Fix access to destroyed QtJniTypes::String object

Doing

  const char *value = QtJniTypes::className<Foo>();

is the same as doing

  const char *value = qstring.toUtf8().constData();

The object owning the data is temporary, and destroyed at the end of the
statement. 'value' becomes a dangling pointer.

With QtJniTypes::String we have the extra problem that strings of
different lengths are different types, so we can't return different ones
from non-template functions. Instead, we have to make a deep copy of the
data into a QByteArray, and then operate on that.

This defeats the purpose of having compile-time string generation in the
first place, but fixes the immediate issue.

A follow-up commit can turn the valueForStaticField helper into a
template function that makes full use of compile-time generated
strings. However, this requires changes in the calling code as well.

Amends 70afc6721d4331fec0e9ccbb580d7c8b3ce5ec04.

Change-Id: I206d58f3a34700042782b77f9a28aa3a61520439
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>
(cherry picked from commit 3d69e7ee4fa5f9b486a072fdc1ba6f697144351b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 23 insertions, 18 deletions

diff --git a/src/bluetooth/android/jni_android.cpp b/src/bluetooth/android/jni_android.cpp
index 6392c07e..b01bbff6 100644
--- a/src/bluetooth/android/jni_android.cpp
+++ b/src/bluetooth/android/jni_android.cpp
@@ -38,25 +38,29 @@ static const char * const javaExtraUuid = "EXTRA_UUID";
  * This function operates on the assumption that each
  * field is of type java/lang/String.
  */
+inline QByteArray classNameForStaticField(JavaNames javaName)
+{
+    switch (javaName) {
+    case JavaNames::BluetoothAdapter: {
+        constexpr auto cn = QtJniTypes::className<QtJniTypes::BluetoothAdapter>();
+        return QByteArray(cn.data(), cn.size());
+    }
+    case JavaNames::BluetoothDevice: {
+        constexpr auto cn = QtJniTypes::className<QtJniTypes::BluetoothDevice>();
+        return QByteArray(cn.data(), cn.size());
+    }
+    default:
+        break;
+    }
+    return {};
+}
+
 QJniObject valueForStaticField(JavaNames javaName, JavaNames javaFieldName)
 {
     //construct key
     //the switch statements are used to reduce the number of duplicated strings
     //in the library
 
-    const char* className;
-    switch (javaName) {
-    case JavaNames::BluetoothAdapter:
-        className = QtJniTypes::className<QtJniTypes::BluetoothAdapter>();
-        break;
-    case JavaNames::BluetoothDevice:
-        className = QtJniTypes::className<QtJniTypes::BluetoothDevice>();
-        break;
-    default:
-        qCWarning(QT_BT_ANDROID) << "Unknown java class name passed to valueForStaticField():" << javaName;
-        return QJniObject();
-    }
-
     const char *fieldName;
     switch (javaFieldName) {
     case JavaNames::ActionAclConnected:
@@ -94,12 +98,13 @@ QJniObject valueForStaticField(JavaNames javaName, JavaNames javaFieldName)
         return QJniObject();
     }
 
-    const size_t offset_class = qstrlen(className);
-    const size_t offset_field = qstrlen(fieldName);
-    QByteArray key(qsizetype(offset_class + offset_field), Qt::Uninitialized);
-    memcpy(key.data(), className, offset_class);
-    memcpy(key.data()+offset_class, fieldName, offset_field);
+    const QByteArray className = classNameForStaticField(javaName);
+    if (className.isEmpty()) {
+        qCWarning(QT_BT_ANDROID) << "Unknown java class name passed to valueForStaticField():" << javaName;
+        return QJniObject();
+    }
 
+    const QByteArray key = className + fieldName;
     JCachedStringFields::iterator it = cachedStringFields()->find(key);
     if (it == cachedStringFields()->end()) {
         QJniEnvironment env;