Add the scheduling module

Three replicas of one application against one PostgreSQL database, proving
duplicate @Scheduled execution and then removing it with ShedLock: 24
executions where 8 were due, then 7 for 7 ticks.

Also measured: @SchedulerLock without @EnableSchedulerLock does nothing and
warns about nothing; spring.task.scheduling.pool.size=1 does not starve a
fixedRate job but delays it and fires 35 of 40 executions in a burst; and a
node whose clock is 40 seconds fast takes a live lock unless the provider
uses usingDbTime().

scheduling/scripts/postgres.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# Starts a throwaway PostgreSQL 14 on port 55432 with no Docker and no root.
+#
+# On a Debian/Ubuntu machine the .deb packages can simply be downloaded and unpacked into a
+# prefix; PostgreSQL does not need to be installed system-wide and refuses to run as root anyway.
+# This is how docs/output/ was produced. If you have Docker, docker-compose.yml is easier.
+#
+#   scripts/postgres.sh start|stop|psql
+set -euo pipefail
+cd "$(dirname "$0")/.."
+PREFIX="$PWD/target/pg"
+PGROOT="$PREFIX/root"
+PGDATA="$PREFIX/data"
+export PATH="$PGROOT/usr/lib/postgresql/14/bin:$PATH"
+export LD_LIBRARY_PATH="$PGROOT/usr/lib/x86_64-linux-gnu:$PGROOT/lib/x86_64-linux-gnu:${LD_LIBRARY_PATH:-}"
+
+install_pg() {
+  mkdir -p "$PREFIX/debs" "$PGROOT"
+  ( cd "$PREFIX/debs"
+    for p in postgresql-14 postgresql-client-14 postgresql-common postgresql-client-common \
+             libpq5 libllvm14 libicu70 libssl3 libxslt1.1 libxml2 libedit2 libbsd0 libmd0 \
+             zlib1g liblz4-1 libzstd1 libtinfo6 libncurses6 libcom-err2 libkrb5-3 libk5crypto3 \
+             libkrb5support0 libgssapi-krb5-2 libkeyutils1 libsasl2-2 libldap-2.5-0 libgnutls30 \
+             libnettle8 libhogweed6 libgmp10 libp11-kit0 libtasn1-6 libidn2-0 libunistring2 libffi8; do
+      apt-get download "$p" >/dev/null 2>&1 || true
+    done
+    for d in *.deb; do dpkg -x "$d" "$PGROOT" 2>/dev/null || true; done )
+}
+
+case "${1:-start}" in
+  start)
+    [ -x "$PGROOT/usr/lib/postgresql/14/bin/initdb" ] || install_pg
+    if [ ! -d "$PGDATA" ]; then
+      initdb -D "$PGDATA" -U app --auth=trust -E UTF8 >/dev/null
+    fi
+    pg_ctl -D "$PGDATA" -o "-p 55432 -k $PREFIX -c listen_addresses=127.0.0.1" \
+           -l "$PREFIX/pg.log" start >/dev/null 2>&1 || true
+    for _ in $(seq 1 30); do pg_isready -h 127.0.0.1 -p 55432 >/dev/null 2>&1 && break; sleep 1; done
+    psql -h 127.0.0.1 -p 55432 -U app -d postgres -tc \
+      "select 1 from pg_database where datname='shedlockdemo'" | grep -q 1 || \
+      psql -h 127.0.0.1 -p 55432 -U app -d postgres -q -c "create database shedlockdemo"
+    pg_isready -h 127.0.0.1 -p 55432
+    ;;
+  stop) pg_ctl -D "$PGDATA" stop -m fast >/dev/null 2>&1 || true ;;
+  psql) shift; psql -h 127.0.0.1 -p 55432 -U app -d shedlockdemo "$@" ;;
+  *) echo "usage: $0 start|stop|psql" >&2; exit 2 ;;
+esac