Add way to set login cookie expiration

[ericdwang]

Proposed change

The cookie_max_age_days option can be used to set how long the login cookie signature is valid for, but there's no way to set when the cookie itself actually expires. Currently it just uses Tornado's default of 30 days, so setting cookie_max_age_days to a value greater than 30 also won't work as expected (the cookie will expire before the signature does).

Alternative options

If not being able to change the cookie expiration is intentional then it'd be helpful to just document that for cookie_max_age_days.

Who would use this feature?

Anyone who wants to change when the login cookie expires or set cookie_max_age_days to a value greater than 30.

(Optional): Suggest a solution

expires_days just needs to be passed in when calling set_cookie() here, similar to how max_age_days is passed in to get_secure_cookie() here. That's what the Tornado docs recommend for changing signed cookie expirations also. This could be a new setting or just the same value as cookie_max_age_days, not sure if there's a benefit to making them different. Happy to open a PR with whatever's decided on!

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[minrk]

I believe you can set this with:

c.JupyterHub.tornado_settings = {"cookie_options": {"expires_days": 7}}

but I agree it probably makes sense to use cookie_max_age_days for the default. Would you like to try a PR?

[rickmcgeer]

I haven't seen a response to @minrk 's suggestion, which I think is a good one. It seems to me that it could be done by modifying BasicHandler.set_hub_cookie to take an extra parameter. I'd be happy to try it and do a PR if anyone is interested in reviewing it. Any comments on the idea?

[minrk]

@rickmcgeer A PR would be very welcome!