QDBusSignature: accept empty strings as validHEADdev

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>

4 files changed, 11 insertions, 6 deletions

diff --git a/src/dbus/qdbusextratypes.cpp b/src/dbus/qdbusextratypes.cpp
index 61f2075443..3354e76577 100644
--- a/src/dbus/qdbusextratypes.cpp
+++ b/src/dbus/qdbusextratypes.cpp
@@ -34,6 +34,8 @@ void QDBusSignature::doCheck()
     if (!QDBusUtil::isValidSignature(m_signature)) {
         qWarning("QDBusSignature: invalid signature \"%s\"", qPrintable(m_signature));
         m_signature.clear();
+    } else if (m_signature.isEmpty()) {
+        m_signature.detach();       // we need it to not be null
     }
 }
 
diff --git a/src/dbus/qdbusextratypes.h b/src/dbus/qdbusextratypes.h
index 1bc0f3086d..1c0826b992 100644
--- a/src/dbus/qdbusextratypes.h
+++ b/src/dbus/qdbusextratypes.h
@@ -77,7 +77,10 @@ class Q_DBUS_EXPORT QDBusSignature
 {
     QString m_signature;
 public:
-    QDBusSignature() noexcept : m_signature() {}
+    QDBusSignature() noexcept
+    {
+        m_signature.detach(); // mark non-null (empty signatures are valid)
+    }
     // compiler-generated copy/move constructor/assignment operators are ok!
     // compiler-generated destructor is ok!
 
diff --git a/src/dbus/qdbusmarshaller.cpp b/src/dbus/qdbusmarshaller.cpp
index b2ed2586fb..d1f1a41ab0 100644
--- a/src/dbus/qdbusmarshaller.cpp
+++ b/src/dbus/qdbusmarshaller.cpp
@@ -120,7 +120,7 @@ inline void QDBusMarshaller::append(const QDBusObjectPath &arg)
 inline void QDBusMarshaller::append(const QDBusSignature &arg)
 {
     QByteArray data = arg.signature().toUtf8();
-    if (!ba && data.isEmpty()) {
+    if (!ba && data.isNull()) {
         error("Invalid signature passed in arguments"_L1);
     } else {
         const char *cdata = data.constData();
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;
     }
 
     /*!