex"])
+
+    assert args
+           |> Enum.chunk_every(2, 1, :discard)
+           |> Enum.any?(fn
+             ["--files", "lib/blogex/seo.ex"] -> true
+             _ -> false
+           end)
+  end
+
+  test "an empty scope fails loudly instead of running over nothing" do
+    path =
+      Path.join(System.tmp_dir!(), "muex-empty-scope-#{System.unique_integer([:positive])}.txt")
+
+    File.write!(path, "\n   \n")
+    on_exit(fn -> File.rm(path) end)
+
+    assert_raise Mix.Error, ~r/empty/, fn ->
+      Blogex.MixProject.read_mutation_scope(path)
+    end
+  end
+
+  defp dep!(deps, name) do
+    case Enum.find(deps, fn
+           {^name, _requirement} -> true
+           {^name, _requirement, _opts} -> true
+           _ -> false
+         end) do
+      {^name, requirement} -> {name, requirement, []}
+      {^name, requirement, opts} -> {name, requirement, opts}
+      nil -> flunk("expected #{inspect(name)} in deps/0")
+    end
+  end
+
+  defp files_arg(args) do
+    args
+    |> Enum.chunk_every(2, 1, :discard)
+    |> Enum.find_value(fn
+      ["--files", files] -> files
+      _ -> nil
+    end)
+  end
+end

blogex/test/mutation_scope_test.exs

@@ -0,0 +1,64 @@
+defmodule Blogex.MutationScopeTest do
+  # The mutation scope files are the single source of truth for what `mix
+  # mutate` / `mix mutate.fast` are allowed to touch. These pins keep the
+  # sensor honest: a scope that quietly shrinks to an empty (or stale) set
+  # would report a score for a check it never ran.
+  use ExUnit.Case
+
+  @root Path.expand("..", __DIR__)
+  @full "mutation_scope.txt"
+  @fast "mutation_scope_fast.txt"
+
+  # Pure modules only. These are excluded deliberately:
+  #   blogex.ex      facade + moduledoc, nothing to mutate
+  #   blogex/blog.ex the `__using__` macro (NimblePublisher expansion)
+  #   router.ex      Phoenix router DSL
+  #   components.ex  Phoenix component DSL
+  #   layout.ex      Phoenix/LiveView layout DSL
+  @shell_modules [
+    "lib/blogex.ex",
+    "lib/blogex/blog.ex",
+    "lib/blogex/router.ex",
+    "lib/blogex/components.ex",
+    "lib/blogex/layout.ex"
+  ]
+
+  test "full scope lists only existing files" do
+    missing = Enum.reject(scope(@full), &File.exists?(Path.join(@root, &1)))
+
+    assert missing == [],
+           "mutation scope names files that do not exist: #{inspect(missing)}"
+  end
+
+  test "full scope is not empty" do
+    assert scope(@full) != []
+  end
+
+  test "fast scope is not empty" do
+    assert scope(@fast) != []
+  end
+
+  test "fast scope is a subset of full scope" do
+    out_of_scope = MapSet.difference(MapSet.new(scope(@fast)), MapSet.new(scope(@full)))
+
+    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 excludes the macro and shell modules" do
+    scoped = MapSet.new(scope(@full) ++ scope(@fast))
+    present = Enum.filter(@shell_modules, &MapSet.member?(scoped, &1))
+
+    assert present == [],
+           "shell/macro modules belong outside the mutation scope: #{inspect(present)}"
+  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

doc/mutation-testing.md

@@ -0,0 +1,163 @@
+# Mutation testing with muex
+
+Mutation testing is a **sensor, not a gate**. It deliberately breaks
+production code one mutant at a time and re-runs the suite: a mutant *killed*
+by a test is evidence the test pins that behaviour; a mutant that *survives*
+is a question for a Lens session. Runs always pass `--fail-at 0`, so a survivor
+is reported and triaged, never a failing build.
+
+Nothing here is wired into `VERIFY_SENSORS`. `make check`, `make verify`,
+coverage floors and `.githooks/pre-push` are unchanged by this yak.
