packages: fix up parse_accept_language()

[allisonkarlitskaya]

Make several fixes to our handling of the Accept-Language: header:

• we used to crash if q= contained invalid values due to an unchecked cast with float(). Catch that error and ignore the entry in that case.

• our handling of English was incorrect. We would look for and fail to find po.en.js, and move on to the next item in the list. That means if the user listed English first, followed by another language which we did support, they'd see Cockpit in that other language, which is unexpected. This is a regression introduced by f4be906. Now we drop all items that sort after English.

• our handling of fallbacks (ie: 'de' from 'de-de') was incorrect. RFC4647 §3.4 says that a "Lookup" should consider items in the order they're found, stripping each item down to its base form, before considering the next item. This passes a gut check, as well: a user who lists de-de, nl probably expects to see a German translation before a Dutch one.

We also now mark the parser code as @lru_cache. This makes sense: within a given run of cockpit-bridge, we're likely to see very many copies of the Accept-Language header, and they're practically always going to be identical. Make sure we now accept and return immutable types to prevent weird side-effects.

While we're at it, up our testing game a bit to make sure we don't mistreat requests for 'English' in the future.

Closes #19526

[allisonkarlitskaya]

Note: this change means that a language list like en-ca, en-gb, en is entirely pointless. After en-ca is considered, en will be directly considered after it, effectively ending the search.

It also means that lists like de-at, de-ch are almost pointless: after we fail to find de-at we'll immediately try to find de (which will typically be there). de-ch won't be considered in that case. This is the correct behaviour per RFC 4647.

[allisonkarlitskaya]

Traceback (most recent call last):
  File "/work/bots/make-checkout-workdir/test/verify/check-storage-stratis", line 869, in testBasic
    m.execute(f"stratis pool stop {self.stop_type_opt} pool0")
                                   ^^^^^^^^^^^^^^^^^^
AttributeError: 'TestStorageStratisNBDE' object has no attribute 'stop_type_opt'

hihi #19525

[martinpitt]

Very nice, thanks for fixing this! I'd like to see some more edge cases covered in tests, but code looks good!

[martinpitt]

One interesting test case would be en;q=0.9, de. Intuitively that should result in "de", but given how my intuition was already fooled before by the "insert fallbacks with no country" before (which feels actively wrong for some cases), we better pin it down?

[martinpitt]

Cheers!