ea235dc9ee
I believe this change is wrong both theoretically and practically. Theoretically, `null` is available on every platform, because `buildInputs = [ null ];` always succeeds and never throws a platform availability error. `null` should be handled consistently with packages that have no explicit list of supported platforms, as it of course has no such list itself. Practically, we use `null` to represent libraries that are always present on a platform and do not require a library (for instance, because they are part of `libc` or the macOS SDK). This has been used for a long time by `libintl` (on all non‐glibc platforms), and is also now used by `libGL` and friends on Darwin. This change broke the check SDL3 does for OpenGL availability on Darwin, causing <https://github.com/NixOS/nixpkgs/issues/407056>, which had to be worked around by <https://github.com/NixOS/nixpkgs/pull/409525>. Both `libintl` and `libGL` should count as available on platforms where their functionality is part of the standard build environment, and a package that is completely unavailable and whose functionality cannot be expected should not use `null`, as it should result in errors if used in a dependency list on an unsupported platform. I accept that overriding with `null` is often a useful way to disable dependencies that don’t have explicit feature flags, but I do not think that making it work better with feature flags conditioned on availability is worth the inconsistency and problems caused by this change. Packages can instead expose the relevant feature flags as arguments that default to the `lib.meta.availableOn` check or, if they want to keep an “override the dependency to `null`” interface, insert an explicit `pkg != null && …` check. Additionally, the pull request was merged over a week after all breaking changes were restricted for the 25.05 release. I believe that the potential problems of dealing with the effects of this change for an entire release cycle – the first release cycle where `libGL` is `null` on Darwin, a change I made before the deadline and before this change to `lib.meta.availableOn` – offset the risks of backporting this revert at such a late stage. It will cause overrides to backwards‐incompatibly revert to the behaviour they had before the change, but since such overrides were not possible until a few weeks ago, I hope that is an acceptable risk compared to the potential issues leaving this in the release can cause, given that it was merged after the deadline and has already broken an existing construction in Nixpkgs. This reverts commit9338d924db. (cherry picked from commit98dbc7cc58)
Nixpkgs is a collection of over 120,000 software packages that can be installed with the Nix package manager. It also implements NixOS, a purely-functional Linux distribution.
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