Exit from VirtioNet:pull if link is nil

[dpino]

Consider the following script that forwards packets from one nic to another one. NICs use VirtioNet driver (Snabb Switch running within a guest):

module(..., package.seeall)

local VirtioNet = require("apps.virtio_net.virtio_net").VirtioNet
function run(args)
   local pci1, pci2 = unpack(args)
   local c = config.new()
   config.app(c, "eth0", VirtioNet, {
      pciaddr=pci1,
   })
   config.app(c, "eth1", VirtioNet, {
      pciaddr=pci2,
   })
   config.link(c, "eth0.tx -> eth1.rx")
   engine.configure(c)
   engine.main()

The program breaks due to an error in core/link.lua:

core/link.lua:80: attempt to index local 'r' (a nil value)
stack traceback:
        core/main.lua:126: in function '__index'
        core/link.lua:80: in function 'nreadable'
        core/link.lua:88: in function 'nwritable'

The problem is that although there's only one link configured, the engine executes VirtioNet:pull in eth1, and the output link is nil, so a checking that exits pull in that case should be added. I noticed the same checking is present in intel_app.lua, and there's a another one for the push case. I couldn't reproduce a similar test for push, so I didn't add it.

In my opinion, a pull in eth1 should never be executed as it's not present on the left side of a link, but maybe I'm missing other reasons while it's being called.

[mention-bot]

By analyzing the blame information on this pull request, we identified @nnikolaev-virtualopensystems to be a potential reviewer

[eugeneia]

LGTM. @nnikolaev-virtualopensystems any objections?

[ghost]

I'm OK with this.