Preserve FieldMask as a message in the CelValue runtime path

common/src/main/java/dev/cel/common/values/BaseProtoCelValueConverter.java

@@ -17,8 +17,6 @@
 import static com.google.common.collect.ImmutableList.toImmutableList;
 import static com.google.common.collect.ImmutableMap.toImmutableMap;
 
-import com.google.common.base.CaseFormat;
-import com.google.common.base.Joiner;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
@@ -29,7 +27,6 @@
 import com.google.protobuf.BytesValue;
 import com.google.protobuf.DoubleValue;
 import com.google.protobuf.Duration;
-import com.google.protobuf.FieldMask;
 import com.google.protobuf.FloatValue;
 import com.google.protobuf.Int32Value;
 import com.google.protobuf.Int64Value;
@@ -44,8 +41,6 @@
 import dev.cel.common.annotations.Internal;
 import dev.cel.common.internal.ProtoTimeUtils;
 import dev.cel.common.internal.WellKnownProto;
-import java.util.ArrayList;
-import java.util.List;
 
 /**
  * {@code BaseProtoCelValueConverter} contains the common logic for converting between native Java
@@ -103,15 +98,6 @@ protected Object fromWellKnownProto(MessageLiteOrBuilder message, WellKnownProto
         return UnsignedLong.valueOf(((UInt32Value) message).getValue());
       case UINT64_VALUE:
         return UnsignedLong.fromLongBits(((UInt64Value) message).getValue());
-      case FIELD_MASK:
-        FieldMask fieldMask = (FieldMask) message;
-        List<String> paths = new ArrayList<>(fieldMask.getPathsCount());
-        for (String path : fieldMask.getPathsList()) {
-          if (!path.isEmpty()) {
-            paths.add(CaseFormat.LOWER_UNDERSCORE.to(CaseFormat.LOWER_CAMEL, path));
-          }
-        }
-        return normalizePrimitive(Joiner.on(",").join(paths));
       case EMPTY:
         return ImmutableMap.of();
       default:

common/src/main/java/dev/cel/common/values/ProtoCelValueConverter.java

@@ -71,6 +71,9 @@ protected Object fromWellKnownProto(MessageLiteOrBuilder msg, WellKnownProto wel
               "Unpacking failed for message: " + message.getDescriptorForType().getFullName(), e);
         }
         return toRuntimeValue(unpackedMessage);
+      case FIELD_MASK:
+        return ProtoMessageValue.create(
+            (Message) message, celDescriptorPool, this, celOptions.enableJsonFieldNames());
       default:
         return super.fromWellKnownProto(message, wellKnownProto);
     }

common/src/main/java/dev/cel/common/values/ProtoLiteCelValueConverter.java

@@ -28,6 +28,7 @@
 import com.google.protobuf.CodedInputStream;
 import com.google.protobuf.ExtensionRegistryLite;
 import com.google.protobuf.MessageLite;
+import com.google.protobuf.MessageLiteOrBuilder;
 import com.google.protobuf.WireFormat;
 import dev.cel.common.annotations.Internal;
 import dev.cel.common.internal.CelLiteDescriptorPool;
@@ -178,12 +179,27 @@ public Object toRuntimeValue(Object value) {
         return ProtoMessageLiteValue.create(msg, descriptor.getProtoTypeName(), this);
       }
 
-      return super.fromWellKnownProto(msg, wellKnownProto);
+      return fromWellKnownProto(msg, wellKnownProto);
     }
 
     return super.toRuntimeValue(value);
   }
 
+  @Override
+  protected Object fromWellKnownProto(MessageLiteOrBuilder msg, WellKnownProto wellKnownProto) {
+    if (wellKnownProto == WellKnownProto.FIELD_MASK) {
+      MessageLite message = (MessageLite) msg;
+      MessageLiteDescriptor descriptor =
+          descriptorPool
+              .findDescriptor(message)
+              .orElseThrow(
+                  () -> new NoSuchElementException("Could not find a descriptor for: " + message));
+      return ProtoMessageLiteValue.create(message, descriptor.getProtoTypeName(), this);
+    }
+
+    return super.fromWellKnownProto(msg, wellKnownProto);
+  }
+
   private Object getDefaultValue(FieldLiteDescriptor fieldDescriptor) {
     EncodingType encodingType = fieldDescriptor.getEncodingType();
     switch (encodingType) {

common/src/test/java/dev/cel/common/values/ProtoLiteCelValueConverterTest.java

@@ -27,6 +27,7 @@
 import com.google.protobuf.DoubleValue;
 import com.google.protobuf.Duration;
 import com.google.protobuf.ExtensionRegistryLite;
+import com.google.protobuf.FieldMask;
 import com.google.protobuf.FloatValue;
 import com.google.protobuf.Int32Value;
 import com.google.protobuf.Int64Value;
@@ -104,6 +105,17 @@ public void fromProtoMessageToCelValue_withWellKnownProto_convertsToPrimitivesFr
     assertThat(adaptedValue).isEqualTo(testCase.value);
   }
 
+  @Test
+  public void fromProtoMessageToCelValue_fieldMask_returnsProtoMessageLiteValue() {
+    FieldMask fieldMask = FieldMask.newBuilder().addPaths("foo").addPaths("bar").build();
+
+    Object adaptedValue = PROTO_LITE_CEL_VALUE_CONVERTER.toRuntimeValue(fieldMask);
+
+    assertThat(adaptedValue).isInstanceOf(ProtoMessageLiteValue.class);
+    assertThat(((ProtoMessageLiteValue) adaptedValue).select("paths"))
+        .isEqualTo(ImmutableList.of("foo", "bar"));
+  }
+
   /** Test cases for repeated_int64: 1L,2L,3L */
   @SuppressWarnings("ImmutableEnumChecker") // Test only
   private enum RepeatedFieldBytesTestCase {

common/src/test/java/dev/cel/common/values/ProtoMessageValueTest.java

@@ -303,6 +303,24 @@ public void selectField_durationOutOfRange_success(int seconds, int nanos) {
         .isEqualTo(Duration.ofSeconds(seconds, nanos));
   }
 
+  @Test
+  public void selectField_fieldMask_returnsProtoMessageValue() {
+    TestAllTypes testAllTypes =
+        TestAllTypes.newBuilder()
+            .setFieldMask(
+                com.google.protobuf.FieldMask.newBuilder().addPaths("foo").addPaths("bar"))
+            .build();
+
+    ProtoMessageValue protoMessageValue =
+        ProtoMessageValue.create(
+            testAllTypes, DefaultDescriptorPool.INSTANCE, PROTO_CEL_VALUE_CONVERTER, false);
+
+    Object selected = protoMessageValue.select("field_mask");
+    assertThat(selected).isInstanceOf(ProtoMessageValue.class);
+    assertThat(((ProtoMessageValue) selected).select("paths"))
+        .isEqualTo(ImmutableList.of("foo", "bar"));
+  }
+
   @SuppressWarnings("ImmutableEnumChecker") // Test only
   private enum SelectFieldJsonValueTestCase {
     NULL(Value.newBuilder().build(), NullValue.NULL_VALUE),