Changed behavior of (*DecodingLayerParser).DecodeLayers on empty data in v1.1.18

[tklauser]

Before v1.1.18, the following code would not print any message, i.e. dlp.DecodeLayers returns a nil error:

import (
    "fmt"

    "github.com/google/gopacket"
    "github.com/google/gopacket/layers"
)

func main() {
        dlp := gopacket.NewDecodingLayerParser(layer.LayerTypeEthernet, &layers.Ethernet{}, &layers.IPv4{}, &layers.TCP{}, &layersUDP{})
        decoded := []gopacket.LayerType{}
        empty := []byte{}
        err := dlp.DecodeLayers(empty, &decoded)
        if err != nil {
                fmt.Printf("Error from dlp parser: %v\n", err)
        }
}

In v1.1.18 this behavior changed and an error is now returned from the first layer, i.e. layers.Ethernet in this case:

Error from dlp parser: Ethernet packet too small

This seems to be due to 04f6565 (PR #660), namely the following change:

@@ -153,23 +304,15 @@ func (l *DecodingLayerParser) DecodeLayers(data []byte, decoded *[]LayerType) (e
        if !l.IgnorePanic {
                defer panicToError(&err)
        }
-       typ := l.first
-       *decoded = (*decoded)[:0] // Truncated decoded layers.
-       for len(data) > 0 {
-               decoder, ok := l.decoders[typ]
-               if !ok {
-                       if l.IgnoreUnsupported {
-                               return nil
-                       }
-                       return UnsupportedLayerType(typ)
-               } else if err = decoder.DecodeFromBytes(data, l.df); err != nil {
-                       return err
+       typ, err := l.decodeFunc(data, decoded)
+       if typ != LayerTypeZero {
+               // no decoder
+               if l.IgnoreUnsupported {
+                       return nil
                }
-               *decoded = append(*decoded, typ)
-               typ = decoder.NextLayerType()
-               data = decoder.LayerPayload()
+               return UnsupportedLayerType(typ)
        }
-       return nil
+       return err
 }

We have code that depends on this behavior and strictly speaking this is an API breakage, so my suggestion would be to keep returning nil for empty data. The other option of course is to check len(data) != 0 before calling (*DecodingLayerParser).DecodeLayers in our but this would be rather cumbersome.

/cc @notti @yerden