quic: add the ability to create an endpoint with a fake network

Add a NewEndpoint function to create an *Endpoint using a net.PacketConn.
We rely on a number of features of *net.UDPConn which aren't provided
by PacketConn, so an Endpoint using anything other than a UDPConn will
be limited. The main use case for providing a non-UDPConn connection
is testing, in particular tests which use testing/synctest and cannot
use a concrete network connection.

Change-Id: I9e62cb8d7d545f64d99103beb9a32f149d4119bf
Reviewed-on: https://go-review.googlesource.com/c/net/+/641498
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Damien Neil <dneil@google.com>

quic/endpoint.go

@@ -73,6 +73,25 @@ func Listen(network, address string, listenConfig *Config) (*Endpoint, error) {
 	return newEndpoint(pc, listenConfig, nil)
 }
 
+// NewEndpoint creates an endpoint using a net.PacketConn as the underlying transport.
+//
+// If the PacketConn is not a *net.UDPConn, the endpoint may be slower and lack
+// access to some features of the network.
+func NewEndpoint(conn net.PacketConn, config *Config) (*Endpoint, error) {
+	var pc packetConn
+	var err error
+	switch conn := conn.(type) {
+	case *net.UDPConn:
+		pc, err = newNetUDPConn(conn)
+	default:
+		pc, err = newNetPacketConn(conn)
+	}
+	if err != nil {
+		return nil, err
+	}
+	return newEndpoint(pc, config, nil)
+}
+
 func newEndpoint(pc packetConn, config *Config, hooks endpointTestHooks) (*Endpoint, error) {
 	e := &Endpoint{
 		listenConfig: config,

quic/udp_packetconn.go

@@ -0,0 +1,69 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.21
+
+package quic
+
+import (
+	"net"
+	"net/netip"
+)
+
+// netPacketConn is a packetConn implementation wrapping a net.PacketConn.
+//
+// This is mostly useful for tests, since PacketConn doesn't provide access to
+// important features such as identifying the local address packets were received on.
+type netPacketConn struct {
+	c         net.PacketConn
+	localAddr netip.AddrPort
+}
+
+func newNetPacketConn(pc net.PacketConn) (*netPacketConn, error) {
+	addr, err := addrPortFromAddr(pc.LocalAddr())
+	if err != nil {
+		return nil, err
+	}
+	return &netPacketConn{
+		c:         pc,
+		localAddr: addr,
+	}, nil
+}
+
+func (c *netPacketConn) Close() error {
+	return c.c.Close()
+}
+
+func (c *netPacketConn) LocalAddr() netip.AddrPort {
+	return c.localAddr
+}
+
+func (c *netPacketConn) Read(f func(*datagram)) {
+	for {
+		dgram := newDatagram()
+		n, peerAddr, err := c.c.ReadFrom(dgram.b)
+		if err != nil {
+			return
+		}
+		dgram.peerAddr, err = addrPortFromAddr(peerAddr)
+		if err != nil {
+			continue
+		}
+		dgram.b = dgram.b[:n]
+		f(dgram)
+	}
+}
+
+func (c *netPacketConn) Write(dgram datagram) error {
+	_, err := c.c.WriteTo(dgram.b, net.UDPAddrFromAddrPort(dgram.peerAddr))
+	return err
+}
+
+func addrPortFromAddr(addr net.Addr) (netip.AddrPort, error) {
+	switch a := addr.(type) {
+	case *net.UDPAddr:
+		return a.AddrPort(), nil
+	}
+	return netip.ParseAddrPort(addr.String())
+}