Merge ~cjwatson/launchpad-buildd:production-deployment-notes into launchpad-buildd:master

[Colin Watson]

Add some notes on the production deployment

[Kristian Glass (doismellburning)]

review: Approve

diff --git a/README b/README
index 7d4c520..d2eb28e 100644
--- a/README
+++ b/README
@@ -28,3 +28,23 @@ To clean up, do:
 fakeroot debian/rules clean
 rm launchpad-buildd*deb
 rm ../launchpad-buildd*changes
+
+Production deployment notes
+***************************
+
+In Launchpad's production build farm, launchpad-buildd is deployed via base
+virtual machine images constructed by taking standard Ubuntu cloud images
+and installing launchpad-buildd in them from
+https://launchpad.net/~canonical-is-sa/+archive/ubuntu/buildd.  This is done
+by
+https://code.launchpad.net/~canonical-sysadmins/canonical-is-charms/launchpad-buildd-image-modifier
+(currently private, sorry).
+
+At present (May 2020), all these base VM images are built from Ubuntu
+xenial, and launchpad-buildd runs on Python 2.  We plan to upgrade these to
+bionic soon, where it will be possible for launchpad-buildd to run on Python
+3; however, it's necessary to support the powerpc architecture until at
+least April 2021 (end of standard maintenance for xenial), and the powerpc
+base images will need to stay on xenial since that architecture is no longer
+supported by bionic.  As a result, launchpad-buildd must run on both Python
+2 and 3 for the time being.
diff --git a/debian/changelog b/debian/changelog
index a6e5033..26d4622 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -26,6 +26,7 @@ launchpad-buildd (190) UNRELEASED; urgency=medium
   * Add a python3-lpbuildd binary package.
   * Fix LXD.run to not default to universal_newlines=True.
   * Run on Python 3 when built for >= bionic.
+  * Add some notes on the production deployment.
 
   [ Dimitri John Ledkov ]
   * lxd: Add riscv64 to arch table.