Backport http-party#1407

Author: roderik

lib/http-proxy/passes/ws-incoming.js

@@ -76,7 +76,7 @@ module.exports = {
    *
    * @api private
    */
-  stream : function stream(req, socket, options, head, server, clb) {
+  stream : async function stream(req, socket, options, head, server, clb) {
 
     var createHttpHeader = function(line, headers) {
       return Object.keys(headers).reduce(function (head, key) {
@@ -105,7 +105,7 @@ module.exports = {
     );
 
     // Enable developers to modify the proxyReq before headers are sent
-    if (server) { server.emit('proxyReqWs', proxyReq, req, socket, options, head); }
+    // if (server) { server.emit('proxyReqWs', proxyReq, req, socket, options, head); }
 
     // Error Handler
     proxyReq.on('error', onOutgoingError);
@@ -146,6 +146,24 @@ module.exports = {
 
       server.emit('open', proxySocket);
       server.emit('proxySocket', proxySocket);  //DEPRECATED.
+
+
+     // Enable developers to modify the proxyReq before headers are sent
+     if (server) {
+      // Provides a way for the event handler to communicate back to the emitter when it finishes its async handling
+      let asyncHandler
+      const asyncContext = (callback) => {
+        asyncHandler = callback
+      }
+
+      server.emit('proxyReqWs', proxyReq, req, socket, options, head, asyncContext);
+
+      if (asyncHandler) {
+        await asyncHandler()
+      }
+    }
+
+    proxyReq.end();
     });
 
     return proxyReq.end(); // XXX: CHECK IF THIS IS THIS CORRECT