net: tryReadStart destroys socket on UV_EALREADY instead of ignoring it

[dirkwa]

Version

v26.0.0-pre (main branch, also affects v24.12.0)

Platform

Linux 6.12.73+deb13-amd64 x86_64

Subsystem

net

What steps will reproduce the bug?

tryReadStart() in lib/net.js destroys the socket when readStart() returns UV_EALREADY. This can be reproduced by calling readStart when reading is already active at the libuv level:

const net = require('net');
const { internalBinding } = require('internal/test/binding');
const { UV_EALREADY } = internalBinding('uv');

const server = net.createServer((conn) => { conn.on('error', () => {}); });
server.listen('/tmp/test-ealready.sock', () => {
  const socket = net.createConnection({ path: '/tmp/test-ealready.sock' });
  socket.on('connect', () => {
    // Simulate readStart returning UV_EALREADY (happens with sockets
    // opened via uv_pipe_open where reading is already active)
    socket._handle.readStart = () => UV_EALREADY;
    socket._handle.reading = false;
    socket._read(16384);

    // Socket is now destroyed even though reading was already active
    console.log('destroyed:', socket.destroyed); // true
    socket.destroy();
  });
  socket.on('error', (e) => console.log('error:', e.code)); // EALREADY
  socket.on('close', () => server.close());
});

In practice, this happens when socket types with no async connect phase (synchronous bind, 'connect' emitted via nextTick) cause multiple readStart calls in the same microtask batch. Discovered while implementing AF_CAN socket support (#62398), but the bug is in tryReadStart itself.

How often does it reproduce? Is there a required condition?

100% reproducible. The condition is readStart() returning UV_EALREADY, which occurs when:

• A socket fd is opened via uv_pipe_open() and readStart is called more than once
• The deferred _read callback (queued while connecting=true) and the stream resume() from a 'data' listener both fire in the same tick

What is the expected behavior? Why is that the expected behavior?

UV_EALREADY means "reading is already in progress." The socket should continue operating normally. tryReadStart should be idempotent — if reading is already active, there's nothing to do.

What do you see instead?

The socket is destroyed with Error: read EALREADY. The 'error' event fires and the socket becomes unusable

Additional information

Fix with regression test: https://github.com/dirkwa/node/tree/fix-tryreadstart-ealready

The fix adds a guard for UV_EALREADY in tryReadStart, making it ignore the "already reading" case instead of destroying the socket. This matches the intent of the existing JS-side reading flag guard in Socket._read.

Refs: #62398