Does not reply on dbus when busy (e.g. downloading)

We ran into the issue that system-image-dbus would sometimes not reply on snappy. After long debugging and deep dive into the go-dbus code it seems that system-image-dbus simply does not reply when its busy, at least in the snappy branch.

This may well be fixed in the 3.0 branch so here is a test and it would be great to know if that works with the curl backend and the 3.0 branch:

Ensure there is something for s-i-dbus to do (e.g. download new image):
$ rm -f /writable/cache/*.tar.xz

$ dbus-send --system --dest=com.canonical.SystemImage --type=method_call --print-reply /Service com.canonical.SystemImage.CheckForUpdate && sleep 1 && dbus-send --system --dest=com.canonical.SystemImage --type=method_call --print-reply /Service com.canonical.SystemImage.DownloadUpdate && sleep 1 && dbus-send --system --dest=com.canonical.SystemImage --type=method_call --reply-timeout=5000 --print-reply /Service com.canonical.SystemImage.Information

Running the above gives me:
"""
method return sender=:1.54 -> dest=:1.53 reply_serial=2
method return sender=:1.54 -> dest=:1.55 reply_serial=2
Error org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
"""

So it appears the curl backend is not quite there for snappy yet.