home-network-k3s-impl/ci/touched-requirements.sh

#!/usr/bin/env bash
# ci/touched-requirements.sh — compute the requirement subset a change "touches".
#
# R-TEST-03 gates a merge on the conformance subset tagged with the requirement
# IDs the change touches. This script derives that subset from the diff so the
# conformance runner can be filtered to it (conformance.sh --requirements ...).
#
# Output (stdout): newline-separated, sorted, unique R-* IDs. Nothing else goes
# to stdout, so callers can `mapfile`/`$(...)` it directly.
#
# SOURCE OF TRUTH (reconciled, PM 2026-08-16): the conformance requirement TAGS are
# authoritative. Each Chainsaw test declares the requirements it verifies via
# `conformance.home.k3s/<R-ID>` labels; the coverage report (conformance/_artifacts/
# coverage/coverage.json) is generated from those tags. This script's job is only to
# select which tagged subset a change TOUCHES, then hand those IDs to
# `conformance.sh --requirements ...` which runs exactly the tests carrying those tags.
#
# Detection = union of:
#   1. R-* tokens on the ADDED/REMOVED LINES of the diff, in non-Markdown files —
#      including the authoritative `conformance.home.k3s/R-*` tags on changed tests, and
#      the R-IDs that policies/ServiceDefinitions self-declare (docs/14 + ADR-0011).
#      Untouched lines and prose are NOT scanned; see changed_diff() for why.
#   2. A coarse path->requirement fallback for changes that don't self-declare IDs.
#   3. For a change anywhere under `conformance/tests/<test>/`: THAT TEST'S OWN requirement
#      labels, read from its chainsaw-test.yaml. See test_requirements_for_path().
# The coarse map (2) NEVER overrides an explicit tag; it only widens coverage for
# untagged changes so nothing silently escapes the gate. (3) is the same principle applied
# to the tests themselves: editing or weakening a test must be gated by the requirements
# that test CERTIFIES, not merely by R-TEST-0x.
set -euo pipefail
CI_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
# shellcheck disable=SC1091
source "${CI_DIR}/lib.sh"

# ---------------------------------------------------------------------------------
# BASE-REF RESOLUTION — and why it now REFUSES rather than guesses.
#
# BUG THIS FIXES (local/CI divergence, 2026-08-31): every failure to establish a
# reliable diff base used to degrade SILENTLY to an EMPTY touched set, which main()
# reported as "no changed files vs base" and exit 0. An empty set means the coverage
# `--require` gate is never applied, so the merge gate printed "MERGE GATE: PASS"
# having checked NOTHING — while CI, with a correct base, computed 17 touched
# requirements and blocked. A false green is the worst outcome this project can
# produce, so every one of those degradations is now a LOUD exit 3:
#
#   * HEAD~1 last resort  REMOVED. It silently narrowed the diff to the last commit
#                         (and on a one-commit repo failed into an empty set).
#   * two-dot fallback    REMOVED. It masked a failing three-dot diff as "no changes".
#   * unresolvable base   exit 3 (was: HEAD~1 or empty).
#   * diff command fails  exit 3 (was: empty set).
#   * HEAD not ahead of   exit 3 (was: exit 0 with an empty set — this is the case
#     the base                that produced the false green: running the gate while
#                             sitting on `main`, or on a branch already merged).
#
# Exit 3 is the ci/README.md "SKIPPED / not-run" code: NOT a pass, NOT a fail. Callers
# (ci/gate.sh) must surface it as an UNDECIDED merge decision. An honest, noisy skip
# beats a quiet false pass.
#
# Order of preference: BASE_REF (explicit) > GITHUB_BASE_REF (Actions PR runs, target
# branch) > origin/main > main. Each candidate is VERIFIED to resolve to a commit
# before it is used, so a shallow checkout that lacks origin/main is reported, not
# papered over.
: "${BASE_REF:=}"
BASE=""            # resolved base ref
BASE_SOURCE=""     # where it came from (for the log line)
BASE_PROBLEM=""    # why resolution failed, when it did

