http: add writeEarlyHints function to ServerResponse

doc/api/http.md

@@ -2127,10 +2127,41 @@ buffer. Returns `false` if all or part of the data was queued in user memory.
 added: v0.3.0
 -->
 
-Sends a HTTP/1.1 100 Continue message to the client, indicating that
+Sends an 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(links[, callback])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `links` {string|Array}
+* `callback` {Function}
+
+Sends an 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.
+The `links` can be a string or an array of strings containing the values
+of the `Link` header. The optional `callback` argument will be called when
+the response message has been written.
+
+**Example**
+
+```js
+const earlyHintsLink = '</styles.css>; rel=preload; as=style';
+response.writeEarlyHints(earlyHintsLink);
+
+const earlyHintsLinks = [
+  '</styles.css>; rel=preload; as=style',
+  '</scripts.js>; rel=preload; as=script',
+];
+response.writeEarlyHints(earlyHintsLinks);
+
+const earlyHintsCallback = () => console.log('early hints message sent');
+response.writeEarlyHints(earlyHintsLinks, earlyHintsCallback);
+```
+
 ### `response.writeHead(statusCode[, statusMessage][, headers])`
 
 <!-- YAML

doc/api/http2.md

@@ -3991,6 +3991,32 @@ Sends a status `100 Continue` to the client, indicating that the request body
 should be sent. See the [`'checkContinue'`][] event on `Http2Server` and
 `Http2SecureServer`.
 
+### `response.writeEarlyHints(links)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `links` {string|Array}
+
+Sends a status `103 Early Hints` to the client with a Link header,
+indicating that the user agent can preload/preconnect the linked resources.
+The `links` can be a string or an array of strings containing the values
+of the `Link` header.
+
+**Example**
+
+```js
+const earlyHintsLink = '</styles.css>; rel=preload; as=style';
+response.writeEarlyHints(earlyHintsLink);
+
+const earlyHintsLinks = [
+  '</styles.css>; rel=preload; as=style',
+  '</scripts.js>; rel=preload; as=script',
+];
+response.writeEarlyHints(earlyHintsLinks);
+```
+
 #### `response.writeHead(statusCode[, statusMessage][, headers])`
 
 <!-- YAML

lib/_http_server.js

@@ -80,7 +80,8 @@ const {
 } = codes;
 const {
   validateInteger,
-  validateBoolean
+  validateBoolean,
+  validateLinkHeaderValue
 } = require('internal/validators');
 const Buffer = require('buffer').Buffer;
 const { setInterval, clearInterval } = require('timers');
@@ -295,6 +296,43 @@ 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') {
+    validateLinkHeaderValue(links, 'links');
+    head += 'Link: ' + links + '\r\n';
+  } else if (ArrayIsArray(links)) {
+    if (!links.length) {
+      return;
+    }
+
+    head += 'Link: ';
+
+    for (let i = 0; i < links.length; i++) {
+      const link = links[i];
+      validateLinkHeaderValue(link, 'links');
+      head += link;
+
+      if (i !== links.length - 1) {
+        head += ', ';
+      }
+    }
+
+    head += '\r\n';
+  } else {
+    throw new ERR_INVALID_ARG_VALUE(
+      'links',
+      links,
+      'must be an array or string of format "</styles.css>; rel=preload; as=style"'
+    );
+  }
+
+  head += '\r\n';
+
+  this._writeRaw(head, 'ascii', cb);
+};
+
 ServerResponse.prototype._implicitHeader = function _implicitHeader() {
   this.writeHead(this.statusCode);
 };

