QDBusSignature: accept empty strings as valid

QDBusSignature holds a D-Bus value of type SIGNATURE, which is zero or
more signatures, not one or more. This changes the default constructor
to create a valid signature, which we denote by not being a null
QString. That means we need to use something other than the default
constructor in our tests for attempting to pass invalid signatures.

[ChangeLog][QtDBus][QDBusSignature] Fixed a bug that caused the class
not to accept an empty string as a valid D-Bus SIGNATURE value.

Fixes: QTBUG-124919
Pick-to: 6.5 6.7
Change-Id: I262c3499666e4f4fbcfbfffd17cb3793dcf2eae3
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/dbus/qdbusutil.cpp b/src/dbus/qdbusutil.cpp
index 827418c487..78338aa054 100644
--- a/src/dbus/qdbusutil.cpp
+++ b/src/dbus/qdbusutil.cpp
@@ -512,14 +512,14 @@ namespace QDBusUtil
     bool isValidSignature(const QString &signature)
     {
         QByteArray ba = signature.toLatin1();
-        const char *data = ba.constData();
-        while (true) {
+        const char *data = ba.constBegin();
+        const char *end = ba.constEnd();
+        while (data != end) {
             data = validateSingleType(data);
             if (!data)
                 return false;
-            if (*data == '\0')
-                return true;
         }
+        return true;
     }
 
     /*!