Open files in OutputStream start to ensure they're always created

Co-authored-by: Nicole Skidmore <nicola.skidmore@cern.ch>
Co-authored-by: Marco Clemencic <marco.clemencic@cern.ch>

GaudiCommonSvc/src/PersistencySvc/OutputStream.cpp

 /***********************************************************************************\
-* (c) Copyright 1998-2019 CERN for the benefit of the LHCb and ATLAS collaborations *
+* (c) Copyright 1998-2023 CERN for the benefit of the LHCb and ATLAS collaborations *
 *                                                                                   *
 * This software is distributed under the terms of the Apache version 2 licence,     *
 * copied verbatim in the file "LICENSE".                                            *
@@ -42,6 +42,13 @@ namespace {
   }
 } // namespace
 
+StatusCode OutputStream::start() {
+  return Algorithm::start().andThen( [this]() {
+    // Open the output file now to ensure it is always written even if empty
+    return m_pConversionSvc->connectOutput( m_outputName, m_outputType );
+  } );
+}
+
 // initialize data writer
 StatusCode OutputStream::initialize() {
 
@@ -126,6 +133,7 @@ StatusCode OutputStream::initialize() {
   m_acceptNames.useUpdateHandler();
   m_requireNames.useUpdateHandler();
   m_vetoNames.useUpdateHandler();
+
   return StatusCode::SUCCESS;
 }
 

GaudiCommonSvc/src/PersistencySvc/OutputStream.h

 /***********************************************************************************\
-* (c) Copyright 1998-2019 CERN for the benefit of the LHCb and ATLAS collaborations *
+* (c) Copyright 1998-2023 CERN for the benefit of the LHCb and ATLAS collaborations *
 *                                                                                   *
 * This software is distributed under the terms of the Apache version 2 licence,     *
 * copied verbatim in the file "LICENSE".                                            *
@@ -139,6 +139,8 @@ protected:
   virtual bool hasInput() const;
 
 public:
+  /// Start OutputStream
+  StatusCode start() override;
   /// Initialize OutputStream
   StatusCode initialize() override;
   /// Terminate OutputStream

GaudiExamples/tests/pytest/test_outputfile_no_events.py

+#####################################################################################
+# (c) Copyright 2023 CERN for the benefit of the LHCb and ATLAS collaborations      #
+#                                                                                   #
+# This software is distributed under the terms of the Apache version 2 licence,     #
+# copied verbatim in the file "LICENSE".                                            #
+#                                                                                   #
+# In applying this licence, CERN does not waive the privileges and immunities       #
+# granted to it by virtue of its status as an Intergovernmental Organization        #
+# or submit itself to any jurisdiction.                                             #
+#####################################################################################
+import os
+
+from GaudiTests import run_gaudi
+
+OUTPUT_FILE_NAME = "empty-output-file.dst"
+
+
+def config():
+    """
+    Configure a job writing to an empty file.
+    """
+    from Configurables import GaudiPersistency
+    from GaudiConfig2 import Configurables as C
+
+    GaudiPersistency()
+
+    out = C.OutputStream(
+        ItemList=["/Event#999"],
+        Output=f"DATAFILE='PFN:{OUTPUT_FILE_NAME}' SVC='Gaudi::RootCnvSvc' OPT='RECREATE'",
+    )
+    app = C.ApplicationMgr(EvtMax=0, OutStream=[out.__opt_value__()])
+
+    return [app, out]
+
+
+def test(tmp_path):
+    """
+    Check that an output file is created even if no event is written to it.
+    """
+    os.chdir(tmp_path)
+
+    if os.path.exists(OUTPUT_FILE_NAME):
+        os.remove(OUTPUT_FILE_NAME)
+
+    run_gaudi(f"{__file__}:config", check=True)
+
+    assert os.path.exists(
+        OUTPUT_FILE_NAME
+    ), f"expected output file {OUTPUT_FILE_NAME} not found"
+    os.remove(OUTPUT_FILE_NAME)