net: add writeQueueSize

doc/api/net.md

@@ -825,6 +825,16 @@ added: v0.5.3
 
 The amount of bytes sent.
 
+### `socket.writeQueueSize`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {integer}
+
+The amount of bytes in libuv write queue.
+
 ### `socket.connect()`
 
 Initiate a connection on a given socket.

lib/net.js

@@ -967,6 +967,9 @@ protoGetter('bytesWritten', function bytesWritten() {
   return bytes;
 });
 
+protoGetter('writeQueueSize', function writeQueueSize() {
+  return this._handle ? this._handle.writeQueueSize : -1;
+});
 
 function checkBindError(err, port, handle) {
   // EADDRINUSE may not be reported until we call listen() or connect().

test/parallel/test-net-connect-write-queue-size.js

@@ -0,0 +1,31 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const tmpdir = require('../common/tmpdir');
+
+tmpdir.refresh();
+
+const socket = new net.Socket();
+assert.strictEqual(socket.writeQueueSize, -1);
+
+const server = net.createServer()
+  .listen(common.PIPE, common.mustCall(() => {
+    // The pipe connection is a synchronous operation
+    // `net.connect` will set `socket.connecting` to true
+    const socket = net.connect(common.PIPE, common.mustCall(() => {
+      socket.destroy();
+      server.close();
+    }));
+    // Set connecting to false here to make `socket.write` can call into
+    // libuv directly (see `_writeGeneric` in `net.js`).
+    // because the socket is connecting currently in libuv, so libuv will
+    // insert the write request into write queue, then we can get the
+    // size of write queue by `socket.writeQueueSize`.
+    socket.connecting = false;
+    const data = 'hello';
+    socket.write(data, 'utf-8', common.mustCall());
+    assert.strictEqual(socket.writeQueueSize, common.isWindows ? 0 : data.length);
+    // Restore it
+    socket.connecting = true;
+  }));