Salsa-CI: Install readline from Buster as it was removed from Sid

The older MariaDB.org packages were built against libreadline5 and depend
on it to install. As it was removed from Sid (and Bullseye), fetch and
install it from Buster.

This can eventually be removed once we stop backwards compat testing for
older MariaDB versions (new ones don't depend on it).

Upstreamed from Debian packaging repository commit
https://salsa.debian.org/mariadb-team/mariadb-10.5/-/commit/4b729db7f89f573dbe92d601a2d984fab158fc4f

debian/salsa-ci.yml

@@ -146,6 +146,12 @@ blhc:
   # Prime the apt cache so later apt commands can run
   apt-get update
 
+# Readline was removed from Debian Sid (and Bullseye) in Feb 2021. To be able to install older
+# versions of MariaDB that depend on it, fetch and install it from Buster.
+.test-install-readline-in-sid-for-backwards-compat: &test-install-readline-in-sid-for-backwards-compat |
+  curl -O http://ftp.de.debian.org/debian/pool/main/r/readline5/libreadline5_5.2+dfsg-3+b13_amd64.deb
+  apt install -y ./libreadline5_5.2+dfsg-3+b13_amd64.deb
+
 .test-verify-initial: &test-verify-initial |
   dpkg -l | grep -iE 'maria|mysql|galera' || true # List installed
   service mysql status || service mariadb status # Early MariaDB 10.5 only had 'mariadb'
@@ -664,6 +670,7 @@ mariadb.org-10.5 to mariadb-10.5 upgrade:
     - curl -sS https://mariadb.org/mariadb_release_signing_key.asc -o /etc/apt/trusted.gpg.d/mariadb.asc
     - echo 'deb http://mirror.one.com/mariadb/repo/10.5/debian sid main' > /etc/apt/sources.list.d/mariadb.list
     - apt-get update
+    - *test-install-readline-in-sid-for-backwards-compat
     # Package libmariadbclient-dev from mariadb.org conficts with libmariadb-dev in Sid, so cannot use wildcard that would include it
     - apt-get install -y 'mariadb*' libmariadb3 'libmariadb-*' 'libmariadbd*'
     - *test-verify-initial
@@ -698,6 +705,7 @@ mariadb.org-10.4 to mariadb-10.5 upgrade:
     - curl -sS https://mariadb.org/mariadb_release_signing_key.asc -o /etc/apt/trusted.gpg.d/mariadb.asc
     - echo 'deb http://mirror.one.com/mariadb/repo/10.4/debian sid main' > /etc/apt/sources.list.d/mariadb.list
     - apt-get update
+    - *test-install-readline-in-sid-for-backwards-compat
     - apt-get install -y mariadb-server-10.4
     # MariaDB.org version of 10.4 and early 10.5 do not install an init file, so
     # it must be installed here manually
@@ -731,6 +739,7 @@ mariadb.org-10.3 to mariadb-10.5 upgrade:
     - curl -sS https://mariadb.org/mariadb_release_signing_key.asc -o /etc/apt/trusted.gpg.d/mariadb.asc
     - echo 'deb http://mirror.one.com/mariadb/repo/10.3/debian sid main' > /etc/apt/sources.list.d/mariadb.list
     - apt-get update
+    - *test-install-readline-in-sid-for-backwards-compat
     - apt-get install -y mariadb-server-10.3
     # Verify initial state before upgrade
     - dpkg -l | grep -iE 'maria|mysql|galera' || true # List installed
@@ -767,6 +776,7 @@ mariadb.org-10.2 to mariadb-10.5 upgrade:
     - curl -sS https://mariadb.org/mariadb_release_signing_key.asc -o /etc/apt/trusted.gpg.d/mariadb.asc
     - echo 'deb http://mirror.one.com/mariadb/repo/10.2/debian sid main' > /etc/apt/sources.list.d/mariadb.list
     - apt-get update
+    - *test-install-readline-in-sid-for-backwards-compat
     - apt-get install -y mariadb-server-10.2
     # Verify initial state before upgrade
     - dpkg -l | grep -iE 'maria|mysql|galera' || true # List installed