Fix OpenAPI 3.0 output of nullable references

When a nullable field or parameter references a non-nullable schema, we
generate a schema for the field which uses anyOf to permit null. Any
informational properties or additional assertions defined on the field
are also put on the field schema and the type property is also retained.

When we were transforming this field schema for OpenAPI 3.0, we were
only doing certain transformations if the type property is not present,
which doesn't handle the case above.

Ensure the transformation to and from OpenAPI 3.0 schemas handles these
nullable references which include the type property.

core/src/main/java/io/smallrye/openapi/runtime/io/media/SchemaIO.java

@@ -191,7 +191,7 @@ private void populateSchemaObject30(Schema schema, O node) {
         }
 
         // Detect {$ref=....,nullable=true} and convert to anyOf[{$ref=...}, {type=null}]
-        if (schema.getRef() != null && schema.getType() == null && SchemaSupport.getNullable(schema) == Boolean.TRUE) {
+        if (schema.getRef() != null && SchemaSupport.getNullable(schema) == Boolean.TRUE) {
             List<Schema> newAnyOfSchemas = new ArrayList<>();
             newAnyOfSchemas.add(OASFactory.createSchema().ref(schema.getRef()));
             newAnyOfSchemas.add(SchemaSupport.nullSchema());
@@ -201,7 +201,11 @@ private void populateSchemaObject30(Schema schema, O node) {
                 schema.addAllOf(OASFactory.createSchema().anyOf(newAnyOfSchemas));
             }
             schema.setRef(null);
-            SchemaSupport.setNullable(schema, null);
+            if (schema.getType() == null) {
+                SchemaSupport.setNullable(schema, null);
+            } else {
+                SchemaSupport.setNullable(schema, Boolean.TRUE);
+            }
         }
 
         // Detect {enum=[null]} and convert to {type=null}
@@ -389,7 +393,7 @@ private ReplacementFields compute30ReplacementFields(Schema schema31) {
         }
 
         // If we have anyOf = [{type=null}, {$ref=...}], remove it and set nullable and allOf = [{$ref=...}]
-        if (result.anyOf != null && result.anyOf.size() == 2 && result.ref == null && result.type == null) {
+        if (result.anyOf != null && result.anyOf.size() == 2 && result.ref == null) {
             Optional<Schema> typeNullSchema = result.anyOf.stream().filter(s -> isSoloTypeNull(s)).findFirst();
             Optional<Schema> refSchema = result.anyOf.stream().filter(s -> isSoloRef(s)).findFirst();
             if (typeNullSchema.isPresent() && refSchema.isPresent()) {

core/src/test/java/io/smallrye/openapi/runtime/scanner/SchemaPropertyTest.java

@@ -93,4 +93,29 @@ static class Turtle extends Reptile {
         String shellPattern;
         Speed speed;
     }
+
+    @Test
+    void testNullableFieldReference() throws Exception {
+        Index index = indexOf(FieldTarget.class, FieldReferrer.class);
+        SmallRyeOpenAPI result1 = SmallRyeOpenAPI.builder()
+                .withConfig(config(Collections.emptyMap()))
+                .withIndex(index)
+                .defaultRequiredProperties(false)
+                .build();
+        printToConsole(result1.model());
+        assertJsonEquals("components.schemas.schemaproperty-nullable.json", result1.model());
+    }
+
+    @org.eclipse.microprofile.openapi.annotations.media.Schema
+    public static class FieldTarget {
+    }
+
+    @org.eclipse.microprofile.openapi.annotations.media.Schema
+    public static class FieldReferrer {
+        @org.eclipse.microprofile.openapi.annotations.media.Schema(nullable = true)
+        private FieldTarget a;
+        @org.eclipse.microprofile.openapi.annotations.media.Schema(nullable = true, description = "b")
+        private FieldTarget b;
+    }
+
 }

core/src/test/resources/io/smallrye/openapi/runtime/io/schemas-with-nullable-reference.json

@@ -13,6 +13,18 @@
           }
         ]
       },
+      "NullRefWithAnnotations": {
+        "description": "Description is an annotation. Scanner generates type as well here.",
+        "type": ["object", "null"],
+        "anyOf": [
+          {
+            "$ref": "#/components/schemas/mySchema"
+          },
+          {
+            "type": "null"
+          }
+        ]
+      },
       "NullRefWithAnyOf": {
         "allOf": [
           {

core/src/test/resources/io/smallrye/openapi/runtime/io/schemas-with-nullable-reference30.json

@@ -11,6 +11,16 @@
           }
         ]
       },
+      "NullRefWithAnnotations": {
+        "description": "Description is an annotation. Scanner generates type as well here.",
+        "type": "object",
+        "allOf": [
+          {
+            "$ref": "#/components/schemas/mySchema"
+          }
+        ],
+        "nullable": true
+      },
       "NullRefWithAnyOf": {
         "allOf": [
           {

core/src/test/resources/io/smallrye/openapi/runtime/scanner/components.schemas.schemaproperty-nullable.json

@@ -0,0 +1,31 @@
+{
+  "openapi" : "3.1.0",
+  "components" : {
+    "schemas" : {
+      "FieldReferrer" : {
+        "type" : "object",
+        "properties" : {
+          "a" : {
+            "anyOf" : [ {
+              "$ref" : "#/components/schemas/FieldTarget"
+            }, {
+              "type" : "null"
+            } ]
+          },
+          "b" : {
+            "description" : "b",
+            "type" : "object",
+            "anyOf" : [ {
+              "$ref" : "#/components/schemas/FieldTarget"
+            }, {
+              "type" : "null"
+            } ]
+          }
+        }
+      },
+      "FieldTarget" : {
+        "type" : "object"
+      }
+    }
+  }
+}