Creating too many fetches at once can return {} data and thus give the illusion of fewer datastreams

[C-Loftus]

If you add force_fetch = True to some of tests and then run everything at once with make test it is possible for some tests to fail by having fewer coverages in the covjson than are expected. This is since the server is presumably overwhelmed and returns an empty response. If you reduce the range or query a smaller subset it works back as expected.

There is not a great way to detect this since the behavior of the server on overload is ill defined.Sometimes it errors, sometimes it returns {} sometimes, gives a maintenance message. Would need to check all these cases.