refactor: Make ThreadSafeMessageBox signal void

The message will always return false (a constant) and the return value
is never used.

Also, annotate ThreadSafeMessageBox in the GUI code as [[nodiscard]],
because it may actually return a value, which is handled for questions
(but not for messages).

src/interfaces/node.h

@@ -219,8 +219,7 @@ public:
     virtual std::unique_ptr<Handler> handleInitMessage(InitMessageFn fn) = 0;
 
     //! Register handler for message box messages.
-    using MessageBoxFn =
-        std::function<bool(const bilingual_str& message, unsigned int style)>;
+    using MessageBoxFn = std::function<void(const bilingual_str& message, unsigned int style)>;
     virtual std::unique_ptr<Handler> handleMessageBox(MessageBoxFn fn) = 0;
 
     //! Register handler for question messages.

src/node/interface_ui.cpp

@@ -13,7 +13,7 @@ using util::MakeUnorderedList;
 CClientUIInterface uiInterface;
 
 struct UISignals {
-    btcsignals::signal<CClientUIInterface::ThreadSafeMessageBoxSig, btcsignals::optional_last_value<bool>> ThreadSafeMessageBox;
+    btcsignals::signal<CClientUIInterface::ThreadSafeMessageBoxSig> ThreadSafeMessageBox;
     btcsignals::signal<CClientUIInterface::ThreadSafeQuestionSig, btcsignals::optional_last_value<bool>> ThreadSafeQuestion;
     btcsignals::signal<CClientUIInterface::InitMessageSig> InitMessage;
     btcsignals::signal<CClientUIInterface::InitWalletSig> InitWallet;
@@ -45,7 +45,7 @@ ADD_SIGNALS_IMPL_WRAPPER(NotifyBlockTip);
 ADD_SIGNALS_IMPL_WRAPPER(NotifyHeaderTip);
 ADD_SIGNALS_IMPL_WRAPPER(BannedListChanged);
 
-bool CClientUIInterface::ThreadSafeMessageBox(const bilingual_str& message, unsigned int style) { return g_ui_signals.ThreadSafeMessageBox(message, style).value_or(false);}
+void CClientUIInterface::ThreadSafeMessageBox(const bilingual_str& message, unsigned int style) { return g_ui_signals.ThreadSafeMessageBox(message, style); }
 bool CClientUIInterface::ThreadSafeQuestion(const bilingual_str& message, const std::string& non_interactive_message, unsigned int style) { return g_ui_signals.ThreadSafeQuestion(message, non_interactive_message, style).value_or(false);}
 void CClientUIInterface::InitMessage(const std::string& message) { return g_ui_signals.InitMessage(message); }
 void CClientUIInterface::InitWallet() { return g_ui_signals.InitWallet(); }

src/node/interface_ui.h

@@ -72,7 +72,7 @@ public:
     btcsignals::connection signal_name##_connect(std::function<signal_name##Sig> fn)
 
     /** Show message box. */
-    ADD_SIGNALS_DECL_WRAPPER(ThreadSafeMessageBox, bool, const bilingual_str& message, unsigned int style);
+    ADD_SIGNALS_DECL_WRAPPER(ThreadSafeMessageBox, void, const bilingual_str& message, unsigned int style);
 
     /** If possible, ask the user a question. If not, falls back to ThreadSafeMessageBox(noninteractive_message, style) and returns false. */
     ADD_SIGNALS_DECL_WRAPPER(ThreadSafeQuestion, bool, const bilingual_str& message, const std::string& noninteractive_message, unsigned int style);

src/noui.cpp

@@ -17,7 +17,7 @@ btcsignals::connection noui_ThreadSafeMessageBoxConn;
 btcsignals::connection noui_ThreadSafeQuestionConn;
 btcsignals::connection noui_InitMessageConn;
 
-bool noui_ThreadSafeMessageBox(const bilingual_str& message, unsigned int style)
+void noui_ThreadSafeMessageBox(const bilingual_str& message, unsigned int style)
 {
     bool fSecure = style & CClientUIInterface::SECURE;
     style &= ~CClientUIInterface::SECURE;
@@ -41,12 +41,12 @@ bool noui_ThreadSafeMessageBox(const bilingual_str& message, unsigned int style)
     }
 
     tfm::format(std::cerr, "%s%s\n", strCaption, message.original);
-    return false;
 }
 
 bool noui_ThreadSafeQuestion(const bilingual_str& /* ignored interactive message */, const std::string& message, unsigned int style)
 {
-    return noui_ThreadSafeMessageBox(Untranslated(message), style);
+    noui_ThreadSafeMessageBox(Untranslated(message), style);
+    return false; // Answer the question with false in the noui context
 }
 
 void noui_InitMessage(const std::string& message)
@@ -61,10 +61,9 @@ void noui_connect()
     noui_InitMessageConn = uiInterface.InitMessage_connect(noui_InitMessage);
 }
 
-bool noui_ThreadSafeMessageBoxRedirect(const bilingual_str& message, unsigned int style)
+void noui_ThreadSafeMessageBoxRedirect(const bilingual_str& message, unsigned int style)
 {
     LogInfo("%s", message.original);
-    return false;
 }
 
 bool noui_ThreadSafeQuestionRedirect(const bilingual_str& /* ignored interactive message */, const std::string& message, unsigned int style)

src/noui.h

@@ -10,7 +10,7 @@
 struct bilingual_str;
 
 /** Non-GUI handler, which logs and prints messages. */
-bool noui_ThreadSafeMessageBox(const bilingual_str& message, unsigned int style);
+void noui_ThreadSafeMessageBox(const bilingual_str& message, unsigned int style);
 /** Non-GUI handler, which logs and prints questions. */
 bool noui_ThreadSafeQuestion(const bilingual_str& /* ignored interactive message */, const std::string& message, unsigned int style);
 /** Non-GUI handler, which only logs a message. */

src/qt/bitcoingui.cpp

@@ -1589,7 +1589,7 @@ void BitcoinGUI::showModalOverlay()
         modalOverlay->toggleVisibility();
 }
 
-static bool ThreadSafeMessageBox(BitcoinGUI* gui, const bilingual_str& message, unsigned int style)
+[[nodiscard]] static bool ThreadSafeMessageBox(BitcoinGUI* gui, const bilingual_str& message, unsigned int style)
 {
     bool modal = (style & CClientUIInterface::MODAL);
     // The SECURE flag has no effect in the Qt GUI.
@@ -1621,7 +1621,7 @@ void BitcoinGUI::subscribeToCoreSignals()
 {
     // Connect signals to client
     m_handler_message_box = m_node.handleMessageBox([this](const bilingual_str& message, unsigned int style) {
-        return ThreadSafeMessageBox(this, message, style);
+        (void)ThreadSafeMessageBox(this, message, style);
     });
     m_handler_question = m_node.handleQuestion([this](const bilingual_str& message, const std::string& /*non_interactive_message*/, unsigned int style) {
         return ThreadSafeMessageBox(this, message, style);