rev() { git -C "$REPO_ROOT" rev-parse --verify -q "$1^{commit}" 2>/dev/null; }

# Loud, structured refusal. Prints to stderr only; stdout stays empty so a caller that
# captures stdout gets NO requirement list rather than a silently-empty one.
#
# $1 is a machine-readable REASON CODE, also written to $TOUCHED_REASON_FILE when that
# env var names a path. ci/gate.sh uses it to tell the two cases apart:
#   on-base      HEAD is the base branch (or already merged) — nothing to gate. Legitimate
#                (a push-to-main CI run lands here), so the gate reports NOT APPLICABLE
#                rather than PASS, and does not fail the job.
#   no-base      no reliable base ref exists in this checkout — the ENVIRONMENT is broken,
#   diff-failed  or git could not produce the diff. A merge decision is impossible, so the
#                gate BLOCKS rather than guessing.
undecidable() {
  local code="$1"; shift
  if [ -n "${TOUCHED_REASON_FILE:-}" ]; then printf '%s\n' "$code" > "$TOUCHED_REASON_FILE"; fi
  err "TOUCHED-REQUIREMENT SET NOT DETERMINABLE (${code})"
  err "  reason: $*"
  err "  consequence: the R-TEST-03 coverage gate CANNOT be applied to this checkout."
  err "  This is exit 3 (SKIPPED / not-run), never a pass — see ci/README.md."
  err "  Fix one of:"
  err "    * check out the feature branch you mean to gate (not the base branch), or"
  err "    * git fetch origin main   (so origin/main exists and is current), or"
  err "    * BASE_REF=<ref> make -C ci gate   to name the base explicitly."
  exit 3
}

resolve_base() {
  local cand
  if [ -n "$BASE_REF" ]; then
    if rev "$BASE_REF" >/dev/null; then BASE="$BASE_REF"; BASE_SOURCE="BASE_REF"; return 0; fi
    BASE_PROBLEM="BASE_REF='${BASE_REF}' does not resolve to a commit in ${REPO_ROOT}"
    return 1
  fi
  if [ -n "${GITHUB_BASE_REF:-}" ]; then
    for cand in "origin/${GITHUB_BASE_REF}" "${GITHUB_BASE_REF}"; do
      if rev "$cand" >/dev/null; then BASE="$cand"; BASE_SOURCE="GITHUB_BASE_REF"; return 0; fi
    done
    BASE_PROBLEM="GITHUB_BASE_REF='${GITHUB_BASE_REF}' is set, but neither 'origin/${GITHUB_BASE_REF}' nor '${GITHUB_BASE_REF}' exists here (shallow checkout? the workflow needs fetch-depth: 0)"
    return 1
  fi
  for cand in origin/main main; do
    if rev "$cand" >/dev/null; then BASE="$cand"; BASE_SOURCE="default"; return 0; fi
  done
  BASE_PROBLEM="no base ref resolves (tried origin/main, main) in ${REPO_ROOT}"
  return 1
}

# Both diff helpers assume BASE is already resolved and validated by main(). They return
# git's exit status so the CALLER can distinguish "no changes" from "the diff failed" —
# conflating those two is exactly what produced the false green.
changed_files() {
  # Three-dot: changes on this branch since it diverged from base.
  git -C "$REPO_ROOT" diff --name-only "${BASE}...HEAD"
}

