fix: scope job derives additively — labeler v5's PUT clobbered mid-job labels

actions/labeler@v5 writes the whole label set (setLabels PUT) even under
sync-labels: false, so a label applied between its read and its write is
silently removed — ceremony#128 lost `release` that way. v6/v7 write the
same way. Replace the step with actions/labels-scope: same labeler.yml
mapping (changed-files/any-glob-to-any-file subset, refused loudly
otherwise), changed paths via the API, and an additive POST as the only
write.

Part of #130.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/labels.yml

@@ -5,8 +5,9 @@ name: labels
 # The caller uses pull_request_target, not pull_request: every PR in this
 # family arrives from a fork, where pull_request runs with a READ-ONLY token
 # and cannot label anything. _target is safe in this workflow because no PR
-# code is ever checked out or executed — labeler reads changed paths via the
+# code is ever checked out or executed — scope reads changed paths and the
-# API, and reconcile checks out the BASE branch only. Keep it that way.
+# path mapping via the API and checks out only the ceremony implementation,
+# and reconcile checks out the BASE branch only. Keep it that way.
 #
 # There is no pull_request_review_target, so a review landing cannot wake this
 # workflow directly — and the */15 cron is advisory: GitHub deprioritises
@@ -28,7 +29,7 @@ env:
 
 jobs:
   scope:
-    # Not on labeled/unlabeled: those events change no paths, so labeler has
+    # Not on labeled/unlabeled: those events change no paths, so scope has
     # nothing new to derive — and label churn is precisely what they are.
     if: >-
       github.event_name == 'pull_request_target' &&
@@ -39,11 +40,41 @@ jobs:
       group: labels-scope-${{ github.event.pull_request.number }}
       cancel-in-progress: true
     steps:
-      - uses: actions/labeler@v5
+      # actions/labeler@v5 held this seat until #130. Even with
+      # sync-labels: false it wrote the WHOLE label set — PUT of
+      # (labels-fetched-at-job-start ∪ derived) — so a label applied while
+      # the job ran was silently removed: ceremony#128 lost its `release`,
+      # the merge door's declared-intent read, two seconds after the
+      # builder set it. v6/v7 write the same way, so the step was replaced
+      # rather than repinned. labels-scope reads the consumer's
+      # .github/labeler.yml and the changed paths via the API, and its
+      # only write is an additive POST of the derived scopes: a label
+      # applied mid-job survives by construction.
+      #
+      # Still no PR code: both checkouts below fetch the ceremony
+      # implementation only. The dogfood checkout rides github.sha — the
+      # base-branch commit the workflow file itself came from, so the
+      # script and workflow can never skew — and doubles as the #11
+      # bootstrap: ceremony's own labels must work before any release tag
+      # exists for the pinned checkout to fetch.
+      - uses: actions/checkout@v4
+        if: github.repository == 'heavy-duty/ceremony'
         with:
-          # labeler reads the consumer's .github/labeler.yml via the API
+          repository: ${{ github.repository }}
-          # additive only — a hand-applied scope must survive the machine
+          ref: ${{ github.sha }}
-          sync-labels: false
+      - uses: actions/checkout@v4
+        if: github.repository != 'heavy-duty/ceremony'
+        with:
+          repository: heavy-duty/ceremony
+          ref: ${{ env.CEREMONY_SELF_REF }}
+      - uses: ./actions/labels-scope
+        env:
+          GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          # the BASE branch commit — a PR must not label itself by editing
+          # the mapping it is judged by
+          CONFIG_REF: ${{ github.sha }}
 
   reconcile:
     runs-on: ubuntu-latest

actions/labels-scope/action.yml

@@ -0,0 +1,8 @@
+name: Derive scope labels
+description: Additively apply path-derived scope:* labels to a PR — the only write is POST (issue #130)
+runs:
+  using: composite
+  steps:
+    - name: derive and add scope labels
+      shell: bash
+      run: bash "$GITHUB_ACTION_PATH/labels-scope.sh"

