Workaround failing test on Alpine Linux

[MichaelChirico]

Stop-gap for #6350 not to block CRAN release.

Idea to check if /etc/alpine-release exists taken from https://serverfault.com/questions/805389/which-os-is-running-in-my-docker-container#comment1290522_805390.

Here's some more references to that file:

• https://endoflife.date/alpine
• https://superuser.com/a/1163235/454554
• https://github.com/search?q=%2F%5C%2Fetc%5C%2Falpine%5B-%5Drelease%2F&type=code

cc @bastistician

[bastistician]

I'd suggest to fix this for these C-locale tests just like in the tests 1590.12/14 below:
if x1==x2 (as seen on Alpine Linux), then test that forderv(c(x2,x1,x1,x2)) gives integer(), otherwise INT(1,4,2,3).

[tdhock]

I'm not sure I really understand the core issue this fixes, but I guess it is OK to skip a couple of tests on one platform, temporarily. I would suggest keeping the issue open and hopefully @bastistician can help suggest a longer term fix/PR?

[MichaelChirico]

it really takes some heavy focus time for me to wrap my head around encoding issues, so I'd rather kick the can here/not hold up release waiting for a fix. leaving the issue open to revisit for 1.17.0

[bastistician]

hopefully @bastistician can help suggest a longer term fix/PR?

Sure, see my suggestion above, i.e., test the forderv() result depending on x1==x2.
Conditioning on Alpine Linux specifically will make my check system happy but won't help other musl-based distros (where I'd expect the same test failures).

[MichaelChirico]

I agree it's better, but it's not a priority at the moment. That code has been in data.table since 1.11.4 (2018: 2b4d73613), with yours the first user report.