#!/usr/bin/env bash
#
# check-workflow-concurrency.sh
#
# Every workflow must declare a concurrency group, so a push supersedes the run
# before it instead of queueing behind it.
#
# Why this is a gate and not a convention. None of the 15 workflows declared
# `concurrency:` until 2026-10-05. A session pushing four versions of a branch
# therefore queued four complete sets of runs, all of which had to drain: one
# PR's 21 contexts settled none in 70 minutes behind a backlog of 25 runs that
# grew faster than it cleared. The repo's CI became unusable for every other
# session, and two PRs were merged on locally-executed equivalents instead of
# on the gate. A gate that cannot run is not a gate.
#
# One omitted `concurrency:` block reintroduces that for the whole repo, and it
# is invisible: the workflow still passes, it just queues forever under load.
# So it is checked rather than remembered.
#
# Runs in seconds with no toolchain, as a step inside an existing job — adding
# a job of its own would spend runner capacity to protect runner capacity.

set -euo pipefail

cd "$(dirname "$0")/.."

fail=0
count=0

for f in .github/workflows/*.yml; do
  count=$((count + 1))
  name=$(basename "$f")

  # A top-level key, so match at column zero: a `concurrency:` nested under a
  # job is a different (and legal) thing that does not bound the workflow.
  if ! grep -qE '^concurrency:' "$f"; then
    echo "::error file=$f::$name declares no top-level \`concurrency:\` block."
    echo "  Add:"
    echo "    concurrency:"
    echo "      group: \${{ github.workflow }}-\${{ github.ref }}"
    echo "      cancel-in-progress: true"
    fail=1
    continue
  fi

  # The group must vary by ref. A fixed group serialises every branch against
  # every other, which is worse than no group at all: one busy branch then
  # blocks the repo instead of only itself.
  if ! grep -A3 -E '^concurrency:' "$f" | grep -qE 'group:.*github\.ref'; then
    echo "::error file=$f::$name has a concurrency group that does not vary by \`github.ref\`."
    echo "  A fixed group serialises unrelated branches against each other."
    fail=1
  fi
done

if [ "$count" -eq 0 ]; then
  echo "::error::no workflows found — this check looked in the wrong place."
  exit 1
fi

if [ "$fail" -ne 0 ]; then
  echo
  echo "checked $count workflow(s): at least one would queue instead of superseding."
  exit 1
fi

echo "workflow concurrency: $count workflow(s), each grouped per ref."