DPL: add support for decompressing directly to shared memory

This PR postpones the read operations which would usually populate an intermediate
RecordBatch and it performs them directly on its subsequent shared memory
serialization. Doing so avoids having the intermediate representation allocate most
of the memory.

For the moment this is only done for the TTree plugin. RNtuple support will come
in a subsequent PR.

Author: ktf

Framework/AnalysisSupport/src/RNTuplePlugin.cxx

@@ -12,6 +12,7 @@
 #include "Framework/RuntimeError.h"
 #include "Framework/RootArrowFilesystem.h"
 #include "Framework/Plugins.h"
+#include "Framework/FairMQResizableBuffer.h"
 #include <ROOT/RNTupleModel.hxx>
 #include <ROOT/RNTupleWriteOptions.hxx>
 #include <ROOT/RNTupleWriter.hxx>
@@ -852,7 +853,10 @@ struct RNTupleObjectReadingImplementation : public RootArrowFactoryPlugin {
     return new RootArrowFactory{
       .options = [context]() { return context->format->DefaultWriteOptions(); },
       .format = [context]() { return context->format; },
-    };
+      .deferredOutputStreamer = [](std::shared_ptr<arrow::dataset::FileFragment> fragment, const std::shared_ptr<arrow::ResizableBuffer>& buffer) -> std::shared_ptr<arrow::io::OutputStream> {
+        auto treeFragment = std::dynamic_pointer_cast<RNTupleFileFragment>(fragment);
+        return std::make_shared<FairMQOutputStream>(buffer);
+      }};
   }
 };