# The ADDED/REMOVED lines of the change, for the R-ID content scan — excluding
# documentation.
#
# THREE scoping rules, all learned from the gate blocking merges it should not have:
#
#  1. Scan the DIFF, not whole file contents. Grepping whole files meant a change
#     inherited every requirement those files had EVER mentioned, on lines the author
#     never touched. Since some of those are legitimately uncovered, `--require-all`
#     then failed and BLOCKED the merge over unrelated prose.
#
#  2. Skip `*.md`. A requirement ID in prose is a REFERENCE, not a declaration —
#     explaining a requirement is not the same as changing behaviour against it.
#     Without this, the decision log cannot record which requirements a change
#     concerned without thereby claiming to have touched them, and the CI-fix commit
#     that explains a mis-attribution re-triggers the very mis-attribution it fixes.
#     Nothing escapes: authoritative declarations (`conformance.home.k3s/R-*` tags,
#     Kyverno policies, ServiceDefinitions) all live in YAML, and the doc subtrees
#     that matter (conformance/, schemas/, catalog/, clusters/, ci/) are covered by
#     the path->requirement fallback regardless of file type.
#
#  3. Skip COMMENT-ONLY lines in non-Markdown files — rule 2's logic, applied to the
#     prose that lives INSIDE code files. A requirement ID cited in a YAML/shell `#`
#     comment is documentation, not a declaration, exactly like an ID in a `*.md` file.
#     Rule 2 drops whole Markdown files, but a `.gitea/workflows/*.yml` or `ci/*.sh`
#     HEADER that explains a requirement would otherwise re-attribute it. Concretely:
#     gate.yml's header cites R-TEST-06 (enforced by the spec-immutability STAGE, not by
#     any Chainsaw test, so a known-uncovered ID); before this rule, editing that comment
#     re-attributed R-TEST-06 and the coverage `--require` check false-blocked the merge —
#     the same self-referential trap rule 2 fixed for `*.md`. Applied in main()'s content
#     scan (see there): after the diff marker is stripped, a line whose first non-blank
#     character is `#` is dropped BEFORE the R-ID grep. Deliberately conservative — only
#     WHOLE-LINE comments are dropped; an R-ID on a real code/spec line is still detected
#     (authoritative declarations — test tags, policies, ServiceDefinitions — are never a
#     bare comment), so this only narrows PROSE, never a declaration.
#
# Removed lines count: deleting a test's requirement tag, or a policy's self-declared
# ID, touches that requirement just as much as adding one.
# `:(exclude)*.md` drops Markdown from the content scan (rule 2 above).
# `:(exclude)ci/tests/` drops this dir's OWN unit-test fixtures: those files embed example
#   R-IDs as DATA (asserting the scan's behaviour), never as declarations — the same
#   "reference, not declaration" logic as rule 2. Their gate coverage is unaffected: any
#   ci/ change (ci/tests/ included) still attributes R-GITOPS-01/R-TEST-02/R-TEST-03 via the
#   path->requirement map below. NOTE: this excludes ci/tests/ only — conformance/tests/
#   carry AUTHORITATIVE `conformance.home.k3s/R-*` labels and are deliberately still scanned.
# Three-dot to match changed_files(): changes on this branch since it diverged from base.
changed_diff() {
  local -a excl=(':(exclude)*.md' ':(exclude)ci/tests/')
  git -C "$REPO_ROOT" diff -U0 "${BASE}...HEAD" -- . "${excl[@]}"
}

