nixos/systemd-boot: fix systemd-boot-builder refusing to update

[waldheinz]

Handling of the string length condition in should_update
was broken, as evident with the log message

leaving systemd-boot 246 in place (250.4 is not newer)

when updating an old NixOS system.

Fixes a30de3b

Sorry, I goofed up following the code in bootctl.c

[waldheinz]

Ping and sorry for the noise, @mweinelt

[mweinelt]

What is it with those length checks anyway? What are you trying to port? I think we can just rely on that last return statement.

>>> "" < "250.4"
True
>>> "246" < "250.4"
True
>>> "246.1" < "250.4"
True
>>> "246.99" < "250.4"
True
False
>>> "250.3" < "250.4"
True
>>> "250.5" < "250.4"
False

[waldheinz]

I tried to implement what that compare_product function does. I really don't know what's the purpose, I just wanted to copy the functionality and failed.

[mweinelt]

The upstream functions returns a tristate (-1, 0, 1) while we return a bool. Our function is also semantically different, which is fine. If you can't come up with a reason to keep the length checks I'm for dropping them.

When should we update?

• to is newer (>) than from

When shouldn't we update?

• to is the same (=) as from
• to is older than (<) from

That's basically only updating when to > from, which is the return statement.

[waldheinz]

I agree, will update the PR tomorrow. It's unlikely that anyone is hurt by the current state because only updates are affected and the old bootloader was obviously good enough for the system.

[github-actions]

Successfully created backport PR #175728 for release-22.05.