#!/usr/bin/env bash
# Moves the floating `v1` tag forward to a gated commit on `main`, and never
# backwards. Run from a clone whose `origin` is this repository.
#
#   release-v1.sh merge <gated-sha>     merge-triggered job: release <gated-sha>
#                                       if it is still main's tip
#   release-v1.sh sweep-check           scheduled sweep: report whether v1 lags
#                                       main (tip=, v1=, needed= to
#                                       $GITHUB_OUTPUT, or stdout without one)
#   release-v1.sh push <gated-sha> <v1-as-read>
#                                       scheduled sweep, after gating the tip
#
# Every push is leased on the v1 value the caller reasoned about. A lost lease
# means another writer moved v1 first: that is a clean skip once v1 is at or
# ahead of <gated-sha>, a retry against the new value while v1 is still behind
# it, and a failure otherwise.
set -euo pipefail

MAX_ATTEMPTS=3

fetch_main() {
  git fetch -q origin +refs/heads/main:refs/remotes/origin/main
  git rev-parse refs/remotes/origin/main
}

# Prints origin's v1 commit, or nothing when origin has no v1.
fetch_v1() {
  if [ -z "$(git ls-remote origin refs/tags/v1)" ]; then
    git update-ref -d refs/release-v1/seen 2>/dev/null || true
    return 0
  fi
  git fetch -q origin +refs/tags/v1:refs/release-v1/seen
  git rev-parse 'refs/release-v1/seen^{commit}'
}

# True when v1 already covers <sha>: at it, or a descendant of it.
covers() {
  local sha="$1" v1="$2"
  [ -n "$v1" ] && git merge-base --is-ancestor "$sha" "$v1"
}

push_leased() {
  local sha="$1" expect="$2" now attempt
  for ((attempt = 1; attempt <= MAX_ATTEMPTS; attempt++)); do
    if git push -q --force-with-lease="refs/tags/v1:$expect" origin "$sha:refs/tags/v1"; then
      echo "v1 moved ${expect:-<absent>} -> $sha"
      return 0
    fi
    now=$(fetch_v1)
    if [ "$now" = "$expect" ]; then
      echo "ERROR: push of v1 -> $sha rejected while v1 was still ${expect:-<absent>} -- not a lost lease" >&2
      return 1
    fi
    if covers "$sha" "$now"; then
      echo "lost the lease: another writer moved v1 to $now, at or ahead of $sha -- nothing to do"
      return 0
    fi
    if [ -n "$now" ] && ! git merge-base --is-ancestor "$now" "$sha"; then
      echo "ERROR: v1 moved to $now, which is neither behind nor ahead of $sha" >&2
      return 1
    fi
    echo "lost the lease: v1 moved to ${now:-<absent>}, still behind $sha -- retrying"
    expect="$now"
  done
  echo "ERROR: lost the lease on v1 $MAX_ATTEMPTS times running" >&2
  return 1
}

cmd="${1:?usage: release-v1.sh merge <sha> | sweep-check | push <sha> <v1-as-read>}"
shift
case "$cmd" in
  merge)
    SHA="${1:?usage: release-v1.sh merge <gated-sha>}"
    TIP=$(fetch_main)
    if [ "$TIP" != "$SHA" ]; then
      echo "main's tip ($TIP) is past this run's gated commit ($SHA) -- deferring; the sweep releases the tip"
      exit 0
    fi
    V1=$(fetch_v1)
    if covers "$SHA" "$V1"; then
      echo "v1 ($V1) already at or ahead of $SHA -- nothing to do"
      exit 0
    fi
    push_leased "$SHA" "$V1"
    ;;
  sweep-check)
    TIP=$(fetch_main)
    V1=$(fetch_v1)
    if covers "$TIP" "$V1"; then NEEDED=false; else NEEDED=true; fi
    echo "main=$TIP v1=${V1:-<absent>} release-needed=$NEEDED"
    printf 'tip=%s\nv1=%s\nneeded=%s\n' "$TIP" "$V1" "$NEEDED" >> "${GITHUB_OUTPUT:-/dev/stdout}"
    ;;
  push)
    push_leased "${1:?usage: release-v1.sh push <gated-sha> <v1-as-read>}" "${2-}"
    ;;
  *)
    echo "release-v1.sh: unknown command '$cmd'" >&2
    exit 2
    ;;
esac