test: add regression test for large write

Fixes: #19562

PR-URL: #19551
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

test/parallel/test-net-bytes-written-large.js

@@ -0,0 +1,67 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+
+// Regression test for https://github.com/nodejs/node/issues/19562:
+// Writing to a socket first tries to push through as much data as possible
+// without blocking synchronously, and, if that is not enough, queues more
+// data up for asynchronous writing.
+// Check that `bytesWritten` accounts for both parts of a write.
+
+const N = 10000000;
+{
+  // Variant 1: Write a Buffer.
+  const server = net.createServer(common.mustCall((socket) => {
+    socket.end(Buffer.alloc(N), common.mustCall(() => {
+      assert.strictEqual(socket.bytesWritten, N);
+    }));
+    assert.strictEqual(socket.bytesWritten, N);
+  })).listen(0, common.mustCall(() => {
+    const client = net.connect(server.address().port);
+    client.resume();
+    client.on('close', common.mustCall(() => {
+      assert.strictEqual(client.bytesRead, N);
+      server.close();
+    }));
+  }));
+}
+
+{
+  // Variant 2: Write a string.
+  const server = net.createServer(common.mustCall((socket) => {
+    socket.end('a'.repeat(N), common.mustCall(() => {
+      assert.strictEqual(socket.bytesWritten, N);
+    }));
+    assert.strictEqual(socket.bytesWritten, N);
+  })).listen(0, common.mustCall(() => {
+    const client = net.connect(server.address().port);
+    client.resume();
+    client.on('close', common.mustCall(() => {
+      assert.strictEqual(client.bytesRead, N);
+      server.close();
+    }));
+  }));
+}
+
+{
+  // Variant 2: writev() with mixed data.
+  const server = net.createServer(common.mustCall((socket) => {
+    socket.cork();
+    socket.write('a'.repeat(N));
+    assert.strictEqual(socket.bytesWritten, N);
+    socket.write(Buffer.alloc(N));
+    assert.strictEqual(socket.bytesWritten, 2 * N);
+    socket.end('', common.mustCall(() => {
+      assert.strictEqual(socket.bytesWritten, 2 * N);
+    }));
+    socket.uncork();
+  })).listen(0, common.mustCall(() => {
+    const client = net.connect(server.address().port);
+    client.resume();
+    client.on('close', common.mustCall(() => {
+      assert.strictEqual(client.bytesRead, 2 * N);
+      server.close();
+    }));
+  }));
+}