feat(cicd_rules): duplicate_cron_schedule (#362) (#406)

Detect workflows whose on.schedule carries redundant cron triggers: 2+ entries on the same day-of-week field, or a daily `* * *` entry strictly covering a day-specific entry at the same HH:MM. Adds scan_/check_ functions, facade defdelegate, tests, and changelog entries.

Verified at source (Elixir 1.14 locally): mix-format clean, isolated compile with zero warnings, and real-module oracles from hypatia#331 (tests.yml subset, verify-proofs.yml three-Monday, security-policy.yml negative).

Closes #362

https://claude.ai/code/session_01J8oLNn6MjKDRRUF65e2jLf

Author: hyperpolymath

CHANGELOG.adoc

@@ -12,6 +12,23 @@ https://semver.org/[Semantic Versioning].
 
 === Added
 
+==== `CicdRules` rule `duplicate_cron_schedule` (2026-05-30, #362)
+
+Flags workflows whose `on.schedule` carries redundant `cron:` triggers: two
+or more entries on the same day-of-week field (e.g. three Monday crons
+inherited from a workflow consolidation), or a daily `* * *` entry that
+strictly covers a day-specific entry at the same `HH:MM`. Root-fixed
+estate-wide in `hypatia#331` (`tests.yml`, `verify-proofs.yml`); distinct
+intentional triggers (different times, or different `if:` gates on the same
+day) are advisory only and never auto-removed.
+
+Surfaced through the facade as
+`Hypatia.Rules.scan_duplicate_cron_schedules/1`; the pure predicate
+`CicdRules.check_duplicate_cron_schedules/2` is covered by
+`test/rules/cicd_rules_duplicate_cron_test.exs` for both sensitivity
+(same-day-of-week + daily-subset oracles) and specificity (different-time
+non-subset, single cron, commented-out lines).
+
 ==== `CicdRules` rule `scorecard_wrapper_missing_job_permissions` (2026-05-30, #390)
 
 New forward-detection rule for a silent-CI-failure class: a

CHANGELOG.md

@@ -20,6 +20,7 @@ this project aims to follow [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- feat(rules): CicdRules `duplicate_cron_schedule` — flag workflows with redundant cron entries on the same day-of-week / daily-subset (#362)
 - feat(rules): CicdRules `scorecard_wrapper_missing_job_permissions` — flag scorecard.yml wrappers that call the standards reusable but omit `security-events: write` on the calling job (#390)
 - feat(rules): AffineScript hand-port pitfalls — HANDLE-as-fn-name + OCaml float ops (#332)
 - feat(rules): wire 4 new rule modules through the facade (#326)

lib/rules/cicd_rules.ex

@@ -930,6 +930,116 @@ defmodule Hypatia.Rules.CicdRules do
     rel == @scorecard_wrapper_path or String.ends_with?(rel, "/" <> @scorecard_wrapper_path)
   end
 
