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-# #914897: tech-ctte: Should debootstrap disable merged `/usr` by default?
-
-## What is "merged `/usr`"
-
-"Merged `/usr`" describes a possible future standard directories scheme in which the `/{bin,sbin,lib*}/` directories have been made superfluous through replacing them by symlinks to their `/usr` equivalents (`/usr/{bin,sbin,lib*}`).
-The motivation to get Debian systems to converge towards such a scheme is vastly documented elsewhere ([FDO's TheCaseForTheUsrMerge][0], [wiki.d.o UsrMerge][1]) but can be summarized as the following points:
-
-* having separate `/` and `/usr` filesystems has been useful in the past for booting without initramfs onto a minimal root filesystem that carried just enough to mount the `/usr` filesystem later in the boot process. Given the evolution of physical hosts' capabilities, initramfs'es have been default in Debian (and elsewhere) for a long time, and most systems no longer have an intermediate state during boot in which they have only `/`, but not `/usr`, mounted. Booting hosts through that intermediate state is not systematically tested in Debian anymore.
-* another use-case is to share system files from `/usr` between hosts (over a network link) or containers (locally) which use different data or configuration. Having all software under `/usr` (instead of spread between `/` and `/usr`) makes the centralized update and the sharing easier.
-* the packaging infrastructure to install files outside of `/usr` (e.g. installing libs under `/lib` instead of `/usr/lib`) is not standard and represents technical debt.
-* given its status as remnant "folklore", the distinction between what _needs_ to be shipped in `/` and what can stay in `/usr` is often interpreted arbitrarily;
-* allowing shipment of identically-named libraries or binaries in different paths can confuse common understanding of paths precedence.
-
-[0]: https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/
-[1]: https://wiki.debian.org/UsrMerge
-
-The arguments against moving the base directories' scheme towards "merged `/usr`" are as follows:
-
-* there's no gain in disrupting something that is not inherently broken;
-* `/{bin,sbin,lib*}/` → `/usr/{bin,sbin,lib*}/` symlinks create confusing views of the system (`/bin/cat` and `/usr/bin/cat` are the same file), and dpkg doesn't support this situation cleanly: [#134758](https://bugs.debian.org/134758).
-* it is possible for distributions to converge towards having all system files in `/usr` in finite time instead of shortcutting this migration with `/{bin,sbin,lib*}/` → `/usr/{bin,sbin,lib*}/` symlinks.
-
-The compatibility symbolic links `/lib` → `/usr/lib` and `/lib64` → `/usr/lib64` are required by the various CPUs' platform ABIs (for example i386 requires `/lib/ld-linux.so.2` to resolve to glibc's `ld.so`, and amd64 requires `/lib64/ld-linux-x86-64.so.2`) so there are no plans to remove them altogether. Similarly, removing `/bin` is not under consideration because it would break the assumption that `/bin/sh` exists, and removing `/sbin` would break the assumption that `/sbin/fsck.*` and `/sbin/mount.*` exist.
-
-## "merged `/usr`" in Debian
-
-In Debian buster, the current testing suite, "merged `/usr`" is only considered for implementation with symlinks (there are no proposals for simply dropping `/{bin,sbin,lib*}`) and is implemented in two main ways:
-
-* existing hosts can be made to have a "merged `/usr`" by installing the [usrmerge][2] package;
-* new hosts get the `/{bin,sbin,lib*}/`→ `/usr/{bin,sbin,lib*}/` symlinks by default when using debootstrap >= 1.0.102.
-
-The usrmerge package contains a `/usr/lib/convert-usrmerge` perl executable that runs in `postinst`, that will move the contents of `/{bin,sbin,lib*}/` and replace these directories with symlinks when empty.
-
-It is also possible to merge `/usr` in other ways, for example with `debootstrap --merged-usr` or by bootstrapping into a chroot that already contains the necessary symlinks.
-
-[2]: https://tracker.debian.org/pkg/usrmerge
-
-## Issues from "merged `/usr`"
-
-Since the default change in debootstrap 1.0.102, some issues have arisen.
