Avoid a deadlock updating the network D-Bus tree

[imobachgs]

Problem

If the network service receives an Apply at the same time as any other request, it may get blocked.

Solution

Updating the tree must be done in a separate task. Otherwise, it could fight with an incoming request for the ObjectServer mutex, blocking the actions dispatching.

Using tokio-console

I decided to use tokio-console to make it easier to find the problem. This tool collects and displays information about the asynchronous tasks in your program. In the screenshot below you can see that the D-Bus Set dispatchers are blocked.

Once you find the problem, it is kind of "obvious", but it may take some time until you figure out what is happening.

To use tokio-console you need to add some instrumentation to your program and, at this point, it implies adding some unstable Tokio APIs. So if you want to give it a try, you can try the code in the tokio-console branch.

[coveralls]

coverage: 74.997% (-0.01%) from 75.009%
when pulling c40e251 on avoid-dbus-tree-deadlock
into e4ae7ed on master.

[teclator]

LGTM

[teclator]

Even better now ;)