frontend/qt: use const char* over char* in the exported Go funcs

`cchar_t` is a typedef for `const char`, which allows us to use `const
char*` instead of `char*` for the string input params in the Go
functions. Then we can can remove the ugly const casts in C++.

`toLocal8bit().constData()` is the documented way to get `const char*`
from q `QString`, see:

https://doc.qt.io/qt-6/qstring.html#qPrintable

frontends/qt/libserver.h

@@ -39,9 +39,9 @@ extern "C" {
 #endif
 
 
-extern void backendCall(int p0, char* p1);
+extern void backendCall(int queryID, cchar_t* s);
 
-extern void handleURI(char* p0);
+extern void handleURI(cchar_t* uri);
 
 extern void serve(
     cppHeapFree cppHeapFreeFn,
@@ -52,7 +52,7 @@ extern void serve(
     getSaveFilenameCallback getSaveFilenameFn
 );
 
-extern void systemOpen(char* p0);
+extern void systemOpen(cchar_t* url);
 
 extern void goLog(cchar_t* msg);
 

frontends/qt/main.cpp

@@ -82,7 +82,7 @@ class BitBoxApp : public SingleApplication
                 // if the BitBoxApp is launched and also when it is already running, in which case
                 // it is brought to the foreground automatically.
 
-                handleURI(const_cast<char*>(openEvent->url().toString().toStdString().c_str()));
+                handleURI(openEvent->url().toString().toLocal8Bit().constData());
             }
         }
 
@@ -124,7 +124,7 @@ class RequestInterceptor : public QWebEngineUrlRequestInterceptor {
         if (onBuyPage) {
             if (info.firstPartyUrl().toString() == info.requestUrl().toString()) {
                 // A link with target=_blank was clicked.
-                systemOpen(const_cast<char*>(info.requestUrl().toString().toStdString().c_str()));
+                systemOpen(info.requestUrl().toString().toLocal8Bit().constData());
                 // No need to also load it in our page.
                 info.block(true);
             }
@@ -387,11 +387,11 @@ int main(int argc, char *argv[])
         [&](int instanceId, QByteArray message) {
             QString arg = QString::fromUtf8(message);
             qDebug() << "Received arg from secondary instance:" << arg;
-            handleURI(const_cast<char*>(arg.toStdString().c_str()));
+            handleURI(arg.toLocal8Bit().constData());
         });
     // Handle URI which the app was launched with in the primary instance.
     if (a.arguments().size() == 2) {
-        handleURI(const_cast<char*>(a.arguments()[1].toStdString().c_str()));
+        handleURI(a.arguments()[1].toLocal8Bit().constData());
     }
 
     return a.exec();

frontends/qt/server/server.go

@@ -90,7 +90,7 @@ func backendCall(queryID C.int, s *C.char) {
 }
 
 //export handleURI
-func handleURI(uri *C.char) {
+func handleURI(uri *C.cchar_t) {
 	bridgecommon.HandleURI(C.GoString(uri))
 }
 
@@ -182,7 +182,7 @@ func serve(
 }
 
 //export systemOpen
-func systemOpen(url *C.char) {
+func systemOpen(url *C.cchar_t) {
 	goURL := C.GoString(url)
 	if err := system.Open(goURL); err != nil {
 		logging.Get().WithGroup("server").WithError(err).Errorf("systemOpen: error opening %v", goURL)

frontends/qt/webclass.h

@@ -21,7 +21,7 @@ class WebClass : public QObject
     Q_OBJECT
 public slots:
     void call(int queryID, const QString& query) {
-        backendCall(queryID, const_cast<char*>(query.toStdString().c_str()));
+        backendCall(queryID, query.toLocal8Bit().constData());
     }
 signals:
     void gotResponse(int queryID, QString response);