+  # ---------------------------------------------------------------------------
+  # Duplicate cron schedules (#362)
+  # ---------------------------------------------------------------------------
+  #
+  # A workflow whose `on.schedule` lists multiple `cron:` entries that fire on
+  # the same day-of-week is usually a consolidation artefact (merged-in source
+  # workflows each carrying their own trigger): wasted runner minutes and log
+  # noise. Two shapes are flagged:
+  #
+  #   1. >= 2 entries sharing the same day-of-week field (e.g. three `* * 1`
+  #      Monday crons inherited from pre-consolidation workflows).
+  #   2. A daily entry (`* * *`) and a day-specific entry at the same HH:MM —
+  #      the specific entry is a strict subset of the daily one.
+  #
+  # Root-fixed estate-wide in hypatia#331 (tests.yml, verify-proofs.yml).
+  # Distinct intentional triggers (different HH:MM, or different `if:` gates on
+  # the same day) are advisory under shape (1) and never auto-removed.
+
+  @doc """
+  Scan `repo_path`'s workflow files for redundant `cron:` schedules (#362).
+
+  Reads each `.github/workflows/*.{yml,yaml}` (root or nested monorepo copy)
+  and returns the findings from `check_duplicate_cron_schedules/2` for each.
+
+  Returns `[%{rule:, severity:, file:, reason:, fix:, crons:}]`.
+  """
+  def scan_duplicate_cron_schedules(repo_path) do
+    Path.wildcard("#{repo_path}/**/*", match_dot: true)
+    |> Enum.reject(&File.dir?/1)
+    |> Enum.map(&Path.relative_to(&1, repo_path))
+    |> Enum.filter(&workflow_file?/1)
+    |> Enum.flat_map(fn rel ->
+      case File.read(Path.join(repo_path, rel)) do
+        {:ok, content} -> check_duplicate_cron_schedules(rel, content)
+        {:error, _} -> []
+      end
+    end)
+  end
+
+  @doc """
+  Pure predicate behind `scan_duplicate_cron_schedules/1`: parse the `cron:`
+  list entries in workflow `content` and return findings for `path`. Lines
+  that are not 5-field cron expressions (and commented-out `# - cron:` lines)
+  are ignored.
+  """
+  def check_duplicate_cron_schedules(path, content) do
+    crons = parse_cron_entries(content)
+
+    same_dow =
+      crons
+      |> Enum.group_by(& &1.dow)
+      |> Enum.filter(fn {_dow, list} -> length(list) >= 2 end)
+      |> Enum.map(fn {dow, list} ->
+        raws = Enum.map(list, & &1.raw)
+
+        %{
+          rule: :duplicate_cron_schedule,
+          severity: :medium,
+          file: path,
+          reason:
+            "#{length(list)} cron entries fire on the same day-of-week (#{dow_label(dow)}): #{Enum.join(raws, ", ")} — likely a consolidation artefact.",
+          fix:
+            "Collapse to a single cron unless the triggers serve distinct purposes (e.g. different `if:` gates); otherwise drop the redundant entries.",
+          crons: raws
+        }
+      end)
+
+    daily = Enum.filter(crons, &(&1.dow == "*"))
+    specific = Enum.reject(crons, &(&1.dow == "*"))
+
+    subsets =
+      for d <- daily, s <- specific, d.min == s.min and d.hour == s.hour do
+        %{
+          rule: :duplicate_cron_schedule,
+          severity: :medium,
+          file: path,
+          reason:
+            "cron `#{s.raw}` is a strict subset of the daily cron `#{d.raw}` (same HH:MM); the daily trigger already covers that day.",
+          fix:
+            "Drop the day-specific entry `#{s.raw}` — the daily `#{d.raw}` already runs at that time.",
+          crons: [d.raw, s.raw]
+        }
+      end
+
+    same_dow ++ subsets
+  end
+
+  defp workflow_file?(rel) do
+    String.contains?(rel, ".github/workflows/") and
+      (String.ends_with?(rel, ".yml") or String.ends_with?(rel, ".yaml"))
+  end
+
+  defp parse_cron_entries(content) do
+    ~r/^\s*-\s*cron:\s*["']?([\d*,\/\- ]+?)["']?\s*(?:#[^\n]*)?$/m
+    |> Regex.scan(content)
+    |> Enum.map(fn [_, expr] -> String.trim(expr) end)
+    |> Enum.map(&parse_one_cron/1)
+    |> Enum.reject(&is_nil/1)
+  end
+
+  defp parse_one_cron(expr) do
+    case String.split(expr, " ", trim: true) do
+      [min, hour, _dom, _mon, dow] -> %{raw: expr, min: min, hour: hour, dow: dow}
+      _ -> nil
+    end
+  end
+
+  defp dow_label("*"), do: "every day"
+  defp dow_label(dow), do: "day-of-week #{dow}"
+
   # ---------------------------------------------------------------------------
   # CI/CD Waste Detection
   # ---------------------------------------------------------------------------

lib/rules/rules.ex

@@ -428,6 +428,12 @@ defmodule Hypatia.Rules do
   """
   defdelegate scan_scorecard_wrapper_permissions(repo_path, opts \\ []), to: CicdRules
 
