lib: support overriding http\s.globalAgent

Overriding `require('http[s]').globalAgent` is now respected by
consequent requests.

In order to achieve that, the following changes were made:

1. Implmentation in `http`: `module.exports.globalAgent` is now defined
through `Object.defineProperty`. Its getter and setter return \ set
`require('_http_agent').globalAgent`.

2. Implementation in `https`: the https `globalAgent` is not the same
as `_http_agent`, and is defined in `https` module itself. Therefore,
the fix here was to simply use `module.exports.globalAgent` to support
mutation.

3. According tests were added for both `http` and `https`, where in
both we create a server, set the default agent to a newly created
instance and make a request to that server. We then assert that the
given instance was actually used by inspecting its sockets property.

Fixes: #23281

lib/http.js

@@ -21,7 +21,7 @@
 
 'use strict';
 
-const { Agent, globalAgent } = require('_http_agent');
+const httpAgent = require('_http_agent');
 const { ClientRequest } = require('_http_client');
 const { methods } = require('_http_common');
 const { IncomingMessage } = require('_http_incoming');
@@ -52,9 +52,8 @@ module.exports = {
   _connectionListener,
   METHODS: methods.slice().sort(),
   STATUS_CODES,
-  Agent,
+  Agent: httpAgent.Agent,
   ClientRequest,
-  globalAgent,
   IncomingMessage,
   OutgoingMessage,
   Server,
@@ -76,3 +75,14 @@ Object.defineProperty(module.exports, 'maxHeaderSize', {
     return maxHeaderSize;
   }
 });
+
+Object.defineProperty(module.exports, 'globalAgent', {
+  configurable: true,
+  enumerable: true,
+  get() {
+    return httpAgent.globalAgent;
+  },
+  set(value) {
+    httpAgent.globalAgent = value;
+  }
+});

lib/https.js

@@ -296,7 +296,7 @@ function request(...args) {
     Object.assign(options, args.shift());
   }
 
-  options._defaultAgent = globalAgent;
+  options._defaultAgent = module.exports.globalAgent;
   args.unshift(options);
 
   return new ClientRequest(...args);

test/parallel/test-http-client-override-global-agent.js

@@ -0,0 +1,26 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+const server = http.Server(common.mustCall((req, res) => {
+  res.writeHead(200);
+  res.end('Hello, World!');
+}));
+
+server.listen(0, common.mustCall(() => {
+  const agent = new http.Agent();
+  const name = agent.getName({ port: server.address().port });
+  http.globalAgent = agent;
+
+  makeRequest();
+  assert(agent.sockets.hasOwnProperty(name)); // agent has indeed been used
+}));
+
+function makeRequest() {
+  const req = http.get({
+    port: server.address().port
+  });
+  req.on('close', () =>
+    server.close());
+}

test/parallel/test-https-client-override-global-agent.js

@@ -0,0 +1,38 @@
+'use strict';
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const assert = require('assert');
+const https = require('https');
+
+if (!common.hasCrypto)
+  common.skip('missing crypto');
+
+// Disable strict server certificate validation by the client
+process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0';
+
+const options = {
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent1-cert.pem')
+};
+
+const server = https.Server(options, common.mustCall((req, res) => {
+  res.writeHead(200);
+  res.end('Hello, World!');
+}));
+
+server.listen(0, common.mustCall(() => {
+  const agent = new https.Agent();
+  const name = agent.getName({ port: server.address().port });
+  https.globalAgent = agent;
+
+  makeRequest();
+  assert(agent.sockets.hasOwnProperty(name)); // agent has indeed been used
+}));
+
+function makeRequest() {
+  const req = https.get({
+    port: server.address().port
+  });
+  req.on('close', () =>
+    server.close());
+}