Add test case for exclusive file lock (#340)

test/lock/run.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+set -e
+
+cd "$(dirname "$0")"
+. ../util.sh
+current_test=$(basename $(pwd))
+
+export LOCAL_DIR=$(mktemp -d)
+
+docker compose up -d --quiet-pull
+sleep 5
+
+ec=0
+
+docker compose exec -e BACKUP_RETENTION_DAYS=7 -e BACKUP_FILENAME=test.tar.gz backup backup & \
+  { set +e; sleep 0.1; docker compose exec -e BACKUP_FILENAME=test2.tar.gz -e LOCK_TIMEOUT=1s backup backup; ec=$?;}
+
+if [ "$ec" = "0" ]; then
+  fail "Subsequent invocation exited 0"
+fi
+pass "Subsequent invocation did not exit 0"
+
+sleep 5
+
+if [ ! -f "${LOCAL_DIR}/test.tar.gz" ]; then
+  fail "Could not find expected tar file"
+fi
+pass "Found expected tar file"
+
+if [ -f "${LOCAL_DIR}/test2.tar.gz" ]; then
+  fail "Subsequent invocation was expected to fail but created archive"
+fi
+pass "Subsequent invocation did not create archive"