Override fromWellKnownProto in ProtoLiteCelValueConverter for FieldMask

Mirror the full converter: intercept FIELD_MASK in the lite converter and wrap
it as a ProtoMessageLiteValue so it is navigable as a message on the lite
runtime. Adds a lite-converter test.

Author: andrewparmet

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 {