* refactor(tunnel): remove nil tunnels hack
This code was originally introduced because a tunnel could be
nil in session.go. I have verified that every invocation of
tunnel.Start is careful to ensure that we have a tunnel name
and that we don't manipulate a nil tunnel.
For this reason, I'd rather remove this tricky bit of code and
further simplify the tunnel code.
Part of https://github.com/ooni/probe/issues/985
* even better docs
This diff breaks the circular dependency between session and
tunnel, by introducing the concept of early session.
An early session is a session that is able to fetch the psiphon
configuration file _only_ if it's embedded in the binary.
This breaks `miniooni --tunnel=psiphon` for users who have
access to the OONI backend. They are not the users we are
writing this feature for, though, so I think this is reasonable.
At the same time, this opens up the possibility of creating
a psiphon tunnel when constructing a session, which is the
approach I was following in https://github.com/ooni/probe-cli/pull/286.
This work is part of https://github.com/ooni/probe/issues/985.
Once this diff is in, I can land https://github.com/ooni/probe-cli/pull/286.
* feat(tunnel): introduce persistent tunnel state dir
This diff introduces a persistent state directory for tunnels, so that
we can bootstrap them more quickly after the first time.
Part of https://github.com/ooni/probe/issues/985
* fix: make tunnel dir optional
We have many tests where it does not make sense to explicitly
provide a tunnel dir because we're not using tunnels.
This should simplify setting up a session.
* fix(tunnel): repair tests
* final changes
* more cleanups
We're trying to remove a circular dependency between the measurement
Session and the tunnel package. To this end, continue to reduce the
dependency scope by providing TorArgs and TorBinary directly.
Part of https://github.com/ooni/probe/issues/985
