#!/usr/bin/env bash
# 011-postgres-observability - Turn on the instrumentation that any future
# tuning depends on (A-tier of the 2026-08 tuning review).
#
#   shared_preload_libraries = 'pg_stat_statements'  query stats (restart)
#   track_io_timing          = on   pg_stat_io timing, cheap on modern Linux
#   log_min_duration_statement = 500ms   slow queries land in the server log
#
# pg_stat_statements extension is created in `iwanhae` (app db) and
# `postgres` (admin db). Inspect with:
#   SELECT calls, round(total_exec_time::numeric,1) ms, rows, query
#     FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 10;
# up:    write drop-in; restart only if the lib is not yet loaded; extensions
# down:  drop extensions (loses collected stats), remove drop-in, restart
set -euo pipefail

PGBIN=/etc/postgresql/18/main
CONF=$PGBIN/conf.d/011-postgres-observability.conf
CLUSTER=$(pg_lsclusters --no-header | awk 'NR==1{print $1"/"$2}')
DBS=(iwanhae postgres)

pg() { runuser -u postgres -- psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }

lib_loaded() { pg 'SHOW shared_preload_libraries' | grep -q pg_stat_statements; }

write_conf() {
  install -d -m 755 "$PGBIN/conf.d"
  cat > "$CONF" <<'EOF'
# managed by sysmig 011-postgres-observability
shared_preload_libraries = 'pg_stat_statements'
track_io_timing = on
log_min_duration_statement = 500ms
EOF
  chown root:postgres "$CONF"
  chmod 644 "$CONF"
}

case "${1:-}" in
  up)
    write_conf
    if lib_loaded; then
      pg_ctlcluster "$CLUSTER" reload
      echo "    cluster reloaded (pg_stat_statements already loaded)"
    else
      pg_ctlcluster "$CLUSTER" restart
      pg_isready -q
      echo "    cluster restarted to load pg_stat_statements"
    fi
    for db in "${DBS[@]}"; do
      runuser -u postgres -- psql -d "$db" -v ON_ERROR_STOP=1 -qc \
        'CREATE EXTENSION IF NOT EXISTS pg_stat_statements'
    done
    lib_loaded || { echo "FATAL: pg_stat_statements not loaded" >&2; exit 1; }
    [[ $(pg 'SHOW track_io_timing') == on ]] || { echo "FATAL: track_io_timing not on" >&2; exit 1; }
    [[ $(pg 'SHOW log_min_duration_statement') == '500ms' ]] || { echo "FATAL: log_min_duration_statement wrong" >&2; exit 1; }
    echo "    extension created in: ${DBS[*]}"
    ;;
  status)
    if [[ $EUID -eq 0 ]]; then
      q() { runuser -u postgres -- psql -d "$1" -v ON_ERROR_STOP=1 -tAc "$2"; }
    elif psql -d postgres -tAc 'SELECT 1' >/dev/null 2>&1; then
      q() { psql -d "$1" -v ON_ERROR_STOP=1 -tAc "$2"; }
    else
      echo "? needs root or peer DB access to verify"
      exit 2
    fi
    [[ -f $CONF ]] || { echo "DRIFT drop-in $CONF missing"; exit 1; }
    if ! q postgres 'SHOW shared_preload_libraries' | grep -q pg_stat_statements; then
      echo "DRIFT pg_stat_statements not preloaded"; exit 1
    fi
    [[ $(q postgres 'SHOW track_io_timing') == on ]] || { echo "DRIFT track_io_timing off"; exit 1; }
    [[ $(q postgres 'SHOW log_min_duration_statement') == '500ms' ]] || { echo "DRIFT log_min_duration_statement"; exit 1; }
    for db in "${DBS[@]}"; do
      [[ $(q "$db" "SELECT count(*) FROM pg_extension WHERE extname='pg_stat_statements'") -ge 1 ]] \
        || { echo "DRIFT extension missing in $db"; exit 1; }
    done
    echo "OK pg_stat_statements + slow-log(500ms) + io timing"
    ;;
  down)
    for db in "${DBS[@]}"; do
      runuser -u postgres -- psql -d "$db" -v ON_ERROR_STOP=1 -qc \
        'DROP EXTENSION IF EXISTS pg_stat_statements' \
        || { echo "cannot drop extension in $db (in use?) - retry later" >&2; exit 1; }
    done
    rm -f "$CONF"
    if lib_loaded; then
      pg_ctlcluster "$CLUSTER" restart
      echo "    extensions dropped, drop-in removed, cluster restarted to unload lib"
    else
      echo "    extensions dropped, drop-in removed (lib was not loaded)"
    fi
    ;;
esac