lib/internal/http2/compat.js

@@ -32,6 +32,7 @@ const {
     HTTP2_HEADER_STATUS,
 
     HTTP_STATUS_CONTINUE,
+    HTTP_STATUS_EARLY_HINTS,
     HTTP_STATUS_EXPECTATION_FAILED,
     HTTP_STATUS_METHOD_NOT_ALLOWED,
     HTTP_STATUS_OK
@@ -55,6 +56,7 @@ const {
 const {
   validateFunction,
   validateString,
+  validateLinkHeaderValue,
 } = require('internal/validators');
 const {
   kSocket,
@@ -844,6 +846,49 @@ class Http2ServerResponse extends Stream {
     });
     return true;
   }
+
+  writeEarlyHints(links) {
+    let linkHeaderValue = '';
+
+    if (typeof links === 'string') {
+      validateLinkHeaderValue(links, 'links');
+      linkHeaderValue += links;
+    } else if (ArrayIsArray(links)) {
+      if (!links.length) {
+        return;
+      }
+
+      linkHeaderValue += '';
+
+      for (let i = 0; i < links.length; i++) {
+        const link = links[i];
+        validateLinkHeaderValue(link, 'links');
+        linkHeaderValue += link;
+
+        if (i !== links.length - 1) {
+          linkHeaderValue += ', ';
+        }
+      }
+    } else {
+      throw new ERR_INVALID_ARG_VALUE(
+        'links',
+        links,
+        'must be an array or string of format "</styles.css>; rel=preload; as=style"'
+      );
+    }
+
+    const stream = this[kStream];
+
+    if (stream.headersSent || this[kState].closed)
+      return false;
+
+    stream.additionalHeaders({
+      [HTTP2_HEADER_STATUS]: HTTP_STATUS_EARLY_HINTS,
+      'Link': linkHeaderValue
+    });
+
+    return true;
+  }
 }
 
 function onServerStream(ServerRequest, ServerResponse,

lib/internal/validators.js

@@ -258,6 +258,21 @@ function validateUnion(value, name, union) {
   }
 }
 
+function validateLinkHeaderValue(value, name) {
+  const linkValueRegExp = /^(?:<[^>]*>;)\s*(?:rel=(")?[^;"]*\1;?)\s*(?:(?:as|anchor|title)=(")?[^;"]*\2)?$/;
+
+  if (
+    typeof value === 'undefined' ||
+    !RegExpPrototypeExec(linkValueRegExp, value)
+  ) {
+    throw new ERR_INVALID_ARG_VALUE(
+      name,
+      value,
+      'must be an array or string of format "</styles.css>; rel=preload; as=style"'
+    );
+  }
+}
+
 module.exports = {
   isInt32,
   isUint32,
@@ -280,4 +295,5 @@ module.exports = {
   validateUndefined,
   validateUnion,
   validateAbortSignal,
+  validateLinkHeaderValue
 };

test/parallel/test-http-early-hints-invalid-argument-type.js

@@ -0,0 +1,33 @@
+'use strict';
+const common = require('../common');
+const assert = require('node:assert');
+const http = require('node:http');
+const debug = require('node:util').debuglog('test');
+
+const testResBody = 'response content\n';
+
+const server = http.createServer(common.mustCall((req, res) => {
+  debug('Server sending early hints...');
+  res.writeEarlyHints({ links: 'bad argument object' });
+
+  debug('Server sending full response...');
+  res.end(testResBody);
+}));
+
+server.listen(0, common.mustCall(() => {
+  const req = http.request({
+    port: server.address().port, path: '/'
+  });
+
+  req.end();
+  debug('Client sending request...');
+
+  req.on('information', common.mustNotCall());
+
+  process.on('uncaughtException', (err) => {
+    debug(`Caught an exception: ${JSON.stringify(err)}`);
+    if (err.name === 'AssertionError') throw err;
+    assert.strictEqual(err.code, 'ERR_INVALID_ARG_VALUE');
+    process.exit(0);
+  });
+}));

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

@@ -0,0 +1,33 @@
+'use strict';
+const common = require('../common');
+const assert = require('node:assert');
+const http = require('node:http');
+const debug = require('node:util').debuglog('test');
+
+const testResBody = 'response content\n';
+
+const server = http.createServer(common.mustCall((req, res) => {
+  debug('Server sending early hints...');
+  res.writeEarlyHints('bad argument value');
+
+  debug('Server sending full response...');
+  res.end(testResBody);
+}));
+
+server.listen(0, common.mustCall(() => {
+  const req = http.request({
+    port: server.address().port, path: '/'
+  });
+
+  req.end();
+  debug('Client sending request...');
+
+  req.on('information', common.mustNotCall());
+
+  process.on('uncaughtException', (err) => {
+    debug(`Caught an exception: ${JSON.stringify(err)}`);
+    if (err.name === 'AssertionError') throw err;
+    assert.strictEqual(err.code, 'ERR_INVALID_ARG_VALUE');
+    process.exit(0);
+  });
+}));