I have enjoyed keeping running logs of my coding work (devlogs) in the past, and am going to start doing those again now.

This ‘holiday’ season, I am spending time teaching myself skills I sortof know about but do not have a deep understanding of.

## JupyterLab extension

I spent the first part of the day (before I started devlogging) working on finishing up a jupyterlab extension I started the day before. It lets you edit notebook metadata. I got started since I wanted to use Jupytext for my work on publishing mybinder.org-analytics.

TypeScript was easy to pick up coming from C#. I wish the phospor / JupyterLab code had more documentation though.

I ran into a bug. While following instructions to set up a JupyterLab dev setup, I somehow managed to delete my source code. Thankfully I got most of it back thanks to a saved copy in vscode. It was a sour start to the morning though.

I’ll get back on to this once the sour taste is gone, and hopefully the bug is fixed :)

## asyncio: what’s next | Yuri Selivanov @ PyBay 2018

I’ve been trying to get a better handle on asyncio. I can use it, but I don’t fully understand it - I am probably leaving bugs everywhere…

From one of the asyncio maintainers. Gave me some impetus to push the default version of Python on mybinder.org to 3.7 :D I’m most excited about getting features from Trio & Curio into the standard library. Was good to hear that nobody can quite figure out exception handling, and not just me.

I discovered aioutils while searching around after this. I’ve copy pasted code that theoretically does the same things as Group and Pool from aioutils, but I’ve no idea if they are right. I’ll be using this library from now!

## Processing Signals

I’m writing a simple process supervisor library to replace the janky parts of nbserverproxy. It should have the following features:

1. Restart processes when they die
2. Propagate signals appropriately
3. Support a sense of ‘readiness’ probes (not liveness)
4. Be very well tested
5. Run on asyncio

This is more difficult than it seems, and am slowly working my way through it. (1) isn’t too difficult.

(2) is a fair bit more difficult. atexit is useless since it doesn’t do anything with SIGTERM. So I need to manage my own SIGTERM handlers. However, this means there needs to be a centralish location of some sort that decides when to exit. This introduces global state, and I don’t like that at all. But unix signals are global, and maybe there’s nothing for me to do here.

I initially created a Supervisor class that holds a bunch of SupervisedProcess’s, but it was still calling sys.exit in it. Since signals are global, I realize there’s no other real way to handle this, and so I made a global handler setup too. This has the additional advantage of being able to remove handlers when a SupervisedProcess dies, avoiding memory leaks and stuff.

Testing this stuff is hard!

I also need to make sure I don’t end up with lots of races. I’m still writing concurrent code, even without threads. Gotta be careefull. Especially with signals thrown in. Although I guess once you get a SIGTERM or SIGINT inconsistent state is not particularly worrysome.