Packet.tpSrc dies silently with invalid argument

[jcollard]

While working through the Frenetic tutorial (http://frenetic-lang.github.io/tutorials/OxMonitor/) I wrote the following:

let is_http_packet (pk : Packet.packet) : bool =
  Packet.tpSrc pk = 80 && Packet.nwProto pk = 6

When a packet pk would arrive without a valid port, the controller would exit silently without saying why it was exiting.

[jnfoster]

Does it work if you add Packet.dlTyp = 0x800?

[jnfoster]

The packet accessors do fail if certain dependencies are not satisfied. This was an explicit design decision vs. returning optional values.

[jcollard]

It works if I change the order to the following:

  let is_http_packet (pk : Packet.packet) : bool =
    Packet.dlTyp pk = 0x800 && 
    Packet.nwProto pk = 6 && 
    Packet.tpSrc pk = 80

It is fine that it fails, that makes sense. It is that it fails silently. There is no message as to why it failed.

[jnfoster]

Yes, that's a bug.

There is an exception thrown by tpSrc:
https://github.com/frenetic-lang/ocaml-packet/blob/master/lib/Packet.ml#L1112

And it should be caught and printed by the top-level Monitor in the Ox controller:
https://github.com/frenetic-lang/ox/blob/master/lib/OxStart.ml#L90

But evidently that's not happening...