Debian: fix overwriting of config settings on upgrade (#4696)

Make sure that users' changes to the config files are preserved.

Fixes #4440.

debian/matrix-synapse-py3.postinst

@@ -8,19 +8,36 @@ USER="matrix-synapse"
 
 case "$1" in
   configure|reconfigure)
-    # Set server name in config file
+
+    # generate template config files if they don't exist
     mkdir -p "/etc/matrix-synapse/conf.d/"
-    db_get matrix-synapse/server-name
+    if [ ! -e "$CONFIGFILE_SERVERNAME" ]; then
+        cat > "$CONFIGFILE_SERVERNAME" <<EOF
+# This file is autogenerated, and will be recreated on upgrade if it is deleted.
+# Any changes you make will be preserved.
 
-    if [ "$RET" ]; then
-        echo "server_name: $RET" > $CONFIGFILE_SERVERNAME
+# The domain name of the server, with optional explicit port.
+# This is used by remote servers to connect to this server,
+# e.g. matrix.org, localhost:8080, etc.
+# This is also the last part of your UserID.
+#
+server_name: ''
+EOF
     fi
 
-    db_get matrix-synapse/report-stats
-    if [ "$RET" ]; then
-        echo "report_stats: $RET" > $CONFIGFILE_REPORTSTATS
+    if [ ! -e "$CONFIGFILE_REPORTSTATS" ]; then
+        cat > "$CONFIGFILE_REPORTSTATS" <<EOF
+# This file is autogenerated, and will be recreated on upgrade if it is deleted.
+# Any changes you make will be preserved.
+
+# Whether to report anonymized homeserver usage statistics.
+report_stats: false
+EOF
     fi
 
+    # update the config files according to whatever is in the debconf database
+    /opt/venvs/matrix-synapse/lib/manage_debconf.pl update
+
     if ! getent passwd $USER >/dev/null; then
       adduser --quiet --system --no-create-home --home /var/lib/matrix-synapse $USER
     fi