+
+## How to run it
+
+Both apps declare two aliases in `mix.exs`; both read a plain-text scope file
+at run time, so the file stays the single source of truth.
+
+| Command | Scope | Blogex wall clock (2026-10-28) |
+|---------|-------|--------------------------------|
+| `mix mutate.fast` | `mutation_scope_fast.txt` (3 modules) | ~3 s |
+| `mix mutate` | `mutation_scope.txt` (8 modules) | ~4 s |
+
+The app declares the same aliases but both scope files are empty (see
+"Scope: app" below), so `mix mutate` there raises a clear Mix error instead of
+running a sensor over nothing.
+
+Extra arguments pass through to muex, so a one-off target is explicit:
+
+```bash
+cd blogex
+mix mutate --files lib/blogex/seo.ex          # one module, ad hoc
+mix mutate.fast --no-optimize                 # full mutant list, slower
+```
+
+The aliases pin `--fail-at 0` first, so an ad-hoc flag cannot turn the sensor
+into a gate. `mix mutate` does not enable `--cover`: muex runs each mutant's
+suite under `MIX_ENV=test`, and coverage would let a code-deleting mutant trip
+the project's coverage floor and abort the run.
+
+## Scope: blogex
+
+| In scope (`mutation_scope.txt`) | Why |
+|---------------------------------|-----|
+| `lib/blogex/link_validator.ex` | Pure link parsing/validation; the yak's example module. |
+| `lib/blogex/seo.ex` | Pure sitemap/JSON-LD/OpenGraph string building. |
+| `lib/blogex/feed.ex` | Pure feed generation. |
+| `lib/blogex/post.ex` | Pure post parsing/visibility helpers. |
+| `lib/blogex/external_links.ex` | Pure HTML rewriting for external links. |
+| `lib/blogex/registry.ex` | Module-registry lookup; no I/O. |
+| `lib/blogex/not_found_error.ex` | Exception definition. |
+| `lib/blogex/link_error.ex` | Exception definition. |
+
+| Out of scope | Why |
+|--------------|-----|
+| `lib/blogex.ex` | Facade + moduledoc: delegation and docs, no logic to mutate. |
+| `lib/blogex/blog.ex` | A `defmacro __using__` that expands `NimblePublisher`; mutating macro bodies produces noise, not behaviour. |
+| `lib/blogex/router.ex` | Phoenix router DSL (`use`, `scope`, `get`); macro-generated. |
+| `lib/blogex/components.ex` | Phoenix component DSL (`use`, `attr`, `slot`, `~H`). |
+| `lib/blogex/layout.ex` | LiveView layout DSL. |
+
+`mutation_scope_fast.txt` is a subset (`link_validator`, `seo`, `post`) so the
+inner loop stays seconds long; a pin in
+`test/mutation_scope_test.exs` fails if it ever drifts outside the audited
+scope. Another pin fails if any scoped path stops existing, so a rename cannot
+silently leave the sensor reporting a score for a reduced set.
+
+## Mutators
+
+The aliases pass an explicit curated set (muex 0.12 CLI names):
+
+```
+arithmetic, boolean, comparison, conditional, function_call, literal, return_value
+```
+
+Macro/`use`-based mutators are deliberately absent. The scoped modules are plain
+functions, and the macro-heavy modules are out of scope; macro mutations produce
+false-positive survivors rather than behavioural questions. The `arithmetic`,
+`boolean`, `comparison` and `conditional` families are the behavioural core;
+`function_call`, `literal` and `return_value` catch removed calls, changed
+constants and stubbed return values.
+
+## Baseline (blogex, 2026-10-28)
+
+Command: `mix mutate` (default muex optimization, `--fail-at 0`).
+Result: **19 mutants, 12 killed, 0 survived, 0 invalid, 7 equivalent — 100%.**
+Wall clock: 3.5 s. The run that found the survivor below was 11 killed / 1
+survived (91.67%) before its pin landed.
