all: make conn.Bind.Open return a slice of receive functions

Instead of hard-coding exactly two sources from which
to receive packets (an IPv4 source and an IPv6 source),
allow the conn.Bind to specify a set of sources.

Beneficial consequences:

* If there's no IPv6 support on a system,
  conn.Bind.Open can choose not to return a receive function for it,
  which is simpler than tracking that state in the bind.
  This simplification removes existing data races from both
  conn.StdNetBind and bindtest.ChannelBind.
* If there are more than two sources on a system,
  the conn.Bind no longer needs to add a separate muxing layer.

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

conn/conn.go

@@ -12,6 +12,11 @@ import (
 	"strings"
 )
 
+// A ReceiveFunc receives a single inbound packet from the network.
+// It writes the data into b. n is the length of the packet.
+// ep is the remote endpoint.
+type ReceiveFunc func(b []byte) (n int, ep Endpoint, err error)
+
 // A Bind listens on a port for both IPv6 and IPv4 UDP traffic.
 //
 // A Bind interface may also be a PeekLookAtSocketFd or BindSocketToInterface,
@@ -19,23 +24,17 @@ import (
 type Bind interface {
 	// Open puts the Bind into a listening state on a given port and reports the actual
 	// port that it bound to. Passing zero results in a random selection.
-	Open(port uint16) (actualPort uint16, err error)
+	// fns is the set of functions that will be called to receive packets.
+	Open(port uint16) (fns []ReceiveFunc, actualPort uint16, err error)
 
 	// Close closes the Bind listener.
+	// All fns returned by Open must return net.ErrClosed after a call to Close.
 	Close() error
 
 	// SetMark sets the mark for each packet sent through this Bind.
 	// This mark is passed to the kernel as the socket option SO_MARK.
 	SetMark(mark uint32) error
 
-	// ReceiveIPv6 reads an IPv6 UDP packet into b.  It reports the number of bytes read,
-	// n, the packet source address ep, and any error.
-	ReceiveIPv6(b []byte) (n int, ep Endpoint, err error)
-
-	// ReceiveIPv4 reads an IPv4 UDP packet into b. It reports the number of bytes read,
-	// n, the packet source address ep, and any error.
-	ReceiveIPv4(b []byte) (n int, ep Endpoint, err error)
-
 	// Send writes a packet b to address ep.
 	Send(b []byte, ep Endpoint) error