emit() does not support structs

[vgurevich]

According to p4_16 specification:

For derived type, emit recursively proceeds on fields:
. . .
If the argument is a struct containing multiple fields, the emit is recursively applied to each component of the struct in the order of their declaration in the struct.

However, the attempt to pass a struct to emit results in the following message:

struct my_headers_t {
    ethernet_t     ethernet;
    vlan_tag_t [2] vlan_tag;
    mpls_t [5]     mpls;
    arp_t          arp;
    arp_ipv4_t     arp_ipv4;
    ipv4_t         ipv4;
    icmp_t         icmp;
}

. . . 

control MyDeparser(
    packet_out      packet,
    in my_headers_t hdr)
{
    apply {
        packet.emit(hdr);
   }
}

results in the following message:

l3_switch.p4(413): error: hdr: emit only supports header and stack arguments, not struct my_headers_t
        packet.emit(hdr);
                    ^^^
l3_switch.p4(110)
struct my_headers_t {
       ^^^^^^^^^^^^

[mihaibudiu]

There is a PR queued with this feature.

[mihaibudiu]

I lied, the PR is not yet submitted, it is only a commit in my repo, until I figure out some issues with BMv2.
I may split it into two PRs if a solution for this is needed soon.

[vgurevich]

It is definitely an extremely useful feature that can relieve a lot of people from the necessity of spending time writing trivial deparsers. Once we allow header_unions in structs it will be doubly-useful and will also make code a lot safer.

I wanted to demonstrate it in the training slides, but... So far, I decided to keep the slide, but put a "Not Implemented yet" note :(