How to extract field from protobuf message without schema - go

Question

According to this issue, the protoreflect package provides APIs for accessing the "unknown fields" of protobuf messages, but I don't see any way to use it if there isn't any existing schema. Basically, I want to perform a "weak decode", similar to what the JSON unmarshaller does if the output is of type map[string]interface{}.
The example from the documentation looks like this:
err := UnmarshalOptions{DiscardUnknown: true}.Unmarshal(b, m)
where b is the input byte slice and m is the output message, which needs to be initialised somehow, as you can see here. I was thinking that dynamicpb can be used for this purpose, but it doesn't look possible without having an existing MessageDescriptor and that's where I got stuck...

Answer 1

I was able to achieve this using the low level protowire package. Here is a full example, where I extract two fields of type uint64 (which happen to be assigned field numbers 4 and 5 in the original schema):
import "google.golang.org/protobuf/encoding/protowire"
func getData(src []byte) (creationTime, expiryTime uint64, err error) {
remaining := src
for len(remaining) > 0 {
fieldNum, wireType, n := protowire.ConsumeTag(remaining)
if n < 0 {
return 0, 0, fmt.Errorf("failed to consume tag: %w", protowire.ParseError(n))
}
remaining = remaining[n:]
switch fieldNum {
case 4: // Expiry time
if wireType != protowire.VarintType {
return 0, 0, fmt.Errorf("unexpected type for expiry time field: %d", wireType)
}
expiryTime, n = protowire.ConsumeVarint(remaining)
case 5: // Creation time
if wireType != protowire.VarintType {
return 0, 0, fmt.Errorf("unexpected type for creation time field: %d", wireType)
}
creationTime, n = protowire.ConsumeVarint(remaining)
default:
n = protowire.ConsumeFieldValue(fieldNum, wireType, remaining)
}
if n < 0 {
return 0, 0, fmt.Errorf("failed to consume value for field %d: %w", fieldNum, protowire.ParseError(n))
}
remaining = remaining[n:]
}
return
}