Bug#914897: #914897: debootstrap, buster: Please disabled merged /usr by default

[Ansgar Burchardt <ansgar@debian.org>]

Ansgar Burchardt writes:
> There were discussions about enabling this by default years ago, I
> don't think minor issues should be a reason to delay this change.
>
> Note that it has been delayed for after the stretch release as there
> were major issues back then (it was enabled by default for a short time
> in stretch).  It took >5 months for someone to find a problem this time
> which is pretty good for any change.

So, I went through all reproducible build failures in unstable without
notes and added notes for differences caused by building in merged-/usr
vs non-merged-/usr packages.  Together with what other people tagged,
about 60 problems were found.  (The oldest rebuild result for unstable
is 17 days old, the merged-/usr variation was deployed before that[1].)

  [1] https://bugs.debian.org/901473#43

There might be some more that as diffoscope had problems or the diff was
too large for some packages (but I'll assume that in this case
merged-/usr is not the only problem the package had).

This should cover all packages that had no other problems and were
reproducible before, that is about 80% of the archive.

Assuming the rate is similar for packages which have other reproducible
build problems, I would expect 60 / 80*20 = 15 more problems.

I haven't looked at build failures, but I would expect these to be
usually not caused by merged-/usr.

So an estimated ~80 packages which might need adjustments for building
correctly in both merged-/usr and non-merged-/usr.  I guess less than
for the average new release of GCC ;-)

The problems are usually easy to fix by passing an explicit path the the
package's configure script at build time; of course there might be some
package where it is more complicated.

Ansgar