net: using File methods on Conn/Listener/PacketConn disables non-blocking I/O and runtime-integrated network poller's deadline features

[jbardin]

Calling a network connection's File() method prevents deadlines from working.

This won't timeout correctly:

c, _ := net.ListenUDP("udp", &net.UDPAddr{IP: net.IP{127, 0, 0, 1}, Port: 9999})
fmt.Println("timing out in 1s")
c.SetReadDeadline(time.Now().Add(time.Second))

// calling File() removes the ability to use deadlines
_, _ = c.File()

buf := make([]byte, 1024)
_, e := c.Read(buf)
fmt.Println(e)

It would be nice if deadlines still worked when a socket is in blocking mode, but if it's not possible this should at least be documented.

[mikioh]

It's documented, and we provide FileConn/FileListener/FilePacketConn functions.

[jbardin]

I understand, but I think it could use a little more precise wording in the documentation as to what "sets the underlying os.File to blocking mode" implies. I wouldn't expect the runtime to start setting RCVTIMEO and SNDTIMEO, but I also couldn't find an stanza stating exactly what the behavior would be.

[mikioh]

use a little more precise wording in the documentation

@jbardin, feel free to send a patch. thanks.
https://golang.org/doc/contribute.html

[mikioh]

See #24942.