Add test case for exclusive file lock (#340)

test/lock/docker-compose.yml

@@ -0,0 +1,23 @@
+version: '3'
+
+services:
+  backup:
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
+    restart: always
+    environment:
+      BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
+      BACKUP_RETENTION_DAYS: '7'
+    volumes:
+      - app_data:/backup/app_data:ro
+      - /var/run/docker.sock:/var/run/docker.sock
+      - ${LOCAL_DIR:-./local}:/archive
+
+  offen:
+    image: offen/offen:latest
+    labels:
+      - docker-volume-backup.stop-during-backup=true
+    volumes:
+      - app_data:/var/opt/offen
+
+volumes:
+  app_data:

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"