Lesson 09 · Skipping a field by wire type

Skipping a field by wire type

A decoder must be able to step over a field it does not care about, and to do that it only needs the wire type, not the schema. Today you build Skip, which consumes exactly one value of each wire type and rejects an unknown one.

The goal

Skip a field value given its wire type, consuming the right number of bytes.

Start here - the target

TO DO

Scenario: Each wire type is skipped by consuming its value

Givena reader positioned at a field value and its wire type

WhenSkip is called for wire type 0 over the bytes 0x96, 0x01

Thenboth varint bytes are consumed and the position advances by 2

Andwire type 5 (I32) advances by 4, wire type 1 (I64) advances by 8, wire type 2 (Len) consumes its length prefix plus that many bytes, and an unknown wire type such as 3 returns an error

Background

Forward and backward compatibility depend on one small ability: when a decoder meets a field it was not built to understand, it can skip it and keep going. Skipping needs only the wire type. A Varint value is skipped by reading a varint and discarding it; an I32 is always 4 bytes and an I64 always 8, so you advance the cursor by a fixed amount without interpreting them; a Len value is skipped by reading its length and stepping over that many bytes, which ReadBytes already does.

Any wire type outside the set 0, 1, 2, 5 - including the retired group markers 3 and 4 - is not something this decoder can consume, so Skip must return an error rather than guess. This makes Skip the natural place to catch a bad wire type, and it is the exact machinery the final chapter reuses to preserve unknown fields: to keep an unknown field you first need to know precisely where its bytes end, which is what advancing correctly by wire type tells you.

Make it work

func (r *Reader) Skip(wire int) error {
  switch wire {
  case WireVarint: _, err := r.ReadVarint(); return err
  case WireI64:    return r.advance(8)
  case WireI32:    return r.advance(4)
  case WireLen:    _, err := r.ReadBytes(); return err
  default:         return fmt.Errorf("bad wire type %d", wire)
  }
}

CheckpointDONE

You can skip any known field and reject an unknown wire type. Commit and stop here.