EAPI 3: Profile Controlled IUSE Injection

This is part of a series on EAPI 3.

EAPI 3 makes some fairly substantial changes to how use flags are handled inside the package manager. For developers, this requires a certain amount of attention; for users, there’s probably not much visible difference.

Historically, core Portage didn’t really do much with USE flags. They were used by conditional dependency specifications, but that was it — things like the use function were just bits of shell script that looked at the USE variable, which mostly just came directly from the user’s configuration. Similarly, IUSE was originally only used for cosmetic purposes — Portage used it to decide which flags to display in output, but didn’t use it to restrict USE in any way. Thus, packages could query use flags that weren’t listed in IUSE and still expect things to work.

USE_EXPAND gets even more complicated. Developers often didn’t bother even trying to list such flags in IUSE, leading to much confusion and strange behaviour.

This is a bit of a nuisance. It makes later user-facing functionality like “has this USE flag changed?” queries much harder to define, and it severely cripples use dependency defaults (was a flag not present for a given version, and thus should the default be used, or was it present but just not listed?). EAPI 3 fixes this.

In EAPI 3, IUSE is strictly enforced. If a flag isn’t listed in IUSE (and isn’t covered by the exceptions detailed later), the package manager will filter it from USE and any relevant USE_EXPAND variable, and make the use functions die if the flag is used in a query.

The problem is, Gentoo developers don’t tend to like the idea of having to explicitly list ARCH flags like x86 or certain USE_EXPAND flags like userland_GNU in IUSE. There are two reasons given for this:

• The flags shouldn’t be user visible.
• It’s lots of effort.

The first argument is mostly invalid; we already have USE_EXPAND_HIDDEN, and EAPI 3 is tweaking the definition of USE_EXPAND to let us treat ARCH as a USE_EXPAND value that isn’t prefixed with arch_ like it otherwise would be, allowing us to hide those too.

The second argument may or may not have merit; Gentoo’s gone for the “at least keep the possibility that we’ll avoid having to make people explicitly list things open” approach (at the expense of use dependency defaults not being usable on anything that’s treated that way), whilst Exherbo’s gone for “list everything”.

Here’s (roughly — see PMS for the gory details) how the compromise works:

• The package manager works out something we’ll call IUSE_EFFECTIVE (which may or may not be an ebuild-visible variable, is not an ebuild-modifiable variable and which will end up in VDB). It contains everything in IUSE, along with profile-injected extra values.
• Any value in the profile IUSE_IMPLICIT variable is treated as being in IUSE_EFFECTIVE. This would let the much-abused build flag carry on working; it has not yet been decided whether any values will be set this way in the profile.
• Any USE_EXPAND or USE_EXPAND_UNPREFIXED variable whose name appears in the profile USE_EXPAND_IMPLICIT variable is treated as having its values implicitly listed. This will probably contain USERLAND, KERNEL, ARCH etc, but not LINGUAS et al. For any USE_EXPAND_IMPLICIT variable FOO, the profile USE_EXPAND_VALUES_FOO variable lists its (unprefixed) implicitly injected values.
• IUSE_EFFECTIVE is thus a fixed, well defined set rather than an infinite set as it is in older EAPIs. Any value that doesn’t end up in IUSE_EFFECTIVE cannot be queried in any way, and is treated as ‘missing’ for use dependency defaults. Any value in IUSE_EFFECTIVE, whether or not it is explicitly in IUSE, is ‘present’ for use dependency defaults.

For developers, all you really need to know is that you need to list things in IUSE if you want to use them, except if they’re in a small group of special values that will likely contain ARCH, USERLAND and KERNEL flags and possibly things like build.