actions/labels-scope/labels-scope.sh

@@ -0,0 +1,158 @@
+#!/usr/bin/env bash
+if [ "${BASH_SOURCE[0]}" = "$0" ]; then
+  set -euo pipefail
+else
+  # Fixture tests source the pure functions and deliberately inspect failures.
+  set -u
+fi
+
+# labels-scope.sh — the additive half of the labels automation: derive
+# scope:* labels from a PR's changed paths and ADD them, touching nothing
+# else. This seat belonged to actions/labeler@v5 until #130: even under
+# `sync-labels: false`, labeler computes (labels-fetched-at-job-start ∪
+# derived) and writes it back with `PUT /issues/{n}/labels`
+# (src/labeler.ts: api.setLabels — a full replace), so any label applied
+# between its read and its write is silently removed. On ceremony#128 the
+# builder's `release` — the merge door's declared-intent read — landed in
+# that window and vanished two seconds later; v6 and v7 write the same
+# way, so the fix is this replacement, not a newer pin.
+#
+# The only write here is `POST /issues/{n}/labels`: GitHub adds the named
+# labels, ignores ones already present, and removes nothing. A label
+# applied while this runs survives by construction.
+#
+# The path mapping stays in the consumer's .github/labeler.yml, read via
+# the API at CONFIG_REF — the base branch, never the PR head, so a PR
+# cannot label itself by editing the mapping. The accepted shape is the
+# one every governed repo uses:
+#
+#   scope:name:
+#     - changed-files:
+#         - any-glob-to-any-file: ["glob", ...]
+#
+# in any YAML spelling (block or flow; a glob list may be a single
+# string). Anything else — all-globs-to-all-files, branch matchers,
+# negations, backslash escapes — is refused loudly rather than
+# half-honoured: this parser exists to make one write additive, not to
+# reimplement minimatch. Globs support `**` (crosses `/`), `*` and `?`
+# (do not); a leading dot is not special; the whole path must match
+# (`README` matches README, never docs/README).
+
+log() { printf 'labels-scope: %s\n' "$*"; }
+
+run() { # every mutation goes through here — DRY_RUN=1 logs instead of doing
+  if [ -n "${DRY_RUN:-}" ]; then log "DRY_RUN: $*"; else "$@"; fi
+}
+
+glob_to_regex() { # $1 = glob (the subset above) → anchored ERE, one line
+  local glob="$1" out="" c i=0 n="${#glob}"
+  while [ "$i" -lt "$n" ]; do
+    c="${glob:i:1}"
+    case "$c" in
+      \*)
+        if [ "${glob:i:2}" = '**' ]; then
+          out="$out.*"
+          i=$((i + 2))
+          continue
+        fi
+        out="$out[^/]*"
+        ;;
+      \?) out="$out[^/]" ;;
+      [a-zA-Z0-9_/-]) out="$out$c" ;;
+      *) out="$out\\$c" ;; # every other byte is literal — ., +, {, (, …
+    esac
+    i=$((i + 1))
+  done
+  printf '^%s$\n' "$out"
+}
+
+parse_labeler_config() { # labeler.yml on stdin → "label<TAB>glob" lines
+  # yq only normalizes YAML to JSON; the shape contract is enforced in jq,
+  # where an unsupported key is a loud error naming the label it sits under.
+  yq -o=json '.' - | jq -r '
+    if type != "object" then
+      error("labeler config: top level must be a map of label -> rules")
+    else . end
+    | to_entries[]
+    | .key as $label
+    | (if (.value | type) != "array" then
+         error("labeler config: \($label): rules must be a list")
+       else .value end)[]
+    | (if type != "object" then
+         error("labeler config: \($label): each rule must be a map")
+       else . end)
+    | ((keys - ["changed-files"]) as $extra
+       | if ($extra | length) > 0 then
+           error("labeler config: \($label): unsupported key(s) \($extra | join(", ")) — the scope job accepts changed-files/any-glob-to-any-file only (#130)")
+         else . end)
+    | .["changed-files"]
+    | (if type == "object" then [.]
+       elif type == "array" then .
+       else error("labeler config: \($label): changed-files must be a list") end)[]
+    | (if type != "object" then
+         error("labeler config: \($label): each changed-files entry must be a map")
+       else . end)
+    | ((keys - ["any-glob-to-any-file"]) as $extra
+       | if ($extra | length) > 0 then
+           error("labeler config: \($label): unsupported matcher(s) \($extra | join(", ")) — the scope job accepts any-glob-to-any-file only (#130)")
+         else . end)
+    | .["any-glob-to-any-file"]
+    | (if type == "string" then [.]
+       elif type == "array" then .
+       else error("labeler config: \($label): any-glob-to-any-file must be a glob or a list of globs") end)[]
+    | (if type != "string" then
+         error("labeler config: \($label): globs must be strings")
+       elif contains("\\") then
+         error("labeler config: \($label): backslash in glob \(.) — escapes are not supported (#130)")
+       else . end)
+    | [$label, .] | @tsv
+  '
+}
+
+derive_labels() { # $1 = "label<TAB>glob" lines, $2 = changed files (one per
+  # line) → matched labels, one per line, config order, deduped
+  local tsv="$1" files="$2" label glob matched=$'\n'
+  [ -n "$files" ] || return 0
+  while IFS="$(printf '\t')" read -r label glob; do
+    [ -n "$label" ] || continue
+    case "$matched" in *$'\n'"$label"$'\n'*) continue ;; esac
+    if printf '%s\n' "$files" | grep -qE -- "$(glob_to_regex "$glob")"; then
+      matched="$matched$label"$'\n'
+      printf '%s\n' "$label"
+    fi
+  done <<<"$tsv"
+}
+
+main() {
+  REPO="${REPO:?set REPO to owner/name}"
+  PR_NUMBER="${PR_NUMBER:?set PR_NUMBER to the pull request number}"
+  CONFIG_REF="${CONFIG_REF:?set CONFIG_REF to the base commit the mapping is read at}"
+  CONFIG_PATH="${CONFIG_PATH:-.github/labeler.yml}"
+
+  local config tsv files labels
+  # No mapping is a consumer that has not adopted scope labels — an
+  # advisory no-op, not a red run (scopes locate, they do not alert). A
+  # mapping that EXISTS but does not parse still fails loudly below.
+  if ! config="$(gh api "repos/$REPO/contents/$CONFIG_PATH?ref=$CONFIG_REF" \
+    --jq '.content' 2>/dev/null | base64 -d)" || [ -z "$config" ]; then
+    log "no $CONFIG_PATH at $CONFIG_REF — nothing to derive"
+    return 0
+  fi
+  tsv="$(parse_labeler_config <<<"$config")"
+  files="$(gh api --paginate "repos/$REPO/pulls/$PR_NUMBER/files" --jq '.[].filename')"
+  labels="$(derive_labels "$tsv" "$files")"
+
+  if [ -z "$labels" ]; then
+    log "#$PR_NUMBER: no scope labels derived"
+    return 0
+  fi
+  local args=()
+  while IFS= read -r label; do args+=(-f "labels[]=$label"); done <<<"$labels"
+  run gh api "repos/$REPO/issues/$PR_NUMBER/labels" "${args[@]}" --silent
+  log "#$PR_NUMBER: scopes -> $(paste -sd, <<<"$labels") (additive POST; already-present names are no-ops)"
+}
+
+# sourced by test/labels-scope.test.sh for the fixture tests; executed in CI
+if [ "${BASH_SOURCE[0]}" = "$0" ]; then
+  main "$@"
+fi