feat: populate dnslookup timing by capturing DNS resolution time on sockets

Captures DNS lookup time via Socket.prototype.emit monkey-patch for the
'lookup' event. When the first request is sent on a socket, the DNS
lookup time is calculated relative to the request start time and stored
in the timing object.

Closes #577

Agent-Logs-Url: https://github.com/node-modules/urllib/sessions/efde376e-b12f-42f6-9490-2ce39519fe7c

Co-authored-by: fengmk2 <156269+fengmk2@users.noreply.github.com>

Author: Copilot

src/diagnosticsChannel.ts

@@ -67,6 +67,15 @@ Socket.prototype.destroy = function (err?: any) {
   return destroySocket.call(this, err);
 };
 
+// capture DNS lookup time on socket via 'lookup' event
+const originalSocketEmit = Socket.prototype.emit;
+Socket.prototype.emit = function (this: SocketExtend, event: string, ...args: any[]) {
+  if (event === 'lookup') {
+    this[symbols.kSocketDnsLookupTime] = performance.now();
+  }
+  return originalSocketEmit.apply(this, [event, ...args] as any);
+} as typeof originalSocketEmit;
+
 function getRequestOpaque(request: DiagnosticsChannel.Request, kHandler?: symbol) {
   if (!kHandler) return;
   const handler = Reflect.get(request, kHandler);
@@ -201,6 +210,13 @@ export function initDiagnosticsChannel(): void {
         opaque[symbols.kRequestStartTime],
         socket[symbols.kSocketStartTime] as number,
       );
+      // kSocketDnsLookupTime - kRequestStartTime = dns lookup time
+      if (socket[symbols.kSocketDnsLookupTime]) {
+        opaque[symbols.kRequestTiming].dnslookup = performanceTime(
+          opaque[symbols.kRequestStartTime],
+          socket[symbols.kSocketDnsLookupTime] as number,
+        );
+      }
     }
   });
 

src/symbols.ts

@@ -9,6 +9,7 @@ const symbols: {
   kSocketConnectHost: symbol;
   kSocketConnectPort: symbol;
   kSocketConnectProtocol: symbol;
+  kSocketDnsLookupTime: symbol;
   kHandledRequests: symbol;
   kHandledResponses: symbol;
   kRequestSocket: symbol;
@@ -30,6 +31,7 @@ const symbols: {
   kSocketConnectHost: Symbol('socket connect params: host'),
   kSocketConnectPort: Symbol('socket connect params: port'),
   kSocketConnectProtocol: Symbol('socket connect params: protocol'),
+  kSocketDnsLookupTime: Symbol('socket dns lookup time'),
   kHandledRequests: Symbol('handled requests per socket'),
   kHandledResponses: Symbol('handled responses per socket'),
   kRequestSocket: Symbol('request on the socket'),

test/options.timing.test.ts

@@ -30,6 +30,8 @@ describe('options.timing.test.ts', () => {
     assert(res.timing.waiting > 0);
     assert(res.timing.queuing > 0);
     assert(res.timing.connected > 0);
+    assert(res.timing.dnslookup > 0);
+    assert(res.timing.dnslookup <= res.timing.connected);
     assert(res.timing.requestHeadersSent > 0);
     assert(res.timing.requestSent > 0);
     assert(res.timing.contentDownload > 0);
@@ -49,6 +51,7 @@ describe('options.timing.test.ts', () => {
     assert(res.timing.waiting > 0);
     assert(res.timing.queuing > 0);
     assert.equal(res.timing.connected, 0);
+    assert.equal(res.timing.dnslookup, 0);
     assert(res.timing.requestHeadersSent > 0);
     assert(res.timing.requestSent > 0);
     assert(res.timing.contentDownload > 0);