Ignore failed move of cache index and warn.

[jgosmann]

Motivation and context:
The replace respectively move can fail on Windows due to some interaction with anti-virus software (see #1200 for details). This PR ignores the failure and produces a warning.

Interactions with other PRs:
I would like to include a link to some documentation on how to disable the cache in the warning, but I need #1130 to be merged for that.

How has this been tested?
I haven't tested this because testing stuff on Windows is always very time consuming and annoying for me. But maybe @arvoelke is so nice to give it a test run? 🙏

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING.rst document.
• [n/a] I have updated the documentation accordingly.
• I have included a changelog entry.
• [n/a] I have added tests to cover my changes. (Hard to do because the code in question is only used on very specific systems and the problem can be only reproduced under certain conditions.)
• All new and existing tests passed.

[jgosmann]

It would probably be nice to have this in a 2.3.1 bugfix release.

[Seanny123]

LGTM, pending on this working on Aaron's system which experienced the problem in the first place.

[arvoelke]

c:\users\aaron\ctn\nengo\nengo\cache.pyc in _write_index(self)
    404             # moved. There is not a lot we could do about this. See
    405             # <https://github.com/nengo/nengo/issues/1200> for more info.
--> 406             warnings.warn(CacheIndexReplaceFailedWarning(self))
    407         if os.path.exists(self.legacy_path):
    408             os.remove(self.legacy_path)

TypeError: expected string or buffer

[jgosmann]

Made a fixup commit, hopefully it works now.

[arvoelke]

It worked. 🎆 Do we have an open issue for documenting how to disable the cache? Point 3 in the warning is currently not so helpful for the average user since googling "nengo disable cache" or searching our pydocs for "cache disable" brings up nothing.

[jgosmann]

👉 Please read “Interactions with other PRs” of the PR description.

[tbekolay]

Pushed some commits to rearrange a bit and add a test. If they're OK with @jgosmann I'll merge; will add a possible 2.3.1 release to the dev meeting agenda.

[jgosmann]

Looks fine to me. Not sure if the added test is necessary or useful. It seems to test implementation details to some degree (whether replace will actually be used) and it almost monkey-patches in the desired behaviour (though, I suppose it is still testing that the exception is converted to a warning).

[jgosmann]

@tbekolay friendly reminder to consider this for inclusion in 2.3.1

Also, there is PR #1264 which seems to do about the same thing? Not sure why I opened that one ... maybe I just forgot that I already fixed the problem ...?

[tbekolay]

Thanks for the ping, will look at this now, thanks!

[tbekolay]

Looks good! I'll add the changelog entry in the merge.