fix: ensure nullability preserved in calcite roundtrip

Author: benbellick

isthmus/src/main/java/io/substrait/isthmus/expression/CallConverters.java

@@ -77,6 +77,7 @@ public class CallConverters {
                     com.google.protobuf.Any.parseFrom(literal.value().toByteArray());
 
                 return Expression.UserDefinedAnyLiteral.builder()
+                    .nullable(t.nullable())
                     .urn(t.urn())
                     .name(t.name())
                     .addAllTypeParameters(t.typeParameters())

isthmus/src/test/java/io/substrait/isthmus/CustomFunctionTest.java

@@ -7,7 +7,6 @@
 import io.substrait.dsl.SubstraitBuilder;
 import io.substrait.expression.Expression.UserDefinedLiteral;
 import io.substrait.expression.ExpressionCreator;
-import io.substrait.extension.ExtensionCollector;
 import io.substrait.extension.SimpleExtension;
 import io.substrait.isthmus.expression.AggregateFunctionConverter;
 import io.substrait.isthmus.expression.FunctionMappings;
@@ -16,9 +15,7 @@
 import io.substrait.isthmus.utils.UserTypeFactory;
 import io.substrait.proto.Expression;
 import io.substrait.proto.Expression.Literal.Builder;
-import io.substrait.relation.ProtoRelConverter;
 import io.substrait.relation.Rel;
-import io.substrait.relation.RelProtoConverter;
 import io.substrait.type.Type;
 import io.substrait.type.TypeCreator;
 import java.io.IOException;
@@ -601,10 +598,24 @@ void customTypesLiteralInFunctionsRoundtrip() {
     RelNode calciteRel = substraitToCalcite.convert(rel1);
     Rel rel2 = calciteToSubstrait.apply(calciteRel);
     assertEquals(rel1, rel2);
+  }
+
+  @Test
+  void customNullableUserDefinedLiteralRoundtrip() {
+    Builder bldr = Expression.Literal.newBuilder();
+    Any anyValue = Any.pack(bldr.setI32(10).build());
+    UserDefinedLiteral nullableLiteral =
+        ExpressionCreator.userDefinedLiteralAny(
+            true, URN, "a_type", java.util.Collections.emptyList(), anyValue);
+
+    Rel rel =
+        b.project(
+            input -> List.of(nullableLiteral),
+            b.remap(1),
+            b.namedScan(List.of("example"), List.of("a"), List.of(N.userDefined(URN, "a_type"))));
 
-    ExtensionCollector extensionCollector = new ExtensionCollector();
-    io.substrait.proto.Rel protoRel = new RelProtoConverter(extensionCollector).toProto(rel1);
-    Rel rel3 = new ProtoRelConverter(extensionCollector, extensionCollection).from(protoRel);
-    assertEquals(rel1, rel3);
+    RelNode calciteRel = substraitToCalcite.convert(rel);
+    Rel relReturned = calciteToSubstrait.apply(calciteRel);
+    assertEquals(rel, relReturned);
   }
 }