bug#38086: RAID installation script with ‘mdadm’ no longer works

["Ludovic Courtès" <ludo@gnu.org>]

An update: this is the last known good test:

  https://berlin.guixsd.org/build/1793057/details

and this is the first known-bad (‘mdadm’ failing with “cannot assemble
multi-zone RAID0 without default_layout setting”):

  https://berlin.guixsd.org/build/1795351/details

We have to resort to an ungly hack to get the evaluation number and
corresponding commit of each build because they aren’t accessible over
HTTP (which is unfortunate!):

--8<---------------cut here---------------start------------->8---
sqlite> select * from builds where rowid = 1793057;
/gnu/store/618hm2w0clcrxz16yww846mgqdc1l4s0-raid-root-os.drv|7863|test.raid-root-os.i686-linux|i686-linux|raid-root-os||0|1570439988|1570459635|1570459744
sqlite> select * from checkouts where evaluation = 7863;
guix-master|7b6a8e23b0de18262a42e44432f955517d71d796|7863|guix|/gnu/store/7sd2lwj83n6kyn66p9bdgs5yvzqnl539-guix-7b6a8e2
sqlite> select * from builds where rowid = 1795351;
/gnu/store/qskl45gw9y9hd8qp7s5451d53pvpc60q-raid-root-os.drv|7867|test.raid-root-os.i686-linux|i686-linux|raid-root-os||2|1570440409|0|1570457622
sqlite> select * from checkouts where evaluation = 7867;
guix-master|7d82e920717f08bceb42bb570d786dff233171e1|7867|guix|/gnu/store/b2cq9zhdsz4qri2xkg3rgwmyri0wyxxb-guix-7d82e92
--8<---------------cut here---------------end--------------->8---

So the commit that introduced the change of behavior of ‘mdadm’ is:

--8<---------------cut here---------------start------------->8---
commit 7d82e920717f08bceb42bb570d786dff233171e1
Date:   Sun Oct 6 06:07:15 2019 +0000

    gnu: linux-libre: Update to 5.3.4.
--8<---------------cut here---------------end--------------->8---

And indeed that brings us back to:

  https://blog.icod.de/2019/10/10/caution-kernel-5-3-4-and-raid0-default_layout/

Hmm alright, nothing new here.  Oh well!

Ludo’.