Create systemd snippet to set process group if necessary

debian/replds-instance-create

 #!/bin/sh
+#
+# Create a new replds instance.
+#
+# The new instance will have its own configuration in
+# /etc/replds/<name>.yml, its data directory in
+# /var/lib/replds/<name>, and its own system user and group.
+#
+# If the '-g <group>' option is passed, the new instance will use the
+# specified UNIX group. Permissions of the data directory will be set
+# accordingly, and the systemd configuration will be modified.
+#
+# You are expected to invoke 'systemctl enable replds@<name>.service'
+# yourself.
 
 instance=
 data_group=
@@ -48,6 +61,17 @@ if [ -n "${data_group}" ]; then
     usermod --append --groups ${data_group} ${instance_user}
 fi
 
+# If data_group is specified, create a customized snippet for systemd
+# to set the data_group for the running process.
+if [ -n "${data_group}" ]; then
+    systemd_dir="/etc/systemd/system/replds@${instance}.service.d"
+    mkdir -p ${systemd_dir}
+    cat >${systemd_dir}/group.conf <<EOF
+[Service]
+Group=${data_group}
+EOF
+fi
+
 # Create the repository directory.
 if [ ! -e ${instance_dir} ]; then
     mkdir -p ${instance_dir}