How to Handle Unmarshaling of Unknown Protobuf Messages?

Protobuf Unmarshal: Handling Unknown Messages

In scenarios where the message type in a protobuf payload is not known in advance, unmarshaling into an interface{} type is not feasible as it requires an implementation of the protobuf message interface.

Alternative Approach

When at least partial information accompanying the protobuf payload is available, such as a string or number, you can use this as a key to instantiate the appropriate concrete protobuf type and then perform the unmarshaling.

Handling Truly Unknown Messages

If all information about the message type is absent, the protowire package can be utilized to extract limited information from the wire format of the protobuf payload.

Risks and Limitations

Parsing unknown proto messages presents certain challenges:

• Ambiguous Representation: The wire format can be ambiguous, making it difficult to determine the exact type of certain data structures.
• Loss of Subfield Information: Wrapped messages can lose context about wire type and tag number, potentially limiting the ability to reconstruct the original message structure.
• Exclusion of Non-Proto Types: Non-proto data such as wrapped JSON or XML may not be captured accurately.

Sample Implementation

The code below provides an example of parsing an unknown protobuf message:

package main

import (
    "fmt"

    "github.com/jhump/protoreflect/desc"
    "github.com/jhump/protoreflect/internal/strategy"
    "github.com/jhump/protoreflect/protopath"
)

// Field represents an individual element parsed from an unknown protobuf message.
type Field struct {
    Tag   Tag
    Val   interface{}
    Length int
}

// Tag represents a field tag with its number and wire type.
type Tag struct {
    Num  int32
    Type int
}

// parseUnknown parses an unknown protobuf message.
func parseUnknown(b []byte, parentMsg *desc.MessageDescriptor) []Field {
    parser := &protowireParser{msg: parentMsg, buf: b}
    return parser.run()
}

type protowireParser struct {
    msg *desc.MessageDescriptor
    buf []byte
}

const wireTypeMask = 0x7

func (p *protowireParser) run() (fields []Field) {
    for len(p.buf) > 0 {
        field, l, err := protowire.ConsumeField(p.buf)
        if err != nil || l < 1 {
            log.Printf("Error parsing field: %v", err)
            p.buf = nil
            return
        }

        tag := Tag{Num: field.Tag, Type: field.WireType & wireTypeMask}

        var v interface{}
        switch field.WireType & wireTypeMask {
        // ... handle different wire types
        }

        fields = append(fields, Field{Tag: tag, Val: v, Length: l})
        p.buf = p.buf[l:]
    }

    return fields
}

Conclusion

Parsing truly unknown protobuf messages is a complex task, but by leveraging the protowire package, you can extract limited information that may be sufficient for your specific scenario. Remember, this approach is not suitable for production-grade applications where accurate representation is critical.

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