Merge #19256: gui: change combiner for signals to optional_last_value

f1a0314 gui: change combiner for signals to optional_last_value (Cory Fields)

Pull request description:

  [`optional_last_value`](https://www.boost.org/doc/libs/1_73_0/doc/html/boost/signals2/optional_last_value.html), which does not throw, has replaced `last_value` as
  Boosts default combiner. Besides being better supported, it also doesn't
  trigger gcc's `-Wmaybe-unitialized` warning, presumably because exceptions no
  longer bubble-up out of signals:

  ```bash
  In file included from ui_interface.cpp:9:
  /bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp: In member function 'boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::result_type boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::operator()(Args ...) [with Combiner = boost::signals2::last_value<bool>; Group = int; GroupCompare = std::less<int>; SlotFunction = boost::function<bool(const bilingual_str&, const std::__cxx11::basic_string<char>&, unsigned int)>; ExtendedSlotFunction = boost::function<bool(const boost::signals2::connection&, const bilingual_str&, const std::__cxx11::basic_string<char>&, unsigned int)>; Mutex = boost::signals2::mutex; R = bool; Args = {const bilingual_str&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned int}]':
  /bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:54:36: warning: '*((void*)& value +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
           if(value) return value.get();
                                      ^
  /bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:43:21: note: '*((void*)& value +1)' was declared here
           optional<T> value;
                       ^~~~~
  /bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp: In member function 'boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::result_type boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::operator()(Args ...) [with Combiner = boost::signals2::last_value<bool>; Group = int; GroupCompare = std::less<int>; SlotFunction = boost::function<bool(const bilingual_str&, const std::__cxx11::basic_string<char>&, const std::__cxx11::basic_string<char>&, unsigned int)>; ExtendedSlotFunction = boost::function<bool(const boost::signals2::connection&, const bilingual_str&, const std::__cxx11::basic_string<char>&, const std::__cxx11::basic_string<char>&, unsigned int)>; Mutex = boost::signals2::mutex; R = bool; Args = {const bilingual_str&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned int}]':
  /bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:54:36: warning: '*((void*)& value +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
           if(value) return value.get();
                                      ^
  /bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:43:21: note: '*((void*)& value +1)' was declared here
           optional<T> value;
                       ^~~~~
  ```

  The change in default happened in [Boost 1.39.0](https://www.boost.org/users/history/version_1_39_0.html) (along with the introduction of the Signals2 library.

  More information is also available here https://www.boost.org/doc/libs/1_73_0/doc/html/signals2/rationale.html#id-1.3.36.9.4:
  > The default combiner for Boost.Signals2 has changed from the last_value combiner used by default in the original Boost.Signals library.
  > This is because last_value requires that at least 1 slot be connected to the signal when it is invoked (except for the last_value<void> specialization).
  >  In a multi-threaded environment where signal invocations and slot connections and disconnections may be happening concurrently, it is difficult to fulfill this requirement. When using optional_last_value, there is no requirement for slots to be connected when a signal is invoked, since in that case the combiner may simply return an empty boost::optional.

ACKs for top commit:
  laanwj:
    ACK f1a0314

Tree-SHA512: 3600f85019a3591b141dc9207f8a7e66d16d9996cf97fdf08f5133a212d55c591955ab835ffbdca20b5d62711578bc305d5525c75546fa957f180192e2a80c1e

Author: laanwj

src/node/ui_interface.cpp

@@ -6,14 +6,14 @@
 
 #include <util/translation.h>
 
-#include <boost/signals2/last_value.hpp>
+#include <boost/signals2/optional_last_value.hpp>
 #include <boost/signals2/signal.hpp>
 
 CClientUIInterface uiInterface;
 
 struct UISignals {
-    boost::signals2::signal<CClientUIInterface::ThreadSafeMessageBoxSig, boost::signals2::last_value<bool>> ThreadSafeMessageBox;
-    boost::signals2::signal<CClientUIInterface::ThreadSafeQuestionSig, boost::signals2::last_value<bool>> ThreadSafeQuestion;
+    boost::signals2::signal<CClientUIInterface::ThreadSafeMessageBoxSig, boost::signals2::optional_last_value<bool>> ThreadSafeMessageBox;
+    boost::signals2::signal<CClientUIInterface::ThreadSafeQuestionSig, boost::signals2::optional_last_value<bool>> ThreadSafeQuestion;
     boost::signals2::signal<CClientUIInterface::InitMessageSig> InitMessage;
     boost::signals2::signal<CClientUIInterface::NotifyNumConnectionsChangedSig> NotifyNumConnectionsChanged;
     boost::signals2::signal<CClientUIInterface::NotifyNetworkActiveChangedSig> NotifyNetworkActiveChanged;
@@ -42,8 +42,8 @@ ADD_SIGNALS_IMPL_WRAPPER(NotifyBlockTip);
 ADD_SIGNALS_IMPL_WRAPPER(NotifyHeaderTip);
 ADD_SIGNALS_IMPL_WRAPPER(BannedListChanged);
 
-bool CClientUIInterface::ThreadSafeMessageBox(const bilingual_str& message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeMessageBox(message, caption, style); }
-bool CClientUIInterface::ThreadSafeQuestion(const bilingual_str& message, const std::string& non_interactive_message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeQuestion(message, non_interactive_message, caption, style); }
+bool CClientUIInterface::ThreadSafeMessageBox(const bilingual_str& message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeMessageBox(message, caption, style).value_or(false);}
+bool CClientUIInterface::ThreadSafeQuestion(const bilingual_str& message, const std::string& non_interactive_message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeQuestion(message, non_interactive_message, caption, style).value_or(false);}
 void CClientUIInterface::InitMessage(const std::string& message) { return g_ui_signals.InitMessage(message); }
 void CClientUIInterface::NotifyNumConnectionsChanged(int newNumConnections) { return g_ui_signals.NotifyNumConnectionsChanged(newNumConnections); }
 void CClientUIInterface::NotifyNetworkActiveChanged(bool networkActive) { return g_ui_signals.NotifyNetworkActiveChanged(networkActive); }

test/lint/lint-includes.sh

@@ -64,7 +64,7 @@ EXPECTED_BOOST_INCLUDES=(
     boost/preprocessor/cat.hpp
     boost/preprocessor/stringize.hpp
     boost/signals2/connection.hpp
-    boost/signals2/last_value.hpp
+    boost/signals2/optional_last_value.hpp
     boost/signals2/signal.hpp
     boost/test/unit_test.hpp
     boost/thread/condition_variable.hpp