https: support disabling session caching

Zero value of `maxCachedSessions` should disable TLS session caching in
`https.Agent`

PR-URL: nodejs#4252
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/https.js

@@ -138,6 +138,10 @@ Agent.prototype._getSession = function _getSession(key) {
 };
 
 Agent.prototype._cacheSession = function _cacheSession(key, session) {
+  // Cache is disabled
+  if (this.maxCachedSessions === 0)
+    return;
+
   // Fast case - update existing entry
   if (this._sessionCache.map[key]) {
     this._sessionCache.map[key] = session;

test/parallel/test-https-agent-disable-session-reuse.js

@@ -0,0 +1,60 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+if (!common.hasCrypto) {
+  console.log('1..0 # Skipped: missing crypto');
+  return;
+}
+
+const TOTAL_REQS = 2;
+
+const https = require('https');
+const crypto = require('crypto');
+
+const fs = require('fs');
+
+const options = {
+  key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
+  cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
+};
+
+const clientSessions = [];
+var serverRequests = 0;
+
+const agent = new https.Agent({
+  maxCachedSessions: 0
+});
+
+const server = https.createServer(options, function(req, res) {
+  serverRequests++;
+  res.end('ok');
+}).listen(common.PORT, function() {
+  var waiting = TOTAL_REQS;
+  function request() {
+    const options = {
+      agent: agent,
+      port: common.PORT,
+      rejectUnauthorized: false
+    };
+
+    https.request(options, function(res) {
+      clientSessions.push(res.socket.getSession());
+
+      res.resume();
+      res.on('end', function() {
+        if (--waiting !== 0)
+          return request();
+        server.close();
+      });
+    }).end();
+  }
+  request();
+});
+
+process.on('exit', function() {
+  assert.equal(serverRequests, TOTAL_REQS);
+  assert.equal(clientSessions.length, TOTAL_REQS);
+  assert.notEqual(clientSessions[0].toString('hex'),
+                  clientSessions[1].toString('hex'));
+});