adding convenience function to check (in a very basic manner) if an API spec is legacy.

funcs.go

@@ -153,3 +153,56 @@ func NameFromPath(path dbus.ObjectPath) (name string, err error) {
 
 	return
 }
+
+/*
+	CheckErrIsFromLegacy takes an error.Error from e.g.:
+
+			Service.SearchItems
+			Collection.CreateItem
+			NewItem
+			Item.ChangeItemType
+			Item.Type
+
+	and (in order) attempt to typeswitch to a *multierr.MultiError, then iterate through
+	the *multierr.MultiError.Errors, attempt to typeswitch each of them to a Dbus.Error, and then finally
+	check if it is regarding a missing Type property.
+
+	This is *very explicitly* only useful for the above functions/methods. If used anywhere else,
+	it's liable to return an incorrect isLegacy even if parsed == true.
+
+	It is admittedly convoluted and obtuse, but this saves a lot of boilerplate for users.
+	It wouldn't be necessary if projects didn't insist on using the legacy draft SecretService specification.
+	But here we are.
+
+	isLegacy is true if this Service's API destination is legacy spec. Note that this is checking for
+	very explicit conditions; isLegacy may return false but it is in fact running on a legacy API.
+	Don't rely on this too much.
+
+	parsed is true if we found an error type we were able to perform logic of determination on.
+*/
+func CheckErrIsFromLegacy(err error) (isLegacy, parsed bool) {
+
+	switch e := err.(type) {
+	case *multierr.MultiError:
+		parsed = true
+		for _, i := range e.Errors {
+			switch e2 := i.(type) {
+			case dbus.Error:
+				if e2.Name == "org.freedesktop.DBus.Error.UnknownProperty" {
+					isLegacy = true
+					return
+				}
+			default:
+				continue
+			}
+		}
+	case dbus.Error:
+		parsed = true
+		if e.Name == "org.freedesktop.DBus.Error.UnknownProperty" {
+			isLegacy = true
+			return
+		}
+	}
+
+	return
+}

item_funcs.go

@@ -85,6 +85,11 @@ func (i *Item) ChangeItemType(newItemType string) (err error) {
 
 	var variant dbus.Variant
 
+	// Legacy spec.
+	if i.collection.service.Legacy {
+		return
+	}
+
 	if strings.TrimSpace(newItemType) == "" {
 		newItemType = DbusDefaultItemType
 	}

service_funcs.go

@@ -257,6 +257,27 @@ func (s *Service) GetSession() (ssn *Session, err error) {
 	return
 }
 
+// Scrapping this idea for now; it would require introspection on a known Item path.
+/*
+	IsLegacy indicates with a decent likelihood of accuracy if this Service is
+	connected to a legacy spec Secret Service (true) or if the spec is current (false).
+
+	It also returns a confidence indicator as a float, which indicates how accurate
+	the guess (because it is a guess) may/is likely to be (as a percentage). For example,
+	if confidence is expressed as 0.25, the result of legacyAPI has a 25% of being accurate.
+*/
+/*
+func (s *Service) IsLegacy() (legacyAPI bool, confidence int) {
+
+	var maxCon int
+
+	// Test 1, property introspection on Item. We're looking for a Type property.
+	DbusInterfaceItem
+
+	return
+}
+*/
+
 // Lock locks an Unlocked Collection or Item (LockableObject).
 func (s *Service) Lock(objects ...LockableObject) (err error) {
 

types.go

@@ -74,7 +74,19 @@ type Service struct {
 		when performing a Service.SearchItems, you probably need to enable this field on the
 		Service returned by NewService. The coverage of this field may expand in the future, but
 		currently it only prevents/suppresses the (non-existent, in legacy spec) Type property
-		from being read or written on Items during NewItem and Collection.CreateItem respectively.
+		from being read or written on Items during e.g.:
+
+			Service.SearchItems
+			Collection.CreateItem
+			NewItem
+			Item.ChangeItemType
+			Item.Type
+
+		It will perform a no-op if enabled in the above contexts to maintain cross-compatability
+		in codebase between legacy and proper current spec systems, avoiding an error return.
+
+		You can use CheckErrIsFromLegacy if Service.Legacy is false and Service.SearchItems returns
+		a non-nil err to determine if this Service is (probably) interfacing with a legacy spec API.
 	*/
 	Legacy bool `json:"is_legacy"`
 }