enable a way to run a task when an io_loop is created

[eastonsuo]

usage:
in extensionApp.initialize_handlers:

self.server_app.startup_tasks.append(function)

[Zsailer]

@pumpkindle, can you give us a bit more detail here—maybe an example of why you'd want this?

[eastonsuo]

@Zsailer hello, one way of using this is when I want to create a background async_task when ioloop is ready, for example to create and supervisor a suprocess when ioloop is ready.(an extension may use this feature for such things)

as far as I can see it, the ioloop will be waiting for request after jupyter server is started, and can not use ioloop util then.

[Zsailer]

Interesting. Maybe what makes sense here is to expose the self.io_loop attribute earlier in ServerApp’s lifecycle—i.e. in the initialize step before extensions are loaded.

This would mean adding a self.io_loop = ioloop.IOLoop.current() somewhere in initialize() before load_server_extension() is called. Then, any extension could add background tasks to the ioloop before the server is actually started.

[eastonsuo]

@Zsailer yes, you're right, expose self.io_loop earlier is a good solution

[Zsailer]

expose self.io_loop earlier is a good solution

@eastonsuo, would you like to update your PR with this fix?

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@40c477b). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #531   +/-   ##
=========================================
  Coverage          ?   76.33%           
=========================================
  Files             ?      110           
  Lines             ?     9816           
  Branches          ?     1067           
=========================================
  Hits              ?     7493           
  Misses            ?     1943           
  Partials          ?      380           

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	90.76% <100.00%> (ø)
jupyter_server/serverapp.py	64.95% <100.00%> (ø)

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[Zsailer]

LGTM