# Coarse path -> requirement map. Deliberately conservative; extend as subtrees
# fill in. Left side is a path prefix (relative to repo root).
path_requirements() {
  case "$1" in
    catalog/*)                 echo "R-GITOPS-02" ;;
    schemas/*)                 echo "R-GITOPS-02" ;;
    # apps/* was UNMAPPED until 2026-09-19, and it is the largest such hole in the map: 49 tracked
    # manifests (apps/wave-0, -1, -2, apps/vms) that describe every workload on the cluster. A change
    # adding or editing an app attributed NOTHING, so the merge gate ran only the whole-repo-scoped
    # checks and none of the requirement-scoped L2 behaviour tests — default-deny enforcement, egress
    # class enforcement, auth-mode enforcement — which are exactly what a new or edited app can
    # violate. Kyverno admission and the L0 egress-path check are real backstops, but "admission will
    # catch it at apply time" is post-merge, and the gate exists to catch it before.
    #
    # ORDER MATTERS: `case` takes the FIRST match, so apps/vms/* must precede apps/*.
    # apps/vms/ are KubeVirt VMs (R-PLAT-03 lifecycle) with root/data volumes (R-STATE-01), still
    # subject to default-deny (R-NET-01).
    apps/vms/*)                echo "R-PLAT-03 R-NET-01 R-STATE-01" ;;
    # Everything else under apps/ is a ServiceDefinition rendered by catalog/render.py: a container
    # workload (R-PLAT-02) rendered from the catalog (R-GITOPS-02), carrying the egress-class
    # (R-NET-05/06/07) and auth-mode (R-IDP-02/R-IDP-06) labels that admission requires, under
    # default-deny (R-NET-01).
    apps/*)                    echo "R-PLAT-02 R-GITOPS-02 R-NET-01 R-NET-05 R-NET-06 R-NET-07 R-IDP-02 R-IDP-06" ;;
    platform/kyverno/*)        echo "R-TEST-01 R-NET-01 R-SEC-01" ;;
    platform/cilium/*)         echo "R-NET-01 R-NET-02 R-NET-05" ;;
    platform/longhorn/*)       echo "R-STATE-01 R-STATE-02" ;;
    platform/cert-manager/*)   echo "R-TLS-01 R-TLS-02" ;;
    platform/externaldns/*)    echo "R-DNS-02" ;;
    platform/velero/*)         echo "R-STATE-03" ;;
    platform/monitoring/*)     echo "R-OBS-01 R-OBS-02" ;;
    # platform/cnpg was UNMAPPED until 2026-09-06. It is the PostgreSQL DR path for keycloak-db /
    # nextcloud-db / semaphore-db (barmanObjectStore WAL archiving + ScheduledBackup), so a change
    # there is an R-IDP-05 / R-STATE-03 change; it now also carries the platform-health alert rules
    # for that path (R-OBS-04). Without this line an edit under platform/cnpg/ contributed ZERO
    # requirements unless it happened to retype an R-ID on a non-comment line, and escaped the gate.
    platform/cnpg/*)           echo "R-IDP-05 R-STATE-03 R-OBS-04" ;;
    # vpn-egress was UNMAPPED until 2026-09-06. platform/vpn-egress/egress-steering.yaml IS the
    # R-NET-04 datapath (the rule that selects between the two egress paths) and the R-NET-05/06/07
    # steering policy — it even self-declares those IDs in its `homelab.keithjackson.dev/requirements`
    # annotation. But the content scan only reads ADDED/REMOVED lines, so any edit that does not
    # happen to retype an R-ID (a selector change, an excludedCIDRs change, a comment) contributed
    # ZERO requirements and escaped the gate entirely. Mapped explicitly; the annotation still wins
    # when a diff does carry the tags.
    platform/vpn-egress/*)     echo "R-NET-04 R-NET-05 R-NET-06 R-NET-07" ;;
    # Keycloak is the whole SSO surface: the realm import (declarative identity config),
    # the SOPS client secrets, and the reconciler that heals them. Added 2026-09-06 after
    # the client-secret incident, in which editing platform/keycloak/ attributed NO
    # requirement, so the L2 keycloak tests were filtered out of the merge gate's touched
    # subset — the change that could break every OIDC login was gated by nothing.
    # R-IDP-05 is in scope because platform/keycloak/ also carries the CNPG DB Cluster and
    # its off-cluster backup credential, not only the realm.
    platform/keycloak/*)       echo "R-IDP-01 R-IDP-02 R-IDP-03 R-IDP-04 R-IDP-05 R-IDP-06" ;;
    platform/oauth2-proxy/*)   echo "R-IDP-02 R-IDP-06" ;;
    platform/image-automation/*) echo "R-UPD-01 R-UPD-02" ;;
    # The C2 UI was UNMAPPED until 2026-09-19. This is the gap named explicitly in changed_diff()'s
    # rule-3 comment above: "path_requirements() has no arm for platform/homepage/* or
    # platform/headlamp/* either, so the ONLY route to R-C2-01/02 was rule 3 (editing the C2 test
    # itself)". homepage + headlamp ARE the command-centre surface (registry: R-C2-01/02 ->
    # docs/12-command-center-ui.md), so editing them must select the C2 tests.
    platform/homepage/*|platform/headlamp/*) echo "R-C2-01 R-C2-02" ;;
    # KubeVirt is the VM platform itself (R-PLAT-03); its virt-* components are also scrape targets.
    platform/kubevirt/*)       echo "R-PLAT-03 R-OBS-01" ;;
    # The shared ingress Gateway: the L2-announced VIP every HTTP service answers on (R-DNS-01), the
    # TLS listeners and their certificates (R-TLS-01/02), under default-deny (R-NET-01).
    platform/gateway/*)        echo "R-DNS-01 R-TLS-01 R-TLS-02 R-NET-01" ;;
    # CoreDNS is cluster DNS resolution (R-DNS-01).
    platform/coredns/*)        echo "R-DNS-01" ;;
    # crafty is a first-class app living under platform/: a container workload with a Longhorn PVC and
    # its OWN L4 LoadBalancer VIP (Minecraft 25565 is not HTTP, so it does not use the Gateway).
    platform/crafty/*)         echo "R-PLAT-02 R-NET-01 R-DNS-01 R-STATE-01" ;;
    # Snapshot machinery is the storage/DR path: CSI snapshots (R-STATE-01) feeding backup (R-STATE-03).
    platform/external-snapshotter/*|platform/backup/*) echo "R-STATE-01 R-STATE-03" ;;
    # runbooks/ IS the R-OBS-04 surface: alert-runbooks-resolvable asserts every alert's runbook_url
    # resolves to a served page, and runbooks/serving/ is the Deployment that serves them. That check
    # is whole-repo-scoped so it runs regardless, but the attribution should still be honest — a
    # runbook change is an observability change.
    runbooks/*)                echo "R-OBS-04" ;;
    # platform-config/ is the env-specific configuration Flux substitutes into and applies.
    platform-config/*|platform-config-cdi/*) echo "R-ENV-02 R-GITOPS-01" ;;
    # platform/kustomization.yaml is the ASSEMBLY: the list of platform components Flux applies.
    # Deleting one line there disables an entire component (cilium, longhorn, velero...) while every
    # file it points at stays perfectly valid on disk, so nothing else in the map would attribute the
    # change. That is the most consequential single-line edit in the repo and it attributed nothing.
    platform/kustomization.yaml) echo "R-GITOPS-01" ;;
    # .sops.yaml holds the encryption rules (which paths must be encrypted, with which keys). A change
    # here can silently stop a new secret path from being encrypted, which is R-SEC-01's whole point.
    .sops.yaml)                echo "R-SEC-01 R-GITOPS-01" ;;
    conformance/*)             echo "R-TEST-01 R-TEST-02 R-TEST-03 R-TEST-04" ;;
    # infra/ was UNMAPPED until 2026-09-12 — the OpenTofu + Ansible that provision the dev
    # nodes. conformance/tests/l0-static/dev-nodes-provisioned-as-code reads NOTHING but
    # infra/, yet an infra/ edit attributed ZERO requirements, so the one check that asserts
    # the nodes are provisioned as code was filtered out of every change that could break it.
    # That test's invariant IS path-scoped (it is deliberately NOT whole-repo-scoped; see its
    # chainsaw-test.yaml), and the correct fix for a path-scoped invariant is a path-map entry.
    infra/*)                   echo "R-ENV-01" ;;
    clusters/*)                echo "R-GITOPS-01 R-ENV-02" ;;
    # bootstrap/ was UNMAPPED until 2026-09-06. The entry STAYS, but its original rationale
    # was written against a file that is not here, and that is corrected on 2026-09-13:
    # it claimed "bootstrap/standup.sh carries the placeholder guards that decide whether
    # `velero` may be enabled against a real backup endpoint". bootstrap/standup.sh HAS NEVER
    # BEEN ON `main` — it was not deleted (no deletion commit exists anywhere), it landed only
    # on unmerged prod-standup branches. `git ls-tree main bootstrap/` lists README.md, flux/,
    # gitea/, sops/. So R-STATE-03 here is ANTICIPATORY, not descriptive.
    #
    # WHY IT IS STILL RIGHT. conformance/tests/l0-static/placeholder-guard-coupling DISCOVERS
    # its guard (any tracked *.sh defining `<name>_is_placeholder()`) instead of hardcoding a
    # path, so the moment a standup guard lands under bootstrap/ that test's coupling half
    # activates — and this entry is what selects the test for the change that lands it.
    # Deleting the entry would mean the change introducing a placeholder guard is gated by
    # nothing, which is the 2026-09-05 class (a prefix glob that stopped matching when the
    # value it guards changed, in a different file) left to reintroduce itself. R-GITOPS-01 is
    # descriptive today: bootstrap/flux/, gitea/ and sops/ ARE the GitOps bootstrap path.
    # Locked down by ci/tests/touched-requirements.test.sh's path-map section.
    bootstrap/*)               echo "R-STATE-03 R-GITOPS-01" ;;
    # R-TEST-06 added 2026-09-12. The comment in changed_diff()'s rule 3 above records why it
    # was ABSENT: R-TEST-06 used to be enforced by the spec-immutability STAGE and carried by no
    # Chainsaw test, so attributing it made the coverage `--require` check false-block. It is now
    # carried by conformance/tests/l0-static/spec-immutability-enforced — whose entire input
    # surface is ci/ — so a ci/ change must attribute it, or the test that proves the
    # spec-immutability mechanism is still load-bearing runs on nothing that can break it.
    .gitea/*|ci/*)             echo "R-GITOPS-01 R-TEST-02 R-TEST-03 R-TEST-06" ;;
    *) : ;;
  esac
}

