net: calling File disables the SetDeadline methods

This essentially applies https://golang.org/cl/81636 to the net package.

The full truth seems too complicated to write in this method's doc, so
I'm going with a simple half truth.

The full truth is that File returns the descriptor in blocking mode,
because that is historically how it worked, and existing programs
would be surprised if the descriptor is suddenly non-blocking. On Unix
systems whether a socket is non-blocking or not is a property of the
underlying file description, not of a particular file descriptor, so
changing the returned descriptor to blocking mode also changes the
existing socket to blocking mode. Blocking mode works fine, althoug I/O
operations now take up a thread. SetDeadline and friends rely on the
runtime poller, and the runtime poller only works if the descriptor is
non-blocking. So it's correct that calling File disables SetDeadline.
The other half of the truth is that if the program is willing to work
with a non-blocking descriptor, it could call
syscall.SetNonblock(f.Fd(), true) to change the descriptor, and
the original socket, to non-blocking mode. At that point SetDeadline
would start working again. I tried to write that in a way that is
short and comprehensible but failed. Since we now have the RawConn
approach to frobbing the descriptor, and hopefully most people can use
that rather than calling File, I decided to punt.

Updates #22934
Fixes #21862

Change-Id: If269da762f6f5a88c334e7b6d6f3998f7e10b11e
Reviewed-on: https://go-review.googlesource.com/82915Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/net.go

@@ -288,6 +288,8 @@ func (c *conn) SetWriteBuffer(bytes int) error {
 // The returned os.File's file descriptor is different from the connection's.
 // Attempting to change properties of the original using this duplicate
 // may or may not have the desired effect.
+//
+// On Unix systems this will cause the SetDeadline methods to stop working.
 func (c *conn) File() (f *os.File, err error) {
 	f, err = c.fd.dup()
 	if err != nil {