m-lab / locate / 21039861677

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refactor(locatetest)!: allow returning valid URLs (#240)

The current implementation of the `locatetest` package does not allow to return valid URLs. As such, mocking a realistic locate response is not possible and testing is less realistic.

To address this, I propose to provide the `*TargetInfo` to the fake server directly. This design allows for maximum flexibility at the cost of slightly more sweating in the tests that use this package, for they would need to provide a fully populated response. (Though I added a constructor for the common case of `ndt7`, which can also serve as a reference implementation for creating the proper mocks elsewhere.)

Yet, this is probably the most flexible compromise in terms of decoupling the ability to run a local, fake locate and specifying what each specific user of `locatetest` needs.

BREAKING CHANGE: users of `locatetest.LocatorV2` now need to specify the full `*TargetInfo` as opposed to just passing the list of servers or use the convenience `NewTargetInfoNdt7` constructor (if that fits your needs). The following code illustrates a reasonable upgrade path that produces a valid ndt7 response:

```Go
// Change this
server := locatetest.NewLocateServerV2(&locatetest.LocatorV2{
    Servers: []string{"127.0.0.1"},
})

// Into this
server := locatetest.NewLocateServerV2(&locatetest.LocatorV2{
    TargetInfo: locatetest.NewTargetInfoNdt7("127.0.0.1"),
})
```

If you need to mock a `/v2/nearest` response for a different experiment, you should adapt `NewTargetInfoNdt7` to your use case. In such a case, we recommend comparing to the actual locate output to make the response realistic.

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