Previously, unless unsupported platforms were allowed, the following
would fail to evaluate (from an "x86_64-linux" system):
pkgsCross.x86_64-freebsd.__splicedPackages.docutils.__spliced.buildHost
It shouldn't have, because the buildHost package ends up being for
Linux. This broke evaluation of e.g. pkgsCross.x86_64-freebsd.libdrm,
because it has docutils in nativeBuildInputs. mkDerivation would try
to go through __spliced.buildHost on docutils to get to the Linux
version, but the check in ensurePythonModules would kick in first,
triggering the meta check because of the equality check in the
implementation of hasPythonModule, which would fail because Python is
not marked as supported on FreeBSD in Nixpkgs at the moment. Thus,
even though they're not supposed to be, the meta checks would be
triggered even though the only attribute being accessed on the
unsupported derivation was __spliced.
We can fix this by using the same mechanism used to implement the meta
checks themselves: lib.extendDerivation. Now, attempting to access
drvPath or outPath on an attribute that fails the validity check will
produce the same error as before, but other accesses will be allowed
through, fixing splicing.
I've tested evaluation of packages that pass and fail the validity
check, and confirmed that the behaviour is still correct.
b682fef8e9