fix(uws): discard any write to an aborted uWS response (#682)

This bug only exists for polling transport connections running on top
of uWS.

If the remote client abruptly disconnects (thus aborting the request)
while the server is waiting on an asynchronous operation such as
compression, the server may attempt to write a response via the aborted
response object. This causes an uncaught exception to be thrown.

lib/userver.ts

@@ -258,6 +258,7 @@ export class uServer extends BaseServer {
 class ResponseWrapper {
   private statusWritten: boolean = false;
   private headers = [];
+  private isAborted = false;
 
   constructor(readonly res: HttpResponse) {}
 
@@ -291,13 +292,17 @@ class ResponseWrapper {
   public getHeader() {}
 
   public writeStatus(status: string) {
+    if (this.isAborted) return;
+
     this.res.writeStatus(status);
     this.statusWritten = true;
     this.writeBufferedHeaders();
     return this;
   }
 
   public writeHeader(key: string, value: string) {
+    if (this.isAborted) return;
+
     if (key === "Content-Length") {
       // the content length is automatically added by uWebSockets.js
       return;
@@ -316,6 +321,8 @@ class ResponseWrapper {
   }
 
   public end(data) {
+    if (this.isAborted) return;
+
     if (!this.statusWritten) {
       // status will be inferred as "200 OK"
       this.writeBufferedHeaders();
@@ -324,10 +331,18 @@ class ResponseWrapper {
   }
 
   public onData(fn) {
+    if (this.isAborted) return;
+
     this.res.onData(fn);
   }
 
   public onAborted(fn) {
-    this.res.onAborted(fn);
+    if (this.isAborted) return;
+
+    this.res.onAborted(() => {
+      // Any attempt to use the UWS response object after abort will throw!
+      this.isAborted = true;
+      fn();
+    });
   }
 }