In the Overview of cross-architecture portability problems, I have dedicated a section to the problems resulting from use of 32-bit time_t type. This design decision, still affecting Gentoo systems…
One thing people reading this should remember is that you cannot guarantee all packages on a Gentoo system will be updated simultaneously. It just can’t be done. Because several of the arches affected by this are old, slow, and less-used (32-bit PowerPC, anyone?), it’s also impossible to test all combinations of USE flags for all arches in advance, so sooner or later someone will have something break in mid-compile. For this change, that could result in an unbootable system, or a badly broken one that can’t continue the upgrade because, for example, Python is broken and so portage can’t run.
The situation really is much more complicated than it would be on a binary distro whose package updates are atomic. Not intractable, but complicated.
That being said, even a completely borked update would not make the system unrecoverable—you boot from live media, copy a known-good toolchain from the install media for that architecture over the borked install, chroot in, and try again (possibly with USE flag tweaks) until you can get at least emerge --emptytree system or similar to run to completion. It’s a major, major pain in the ass, though, and I can understand why the developers want to reduce the number of systems that have to be handled in that way to as few as possible.
One thing people reading this should remember is that you cannot guarantee all packages on a Gentoo system will be updated simultaneously. It just can’t be done. Because several of the arches affected by this are old, slow, and less-used (32-bit PowerPC, anyone?), it’s also impossible to test all combinations of USE flags for all arches in advance, so sooner or later someone will have something break in mid-compile. For this change, that could result in an unbootable system, or a badly broken one that can’t continue the upgrade because, for example, Python is broken and so portage can’t run.
The situation really is much more complicated than it would be on a binary distro whose package updates are atomic. Not intractable, but complicated.
That being said, even a completely borked update would not make the system unrecoverable—you boot from live media, copy a known-good toolchain from the install media for that architecture over the borked install, chroot in, and try again (possibly with USE flag tweaks) until you can get at least
emerge --emptytree systemor similar to run to completion. It’s a major, major pain in the ass, though, and I can understand why the developers want to reduce the number of systems that have to be handled in that way to as few as possible.