# ---------------------------------------------------------------------------------
# A CHANGED TEST ATTRIBUTES THE REQUIREMENTS IT CERTIFIES (detection rule 3).
#
# THE GAP THIS CLOSES (2026-09-12). The path map keys `conformance/*` to
# R-TEST-01..04 — the requirements about the HARNESS. It says nothing about what a given
# test PROVES. So editing `conformance/tests/l2-behavioral/tenant-isolation/check.sh` to
# weaken or delete an assertion attributed R-TEST-01..04 and nothing else: the merge gate
# then ran the harness's self-tests and NOT the weakened test's own requirement subset.
# Weakening a test is the one change that most needs the requirement it certifies to be
# re-verified, and it was the change least likely to select it.
#
# The `conformance.home.k3s/R-*: "true"` labels on a test ARE the authoritative declaration of
# what it verifies (lib/coverage.py reads the same labels); this reads them directly. Note the
# content scan in main() only sees ADDED/REMOVED lines, so it catches a change to the LABEL
# BLOCK but not a change to the test BODY — which is exactly the case that matters here.
#
# WHY IT READS HEAD *OR* THE BASE: a change that DELETES a test, or deletes its
# chainsaw-test.yaml, must still attribute what that test used to certify — otherwise removing
# the only verifier of a requirement is the cheapest way to make the gate stop checking it.
# HEAD first (the post-change truth), base as the fallback (the deleted case). Fail-closed:
# this function can only ADD requirements, never remove one.
: "${CONF_TEST_LABEL_PREFIX:=conformance.home.k3s}"   # == CONF_LABEL_PREFIX, conformance/lib/common.sh

