ipn/ipn{auth,server}: update ipnauth.Actor to carry a context

The context carries additional information about the actor, such as the
request reason, and is canceled when the actor is done.

Additionally, we implement three new ipn.Actor types that wrap other actors
to modify their behavior:
 - WithRequestReason, which adds a request reason to the actor;
 - WithoutClose, which narrows the actor's interface to prevent it from being
   closed;
 - WithPolicyChecks, which adds policy checks to the actor's CheckProfileAccess
   method.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnauth/actor.go

@@ -4,9 +4,11 @@
 package ipnauth
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 
+	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/ipn"
 )
 
@@ -32,6 +34,11 @@ type Actor interface {
 	// a connected LocalAPI client. Otherwise, it returns a zero value and false.
 	ClientID() (_ ClientID, ok bool)
 
+	// Context returns the context associated with the actor.
+	// It carries additional information about the actor
+	// and is canceled when the actor is done.
+	Context() context.Context
+
 	// CheckProfileAccess checks whether the actor has the necessary access rights
 	// to perform a given action on the specified Tailscale profile.
 	// It returns an error if access is denied.
@@ -102,3 +109,27 @@ func (id ClientID) MarshalJSON() ([]byte, error) {
 func (id *ClientID) UnmarshalJSON(b []byte) error {
 	return json.Unmarshal(b, &id.v)
 }
+
+type actorWithRequestReason struct {
+	Actor
+	ctx context.Context
+}
+
+// WithRequestReason returns an [Actor] that wraps the given actor and
+// carries the specified request reason in its context.
+func WithRequestReason(actor Actor, requestReason string) Actor {
+	ctx := apitype.RequestReasonKey.WithValue(actor.Context(), requestReason)
+	return &actorWithRequestReason{Actor: actor, ctx: ctx}
+}
+
+// Context implements [Actor].
+func (a *actorWithRequestReason) Context() context.Context { return a.ctx }
+
+type withoutCloseActor struct{ Actor }
+
+// WithoutClose returns an [Actor] that does not expose the [ActorCloser] interface.
+// In other words, _, ok := WithoutClose(actor).(ActorCloser) will always be false,
+// even if the original actor implements [ActorCloser].
+func WithoutClose(actor Actor) Actor {
+	return withoutCloseActor{actor}
+}