src: add writeEarlyHints function to ServerResponse

doc/api/http.md

@@ -2131,6 +2131,11 @@ Sends a HTTP/1.1 100 Continue message to the client, indicating that
 the request body should be sent. See the [`'checkContinue'`][] event on
 `Server`.
 
+### `response.writeEarlyHints()`
+
+Sends a HTTP/1.1 103 Early Hints message to the client with a Link header,
+indicating that the user agent can preload/preconnect the linked resources.
+
 ### `response.writeHead(statusCode[, statusMessage][, headers])`
 
 <!-- YAML

lib/_http_server.js

@@ -295,6 +295,20 @@ ServerResponse.prototype.writeProcessing = function writeProcessing(cb) {
   this._writeRaw('HTTP/1.1 102 Processing\r\n\r\n', 'ascii', cb);
 };
 
+ServerResponse.prototype.writeEarlyHints = function writeEarlyHints(links, cb) {
+  let head = 'HTTP/1.1 103 Early Hints\r\n';
+
+  if (typeof links === 'string') {
+    head += 'Link: ' + links + '\r\n';
+  } else if (ArrayIsArray(links)) {
+    head += 'Link: ' + links.join(',') + '\r\n';
+  }
+
+  head += '\r\n';
+
+  this._writeRaw(head, 'ascii', cb);
+};
+
 ServerResponse.prototype._implicitHeader = function _implicitHeader() {
   this.writeHead(this.statusCode);
 };

test/parallel/test-http-early-hints-multiple-links.js

@@ -0,0 +1,54 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const http = require('http');
+const debug = require('util').debuglog('test');
+
+const testResBody = 'response content\n';
+
+const server = http.createServer((req, res) => {
+  debug('Server sending early hints...');
+
+  res.writeEarlyHints([
+    '</styles.css>; rel=preload; as=style',
+    '</scripts.js>; rel=preload; as=script',
+  ]);
+
+  debug('Server sending full response...');
+  res.writeHead(200, {
+    'Content-Type': 'text/plain',
+    'ABCD': '1'
+  });
+
+  res.end(testResBody);
+});
+
+server.listen(0, function() {
+  const req = http.request({
+    port: this.address().port,
+    path: '/'
+  });
+
+  req.end();
+  debug('Client sending request...');
+
+  req.on('information', (res) => {
+    assert.strictEqual(res.headers.link, '</styles.css>; rel=preload; as=style,</scripts.js>; rel=preload; as=script');
+  });
+
+  req.on('response', function(res) {
+    let body = '';
+
+    assert.strictEqual(
+      res.statusCode, 200,
+      `Final status code was ${res.statusCode}, not 200.`
+    );
+    res.setEncoding('utf8');
+    res.on('data', function(chunk) { body += chunk; });
+    res.on('end', function() {
+      debug('Got full response.');
+      assert.strictEqual(body, testResBody);
+      server.close();
+    });
+  });
+});

test/parallel/test-http-early-hints.js

@@ -0,0 +1,51 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const http = require('http');
+const debug = require('util').debuglog('test');
+
+const testResBody = 'response content\n';
+
+const server = http.createServer((req, res) => {
+  debug('Server sending early hints...');
+
+  res.writeEarlyHints('</styles.css>; rel=preload; as=style');
+
+  debug('Server sending full response...');
+  res.writeHead(200, {
+    'Content-Type': 'text/plain',
+    'ABCD': '1'
+  });
+
+  res.end(testResBody);
+});
+
+server.listen(0, function() {
+  const req = http.request({
+    port: this.address().port,
+    path: '/'
+  });
+
+  req.end();
+  debug('Client sending request...');
+
+  req.on('information', (res) => {
+    assert.strictEqual(res.headers.link, '</styles.css>; rel=preload; as=style');
+  });
+
+  req.on('response', function(res) {
+    let body = '';
+
+    assert.strictEqual(
+      res.statusCode, 200,
+      `Final status code was ${res.statusCode}, not 200.`
+    );
+    res.setEncoding('utf8');
+    res.on('data', function(chunk) { body += chunk; });
+    res.on('end', function() {
+      debug('Got full response.');
+      assert.strictEqual(body, testResBody);
+      server.close();
+    });
+  });
+});