websocket: fix write back-pressure

Websocket transport is eagerly writing to underlaying websocket
without respecting back-pressure.
When an event is emitted to multiple clients, socket.io adapter
sends the same packet object to all socket clients.
These packet objects are shared for all clients inside room.

Once the packet is sent to transport,
transport prepares buffer with transport headers and packet data
and the sharing among clients is lost.

This change significantly reduces memory usage when
many packets are emitted to many clients in a burst.

    This change causes that buffered data is sent to clients
    more evenly packet by packet.

lib/transports/websocket.js

@@ -63,33 +63,35 @@ class WebSocket extends Transport {
  * @api private
  */
  send(packets) {
- for (let i = 0; i < packets.length; i++) {
- const packet = packets[i];
+ const packet = packets.shift();
+ if (typeof packet === 'undefined') {
+ this.writable = true;
+ this.emit("drain");
+ return;
+ }
 
-  // always creates a new object since ws modifies it
-  const opts = {};
-  if (packet.options) {
-  opts.compress = packet.options.compress;
-  }
+ // always creates a new object since ws modifies it
+ const opts = {};
+ if (packet.options) {
+ opts.compress = packet.options.compress;
+ }
 
- this.parser.encodePacket(packet, this.supportsBinary, data => {
- if (this.perMessageDeflate) {
- const len =
- "string" === typeof data ? Buffer.byteLength(data) : data.length;
- if (len < this.perMessageDeflate.threshold) {
- opts.compress = false;
- }
+ this.parser.encodePacket(packet, this.supportsBinary, data => {
+ if (this.perMessageDeflate) {
+ const len =
+ "string" === typeof data ? Buffer.byteLength(data) : data.length;
+ if (len < this.perMessageDeflate.threshold) {
+ opts.compress = false;
  }
- debug('writing "%s"', data);
- this.writable = false;
+ }
+ debug('writing "%s"', data);
+ this.writable = false;
 
- this.socket.send(data, opts, err => {
- if (err) return this.onError("write error", err.stack);
- this.writable = true;
- this.emit("drain");
- });
+ this.socket.send(data, opts, err => {
+ if (err) return this.onError("write error", err.stack);
+ this.send(packets);
  });
- }
+ });
  }
 
  /**

test/server.js

@@ -1681,7 +1681,9 @@ describe("server", () => {
  conn.send("a");
  conn.send("b");
  conn.send("c");
- conn.close();
+ setTimeout(() => {
+ conn.close();
+ }, 50);
  });
 
  socket.on("open", () => {