+## 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 unuseable 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 existance 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 desireable situation
+
+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 five possible situations at the time of bullseye (buster + 1):
+
+* `none`: "merged `/usr`" has been happened
+* `weak`: both directory schemes are allowed, packages only built on classical hosts
+* `hard`: both directory schemes are allowed, packages can be built anywhere
+* `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