lib/protocol: Use DeviceID in protocol messages, with custom marshalling

This makes the device ID a real type that can be used in the protobuf
schema. That avoids the juggling back and forth from []byte in a bunch
of places and simplifies the code.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3695

lib/protocol/deviceid_test.go

@@ -1,5 +1,8 @@
 // Copyright (C) 2014 The Protocol Authors.
 
+//go:generate go run ../../script/protofmt.go deviceid_test.proto
+//go:generate protoc --proto_path=../../../../../:../../../../gogo/protobuf/protobuf:. --gogofast_out=. deviceid_test.proto
+
 package protocol
 
 import "testing"
@@ -96,3 +99,53 @@ func TestDeviceIDFromBytes(t *testing.T) {
 		t.Errorf("Wrong device ID, got %q, want %q", id1, formatted)
 	}
 }
+
+func TestNewDeviceIDMarshalling(t *testing.T) {
+	// The new DeviceID.Unmarshal / DeviceID.MarshalTo serialization should
+	// be message compatible with how we used to serialize DeviceIDs.
+
+	// Create a message with a device ID in old style bytes format
+
+	id0, _ := DeviceIDFromString(formatted)
+	msg0 := TestOldDeviceID{id0[:]}
+
+	//  Marshal it
+
+	bs, err := msg0.Marshal()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Unmarshal using the new DeviceID.Unmarshal
+
+	var msg1 TestNewDeviceID
+	if err := msg1.Unmarshal(bs); err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify it's the same
+
+	if msg1.Test != id0 {
+		t.Error("Mismatch in old -> new direction")
+	}
+
+	// Marshal using the new DeviceID.MarshalTo
+
+	bs, err = msg1.Marshal()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create an old style message and and attempt unmarshal
+
+	var msg2 TestOldDeviceID
+	if err := msg2.Unmarshal(bs); err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify it's the same
+
+	if DeviceIDFromBytes(msg2.Test) != id0 {
+		t.Error("Mismatch in old -> new direction")
+	}
+}