fix: send volatile packets with binary attachments

The binary attachments of volatile packets were discarded (only the
header packet was sent) due to a bug introduced by [1].

Related: #3919

[1]: dc381b7

lib/client.ts

@@ -209,13 +209,11 @@ export class Client<
     const encodedPackets = opts.preEncoded
       ? (packet as any[]) // previous versions of the adapter incorrectly used socket.packet() instead of writeToEngine()
       : this.encoder.encode(packet as Packet);
-    for (const encodedPacket of encodedPackets) {
-      this.writeToEngine(encodedPacket, opts);
-    }
+    this.writeToEngine(encodedPackets, opts);
   }
 
   private writeToEngine(
-    encodedPacket: String | Buffer,
+    encodedPackets: Array<String | Buffer>,
     opts: WriteOptions
   ): void {
     if (opts.volatile && !this.conn.transport.writable) {
@@ -224,7 +222,12 @@ export class Client<
       );
       return;
     }
-    this.conn.write(encodedPacket, opts);
+    const packets = Array.isArray(encodedPackets)
+      ? encodedPackets
+      : [encodedPackets];
+    for (const encodedPacket of packets) {
+      this.conn.write(encodedPacket, opts);
+    }
   }
 
   /**

test/socket.io.ts

@@ -1382,6 +1382,32 @@ describe("socket.io", () => {
       }, 200);
     });
 
+    it("should emit only one consecutive volatile event with binary (ws)", (done) => {
+      const srv = createServer();
+      const sio = new Server(srv, { transports: ["websocket"] });
+
+      let counter = 0;
+      srv.listen(() => {
+        sio.on("connection", (s) => {
+          // Wait to make sure there are no packets being sent for opening the connection
+          setTimeout(() => {
+            s.volatile.emit("ev", Buffer.from([1, 2, 3]));
+            s.volatile.emit("ev", Buffer.from([4, 5, 6]));
+          }, 20);
+        });
+
+        const socket = client(srv, { transports: ["websocket"] });
+        socket.on("ev", () => {
+          counter++;
+        });
+      });
+
+      setTimeout(() => {
+        expect(counter).to.be(1);
+        done();
+      }, 200);
+    });
+
     it("should emit regular events after trying a failed volatile event (polling)", (done) => {
       const srv = createServer();
       const sio = new Server(srv, { transports: ["polling"] });