#!/usr/bin/env bash
# 012-tune-cpu - Cap query parallelism for the shared-core 2 vCPU e2-micro
# (C-tier of the 2026-08 tuning review).
#
#   max_parallel_workers_per_gather = 1  one query must not take every core;
#                                        other sessions still get CPU
#   max_parallel_workers             = 2  there are only 2 cores to begin with
#   jit                              = off  JIT compile cost outweighs gains for
#                                        short queries / low TPS / shared CPU
#
# All three are reloadable GUCs - no restart, no downtime.
# up:    write drop-in; reload
# down:  remove drop-in; reload
set -euo pipefail

PGBIN=/etc/postgresql/18/main
CONF=$PGBIN/conf.d/012-tune-cpu.conf
CLUSTER=$(pg_lsclusters --no-header | awk 'NR==1{print $1"/"$2}')

# desired state as "guc value" pairs; SHOW renders exactly these strings
WANT=(
  'max_parallel_workers_per_gather 1'
  'max_parallel_workers 2'
  'jit off'
)

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

write_conf() {
  install -d -m 755 "$PGBIN/conf.d"
  cat > "$CONF" <<'EOF'
# managed by sysmig 012-tune-cpu - parallelism caps for shared-core 2 vCPU
max_parallel_workers_per_gather = 1
max_parallel_workers = 2
jit = off
EOF
  chown root:postgres "$CONF"
  chmod 644 "$CONF"
}

case "${1:-}" in
  up)
    write_conf
    pg_ctlcluster "$CLUSTER" reload
    echo "    cluster reloaded"
    for kv in "${WANT[@]}"; do
      read -r guc want <<<"$kv"
      got=$(pg "SHOW $guc")
      [[ $got == "$want" ]] || { echo "FATAL: $guc is '$got', expected '$want'" >&2; exit 1; }
    done
    echo "    ${#WANT[@]} cpu/parallel settings active"
    ;;
  status)
    if [[ $EUID -eq 0 ]]; then
      q() { runuser -u postgres -- psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
    elif psql -d postgres -tAc 'SELECT 1' >/dev/null 2>&1; then
      q() { psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
    else
      echo "? needs root or peer DB access to verify"
      exit 2
    fi
    [[ -f $CONF ]] || { echo "DRIFT drop-in $CONF missing"; exit 1; }
    bad=()
    for kv in "${WANT[@]}"; do
      read -r guc want <<<"$kv"
      got=$(q "SHOW $guc" 2>/dev/null)
      [[ $got == "$want" ]] || bad+=("$guc=$got(want $want)")
    done
    if ((${#bad[@]})); then echo "DRIFT ${bad[*]}"; exit 1; fi
    echo "OK parallel capped (per_gather=1, workers=2), jit off"
    ;;
  down)
    rm -f "$CONF"
    pg_ctlcluster "$CLUSTER" reload
    echo "    drop-in removed, cluster reloaded (defaults restored)"
    ;;
esac