wgengine/magicsock: factor out more separable parts

Updates #8720

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/blockforever_conn.go

@@ -0,0 +1,53 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package magicsock
+
+import (
+	"errors"
+	"net"
+	"net/netip"
+	"sync"
+	"time"
+)
+
+// blockForeverConn is a net.PacketConn whose reads block until it is closed.
+type blockForeverConn struct {
+	mu     sync.Mutex
+	cond   *sync.Cond
+	closed bool
+}
+
+func (c *blockForeverConn) ReadFromUDPAddrPort(p []byte) (n int, addr netip.AddrPort, err error) {
+	c.mu.Lock()
+	for !c.closed {
+		c.cond.Wait()
+	}
+	c.mu.Unlock()
+	return 0, netip.AddrPort{}, net.ErrClosed
+}
+
+func (c *blockForeverConn) WriteToUDPAddrPort(p []byte, addr netip.AddrPort) (int, error) {
+	// Silently drop writes.
+	return len(p), nil
+}
+
+func (c *blockForeverConn) LocalAddr() net.Addr {
+	// Return a *net.UDPAddr because lots of code assumes that it will.
+	return new(net.UDPAddr)
+}
+
+func (c *blockForeverConn) Close() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.closed {
+		return net.ErrClosed
+	}
+	c.closed = true
+	c.cond.Broadcast()
+	return nil
+}
+
+func (c *blockForeverConn) SetDeadline(t time.Time) error      { return errors.New("unimplemented") }
+func (c *blockForeverConn) SetReadDeadline(t time.Time) error  { return errors.New("unimplemented") }
+func (c *blockForeverConn) SetWriteDeadline(t time.Time) error { return errors.New("unimplemented") }