## What is "merged `/usr`"
-"Merged `/usr`" describes a possible future standard directories scheme in which the `/{bin,sbin,lib}/` directories have been made superfluous, either through replacing them by symlinks to their `/usr` equivalents (/usr/{bin,sbin,lib}) or by removing them entirely.
+"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.
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).
+* `/{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).
[0]: https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/
[1]: https://wiki.debian.org/UsrMerge
+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 distinct ways:
+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.
+* 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.
-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
Various valid long-term desireable 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):
+These are the five possible situations at the time of bullseye (buster + 1):
* `none`: "merged `/usr`" has been reverted
* `weak`: both directory schemes are allowed, packages only built on classical hosts
* `middle`: both directory schemes are allowed, packages can be built anywhere
* `hard`: both directory schemes are allowed, packages only built on "merged `/usr`" hosts
* `all`: only "merged `/usr`" directory schemes are allowed, packages only built on "merged `/usr`" hosts
-* `full`: only "merged `/usr`" directory schemes are allowed, symlinks have been removed
It can be summarized by the following table:
| middle | yes | yes | yes | yes | yes | no | no |
| hard | yes | yes | yes | no | yes | no | no |
| all | no | yes | yes | no | yes | yes | no |
-| full | no | yes | no | no | yes | yes | yes |
```
## Immediate actions
Given that hosts with different top-level directory schemes already exist; there are various ways forward that would allow for Debian to converge to a desireable situation:
* Flag-day to get all hosts on "merged `/usr`", through a base-files version; probably in buster+1 (bullseye)
-* Disallow "merged `/usr`", let users who ran `usrmerge` on their own
+* Disallow "merged `/usr`", leave users who already have merged `/usr` in an unsupported situation
* Let Debian converge to a situation where non-"merged `/usr`" Debian hosts are equivalent to symlinked "merged `/usr`" hosts; do this through upgrading all packages shipping files outside of /usr (but exceptions) to stop doing this. Could be achieved by setting policy for buster+1 (should) and buster+2 (must), or maybe even shorter. This would make the symlink "shortcut" migration redundant.
* Support both "merged `/usr`" and non-"merged `/usr`" systems forever: this implies that our packaging tools need to either support countering effects of "merged `/usr`" (e.g. through manipulating PATH for builds to detect files only in their .deb paths) or identifying tainted packages, and letting installing users decide (warn or error out at install time).
projects / debian-ctte.git / blobdiff