bindnode: support parsing stringprefix unions that have no explicit d…

…elimiter.

In this case, the parsing checks each complete prefix.

Errors are also returned when data is nonmatching,
rather than panicking.

node/bindnode/repr.go

@@ -578,15 +578,29 @@ func (w *_assemblerRepr) AssignString(s string) error {
 		}
 		panic("TODO: GetMember result is missing?")
 	case schema.UnionRepresentation_Stringprefix:
-		parts := strings.SplitN(s, stg.GetDelim(), 2)
-		if len(parts) != 2 {
-			panic("TODO: bad format")
+		hasDelim := stg.GetDelim() != ""
+
+		var prefix, remainder string
+		if hasDelim {
+			parts := strings.SplitN(s, stg.GetDelim(), 2)
+			if len(parts) != 2 {
+				return fmt.Errorf("data doesn't match union type: type %s expects delimiter %q and it was not found in the data", w.schemaType.Name(), stg.GetDelim())
+			}
+			prefix, remainder = parts[0], parts[1]
 		}
-		name, value := parts[0], parts[1]
+
 		members := w.schemaType.(*schema.TypeUnion).Members()
 		for idx, member := range members {
-			if stg.GetDiscriminant(member) != name {
-				continue
+			descrm := stg.GetDiscriminant(member)
+			if hasDelim {
+				if stg.GetDiscriminant(member) != prefix {
+					continue
+				}
+			} else {
+				if !strings.HasPrefix(s, descrm) {
+					continue
+				}
+				remainder = s[len(descrm):]
 			}
 
 			w2 := *w
@@ -603,10 +617,9 @@ func (w *_assemblerRepr) AssignString(s string) error {
 				unionSetMember(w.val, idx, valPtr)
 				return nil
 			}
-
-			return w2.AssignString(value)
+			return w2.AssignString(remainder)
 		}
-		panic("TODO: GetMember result is missing?")
+		return fmt.Errorf("data doesn't match union type: type %s couldn't match any of its prefixes to the data %q", w.schemaType.Name(), s)
 	case nil:
 		return (*_assembler)(w).AssignString(s)
 	default: