http: process 100, 102-199 according to specs.

Adding ServerResponse.writeProcessing to send 102 status codes.

Added an `'information'` event to ClientRequest to handle
1xx status codes except 101 Upgrade.
101 Upgrade is excluded due to its non-informational
processing according to RFC7231, Section 6.2.2.

This affects several modules downstream that use the http
module, e.g., node-fetch, all of whom violate HTTP RFCs
due to this module. As such, this could introduce a
breaking change for downstream if HTTP standards were
ignored in an ad-hoc fashion.

See also RFC2518 RFC8297.

PR-URL: https://github.com/nodejs/node/pull/18033
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

doc/api/http.md

@@ -400,6 +400,37 @@ Emitted when the server sends a '100 Continue' HTTP response, usually because
 the request contained 'Expect: 100-continue'. This is an instruction that
 the client should send the request body.
 
+### Event: 'information'
+<!-- YAML
+added: REPLACEME
+-->
+
+Emitted when the server sends a 1xx response (excluding 101 Upgrade). This
+event is emitted with a callback containing an object with a status code.
+
+```js
+const http = require('http');
+
+const options = {
+  hostname: '127.0.0.1',
+  port: 8080,
+  path: '/length_request'
+};
+
+// Make a request
+const req = http.request(options);
+req.end();
+
+req.on('information', (res) => {
+  console.log('got information prior to main response: ' + res.statusCode);
+});
+```
+
+101 Upgrade statuses do not fire this event due to their break from the
+traditional HTTP request/response chain, such as web sockets, in-place TLS
+upgrades, or HTTP 2.0. To be notified of 101 Upgrade notices, listen for the
+[`'upgrade'`][] event instead.
+
 ### Event: 'response'
 <!-- YAML
 added: v0.1.0
@@ -1384,6 +1415,14 @@ which has been transmitted are equal or not.
 Attempting to set a header field name or value that contains invalid characters
 will result in a [`TypeError`][] being thrown.
 
+### response.writeProcessing()
+<!-- YAML
+added: REPLACEME
+-->
+
+Sends a HTTP/1.1 102 Processing message to the client, indicating that
+the request body should be sent.
+
 ## Class: http.IncomingMessage
 <!-- YAML
 added: v0.1.17
@@ -1937,6 +1976,7 @@ not abort the request or do anything besides add a `timeout` event.
 [`'checkContinue'`]: #http_event_checkcontinue
 [`'request'`]: #http_event_request
 [`'response'`]: #http_event_response
+[`'upgrade'`]: #http_event_upgrade
 [`Agent`]: #http_class_http_agent
 [`Duplex`]: stream.html#stream_class_stream_duplex
 [`EventEmitter`]: events.html#events_class_eventemitter

lib/_http_client.js

@@ -447,16 +447,25 @@ function socketOnData(d) {
       socket.destroy();
     }
   } else if (parser.incoming && parser.incoming.complete &&
-             // When the status code is 100 (Continue), the server will
-             // send a final response after this client sends a request
-             // body. So, we must not free the parser.
-             parser.incoming.statusCode !== 100) {
+             // When the status code is informational (100, 102-199),
+             // the server will send a final response after this client
+             // sends a request body, so we must not free the parser.
+             // 101 (Switching Protocols) and all other status codes
+             // should be processed normally.
+             !statusIsInformational(parser.incoming.statusCode)) {
     socket.removeListener('data', socketOnData);
     socket.removeListener('end', socketOnEnd);
     freeParser(parser, req, socket);
   }
 }
 
+function statusIsInformational(status) {
+  // 100 (Continue)    RFC7231 Section 6.2.1
+  // 102 (Processing)  RFC2518
+  // 103 (Early Hints) RFC8297
+  // 104-199 (Unassigned)
+  return (status < 200 && status >= 100 && status !== 101);
+}
 
 // client
 function parserOnIncomingClient(res, shouldKeepAlive) {
@@ -480,10 +489,16 @@ function parserOnIncomingClient(res, shouldKeepAlive) {
     return 2;  // Skip body and treat as Upgrade.
   }
 
-  if (res.statusCode === 100) {
-    // restart the parser, as this is a continue message.
+  if (statusIsInformational(res.statusCode)) {
+    // Restart the parser, as this is a 1xx informational message.
     req.res = null; // Clear res so that we don't hit double-responses.
+    // Maintain compatibility by sending 100-specific events
+    if (res.statusCode === 100) {
       req.emit('continue');
+    }
+    // Send information events to all 1xx responses except 101 Upgrade.
+    req.emit('information', { statusCode: res.statusCode });
+
     return 1;  // Skip body but don't treat as Upgrade.
   }
 

lib/_http_server.js

@@ -188,6 +188,10 @@ ServerResponse.prototype.writeContinue = function writeContinue(cb) {
   this._sent100 = true;
 };
 
+ServerResponse.prototype.writeProcessing = function writeProcessing(cb) {
+  this._writeRaw(`HTTP/1.1 102 Processing${CRLF}${CRLF}`, 'ascii', cb);
+};
+
 ServerResponse.prototype._implicitHeader = function _implicitHeader() {
   this.writeHead(this.statusCode);
 };

test/parallel/test-http-information-processing.js

@@ -0,0 +1,52 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const http = require('http');
+const Countdown = require('../common/countdown');
+
+const test_res_body = 'other stuff!\n';
+const countdown = new Countdown(3, () => server.close());
+
+const server = http.createServer((req, res) => {
+  console.error('Server sending informational message #1...');
+  res.writeProcessing();
+  console.error('Server sending informational message #2...');
+  res.writeProcessing();
+  console.error('Server sending full response...');
+  res.writeHead(200, {
+    'Content-Type': 'text/plain',
+    'ABCD': '1'
+  });
+  res.end(test_res_body);
+});
+
+server.listen(0, function() {
+  const req = http.request({
+    port: this.address().port,
+    path: '/world'
+  });
+  req.end();
+  console.error('Client sending request...');
+
+  let body = '';
+
+  req.on('information', function(res) {
+    console.error('Client got 102 Processing...');
+    countdown.dec();
+  });
+
+  req.on('response', function(res) {
+    assert.strictEqual(countdown.remaining, 1,
+                       'Full response received before all 102 Processing');
+    assert.strictEqual(200, res.statusCode,
+                       `Final status code was ${res.statusCode}, not 200.`);
+    res.setEncoding('utf8');
+    res.on('data', function(chunk) { body += chunk; });
+    res.on('end', function() {
+      console.error('Got full response.');
+      assert.strictEqual(body, test_res_body);
+      assert.ok('abcd' in res.headers);
+      countdown.dec();
+    });
+  });
+});