fix: processQueue never recovers from exceptions

packages/apps-engine/deno-runtime/lib/messenger.ts

@@ -1,4 +1,4 @@
-import { writeAll } from "https://deno.land/std@0.216.0/io/write_all.ts";
+import { writeAll } from 'https://deno.land/std@0.216.0/io/write_all.ts';
 
 import * as jsonrpc from 'jsonrpc-lite';
 
@@ -33,26 +33,52 @@ const COMMAND_PONG = '_zPONG';
 
 export const RPCResponseObserver = new EventTarget();
 
+class ProcessingLock {
+    private isProcessing = false;
+
+    lock() {
+        const granted = this.isProcessing === false;
+
+        if (!granted) {
+            return {
+                granted,
+                [Symbol.dispose]: () => {},
+            };
+        }
+
+        this.isProcessing = true;
+
+        return {
+            granted,
+            [Symbol.dispose]: () => {
+                this.isProcessing = false;
+            },
+        };
+    }
+    [Symbol.dispose]() {
+        this.isProcessing = false;
+    }
+}
+
 export const Queue = new (class Queue {
     private queue: Uint8Array[] = [];
-    private isProcessing = false;
+    private isProcessingLock = new ProcessingLock();
 
     private async processQueue() {
-        if (this.isProcessing) {
+        using processing = this.isProcessingLock.lock();
+
+        if (!processing.granted) {
             return;
         }
 
-        this.isProcessing = true;
-
         while (this.queue.length) {
-            const message = this.queue.shift();
+            const [message] = this.queue;
 
             if (message) {
                 await Transport.send(message);
             }
+            this.queue.shift();
         }
-
-        this.isProcessing = false;
     }
 
     public enqueue(message: jsonrpc.JsonRpc | typeof COMMAND_PONG) {
@@ -63,7 +89,7 @@ export const Queue = new (class Queue {
     public getCurrentSize() {
         return this.queue.length;
     }
-});
+})();
 
 export const Transport = new (class Transporter {
     private selectedTransport: Transporter['stdoutTransport'] | Transporter['noopTransport'];