Fix crash on simultaneous IPC calls using the same thread

This error occurs when non-libmultiprocess clients make simultaenous IPC calls to the server using the same Server thread.
The libmultiprocess Cpp client ensures a 1-to-1 mapping of client-to-server threads, so simultaneous calls using the same thread cannot be made.

Author: Eunovo

include/mp/proxy-io.h

@@ -281,12 +281,13 @@ struct Waiter
     Waiter() = default;
 
     template <typename Fn>
-    void post(Fn&& fn)
+    bool post(Fn&& fn)
     {
         const Lock lock(m_mutex);
-        assert(!m_fn);
+        if (m_fn) return false;
         m_fn = std::forward<Fn>(fn);
         m_cv.notify_all();
+        return true;
     }
 
     template <class Predicate>

include/mp/type-context.h

@@ -152,7 +152,12 @@ auto PassField(Priority<1>, TypeList<>, ServerContext& server_context, const Fn&
                 const auto& thread = static_cast<ProxyServer<Thread>&>(*thread_server);
                 server.m_context.loop->log()
                     << "IPC server post request  #" << req << " {" << thread.m_thread_context.thread_name << "}";
-                thread.m_thread_context.waiter->post(std::move(invoke));
+                if (!thread.m_thread_context.waiter->post(std::move(invoke))) {
+                    server.m_context.loop->log()
+                        << "IPC server error request #" << req
+                        << " {" << thread.m_thread_context.thread_name << "}" << ", thread busy";
+                    throw std::runtime_error("thread busy");
+                }
             } else {
                 server.m_context.loop->log()
                     << "IPC server error request #" << req << ", missing thread to execute request";