file_text_head_or_base() {
  local path="$1"
  git -C "$REPO_ROOT" show "HEAD:${path}" 2>/dev/null && return 0
  git -C "$REPO_ROOT" show "${BASE}:${path}" 2>/dev/null && return 0
  return 0
}

# test_requirements_for_path <changed-path> -> space-separated R-IDs (possibly empty).
# Walks UP from the changed file to the nearest ancestor directory carrying a
# chainsaw-test.yaml, so it works for any file in a test dir (check.sh, mutations.py,
# fixtures, the chainsaw-test.yaml itself) and for nested test layouts.
test_requirements_for_path() {
  local f="$1" dir yaml text
  case "$f" in conformance/tests/*/*) ;; *) return 0 ;; esac
  dir="${f%/*}"
  while :; do
    case "$dir" in
      conformance/tests|conformance|.|"") return 0 ;;
      */*) ;;
      *) return 0 ;;
    esac
    yaml="${dir}/chainsaw-test.yaml"
    text="$(file_text_head_or_base "$yaml")"
    if [ -n "$text" ]; then
      # Whole-line `#` comments are dropped first — same "reference, not declaration" rule the
      # content scan applies (changed_diff() rule 3): a label shown inside a header example is
      # documentation. Only a real YAML mapping line counts.
      printf '%s\n' "$text" \
        | grep -vE '^[[:space:]]*#' \
        | sed -nE "s#^[[:space:]]*${CONF_TEST_LABEL_PREFIX}/(R-[A-Z0-9]+-[0-9]+):[[:space:]]*[\"']?true.*#\1#p" \
        | tr '\n' ' '
      return 0
    fi
    dir="${dir%/*}"
  done
}

main() {
  local files reqs="" rc=0 base_sha head_sha local_main merge_base

  resolve_base || undecidable no-base "$BASE_PROBLEM"
  base_sha="$(rev "$BASE")" || undecidable no-base "resolved base '${BASE}' vanished"
  head_sha="$(rev HEAD)"    || undecidable no-base "HEAD does not resolve to a commit (empty repo?)"
  log "diff base: ${BASE} (${BASE_SOURCE}) @ ${base_sha:0:12} ... HEAD @ ${head_sha:0:12}"

