Custom http agent support (#810)

lib/Connection.js

@@ -46,18 +46,20 @@ class Connection {
       throw new ConfigurationError(`Invalid protocol: '${this.url.protocol}'`)
     }
 
-    // Probably there is a bug in Node Core
-    // see https://github.com/nodejs/node/issues/26357
-    const keepAliveFalse = opts.agent && opts.agent.keepAlive === false
-    const agentOptions = Object.assign({}, {
-      keepAlive: true,
-      keepAliveMsecs: 1000,
-      maxSockets: keepAliveFalse ? Infinity : 256,
-      maxFreeSockets: 256
-    }, opts.agent)
-    this.agent = this.url.protocol === 'http:'
-      ? new http.Agent(agentOptions)
-      : new https.Agent(Object.assign({}, agentOptions, this.ssl))
+    if (typeof opts.agent === 'function') {
+      this.agent = opts.agent()
+    } else {
+      const keepAliveFalse = opts.agent && opts.agent.keepAlive === false
+      const agentOptions = Object.assign({}, {
+        keepAlive: true,
+        keepAliveMsecs: 1000,
+        maxSockets: keepAliveFalse ? Infinity : 256,
+        maxFreeSockets: 256
+      }, opts.agent)
+      this.agent = this.url.protocol === 'http:'
+        ? new http.Agent(agentOptions)
+        : new https.Agent(Object.assign({}, agentOptions, this.ssl))
+    }
 
     this.makeRequest = this.url.protocol === 'http:'
       ? http.request