net: add new net.server.listen tracing channel

doc/api/diagnostics_channel.md

@@ -1131,6 +1131,26 @@ Emitted when a new TCP or pipe client socket is created.
 
 Emitted when a new TCP or pipe connection is received.
 
+`tracing:net.server.listen:asyncStart`
+
+* `server` {net.Server}
+* `options` {Object}
+
+Emitted when [`net.Server.listen()`][] is invoked, before the port or pipe is actually setup.
+
+`tracing:net.server.listen:asyncEnd`
+
+* `server` {net.Server}
+
+Emitted when [`net.Server.listen()`][] has completed and thus the server is ready to accept connection.
+
+`tracing:net.server.listen:error`
+
+* `server` {net.Server}
+* `error` {Error}
+
+Emitted when [`net.Server.listen()`][] is returning an error.
+
 #### UDP
 
 `udp.socket`
@@ -1179,5 +1199,6 @@ Emitted when a new thread is created.
 [`diagnostics_channel.unsubscribe(name, onMessage)`]: #diagnostics_channelunsubscribename-onmessage
 [`end` event]: #endevent
 [`error` event]: #errorevent
+[`net.Server.listen()`]: net.md#serverlisten
 [`start` event]: #startevent
 [context loss]: async_context.md#troubleshooting-context-loss

lib/net.js

@@ -151,6 +151,7 @@ const kPerfHooksNetConnectContext = Symbol('kPerfHooksNetConnectContext');
 const dc = require('diagnostics_channel');
 const netClientSocketChannel = dc.channel('net.client.socket');
 const netServerSocketChannel = dc.channel('net.server.socket');
+const netServerListen = dc.tracingChannel('net.server.listen');
 
 const {
   hasObserver,
@@ -1879,6 +1880,11 @@ function setupListenHandle(address, port, addressType, backlog, fd, flags) {
 
     if (typeof rval === 'number') {
       const error = new UVExceptionWithHostPort(rval, 'listen', address, port);
+
+      if (netServerListen.hasSubscribers) {
+        netServerListen.error.publish({ server: this, error });
+      }
+
       process.nextTick(emitErrorNT, this, error);
       return;
     }
@@ -1898,6 +1904,11 @@ function setupListenHandle(address, port, addressType, backlog, fd, flags) {
     const ex = new UVExceptionWithHostPort(err, 'listen', address, port);
     this._handle.close();
     this._handle = null;
+
+    if (netServerListen.hasSubscribers) {
+      netServerListen.error.publish({ server: this, error: ex });
+    }
+
     defaultTriggerAsyncIdScope(this[async_id_symbol],
                                process.nextTick,
                                emitErrorNT,
@@ -1906,6 +1917,10 @@ function setupListenHandle(address, port, addressType, backlog, fd, flags) {
     return;
   }
 
+  if (netServerListen.hasSubscribers) {
+    netServerListen.asyncEnd.publish({ server: this });
+  }
+
   // Generate connection key, this should be unique to the connection
   this._connectionKey = addressType + ':' + address + ':' + port;
 
@@ -1993,6 +2008,10 @@ Server.prototype.listen = function(...args) {
     throw new ERR_SERVER_ALREADY_LISTEN();
   }
 
+  if (netServerListen.hasSubscribers) {
+    netServerListen.asyncStart.publish({ server: this, options });
+  }
+
   if (cb !== null) {
     this.once('listening', cb);
   }

test/parallel/test-diagnostics-channel-net.js

@@ -5,21 +5,87 @@ const net = require('net');
 const dc = require('diagnostics_channel');
 
 const isNetSocket = (socket) => socket instanceof net.Socket;
+const isNetServer = (server) => server instanceof net.Server;
 
-dc.subscribe('net.client.socket', common.mustCall(({ socket }) => {
-  assert.strictEqual(isNetSocket(socket), true);
-}));
+function testDiagnosticChannel(subscribers, test, after) {
+  dc.tracingChannel('net.server.listen').subscribe(subscribers);
 
-dc.subscribe('net.server.socket', common.mustCall(({ socket }) => {
-  assert.strictEqual(isNetSocket(socket), true);
-}));
+  test(common.mustCall(() => {
+    dc.tracingChannel('net.server.listen').unsubscribe(subscribers);
+    after?.();
+  }));
+}
 
-const server = net.createServer(common.mustCall((socket) => {
-  socket.destroy();
-  server.close();
-}));
+const testSuccessfullListen = common.mustCall(() => {
+  let cb;
+  const server = net.createServer(common.mustCall((socket) => {
+    socket.destroy();
+    server.close();
+    cb();
+  }));
 
-server.listen(() => {
-  const { port } = server.address();
-  net.connect(port);
+  dc.subscribe('net.client.socket', common.mustCall(({ socket }) => {
+    assert.strictEqual(isNetSocket(socket), true);
+  }));
+
+  dc.subscribe('net.server.socket', common.mustCall(({ socket }) => {
+    assert.strictEqual(isNetSocket(socket), true);
+  }));
+
+  testDiagnosticChannel(
+    {
+      asyncStart: common.mustCall(({ server: currentServer, options }) => {
+        assert.strictEqual(isNetServer(server), true);
+        assert.strictEqual(currentServer, server);
+        assert.strictEqual(options.customOption, true);
+      }),
+      asyncEnd: common.mustCall(({ server: currentServer }) => {
+        assert.strictEqual(isNetServer(server), true);
+        assert.strictEqual(currentServer, server);
+      }),
+      error: common.mustNotCall()
+    },
+    common.mustCall((callback) => {
+      cb = callback;
+      server.listen({ port: 0, customOption: true }, () => {
+        const { port } = server.address();
+        net.connect(port);
+      });
+    }),
+    testFailingListen
+  );
 });
+
+const testFailingListen = common.mustCall(() => {
+  const originalServer = net.createServer(common.mustNotCall());
+
+  originalServer.listen(() => {
+    const server = net.createServer(common.mustNotCall());
+
+    testDiagnosticChannel(
+      {
+        asyncStart: common.mustCall(({ server: currentServer, options }) => {
+          assert.strictEqual(isNetServer(server), true);
+          assert.strictEqual(currentServer, server);
+          assert.strictEqual(options.customOption, true);
+        }),
+        asyncEnd: common.mustNotCall(),
+        error: common.mustCall(({ server: currentServer }) => {
+          assert.strictEqual(isNetServer(server), true);
+          assert.strictEqual(currentServer, server);
+        }),
+      },
+      common.mustCall((callback) => {
+        server.on('error', () => {});
+        server.listen({ port: originalServer.address().port, customOption: true });
+        callback();
+      }),
+      common.mustCall(() => {
+        originalServer.close();
+        server.close();
+      })
+    );
+  });
+});
+
+testSuccessfullListen();