Fix failures in Debian packaging (#9079)

Debian package builds were failing for two reasons:

 1. Python versions prior to 3.7 throw exceptions when attempting to print
    Unicode characters under a "C" locale. (#9076)

 2. We depended on `dh-systemd` which no longer exists in Debian Bullseye, but
    is necessary in Ubuntu Xenial. (#9073)

Setting `LANG="C.UTF-8"` in the build environment fixes the first issue.
See also: https://bugs.python.org/issue19846

The second issue is a bit trickier. The dh-systemd package was merged into
debhelper version 9.20160709 and a transitional package left in its wake.

The transitional dh-systemd package was removed in Debian Bullseye.

However, Ubuntu Xenial ships an older debhelper, and still needs dh-systemd.

Thus, builds were failing on Bullseye since we depended on a package which had
ceased existing, but we couldn't remove it from the debian/control file and our
build scripts because we still needed it for Ubuntu Xenial.

We can fix the debian/control issue by listing dh-systemd as an alternative to
the newer versions of debhelper. Since dh-systemd declares that it depends on
debhelper, Ubuntu Xenial will select its older dh-systemd which will in turn
pull in its older debhelper, resulting in no change from the status quo. All
other supported releases will satisfy the debhelper dependency constraint and
skip the dh-systemd alternative.

Build scripts were fixed by unconditionally attempting to install dh-systemd on
all releases and suppressing failures.

Once we drop support for Ubuntu Xenial, we can revert most of this commit and
rely on the version constraint on debhelper in debian/control.

Fixes #9076
Fixes #9073

Signed-off-by: Dan Callahan <danc@element.io>

debian/changelog

@@ -1,3 +1,10 @@
+matrix-synapse-py3 (1.24.0+nmu1) UNRELEASED; urgency=medium
+
+  * Update dependencies to account for the removal of the transitional
+    dh-systemd package from Debian Bullseye.
+
+ -- Dan Callahan <danc@element.io>  Tue, 12 Jan 2021 12:08:33 +0000
+
 matrix-synapse-py3 (1.24.0) stable; urgency=medium
 
   * New synapse release 1.24.0.

debian/control

@@ -3,9 +3,11 @@ Section: contrib/python
 Priority: extra
 Maintainer: Synapse Packaging team <packages@matrix.org>
 # keep this list in sync with the build dependencies in docker/Dockerfile-dhvirtualenv.
+# TODO: Remove the dependency on dh-systemd after dropping support for Ubuntu xenial
+#       On all other supported releases, it's merely a transitional package which
+#       does nothing but depends on debhelper (> 9.20160709)
 Build-Depends:
- debhelper (>= 9),
- dh-systemd,
+ debhelper (>= 9.20160709) | dh-systemd,
  dh-virtualenv (>= 1.1),
  libsystemd-dev,
  libpq-dev,

docker/Dockerfile-dhvirtualenv

@@ -50,17 +50,22 @@ FROM ${distro}
 ARG distro=""
 ENV distro ${distro}
 
+# Python < 3.7 assumes LANG="C" means ASCII-only and throws on printing unicode
+# http://bugs.python.org/issue19846
+ENV LANG C.UTF-8
+
 # Install the build dependencies
 #
 # NB: keep this list in sync with the list of build-deps in debian/control
 # TODO: it would be nice to do that automatically.
+# TODO: Remove the dh-systemd stanza after dropping support for Ubuntu xenial
+#       it's a transitional package on all other, more recent releases
 RUN apt-get update -qq -o Acquire::Languages=none \
     && env DEBIAN_FRONTEND=noninteractive apt-get install \
         -yqq --no-install-recommends -o Dpkg::Options::=--force-unsafe-io \
         build-essential \
         debhelper \
         devscripts \
-        dh-systemd \
         libsystemd-dev \
         lsb-release \
         pkg-config \
@@ -70,7 +75,10 @@ RUN apt-get update -qq -o Acquire::Languages=none \
         python3-venv \
         sqlite3 \
         libpq-dev \
-        xmlsec1
+        xmlsec1 \
+    && ( env DEBIAN_FRONTEND=noninteractive apt-get install \
+         -yqq --no-install-recommends -o Dpkg::Options::=--force-unsafe-io \
+         dh-systemd || true )
 
 COPY --from=builder /dh-virtualenv_1.2~dev-1_all.deb /