Avoid memory churn when looking up channel

Author: ktf

Framework/Core/include/Framework/ExternalFairMQDeviceProxy.h

@@ -22,7 +22,7 @@ namespace o2::framework
 
 /// A callback function to retrieve the fair::mq::Channel name to be used for sending
 /// messages of the specified OutputSpec
-using ChannelRetriever = std::function<std::string(OutputSpec const&, DataProcessingHeader::StartTime)>;
+using ChannelRetriever = std::function<std::string const&(OutputSpec const&, DataProcessingHeader::StartTime)>;
 /// The callback which actually does the heavy lifting of converting the input data into
 /// DPL messages. The callback is invoked with the following parameters:
 /// @param timingInfo is the timing information of the current timeslice
@@ -144,6 +144,6 @@ DataProcessorSpec specifyFairMQDeviceMultiOutputProxy(char const* label,
                                                       const char* defaultChannelConfig,
                                                       ChannelSelector channelSelector = defaultOutputProxyChannelSelector);
 
-} // namespace o2
+} // namespace o2::framework
 
 #endif // FRAMEWORK_RAWDEVICESOURCE_H

Framework/Core/src/ExternalFairMQDeviceProxy.cxx

@@ -752,14 +752,15 @@ DataProcessorSpec specifyExternalFairMQDeviceProxy(char const* name,
                         outputChannels = std::move(outputChannels)](ServiceRegistryRef ref, TimingInfo& timingInfo, fair::mq::Parts& inputs, int, size_t ci, bool newRun) -> bool {
       auto* device = ref.get<RawDeviceService>().device();
       // pass a copy of the outputRoutes
-      auto channelRetriever = [&outputRoutes](OutputSpec const& query, DataProcessingHeader::StartTime timeslice) -> std::string {
+      auto channelRetriever = [&outputRoutes](OutputSpec const& query, DataProcessingHeader::StartTime timeslice) -> std::string const& {
+        static std::string emptyChannel = "";
         for (auto& route : outputRoutes) {
           LOG(debug) << "matching: " << DataSpecUtils::describe(query) << " to route " << DataSpecUtils::describe(route.matcher);
           if (DataSpecUtils::match(route.matcher, query) && ((timeslice % route.maxTimeslices) == route.timeslice)) {
             return route.channel;
           }
         }
-        return {""};
+        return emptyChannel;
       };
 
       std::string const& channel = channels[ci];

Framework/Core/test/benchmark_ExternalFairMQDeviceProxies.cxx

@@ -483,7 +483,7 @@ std::vector<DataProcessorSpec> defineDataProcessing(ConfigContext const& config)
     // Note: we want to run both the output and input proxy in the same workflow and thus we need
     // different data identifiers and change the data origin in the forwarding
     OutputSpec query{"PRX", dh->dataDescription, dh->subSpecification};
-    auto channelName = channelRetriever(query, dph->startTime);
+    auto const& channelName = channelRetriever(query, dph->startTime);
     bool isData = DataSpecUtils::match(OutputSpec{"TST", "DATA", 0}, dh->dataOrigin, dh->dataDescription, dh->subSpecification);
     // for the configured data channel we require the channel name, the EOS message containing
     // the forwarded SourceInfoHeader created by the output proxy will be skipped here since the