perf(websocket): fix write back-pressure (#618)

This change should reduce memory usage when many packets are emitted to
many clients in a burst.

Co-authored-by: Branislav Katreniak <bkatreniak@slido.com>

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", () => {