perf(eio): optimize polling request body buffering

Avoid repeated buffer concatenation while reading binary polling payloads by
accumulating chunks and concatenating once after the request ends. Track payload
size incrementally and stop processing when the HTTP buffer limit is exceeded.

Author: darrachequesne

packages/engine.io/lib/transports/polling.ts

@@ -129,37 +129,49 @@ export class Polling extends Transport {
     this.dataReq = req;
     this.dataRes = res;
 
-    let chunks = isBinary ? Buffer.concat([]) : "";
+    const buffers: Buffer[] = [];
+    let stringChunks = "";
+    let contentLength = 0;
+    let exceededMaxHttpBufferSize = false;
 
     const cleanup = () => {
       req.removeListener("data", onData);
       req.removeListener("end", onEnd);
       req.removeListener("close", onClose);
-      this.dataReq = this.dataRes = chunks = null;
+      this.dataReq = this.dataRes = null;
     };
 
     const onClose = () => {
       cleanup();
       this.onError("data request connection closed prematurely");
     };
 
-    const onData = (data) => {
-      let contentLength;
-      if (isBinary) {
-        chunks = Buffer.concat([chunks, data]);
-        contentLength = chunks.length;
-      } else {
-        chunks += data;
-        contentLength = Buffer.byteLength(chunks);
-      }
+    const onData = (chunk) => {
+      contentLength += isBinary ? chunk.length : Buffer.byteLength(chunk);
 
       if (contentLength > this.maxHttpBufferSize) {
+        exceededMaxHttpBufferSize = true;
         res.writeHead(413).end();
         cleanup();
+        return;
+      }
+
+      if (isBinary) {
+        buffers.push(chunk);
+      } else {
+        stringChunks += chunk;
       }
     };
 
     const onEnd = () => {
+      if (exceededMaxHttpBufferSize) {
+        return;
+      }
+
+      const chunks = isBinary
+        ? Buffer.concat(buffers, contentLength)
+        : stringChunks;
+
       this.onData(chunks);
 
       const headers = {