Remove generated message populators from pkg/ttnpb

[htdvisser]

Summary:

The generated message populators in pkg/ttnpb cause more problems than they solve. In my experience they're only causing flaky tests because of failed validations and non-deterministic code coverage because of randomly taken branches.

What do you see now?

• A mix of generated populators (that usually build garbage) and handwritten populators (that to a slightly better job)
• Randomly failing (or randomly passing) Travis builds
• Changes in code coverage without any code changes

What do you want to see instead?

The end goal is that our tests become deterministic, so ideally I'd want to see (handwritten or rule-aware) populators that have correct and deterministic output for a PRNG with the same seed.

How do you propose to implement this?

• Investigate if we can get more deterministic results with a PRNG with a static seed.
• Get rid of generated populators that build invalid structs
• Investigate if we can generate populators with the validation rules as constraints

What can you do yourself and what do you need help with?

I think it's best if @rvolosatovs looked into this, because protoc and ttnpb is his domain. Let me know if you need any help.

[rvolosatovs]

Populators are fully deterministic.
The issue is that in some tests we run those concurrently, which is where the non-determinism comes from.

[nejraselimovic]

@htdvisser is this important enough to leave it scheduled for Q2?

[htdvisser]

We're slowly starting the big refactoring of pkg/ttnpb. The end goal is to migrate away from gogo/protobuf completely, and removing the message populators (that are generated by gogo/protobuf) is one part of that.

I don't really enjoy doing these kind of chores, so I'd rather just get it over with than keep pushing it forward.

[htdvisser]

Pull request #4450 removed all unused populators, so now it's time to remove all usage of the remaining ones, and remove those too. From what I can see, we still use populators in these places:

• pkg/applicationserver/io/pubsub - @adriansmares
• pkg/encoding/lorawan -- @johanstokking – Remove populators from tests #4452
• pkg/gatewayserver/io/udp -- @KrishnaIyer Remove populators from UDP tests #4623
• pkg/identityserver/store -- @htdvisser - Remove populators for Application, Client, Gateway, Organization and User #4488
• pkg/joinserver -- @johanstokking – Remove populators from tests #4452
• pkg/networkserver -- @adriansmares
• pkg/networkserver/mac -- @adriansmares

[johanstokking]

@KrishnaIyer for removing populators from pkg/gatewayserver/io/udp; just remove generatePushData() and instantiate uplink messages as needed.

@adriansmares the stuff in pkg/networkservers seems really easy to clean up; the populators are just there to have a valid state, not to do anything with it.