Add :shutdown op

doc/pods.md

@@ -178,6 +178,12 @@ message related to the request with `id` `1d17f8fe-4f70-48bf-b6a9-dc004e52d056`.
 
 Now you know most there is to know about the pod protocol!
 
+#### shutdown
+
+When babashka is about to exit, it sends a `{"op" "shutdown"}` message and waits
+for the pod process to end. This gives the pod a chance to clean up resources
+before it exits.
+
 #### out and err
 
 Pods may send messages with an `out` and `err` string value. Babashka prints

src/babashka/impl/pods.clj

@@ -75,6 +75,9 @@
         (binding [*out* @sci/err]
           (prn e))))))
 
+(defn next-id []
+  (str (java.util.UUID/randomUUID)))
+
 (defn invoke [pod pod-var args async?]
   (let [stream (:stdin pod)
         format (:format pod)
@@ -82,7 +85,7 @@
         write-fn (case format
                    :edn pr-str
                    :json cheshire/generate-string)
-        id (str (java.util.UUID/randomUUID))
+        id (next-id)
         chan (async/chan)
         _ (swap! chans assoc id chan)
         _ (write stream {"id" id
@@ -105,8 +108,13 @@
          stdin (.getOutputStream p)
          stdout (.getInputStream p)
          stdout (java.io.PushbackInputStream. stdout)
-         _ (add-shutdown-hook! #(.destroy p))
+         _ (add-shutdown-hook!
-         _ (write stdin {"op" "describe"})
+            (fn []
+              (write stdin {"op" "shutdown"
+                            "id" (next-id)})
+              (.waitFor p)))
+         _ (write stdin {"op" "describe"
+                         "id" (next-id)})
          reply (read stdout)
          format (-> (get reply "format") bytes->string keyword)
          pod {:process p

test-resources/pod.clj

@@ -111,7 +111,8 @@
                               (write
                                {"status" ["done"]
                                 "id" id})))
-                        (recur)))))))))
+                        (recur))
+              :shutdown (System/exit 0))))))))
 
 (let [cli-args (set *command-line-args*)]
   (if (contains? cli-args "--run-as-pod")