Update meta-data evolution validator to check presence rather than optional.

Also, changing this in either direction is problematic as to whether default values are indexed as nulls.

Author: MMcM

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/metadata/MetaDataEvolutionValidator.java

@@ -274,13 +274,12 @@ private void validateField(@Nonnull FieldDescriptor oldFieldDescriptor, @Nonnull
         if (oldFieldDescriptor.isRequired() && !newFieldDescriptor.isRequired()) {
             throw new MetaDataException("required field is no longer required",
                     LogMessageKeys.FIELD_NAME, oldFieldDescriptor.getName());
-        } else if (oldFieldDescriptor.isOptional() && !newFieldDescriptor.isOptional()) {
-            // TODO: In theory, optional -> repeated is okay, but only if the field is not indexed
-            throw new MetaDataException("optional field is no longer optional",
-                    LogMessageKeys.FIELD_NAME, oldFieldDescriptor.getName());
         } else if (oldFieldDescriptor.isRepeated() && !newFieldDescriptor.isRepeated()) {
             throw new MetaDataException("repeated field is no longer repeated",
                     LogMessageKeys.FIELD_NAME, oldFieldDescriptor.getName());
+        } else if (oldFieldDescriptor.hasPresence() != newFieldDescriptor.hasPresence()) {
+            throw new MetaDataException("field changed whether default values are stored if set explicitly",
+                    LogMessageKeys.FIELD_NAME, oldFieldDescriptor.getName());
         }
         if (oldFieldDescriptor.getType().equals(FieldDescriptor.Type.ENUM)) {
             validateEnum(newFieldDescriptor.getName(), oldFieldDescriptor.getEnumType(), newFieldDescriptor.getEnumType());

fdb-record-layer-core/src/test/java/com/apple/foundationdb/record/metadata/MetaDataEvolutionValidatorTest.java

@@ -857,12 +857,17 @@ public void fieldLabelChanged() {
                 DescriptorProtos.FieldDescriptorProto.Label.LABEL_REQUIRED
         );
         for (int i = 0; i < labels.size(); i++) {
-            final int itr = i;
-            final DescriptorProtos.FieldDescriptorProto.Label label = labels.get(itr);
-            final String labelText = label.name().substring(label.name().indexOf('_') + 1).toLowerCase(Locale.ROOT);
-            final String errMsg = labelText + " field is no longer " + labelText;
+            final DescriptorProtos.FieldDescriptorProto.Label label = labels.get(i);
+            final String errMsg;
+            if (label == DescriptorProtos.FieldDescriptorProto.Label.LABEL_OPTIONAL) {
+                errMsg = "field changed whether default values are stored if set explicitly";
+            } else {
+                final String labelText = label.name().substring(label.name().indexOf('_') + 1).toLowerCase(Locale.ROOT);
+                errMsg = labelText + " field is no longer " + labelText;
+            }
+            final DescriptorProtos.FieldDescriptorProto.Label newLabel = labels.get((i + 1) % labels.size());
             FileDescriptor updatedFile = mutateField("MySimpleRecord", "str_value_indexed", oldFile,
-                    field -> field.setLabel(labels.get((itr + 1) % labels.size())));
+                    field -> field.setLabel(newLabel));
             assertInvalid(errMsg, oldFile, updatedFile);
 
             oldFile = updatedFile;

fdb-record-layer-core/src/test/proto/test_records_nulls_3_explicit.proto

@@ -0,0 +1,37 @@
+/*
+ * test_records_nulls_3_explicit.proto
+ *
+ * This source file is part of the FoundationDB open source project
+ *
+ * Copyright 2026 Apple Inc. and the FoundationDB project authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+syntax = "proto3";
+
+package com.apple.foundationdb.record.testnulls3explicit;
+
+option java_package = "com.apple.foundationdb.record";
+option java_outer_classname = "TestRecordsNulls3ExplicitProto";
+
+import "record_metadata_options.proto";
+
+message MyNullRecord {
+  optional string name = 1 [(field).primary_key = true];
+  optional string string_value = 2 [(field).index = {}];
+  optional int32 int_value = 3 [(field).index = {}];
+}
+
+message RecordTypeUnion {
+  optional MyNullRecord _MyNullRecord = 1;
+}