-Due to the fact that some buster/sid hosts have the "merged `/usr`" symlinks in place, it has been observed that some binary packages carried some traces of these differences (notably official packages built on Debian buildd hosts which had been resetup).
-Some such differences can actually render the built packages unusable on non-"merged `/usr`" systems.
-For example, if `cat` is detected at build-time in `/usr/bin/cat` (where coreutils ships `/bin/cat`), a binary hardcoding that path will try to use `/usr/bin/cat` after installation, but that path doesn't exist in non-"merged `/usr`" systems.
-In order to mitigate this, debootstrap has been modified to let its "buildd" variant be non-"merged `/usr`", the Debian buildds have been resetup and the affected packages rebuilt.
-
-The lesson here is that with the existence of (any of) the usrmerge and the debootstrap default change, "merged `/usr`" Debian systems exist already, and that packages built on hosts with such directory schemes can _potentially_ be broken on non-"merged `/usr`" systems.
-At this point, the two variants have to be supported, at least as installation targets of Debian packages.
-
-Two initiatives are worth mentioning at this point:
-* [a patch](https://lists.debian.org/20181202212535.GC11687@gaara.hadrons.org) has been proposed for dpkg-buildpackage to mark packages built on "merged `/usr`" hosts with a `Build-Tainted-By: merged-usr-via-symlinks`;
-* the reproducible builds team has added a "merged `/usr`" variation to their setup, and have then [tagged](https://tests.reproducible-builds.org/debian/issues/unstable/paths_vary_due_to_usrmerge_issue.html) the Debian packages from unstable which had differences due to "merged `/usr`". It seems that ~61 packages were affected by differing builds; 32 from these have been fixed in unstable already.
-
-## The long-term desirable situation
-
-Various valid long-term desirable situations coexist, and while discussing immediate countermeasures, it is useful to keep the long-term outcome that those are most likely to produce.
-
-These are the six possible situations at the time of bullseye (buster + 1):
-
-* `none`: "merged `/usr`" has been reverted
-* `empty`: "merged `/usr`" has been reverted, `/usr` is empty (but the mandatory files)
-* `weak`: both directory schemes are allowed, all packages only built on classical hosts
-* `middle`: both directory schemes are allowed, all packages can be built on either
-* `hard`: both directory schemes are allowed, packages can be built on either, official packages only built on "merged `/usr`" hosts
-* `all`: only "merged `/usr`" directory schemes are allowed, packages only built on "merged `/usr`" hosts
-
-It can be summarized by the following table:
-
-```
-| | Host types that are allowed | Are merged `/usr` | Official packages are built on | Packages built on … can break on the other |
-| Codename | classical hosts | merged `/usr` hosts | symlinks allowed | classical hosts | merged `/usr` hosts | classical hosts | merged `/usr` hosts |
-|----------|-----------------|---------------------|-------------------|—----------------|---------------------|---------------------|----------------------|
-| none | yes | no | no | yes | no | yes | yes |
-| empty | yes | no | no | yes | no | yes | yes |
-| weak | yes | yes | yes | yes | no | no | yes |
-| middle | yes | yes | yes | yes | yes | no | no |
-| hard | yes | yes | yes | no | yes | no | no |
-| all | no | yes | yes | no | yes | yes | no |
-```
-
-The current state of buster is `weak`.
-
-=== Resolution ===
-
-The Technical Committee resolves to decline to override the debootstrap maintainers.
-
-Furthermore, using its §6.1.5 "Offering advice" power, the Technical Committee considers that the desirable solution at the time of `bullseye` is:
-
-* W: `weak`: both directory schemes are allowed, but packages should only be built on hosts with classical directory schemes (or in such chroots)
-
-* M: `middle`: both directory schemes are allowed, and packages (including official packages) can be built on hosts with either classical or "merged `/usr`" directory schemes
-
-* H: `hard`: both directory schemes are allowed, but packages should only be built on hosts with "merged `/usr`" directory schemes (or in such chroots)
-
-* FD: Further Discussion
-
-=== End Resolution ===
projects / debian-ctte.git / blobdiff