How to Unmarshal Unknown Protocol Buffers in Go?

Unmarshaling Unknown Protocol Buffers in Go

When working with protocol buffers (protobuf), it's possible to encounter situations where the type of message received is unknown. This can arise when listening for messages from multiple sources or when dealing with dynamic or unpredictable message types.

Protobuf's default Unmarshal function requires a specific message type to be passed as an argument, but this is not feasible when the message type is unknown. A common approach is to attempt to unmarshal into an interface{}, but this is not supported by the Protobuf library.

Addressing the Limitation

To handle unknown messages, it's necessary to implement a custom approach. One solution is to employ the protowire package, which provides low-level access to the wire representation of protobuf messages.

Parsing Unknown Messages

The following code snippet demonstrates how to parse and extract information from unknown protobuf messages:

type Field struct {
    Tag
    Val
}

type Tag struct {
    Num int32
    Type protowire.Type
}

type Val struct {
    Payload interface{}
    Length int
}

func parseUnknown(b []byte) []Field {
    // ...
}

The parseUnknown function iterates through the byte array, consuming fields and extracting basic information about each field's tag and value.

Extracting Payload

The extracted value payload can be stored in a variety of data structures, depending on the expected type and contents of the field. For instance:

switch t {
case protowire.VarintType:
    v, vlen := protowire.ConsumeVarint(b[taglen:fieldlen])
case protowire.Fixed64Type:
    v, vlen := protowire.ConsumeFixed64(b[taglen:fieldlen])
// ...
}

Limitations and Considerations

This approach has certain limitations:

• Ambiguity: The wire representation of protobuf messages can be ambiguous, making it difficult to determine the exact type of value in some cases.
• Unknown Fields: Any unknown or unsupported field types will not be parsed or preserved.
• Sub-messages: Sub-messages are parsed recursively, but the type information is lost.
• Repeated Fields: Repeated fields are not explicitly handled.

Alternatives to Parsing Unknown Messages

In most scenarios, it's preferable to approach the issue of unknown messages from a different perspective, such as:

• Mapping to a Generic Message: Create a generic message type that can encapsulate all possible fields and map the unknown message to this generic type.
• Using Any Proto: Consider using the Any proto type to encapsulate unknown messages and preserve type information for later retrieval.

The approach described in this article provides a way to extract basic information from unknown protobuf messages, but it should be used as a fallback solution when other options are not feasible.

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