util/deephash: implement SelfHasher to allow types to hash themselves

Updates: corp#16409
Signed-off-by: Tom DNetto <tom@tailscale.com>

util/deephash/types.go

@@ -7,11 +7,25 @@ import (
 	"net/netip"
 	"reflect"
 	"time"
+
+	"tailscale.com/util/hashx"
 )
 
+// SelfHasher is the interface implemented by types that can compute their own hash
+// by writing values through the given parameter.
+//
+// Implementations of Hash MUST NOT call `Reset` or `Sum` on the provided argument.
+//
+// This interface should not be considered stable and is likely to change in the
+// future.
+type SelfHasher interface {
+	Hash(*hashx.Block512)
+}
+
 var (
-	timeTimeType  = reflect.TypeOf((*time.Time)(nil)).Elem()
-	netipAddrType = reflect.TypeOf((*netip.Addr)(nil)).Elem()
+	timeTimeType   = reflect.TypeOf((*time.Time)(nil)).Elem()
+	netipAddrType  = reflect.TypeOf((*netip.Addr)(nil)).Elem()
+	selfHasherType = reflect.TypeOf((*SelfHasher)(nil)).Elem()
 )
 
 // typeIsSpecialized reports whether this type has specialized hashing.
@@ -21,6 +35,11 @@ func typeIsSpecialized(t reflect.Type) bool {
 	case timeTimeType, netipAddrType:
 		return true
 	default:
+		if t.Kind() != reflect.Pointer && t.Kind() != reflect.Interface {
+			if t.Implements(selfHasherType) || reflect.PointerTo(t).Implements(selfHasherType) {
+				return true
+			}
+		}
 		return false
 	}
 }