http: improve performance with known-length calls to end()

This boosts RPS performance for the common API case where you call
`res.end(data)` with the entire response by up to 9%.

Signed-off-by: Tim Perry <pimterry@gmail.com>

Author: pimterry

benchmark/http/end-string.js

@@ -0,0 +1,35 @@
+// Responses sent as a single res.end(string) with a known Content-Length -
+// the shape a JSON or HTML endpoint produces.
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  len: [4, 64, 1024, 16384, 102400],
+  c: [50],
+  duration: 5,
+});
+
+function main({ len, c, duration }) {
+  const http = require('http');
+  const body = 'a'.repeat(len);
+  const headers = {
+    'Content-Type': 'text/plain',
+    'Content-Length': `${len}`,
+  };
+
+  const server = http.createServer((req, res) => {
+    res.writeHead(200, headers);
+    res.end(body);
+  });
+
+  server.listen(0, () => {
+    bench.http({
+      connections: c,
+      duration,
+      port: server.address().port,
+    }, () => {
+      server.close();
+    });
+  });
+}

lib/_http_outgoing.js

@@ -1033,6 +1033,25 @@ function write_(msg, chunk, encoding, callback, fromEnd) {
 }
 
 
+// If this last write can be delivered immediately as the final chunk, this
+// prepares to do so, and then returns true. If not, it returns false and
+// a separate _send call and tick will be required to finish up.
+function maybePrepareFinalChunk(msg, chunk, encoding) {
+  if (typeof chunk !== 'string' && !isUint8Array(chunk))
+    return false;
+
+  if (msg.destroyed || msg.strictContentLength)
+    return false;
+
+  if (!msg._header) {
+    msg._contentLength = typeof chunk === 'string' ?
+      Buffer.byteLength(chunk, encoding) : chunk.byteLength;
+    msg._implicitHeader();
+  }
+
+  return !!msg._header && msg._hasBody && !msg.chunkedEncoding;
+}
+
 function connectionCorkNT(conn) {
   conn.uncork();
 }
@@ -1142,6 +1161,8 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
     encoding = null;
   }
 
+  let finishCallback = null;
+
   if (chunk) {
     if (this.finished) {
       onError(this,
@@ -1154,7 +1175,18 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
       this[kSocket].cork();
     }
 
-    write_(this, chunk, encoding, null, true);
+    if (maybePrepareFinalChunk(this, chunk, encoding)) {
+      // If just one final write is required, with nothing to follow, we
+      // attach finish to the write to avoid a separate send() & tick step
+      // later on - this is purely a performance optimization.
+      if (typeof callback === 'function') {
+        queueEndCallback(this, callback);
+        callback = undefined;
+      }
+      finishCallback = onFinish.bind(undefined, this);
+    }
+
+    write_(this, chunk, encoding, finishCallback, true);
   } else if (this.finished) {
     if (typeof callback === 'function') {
       queueEndCallback(this, callback);
@@ -1176,14 +1208,17 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
     throw new ERR_HTTP_CONTENT_LENGTH_MISMATCH(this[kBytesWritten], this._contentLength);
   }
 
-  const finish = onFinish.bind(undefined, this);
+  if (finishCallback === null) {
+    // If we didn't early finish, send the last data and schedule 'finish' now:
+    finishCallback = onFinish.bind(undefined, this);
 
-  if (this._hasBody && this.chunkedEncoding) {
-    this._send('0\r\n' + this._trailer + '\r\n', 'latin1', finish);
-  } else if (!this._headerSent || this.writableLength || chunk) {
-    this._send('', 'latin1', finish);
-  } else {
-    process.nextTick(finish);
+    if (this._hasBody && this.chunkedEncoding) {
+      this._send('0\r\n' + this._trailer + '\r\n', 'latin1', finishCallback);
+    } else if (!this._headerSent || this.writableLength || chunk) {
+      this._send('', 'latin1', finishCallback);
+    } else {
+      process.nextTick(finishCallback);
+    }
   }
 
   if (this[kSocket]) {

test/parallel/test-http-server-response-standalone.js

@@ -15,18 +15,11 @@ const res = new ServerResponse({
   httpVersionMinor: 1
 });
 
-let firstChunk = true;
-
 const ws = new Writable({
   write: common.mustCall((chunk, encoding, callback) => {
-    if (firstChunk) {
-      assert(chunk.toString().endsWith('hello world'));
-      firstChunk = false;
-    } else {
-      assert.strictEqual(chunk.length, 0);
-    }
+    assert(chunk.toString().endsWith('hello world'));
     setImmediate(callback);
-  }, 2)
+  }, 1)
 });
 
 res.assignSocket(ws);