pang-wu commented on code in PR #40686:
URL: https://github.com/apache/spark/pull/40686#discussion_r1160291713
##########
connector/protobuf/src/main/scala/org/apache/spark/sql/protobuf/ProtobufDeserializer.scala:
##########
@@ -288,7 +289,21 @@ private[sql] class ProtobufDeserializer(
var skipRow = false
while (i < validFieldIndexes.length && !skipRow) {
val field = validFieldIndexes(i)
- val value = if (field.isRepeated || field.hasDefaultValue ||
record.hasField(field)) {
+
+ // If `materializeZeroValues` is true, the written field will contain
the
+ // default zero value for its type (e.g. 0 for int, "" for string, etc:
+ // https://protobuf.dev/programming-guides/proto3/#default).
+ // Otherwise, the field will be null unless the serialized proto
explicitly
+ // contains the field, or it has an explicit default (
+ // proto2 only
https://protobuf.dev/programming-guides/proto2/#optional)
+ // Note that in proto3, the serialized proto will not contain the field
+ // if the value was explicitly set to its zero value. See:
+ //
https://protobuf.dev/programming-guides/field_presence/#presence-in-proto3-apis
Review Comment:
@viirya I don't think this is a bug, according to proto3 default value spec,
> When a message is parsed, if the encoded message does not contain a
particular singular element, the corresponding field in the parsed object is
set to the default value for that field. These defaults are type-specific:
>
> For strings, the default value is the empty string.
> For bytes, the default value is empty bytes.
> For bools, the default value is false.
> For numeric types, the default value is zero.
> For [enums](https://protobuf.dev/programming-guides/proto3/#enum), the
default value is the first defined enum value, which must be 0.
> For message fields, the field is not set. Its exact value is
language-dependent. See the [generated code
guide](https://protobuf.dev/reference/) for details.
>
What that mean is the absent optional field's value of a proto3 message
should be the default instead of null (0 for int, for example) if they are
retrieved by the client -- this is very confusing (I know), it basically says
by default (not using oneof keyword) there is no way to distinguish between
absent field and fields populated with default value (like int64 field set to
0). Protobuf 3.15 somewhat bring this back, more details could be found
[here](https://stackoverflow.com/questions/42622015/how-to-define-an-optional-field-in-protobuf-3)
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