Enable Decode As for custom Lua dissector payload

edit

I have a custom dissector written in Lua. One of the fields in my protocol is a payload which may be possible to analyze with a secondary dissector. I would like the user to be able to choose the secondary dissector in the Decode As dialog.

local proto = Proto("myproto", "My Protocol")
proto.fields.payload = ProtoField.bytes("myproto.payload", "Payload")

My understanding is that I need to create and use a DissectorTable:

local dissector_table = DissectorTable.new(
    "myproto.payload",
    nil, -- use default description
    ftypes.NONE
)

function proto.dissector(tvb, pinfo, tree)
    -- Dispatch payload to sub-dissector
    local dissector = dissector_table:get_dissector()
    if dissector then
        dissector:call(tvb:range(0), pinfo, tree)
    end
end

Then, in the second dissector, I write:

local proto2 = Proto("my2ndproto", "My 2nd Protocol")
...

pcall(require, "myproto") -- make sure we load second
local dt = DissectorTable.get("myproto.payload")
if dt then
    dt:add_for_decode_as(proto2)
end

This gets me to the point where I can set up a rule in the Decode As dialog, to dissect myproto.payload with MY2NDPROTO.

However, dissector_table:get_dissector() always returns nil.