  # Advisory: a never-fetched origin/main silently WIDENS the set (over-broad, never a
  # false green) — surfaced so an unexpectedly large touched set has an explanation.
  if [ "$BASE" = "origin/main" ] && local_main="$(rev main)" && [ -n "$local_main" ] \
     && [ "$local_main" != "$base_sha" ]; then
    warn "local 'main' (${local_main:0:12}) != origin/main (${base_sha:0:12}) — if origin/main"
    warn "  was not fetched recently the touched set may be stale: git fetch origin main"
  fi

  merge_base="$(git -C "$REPO_ROOT" merge-base "$BASE" HEAD 2>/dev/null)" \
    || undecidable no-base "no merge base between '${BASE}' and HEAD (unrelated histories, or a shallow clone)"
  if [ "$merge_base" = "$head_sha" ]; then
    undecidable on-base "HEAD is not ahead of '${BASE}' (merge-base == HEAD): this checkout contains no branch commits to gate. The empty touched set would be empty BY DEFAULT, not by verification."
  fi

  files="$(changed_files)" || rc=$?
  [ "$rc" -eq 0 ] || undecidable diff-failed "git diff --name-only ${BASE}...HEAD failed (rc=${rc})"
  if [ -z "$files" ]; then
    warn "HEAD is ahead of ${BASE} but the three-dot diff is empty (self-cancelling commits?)"
    warn "  -> genuinely empty touched-requirement set; base WAS established."
    exit 0
  fi

  while IFS= read -r f; do
    [ -n "$f" ] || continue
    reqs+=" $(path_requirements "$f")"
    # Detection rule 3: a change under conformance/tests/<test>/ attributes THAT TEST'S own
    # requirement labels, so weakening a test is gated by what the test certifies.
    reqs+=" $(test_requirements_for_path "$f")"
  done <<< "$files"

  # Self-declared R-IDs on the ADDED/REMOVED lines of the change (see changed_diff).
  #
  # THE ID PATTERN IS `R-[A-Z0-9]+-[0-9]+`, NOT `R-[A-Z]+-[0-9]+`. The WORKSTREAM segment may
  # contain a DIGIT: `R-C2-01` and `R-C2-02` (command & control UI) are the only two such IDs
  # in conformance/requirements/registry.yaml today, and the stricter pattern silently failed
  # to match BOTH of them — `[A-Z]+` consumes the `C` and then demands a `-` where the `2` is.
  # Every other consumer in the repo already allows the digit: test_requirements_for_path()
  # twenty lines above uses `R-[A-Z0-9]+-[0-9]+`, as do conformance/lib/coverage.py,
  # conformance/lib/status.py and conformance/lib/run_l1_offline.py. This grep was the lone
  # dissenter, and it dissented by UNDER-attributing — the fail-OPEN direction. A change that
  # retyped a `conformance.home.k3s/R-C2-01` tag, or a manifest self-declaring R-C2-02 from a
  # path the map keys elsewhere, contributed those IDs to NOBODY, so the C2 tests were
  # filtered out of the merge gate. Same defect class as the map's `*) : ;;` arm: a miss that
  # reads as "nothing touched". path_requirements() has no arm for platform/homepage/* or
  # platform/headlamp/* either, so before this fix the ONLY route to R-C2-01/02 was rule 3
  # (editing the C2 test itself). Regression-tested in ci/tests/touched-requirements.test.sh.
  # `^[+-]` keeps hunk headers (`@@`) and the `+++`/`---` file headers out of scope.
  # `sed 's/^[+-]//'` then strips the single-char diff marker (safe: -U0 emits one), so
  #   what remains is the SOURCE line, and `grep -vE '^[[:space:]]*#'` drops whole-line
  #   YAML/shell comments (rule 3 in changed_diff's header) BEFORE the R-ID grep — a
  #   requirement cited only in a `#` comment is prose, not a declaration.
  local diff_txt
  diff_txt="$(changed_diff)" || rc=$?
  [ "$rc" -eq 0 ] || undecidable diff-failed "git diff -U0 ${BASE}...HEAD failed (rc=${rc})"
  reqs+=" $(printf '%s\n' "$diff_txt" \
    | grep -E '^[+-]' \
    | grep -vE '^(\+\+\+|---)' \
    | sed -E 's/^[+-]//' \
    | grep -vE '^[[:space:]]*#' \
    | grep -hoE 'R-[A-Z0-9]+-[0-9]+' \
    | tr '\n' ' ' || true)"

  # Normalize: split, unique, sort. Emit to stdout.
  printf '%s\n' $reqs | sed '/^$/d' | sort -u
}
main "$@"