Fix pickle protocol

[olebole]

Protocol version 4 is supported since Python 3.4 and the default starting with Python 3.8.

Fixing the protocol version allows to downgrade the Python version. To make it more robust, also trigger a re-build of the cache entry if the cache was written by an unsupported protocol version.

Fixes #142.

[jehturner]

Thanks, Ole!

This fixes my problem with starting PyRAF under 3.7 after initially creating the cache under 3.9 (both with this branch). However, if I start PyRAF under 3.7 with an existing cache entry for "gemini" that was created using 2.2.1 under Python 3.8, I get this when trying to load the package:

PyRAF 2.2.1.dev53+g50abdfe
Python/CL command line wrapper
  .help describes executive commands
--> gemini
Traceback (innermost last):
  File "<CL script CL1>", line 1, in <module>
KeyError: 'fd14f25516d1b332bba797340d93d9494c'

Not sure how much we care about that (I have a meeting now, so don't have much time to think about it) but it doesn't seem as intended?

[jehturner]

The above error (from the new try ... except, at L113) only occurs when using PyRAF 2.2.1 on Python 3.8/3.9, followed by this branch on 3.7. The comment suggests that the KeyError will cause the cache entry to be rebuilt, but apparently that is not happening?

[olebole]

The idea was that when the id is not there, the old version is overwritten by one with a new pickle version. This however appears not to work.
Then, probably the best solution is to add the pickle protocol to the version string. Can you try this out again?

[jehturner]

That version seems fine, thanks (on Linux). I suppose it will mean more old duplicate entries sitting around, but I never really see more than a few MB and people can delete them if they want to.

[olebole]

That is true; however this will happen also when we introduce another incompatible change (which changes the version of the cache). And I think that we will very rarely change the pickle protocol in future.

[temuller]

Hi! I don't think this has been added to the latest version, right? pip installs pyraf v2.2.1, but this fix is not included.

[olebole]

No, it is not included in 2.2.1. I didn't make a new release because downgrading Python seemed a rare case, and one can work around the problem by removing the cache after downgrading.