fix(cmd-api-server): shutdown hook was not waiting for promises

1. The library we use for having async functions handle resource deallocation
was not being used correctly by the API server.
2. The library only supports callback style asynchronous code and the API
server assumed that it could return a promise from the shutdown hooks which
lead to the problem that our shutdown hooks were not being awaited for.

Signed-off-by: Peter Somogyvari <peter.somogyvari@accenture.com>

packages/cactus-cmd-api-server/src/main/typescript/api-server.ts

@@ -5,7 +5,7 @@ import type { Server as SecureServer } from "https";
 import type { Request, Response, RequestHandler } from "express";
 import type { ServerOptions as SocketIoServerOptions } from "socket.io";
 import type { Socket as SocketIoSocket } from "socket.io";
-import exitHook from "async-exit-hook";
+import exitHook, { IAsyncExitHookDoneCallback } from "async-exit-hook";
 import os from "os";
 import path from "path";
 import tls from "tls";
@@ -208,6 +208,23 @@ export class ApiServer {
       level: options.config.logLevel,
     });
 
+    if (this.enableShutdownHook) {
+      exitHook((onHookDone: IAsyncExitHookDoneCallback) => {
+        this.log.info("Starting async-exit-hook for cmd-api-server ...");
+        this.shutdown()
+          .catch((ex: unknown) => {
+            this.log.warn("Failed async-exit-hook for cmd-api-server", ex);
+            throw ex;
+          })
+          .finally(() => {
+            this.log.info("Concluded async-exit-hook for cmd-api-server ...");
+            onHookDone();
+          });
+        this.log.info("Started async-exit-hook for cmd-api-server OK");
+      });
+      this.log.info("Registered async-exit-hook for cmd-api-server shutdown.");
+    }
+
     const defaultPluginsPath = path.join(
       os.tmpdir(),
       "org",