Add flow control

Not in the "async/fibers/coro" sense of flow control, but in the TCP
backpressure sense.

Pause the stream when a write isn't flushed, and then resume it once the
writable stream drains.

lib/node-http-proxy.js

@@ -539,7 +539,7 @@ HttpProxy.prototype.proxyRequest = function (req, res, options) {
     response.on('data', function (chunk) {
       if (req.method !== 'HEAD' && res.writable) {
         try {
-          res.write(chunk);
+          var flushed = res.write(chunk);
         } catch (er) {
           console.error("res.write error: %s", er.message);
           try {
@@ -547,7 +547,14 @@ HttpProxy.prototype.proxyRequest = function (req, res, options) {
           } catch (er) {
             console.error("res.end error: %s", er.message);
           }
+          return;
+        }
       }
+      if (!flushed) {
+        response.pause();
+        res.once('drain', function () {
+          response.resume();
+        });
       }
     });
 
@@ -578,7 +585,13 @@ HttpProxy.prototype.proxyRequest = function (req, res, options) {
   // `req` write it to the `reverseProxy` request.
   req.on('data', function (chunk) {
     if (!errState) {
-      reverseProxy.write(chunk);
+      var flushed = reverseProxy.write(chunk);
+      if (!flushed) {
+        req.pause();
+        reverseProxy.once('drain', function () {
+          req.resume();
+        });
+      }
     }
   });
 
@@ -646,7 +659,13 @@ HttpProxy.prototype._forwardRequest = function (req) {
 
   // Chunk the client request body as chunks from the proxied request come in
   req.on('data', function (chunk) {
-    forwardProxy.write(chunk);
+    var flushed = forwardProxy.write(chunk);
+    if (!flushed) {
+      req.pause();
+      forwardProxy.once('drain', function () {
+        req.resume();
+      });
+    }
   })
 
   // At the end of the client request, we are going to stop the proxied request
@@ -741,7 +760,13 @@ HttpProxy.prototype.proxyWebSocketRequest = function (req, socket, head, options
       if (reverseProxy.incoming.socket.writable) {
         try {
           self.emit('websocket:outgoing', req, socket, head, data);
-          reverseProxy.incoming.socket.write(data);
+          var flushed = reverseProxy.incoming.socket.write(data);
+          if (!flushed) {
+            proxySocket.pause();
+            reverseProxy.incoming.socket.once('drain', function () {
+              proxySocket.resume();
+            });
+          }
         }
         catch (e) {
           detach();
@@ -758,7 +783,13 @@ HttpProxy.prototype.proxyWebSocketRequest = function (req, socket, head, options
     reverseProxy.incoming.socket.on('data', listeners.onOutgoing = function(data) {
       try {
         self.emit('websocket:incoming', reverseProxy, reverseProxy.incoming, head, data);
-        proxySocket.write(data);
+        var flushed = proxySocket.write(data);
+        if (!flushed) {
+          reverseProxy.incoming.socket.pause();
+          proxySocket.once('drain', function () {
+            reverseProxy.incoming.socket.resume();
+          });
+        }
       }
       catch (e) {
         detach();
@@ -918,7 +949,14 @@ HttpProxy.prototype.proxyWebSocketRequest = function (req, socket, head, options
         //
         self.emit('websocket:handshake', req, socket, head, sdata, data);
         socket.write(sdata);
-        socket.write(data);
+        var flushed = socket.write(data);
+        if (!flushed) {
+          reverseProxy.socket.pause();
+          socket.once('drain', function () {
+            reverseProxy.socket.resume();
+          });
+        }
+
       }
       catch (ex) {
         proxyError(ex)
@@ -935,9 +973,9 @@ HttpProxy.prototype.proxyWebSocketRequest = function (req, socket, head, options
   reverseProxy.on('error', proxyError);
 
   try {
-    //
     // Attempt to write the upgrade-head to the reverseProxy request.
-    //
+    // This is small, and there's only ever one of it.
+    // No need for pause/resume.
     reverseProxy.write(head);
   }
   catch (ex) {