+  @doc """
+  Scan workflows for redundant `cron:` schedules firing on the same
+  day-of-week (#362).
+  """
+  defdelegate scan_duplicate_cron_schedules(repo_path), to: CicdRules
+
   @doc """
   Run baseline-health checks (BH001-BH007): missing required_status_checks
   on main, deferred-migration TODOs in dep manifests, persistent >24h red

test/rules/cicd_rules_duplicate_cron_test.exs

@@ -0,0 +1,75 @@
+# SPDX-License-Identifier: MPL-2.0
+
+defmodule Hypatia.Rules.CicdRules.DuplicateCronTest do
+  use ExUnit.Case, async: true
+
+  alias Hypatia.Rules.CicdRules
+
+  # #362 — multiple `cron:` entries firing on the same day-of-week are a
+  # consolidation-artefact waste pattern. Two shapes: same exact day-of-week
+  # field (>= 2 entries), and a daily `* * *` entry strictly covering a
+  # day-specific entry at the same HH:MM. Oracles from hypatia#331.
+
+  @three_monday "on:\n  schedule:\n    - cron: 0 5 * * 1\n    - cron: 30 4 * * 1\n    - cron: 0 4 * * 1\n"
+  @daily_plus_monday "on:\n  schedule:\n    - cron: \"0 3 * * *\"\n    - cron: \"0 3 * * 1\"\n"
+  @daily_plus_other_time "on:\n  schedule:\n  - cron: 0 2 * * *\n  - cron: 0 3 * * 1\n"
+  @single "on:\n  schedule:\n    - cron: '0 3 * * *'\n"
+  @commented "on:\n  schedule:\n    - cron: 0 4 * * 1\n    # - cron: 0 5 * * 1\n"
+
+  defp wf(dir, name, body) do
+    path = Path.join([dir, ".github/workflows", name])
+    File.mkdir_p!(Path.dirname(path))
+    File.write!(path, body)
+  end
+
+  setup do
+    dir = Path.join(System.tmp_dir!(), "hyp-cron-#{:erlang.unique_integer([:positive])}")
+    File.mkdir_p!(dir)
+    on_exit(fn -> File.rm_rf!(dir) end)
+    {:ok, dir: dir}
+  end
+
+  describe "check_duplicate_cron_schedules/2 — sensitivity" do
+    test "three crons on the same day-of-week (verify-proofs.yml oracle)" do
+      assert [finding] =
+               CicdRules.check_duplicate_cron_schedules("verify-proofs.yml", @three_monday)
+
+      assert finding.rule == :duplicate_cron_schedule
+      assert finding.severity == :medium
+      assert length(finding.crons) == 3
+    end
+
+    test "daily strictly covers a same-time day-specific entry (tests.yml oracle)" do
+      assert [finding] = CicdRules.check_duplicate_cron_schedules("tests.yml", @daily_plus_monday)
+      assert finding.rule == :duplicate_cron_schedule
+      assert finding.reason =~ "subset"
+    end
+  end
+
+  describe "check_duplicate_cron_schedules/2 — specificity" do
+    test "daily + different-time day-specific is NOT a subset (security-policy.yml)" do
+      assert CicdRules.check_duplicate_cron_schedules(
+               "security-policy.yml",
+               @daily_plus_other_time
+             ) ==
+               []
+    end
+
+    test "a single cron is never flagged" do
+      assert CicdRules.check_duplicate_cron_schedules("x.yml", @single) == []
+    end
+
+    test "commented-out cron lines are ignored" do
+      assert CicdRules.check_duplicate_cron_schedules("x.yml", @commented) == []
+    end
+  end
+
+  describe "scan_duplicate_cron_schedules/1" do
+    test "walks workflow files and reports only the offending one", %{dir: dir} do
+      wf(dir, "clean.yml", @single)
+      wf(dir, "dirty.yml", @three_monday)
+      assert [finding] = CicdRules.scan_duplicate_cron_schedules(dir)
+      assert finding.file == ".github/workflows/dirty.yml"
+    end
+  end
+end