[ntuple] Refine name conflict resolution

The fields from the auxiliary processor get added to the model of the
primary processor wrapped in an untyped collection with the name of the
auxiliary processor. Therefore, instead of checking that the primary
and auxiliary processors don't share the same name, we should check that
the primary processor doesn't already have a field with the name of the
auxiliary processor.

Author: enirolf

tree/ntuple/src/RNTupleProcessor.cxx

@@ -96,12 +96,6 @@ ROOT::Experimental::RNTupleProcessor::CreateJoin(RNTupleOpenSpec primaryNTuple,
       throw RException(R__FAIL("join fields must be unique"));
    }
 
-   // Ensure no name clash exists between the primary and auxiliary ntuple.
-   if ((!processorName.empty() && processorName == auxNTuple.fNTupleName) ||
-       (primaryNTuple.fNTupleName == auxNTuple.fNTupleName)) {
-      throw ROOT::RException(R__FAIL("joining RNTuples with the same name is not allowed"));
-   }
-
    std::unique_ptr<RNTupleProcessor> primaryProcessor;
    if (primaryModel)
       primaryProcessor = Create(primaryNTuple, primaryModel->Clone(), processorName);
@@ -369,6 +363,17 @@ ROOT::Experimental::RNTupleJoinProcessor::RNTupleJoinProcessor(std::unique_ptr<R
       auxModel = fAuxiliaryProcessor->GetModel().Clone();
    }
 
+   // If the primaryProcessor has a field with the name of the auxProcessor (either as a "proper" field or because the
+   // primary processor itself is a join where its auxProcessor bears the same name as the current auxProcessor), there
+   // will be name conflicts, so error out.
+   if (primaryModel->GetFieldNames().find(fAuxiliaryProcessor->GetProcessorName()) !=
+       primaryModel->GetFieldNames().end()) {
+      throw RException(R__FAIL("a field or nested auxiliary processor named \"" +
+                               fAuxiliaryProcessor->GetProcessorName() +
+                               "\" is already present in the model of the primary processor; rename the auxiliary "
+                               "processor to avoid conflicts"));
+   }
+
    SetModel(std::move(primaryModel), std::move(auxModel));
 
    fModel->Freeze();

tree/ntuple/test/ntuple_processor.cxx

@@ -384,3 +384,18 @@ TEST_F(RNTupleProcessorTest, JoinedJoinComposedAuxiliary)
       EXPECT_THAT(err.what(), testing::HasSubstr("auxiliary RNTupleJoinProcessors are currently not supported"));
    }
 }
+
+TEST_F(RNTupleProcessorTest, JoinedJoinComposedSameName)
+{
+   auto primaryProc =
+      RNTupleProcessor::CreateJoin({fNTupleNames[0], fFileNames[0]}, {fNTupleNames[1], fFileNames[1]}, {});
+
+   try {
+      auto auxProc = RNTupleProcessor::Create({fNTupleNames[2], fFileNames[2]});
+      auto proc = RNTupleProcessor::CreateJoin(std::move(primaryProc), std::move(auxProc), {"i"});
+   } catch (const ROOT::RException &err) {
+      EXPECT_THAT(err.what(), testing::HasSubstr("a field or nested auxiliary processor named \"ntuple_aux\" "
+                                                 "is already present in the model of the primary processor; rename "
+                                                 "the auxiliary processor to avoid conflicts"));
+   }
+}

tree/ntuple/test/ntuple_processor_join.cxx

@@ -85,13 +85,6 @@ class RNTupleJoinProcessorTest : public testing::Test {
 
 TEST_F(RNTupleJoinProcessorTest, Aligned)
 {
-   try {
-      auto proc = RNTupleProcessor::CreateJoin({fNTupleNames[0], fFileNames[0]}, {fNTupleNames[0], fFileNames[0]}, {});
-      FAIL() << "ntuples with the same name cannot be joined horizontally";
-   } catch (const ROOT::RException &err) {
-      EXPECT_THAT(err.what(), testing::HasSubstr("joining RNTuples with the same name is not allowed"));
-   }
-
    auto proc = RNTupleProcessor::CreateJoin({fNTupleNames[1], fFileNames[1]}, {fNTupleNames[2], fFileNames[2]}, {});
 
    int nEntries = 0;