writing on closed connection without getting error !

[mahditakrim]

After I close the client connection, I write message on it with conn.WriteJSON() and here is what happens:

• the first attempt to write returns just fine however the connection is closed and I expect to have error.
• the second attempt to write returns error which is "write tcp 127.0.0.1:9090->127.0.0.1:41274: write: broken pipe"

So the question is why the very first write on a closed connection does not return error ?!

[corani]

The behavior you're seeing has two contributing causes.

1. The first net.Conn.Write() after net.Conn.Close() can succeed at the OS level. Close() initiates a TCP half-close by sending a FIN segment (RFC 9293 §3.6), but the kernel accepts the subsequent write into the send buffer and returns success immediately; it hasn't yet received the peer's RST in response to the FIN. The error only surfaces on the next write, once that RST arrives.

2. conn.Close() calls net.Conn.Close() but sets no internal flag that the connection was closed. WriteJSON will therefore proceed to write, and if the OS hasn't yet received a RST, the write may succeed if the entire message fits in the send buffer.

The write path does check an internal writeErr field before attempting a write, and once a write fails writeFatal sets that field so subsequent writes fail immediately without touching the network. Close() could call writeFatal to set this field, which would make writes after Close() fail fast, without ever reaching the OS.

[corani]

A possible fix could be to add c.writeFatal(ErrCloseSent) before calling c.conn.Close() in

websocket/conn.go

Lines 346 to 354 in 82c8017

	func (c *Conn) Close() error {
	if c == nil {
	return ErrNilConn
	}
	if c.conn == nil {
	return ErrNilNetConn
	}
	return c.conn.Close()
	}