net: reading a connectionless datagram socket after a CloseRead still blocks indefinitely #15250
Description
I was trying to cleanly close a Unix socket (i.e. stop receiving traffic, read all the socket buffer and then close the socket) but found it a bit cumbersome.
The issue is that the following code blocks indefinitely:
package main
import (
"fmt"
"net"
)
func main() {
var socket = "/tmp/test.sock"
addr, err := net.ResolveUnixAddr("unixgram", socket)
if err != nil {
panic(err)
}
conn, err := net.ListenUnixgram("unixgram", addr)
if err != nil {
panic(err)
}
w, err := net.DialUnix("unixgram", nil, conn.LocalAddr().(*net.UnixAddr))
if err != nil {
panic(err)
}
if _, err := w.Write([]byte("foo")); err != nil {
panic(err)
}
if err := conn.CloseRead(); err != nil {
panic(err)
}
buf := make([]byte, 10)
n, err := conn.Read(buf)
if err != nil {
panic(err)
}
fmt.Println(string(buf[:n])) // Prints foo.
n, err = conn.Read(buf) // Hangs indefinitely. Should it return io.EOF?
if err != nil {
panic(err)
}
fmt.Println(string(buf[:n]))
}
To read all the socket buffer without blocking indefinitely you have to either:
- use
conn.Close()after a timeout to unblockconn.Read() - use
conn.File()andsyscall.SetNonblock()to read the connection buffer in a non-blocking way
I don't like 1. since it would make my tests slow or flaky. 2. works well but it makes the code more complex.
Since after a CloseRead(), a socket cannot receive more data I think it would make sense that Read returns an io.EOF when it reaches the end of the connection buffer.