If we received a wg engine status while processing an auth URL, there was a
race condition where the authURL could be reset to "" immediately after we
set it.
To fix this we need to check that we are moving from a non-Running state to
a Running state rather than always resetting the URL when we "move" into a
Running state even if that is the current state.
We also need to make sure that we do not return from stopEngineAndWait until
the engine is stopped: before, we would return as soon as we received any
engine status update, but that might have been an update already in-flight
before we asked the engine to stop. Now we wait until we see an update that
is indicative of a stopped engine, or we see that the engine is unblocked
again, which indicates that the engine stopped and then started again while
we were waiting before we checked the state.
Updates #17388
Signed-off-by: James Sanderson <jsanderson@tailscale.com>
Co-authored-by: Nick Khyl <nickk@tailscale.com>
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