test: check socketOnDrain where needPause is false

Adds a tests that checks if we can start reading again
from a socket backing an incoming http request.

PR-URL: nodejs#17654
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

test/parallel/test-http-hightwatermark.js

@@ -0,0 +1,52 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const http = require('http');
+
+// These test cases to check socketOnDrain where needPause becomes false.
+// When send large response enough to exceed highWaterMark, it expect the socket
+// to be paused and res.write would be failed.
+// And it should be resumed when outgoingData falls below highWaterMark.
+
+let requestReceived = 0;
+
+const server = http.createServer(function(req, res) {
+  const id = ++requestReceived;
+  const enoughToDrain = req.connection.writableHighWaterMark;
+  const body = 'x'.repeat(enoughToDrain);
+
+  if (id === 1) {
+    // Case of needParse = false
+    req.connection.once('pause', common.mustCall(() => {
+      assert(req.connection._paused, '_paused must be true because it exceeds' +
+                                     'highWaterMark by second request');
+    }));
+  } else {
+    // Case of needParse = true
+    const resume = req.connection.parser.resume.bind(req.connection.parser);
+    req.connection.parser.resume = common.mustCall((...args) => {
+      const paused = req.connection._paused;
+      assert(!paused, '_paused must be false because it become false by ' +
+                      'socketOnDrain when outgoingData falls below ' +
+                      'highWaterMark');
+      return resume(...args);
+    });
+  }
+  assert(!res.write(body), 'res.write must return false because it will ' +
+                           'exceed highWaterMark on this call');
+  res.end();
+}).on('listening', () => {
+  const c = net.createConnection(server.address().port, () => {
+    c.write('GET / HTTP/1.1\r\n\r\n');
+    c.write('GET / HTTP/1.1\r\n\r\n');
+    c.end();
+  });
+
+  c.on('data', () => {});
+  c.on('end', () => {
+    server.close();
+  });
+});
+
+server.listen(0);