types/key,wgengine/magicsock,control/controlclient,ipn: add debug disco key rotation

Adds the ability to rotate discovery keys on running clients, needed for
testing upcoming disco key distribution changes.

Introduces key.DiscoKey, an atomic container for a disco private key,
public key, and the public key's ShortString, replacing the prior
separate atomic fields.

magicsock.Conn has a new RotateDiscoKey method, and access to this is
provided via localapi and a CLI debug command.

Note that this implementation is primarily for testing as it stands, and
regular use should likely introduce an additional mechanism that allows
the old key to be used for some time, to provide a seamless key rotation
rather than one that invalidates all sessions.

Updates tailscale/corp#34037

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/disco_atomic.go

@@ -0,0 +1,58 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package magicsock
+
+import (
+	"sync/atomic"
+
+	"tailscale.com/types/key"
+)
+
+type discoKeyPair struct {
+	private key.DiscoPrivate
+	public  key.DiscoPublic
+	short   string // public.ShortString()
+}
+
+// discoAtomic is an atomic container for a disco private key, public key, and
+// the public key's ShortString. The private and public keys are always kept
+// synchronized.
+//
+// The zero value is not ready for use. Use [Set] to provide a usable value.
+type discoAtomic struct {
+	pair atomic.Pointer[discoKeyPair]
+}
+
+// Pair returns the private and public keys together atomically.
+// Code that needs both the private and public keys synchronized should
+// use Pair instead of calling Private and Public separately.
+func (dk *discoAtomic) Pair() (key.DiscoPrivate, key.DiscoPublic) {
+	p := dk.pair.Load()
+	return p.private, p.public
+}
+
+// Private returns the private key.
+func (dk *discoAtomic) Private() key.DiscoPrivate {
+	return dk.pair.Load().private
+}
+
+// Public returns the public key.
+func (dk *discoAtomic) Public() key.DiscoPublic {
+	return dk.pair.Load().public
+}
+
+// Short returns the short string of the public key (see [DiscoPublic.ShortString]).
+func (dk *discoAtomic) Short() string {
+	return dk.pair.Load().short
+}
+
+// Set updates the private key (and the cached public key and short string).
+func (dk *discoAtomic) Set(private key.DiscoPrivate) {
+	public := private.Public()
+	dk.pair.Store(&discoKeyPair{
+		private: private,
+		public:  public,
+		short:   public.ShortString(),
+	})
+}