Generate a secret key file, and load SECRET_KEY from said file.

[aronasorman]

Fixes #4213. We already have said vulnerability wherein the SECRETT_KEY is both public and the same for each installation. With the introduction of sessions-in-cookies it's a double whammy.

So now the solution is to create a secret key file for each installation if it's not detected, and load that as the value of SECRET_KEY.

Assigning @jamalex, but @benjaoming please take a look too.

[benjaoming]

Why the backport for 0.14?

[benjaoming]

Sorry, why is this not a backport but straight for 0.14?

[aronasorman]

re: backport -- I just plan to cherry pick this single commit to 0.15. Unless you have other preferred method of propagating this change?

[aronasorman]

from django.conf import settings
>>> settings.SECRET_KEY
'MjFiOTJmYzYtNGYyYS00MWViLTk5ZmMtZTUxZGU3NGU1NTZl'

[jamalex]

Looks good on my side, but made one suggestion for robustness/pythonicness.

[aronasorman]

@jamalex @benjaoming comments addressed. Main changes:

• moved secret key functions to its own file. it's under kalite/project/settings/_utils.py.
• changed the way we detect SECRET_KEY_FILE to catching exceptions and then generating secret keys from that.
• saving the secret key is optional. If we can't save it, then we just keep generating a new one every server restart. This will mean that users are logged out every restart, which @EdDixon wants. Might wanna add a flag for that moving forward.

[aronasorman]

@benjaoming added sys.stderr.write calls.

[jamalex]

Looks good to me -- might be good to test 1 or 2 weird edge cases before merging.

[aronasorman]

Tried with secretkey.txt as a directory. It prints out this in stderr, then continues:

In the future, it is recommended not to keep your own settings module in the kalite code base but to put the file somewhere else in your python path, for instance in the current directory when running 'kalite --settings=my_module'.
  RemovedInKALite_v015_Warning
Error reading secret key file. Generating one now. Error was: [Errno 21] Is a directory: '/home/aron/src/github.com/aronasorman/ka-lite/secretkey.txt'
Error writing secret key file. Error was: [Errno 21] Is a directory: '/home/aron/src/github.com/aronasorman/ka-lite/secretkey.txt'

It's expected behaviour.