+
+| Module | Mutants | Killed | Survived | Equivalent |
+|--------|--------:|-------:|---------:|-----------:|
+| `link_validator.ex` | 7 | 4 | 0 | 3 |
+| `external_links.ex` | 6 | 5 | 0 | 1 |
+| `registry.ex` | 4 | 1 | 0 | 3 |
+| `post.ex` | 1 | 1 | 0 | 0 |
+| `seo.ex` | 1 | 1 | 0 | 0 |
+| `feed.ex` | 0 | 0 | 0 | 0 |
+| `not_found_error.ex` | 0 | 0 | 0 | 0 |
+| `link_error.ex` | 0 | 0 | 0 | 0 |
+
+muex's file analyser gives `not_found_error.ex` a complexity score of 0 and
+skips it under the default filter. The scope file still lists it: it is pure
+code and stays audited if it grows logic. `feed.ex` is analysed but the curated
+mutator set generates nothing for it.
+
+### Survivors found and killed
+
+| Mutant | Resolution |
+|--------|------------|
+| `link_validator.ex:201` — `Boolean: and to or` in `validate_body/3`'s guard `when is_binary(body) and is_atom(blog)` | Genuine: the mutant let a string blog id through the guard. Pinned by two `FunctionClauseError` tests in `test/blogex/link_validator_test.exs`; `mix mutate.fast` then reports 0 survivors. |
+
+## False-positive classes
+
+These are not gaps in behaviour; do not weaken a test to "kill" them.
+
+- **Commutative argument swaps.** `function_call` mutates `a and b` to
+  `b and a`, `a == b` to `b == a`, and `a || b` to `b || a`. These compile to
+  the same behaviour, and muex's trivial-compiler-equivalence check marks them
+  `equivalent` (7 of the 19 baseline mutants). They are noise from the
+  mutator, not survivors.
+- **Removed log lines.** A removed `Logger` call changes no return value; the
+  suite cannot observe it. Such a mutant is documented here instead of adding
+  a log-capture assertion that pins an implementation detail.
+- **`and`/`or` in guards is *not* automatically a false positive.** The
+  baseline survivor above was a real guard-contract gap; only commutative
+  rewrites are equivalent.
+- **Invalid mutants.** Mutated code that does not compile is counted
+  `invalid`, not `survived`, and excluded from the score.
+
+## Deeper runs
+
+muex's default `balanced` optimization reduced 1304 generated mutants to 12
+tested ones (99.1%) on the blogex full scope. That is what keeps the sensor in
+seconds, but it is sampled, not exhaustive. `mix mutate --no-optimize` tests
+everything; on the three-module fast scope it took ~2 min 15 s and produced
+434 mutants (329 killed, 19 survived, 83 invalid, 3 equivalent) before the
+guard pin. Use it when triaging a specific module, not in a loop.
+
+Re-baseline after a yak that changes the scoped modules (notably the Boundary
+work, which adds `use` lines inside them); do not compare scores across such a
+change.
+
+## Scope: app
+
+**Deliberately empty; no app baseline was run.** The candidate pure modules
+are `Firehose.Accounts.User`, `Firehose.Accounts.Scope` and
+`Firehose.Accounts.UserToken`, but every test that exercises them lives in
+`test/firehose/accounts_test.exs`, which uses `Firehose.DataCase`, fixtures and
+`Repo`; the SQL sandbox needs PostgreSQL. The yak's Q45 decision was: use those
+modules only after confirming their tests need no database, otherwise leave the
+app scope empty. They need one, so the scope is empty.
+
+Consequences, all pinned by `app/test/mutation_scope_test.exs` and
+`app/test/mutation_config_test.exs`:
+
+- Both app scope files list nothing, so `mix mutate` / `mix mutate.fast` raise
+  `mutation scope … is empty; refusing to run the mutation sensor over nothing`.
+- Ad-hoc `--files` arguments do not bypass the guard: the scope is validated
+  before muex arguments are built.
+- No scope entry may ever begin with `lib/firehose_web` (controllers,
+  LiveViews, components).
+
+There is no app mutation baseline to report. Do not add one until the scoped
+modules have tests that run without the database; change the scope file and
+these pins in the same commit.