Add raft tests; backwards compatible time.Time

[swenson]

We are concerned with binary backwards compatibility for this project, especially as the upstream has a history of making breaking changes.

With that in mind, I collected a bunch of data from https://github.com/hashicorp/raft for v0.5.5, v1.1.5, and v1.1.6 of go-msgpack to ensure that, moving forward, we can always decode old raft logs.

For the tests to pass, we have to support decoding time.Time using the encoding/binary marshalling format, which was used in certain v1.1.6.

Also added in the go.sum file, which is needed to build with newer versions of Go.

[banks]

I absolutely love this! This library has caused so much pain over the years just fixing this one issue that caused us to diverge is fantastic.

I love the testing methodology too. I have a couple of suggestions that might make them cleaner to read and maintain:

1. All the binary examples are great but make the tests pretty hard to read. I think in Consul we've been slowly moving large example-based tests like this to use testdata instead where each example payload is a separate file in ./testdata and then the actual test code can be much more easily read and worked on.
2. I wonder if, in addition to these we could add fuzz tests that randomly populate each struct type and then try the full matrix of encode/decode pairs. I've used go-fuzz for this a bunch recently. I did submit a PR for it recently related to how it fuzzes time.Time though so might be worth ensuring that fix is in the version used. This could also be a later PR along with possibly other test structs with field types we don't have in raft?

Just som suggestions. the actual PR here looks awesome already!

[swenson]

@banks excellent ideas! I'll work on simplifying the tests, and this is great excuse for me to learn the fuzzing framework in the latest versions of Go. :)

[banks]

Go's recent fuzzing improvement are awesome, but they probably won't help for this type of test yet as they can only fuzz a few simple types. go-fuzz on the other hand uses reflection to populate any arbtrary nested struct which is great for this style of test still. Here's an example: https://github.com/hashicorp/raft-wal/blob/main/codec_test.go#L20 p.s. the time comment in that test was merged upstream now too so might not be needed any more if they released that or you use head.

[swenson]

@banks I moved all of the verbose test data to separate files in internal/testdata. I left them as Go files since they need to keep the type information, and it makes them slightly easier to read/write.

[swenson]

(I think I'll do fuzzing in a separate PR)

[tomhjp]

I didn't have any of the historical context for this issue, but I've dug around to educate myself and this looks good to me AFAICT. The tests are great 👍

[swenson]

Thanks!

[kisunji]

Late to the party but looks great!