Python test script

[sflow]

It looks like it would be fairly straightforward to add a python test script that:

(1) enables sFlow with sampling-rate 1 and polling-interval 1 (2) sends a packet (3) waits a second or so (4) examines the output from "show sflow" to confirm that we have 1 packet sample and either 1 or 2 counter samples (5) turns off sflow again

Does that make sense?

(That would have been enough to catch the problem in issue #9, at least.)

We could expand on it later to include:

• a test for the VAPI queries to linux-cp
• a test for samples of different sizes with different values of header-bytes
• a test to make sure someone ran "modprobe psample" somewhere
• a test to confirm that when we send 100000 packets with sampling-rate=1:100 then the "sample-pool" count is 1000000 and the sample-count is within some probability band.

[sflow]

Added test/test_sflow.py. Adapted from an example in the VPP documentation. It does the first few steps above. It's not clear what sort of warning we should raise if mod_psample has not been loaded into the kernel. It seems like the test should not require that step. On a production VPP device the modprobe might only be done at boot time, so arguably it's out of scope for the VPP test suite? Otherwise we might have to check in something like the github.com/sflow/psampletest code just so a test can use it to verify the NetLink output...

...OK so maybe that doesn't sound completely crazy, but I'm hoping we don't have to go there for now.

The part that is most likely to need maintenance in the future is probably the VAPI query to linux-cp, so may need to learn how a test for one plugin can require that another be loaded.