[vezzy-fnord]

Right, if the problem is slow starters (say, RabbitMQ), then restoring from an image where initialization has already been complete would resolve it. For more complicated cases where there is a race with acquiring some resource, then a readiness notification mechanism could be supported as a niche feature. I'm still not convinced of it in the general case.

[skarnet]

But how would you go with restoring such an image? It won't be instant, there will still be a window where the service manager is continuing its work but the image hasn't been restored yet. One way or the other, you need a synchronization mechanism, so how would you design such a mechanism without support from the daemon?

You don't need a complicated case to have a race with acquiring some resource. A server opening a socket is providing a resource; clients connecting to that socket are using it. Clients should not be started before the server is listening to the socket. How would you solve this case - are you advocating socket activation by the service manager? ;)