Fix issue of event loop thread and synchronous method call thread polling on the same D-Bus connection

Synchronous D-Bus method calls are now done in terms of blocking asynchronous calls.

src/Proxy.cpp

@@ -64,19 +64,36 @@ MethodCall Proxy::createMethodCall(const std::string& interfaceName, const std::
     return connection_->createMethodCall(destination_, objectPath_, interfaceName, methodName);
 }
 
-AsyncMethodCall Proxy::createAsyncMethodCall(const std::string& interfaceName, const std::string& methodName)
-{
-    return AsyncMethodCall{Proxy::createMethodCall(interfaceName, methodName)};
-}
-
 MethodReply Proxy::callMethod(const MethodCall& message, uint64_t timeout)
 {
+    // Sending method call synchronously is the only operation that blocks, waiting for the method
+    // reply message among the incoming message on the sd-bus connection socket. But typically there
+    // already is somebody that generally handles incoming D-Bus messages -- the connection event loop
+    // running typically in its own thread. We have to avoid polling on socket from several threads.
+    // So we have to branch here: either we are within the context of the event loop thread, then we
+    // can send the message simply via sd_bus_call, which blocks. Or we are in another thread, then
+    // we can perform the send operation of the method call message from here (because that is thread-
+    // safe like other sd-bus API accesses), but the incoming reply we have to get through the event
+    // loop thread, because this is be the only rightful listener on the sd-bus connection socket.
+    // So, technically, we use async means to wait here for reply received by the event loop thread.
+
     SDBUS_THROW_ERROR_IF(!message.isValid(), "Invalid method call message provided", EINVAL);
 
-    return message.send(timeout);
+    // If we don't need to wait for any reply, we can send the message now irrespective of the context
+    if (message.doesntExpectReply())
+        return message.send(timeout);
+
+    // If we are in the context of event loop thread, we can send the D-Bus call synchronously
+    // and wait blockingly for the reply, because we are the exclusive listeners on the socket
+    auto reply = connection_->tryCallMethodSynchronously(message, timeout);
+    if (reply.isValid())
+        return reply;
+
+    // Otherwise we send the call asynchronously and do blocking wait for the reply from the event loop thread
+    return sendMethodCallMessageAndWaitForReply(message, timeout);
 }
 
-void Proxy::callMethod(const AsyncMethodCall& message, async_reply_handler asyncReplyCallback, uint64_t timeout)
+void Proxy::callMethod(const MethodCall& message, async_reply_handler asyncReplyCallback, uint64_t timeout)
 {
     SDBUS_THROW_ERROR_IF(!message.isValid(), "Invalid async method call message provided", EINVAL);
 
@@ -88,6 +105,49 @@ void Proxy::callMethod(const AsyncMethodCall& message, async_reply_handler async
     pendingAsyncCalls_.addCall(callData->slot.get(), std::move(callData));
 }
 
+MethodReply Proxy::sendMethodCallMessageAndWaitForReply(const MethodCall& message, uint64_t timeout)
+{
+    /*thread_local*/ SyncCallReplyData syncCallReplyData;
+
+    async_reply_handler asyncReplyCallback = [&syncCallReplyData](MethodReply& reply, const Error* error)
+    {
+        syncCallReplyData.sendMethodReplyToWaitingThread(reply, error);
+    };
+    auto callback = (void*)&Proxy::sdbus_async_reply_handler;
+    AsyncCalls::CallData callData{*this, std::move(asyncReplyCallback), {}};
+
+    message.send(callback, &callData, timeout, dont_request_slot);
+
+    return syncCallReplyData.waitForMethodReply();
+}
+
+void Proxy::SyncCallReplyData::sendMethodReplyToWaitingThread(MethodReply& reply, const Error* error)
+{
+    SCOPE_EXIT{ cond_.notify_one(); };
+    std::unique_lock lock{mutex_};
+    SCOPE_EXIT{ arrived_ = true; };
+
+    //error_ = nullptr; // Necessary if SyncCallReplyData instance is thread_local
+
+    if (error == nullptr)
+        reply_ = std::move(reply);
+    else
+        error_ = std::make_unique<Error>(*error);
+}
+
+MethodReply Proxy::SyncCallReplyData::waitForMethodReply()
+{
+    std::unique_lock lock{mutex_};
+    cond_.wait(lock, [this](){ return arrived_; });
+
+    //arrived_ = false; // Necessary if SyncCallReplyData instance is thread_local
+
+    if (error_)
+        throw *error_;
+
+    return std::move(reply_);
+}
+
 void Proxy::registerSignalHandler( const std::string& interfaceName
                                  , const std::string& signalName
                                  , signal_handler signalHandler )
@@ -122,7 +182,7 @@ void Proxy::registerSignalHandlers(sdbus::internal::IConnection& connection)
             slot = connection.registerSignalHandler( objectPath_
                                                    , interfaceName
                                                    , signalName
-                                                   , &Proxy::sdbus_signal_callback
+                                                   , &Proxy::sdbus_signal_handler
                                                    , this );
         }
     }
@@ -141,7 +201,13 @@ int Proxy::sdbus_async_reply_handler(sd_bus_message *sdbusMessage, void *userDat
     assert(asyncCallData->callback);
     auto& proxy = asyncCallData->proxy;
 
-    SCOPE_EXIT{ proxy.pendingAsyncCalls_.removeCall(asyncCallData->slot.get()); };
+    SCOPE_EXIT
+    {
+        // Slot may be null if we're doing blocking synchronous call implemented by means of asynchronous call,
+        // because in that case the call data is still alive on the stack, we don't need to manage it separately.
+        if (asyncCallData->slot)
+            proxy.pendingAsyncCalls_.removeCall(asyncCallData->slot.get());
+    };
 
     auto message = Message::Factory::create<MethodReply>(sdbusMessage, &proxy.connection_->getSdBusInterface());
 
@@ -159,7 +225,7 @@ int Proxy::sdbus_async_reply_handler(sd_bus_message *sdbusMessage, void *userDat
     return 1;
 }
 
-int Proxy::sdbus_signal_callback(sd_bus_message *sdbusMessage, void *userData, sd_bus_error */*retError*/)
+int Proxy::sdbus_signal_handler(sd_bus_message *sdbusMessage, void *userData, sd_bus_error */*retError*/)
 {
     auto* proxy = static_cast<Proxy*>(userData);
     assert(proxy != nullptr);