Allow multiple back-to-back graceful upgrades about tableflip

[lmb]

I'm glad it's useful :) Doing that will be really difficult I think.

From the POV of the library:

• How are you going to manage the old processes? What will enforce timeouts / maximum drain times?
• How can we ensure that unused Unix sockets are being cleaned up? With multiple processes is seems impossible to figure out when something is unused.

As someone deploying such an application:

• How many concurrent processes are the limit?
• If you expose metrics via a HTTP endpoint or similar you'll probably share this between processes as well. Which process gets to expose its metrics?
• How do you deal with bugs or vulnerabilities in old processes? How do you know which processes are vulnerable?

Ultimately, I think that the tableflip model isn't suitable for this. I guess you could build a separate process that holds sockets and spawns new versions of your service as you go along, but at that point you're reinventing systemd to a degree. I guess you could use tableflip to build this supervisory process if you wanted to.