Add worker wrapper to fix Deb incompatibility

Celery 5.x introduced a new worker argument format that is not
backwards-compatible with the older Celery 4.x format. This created a
conundrum since we use one service unit for both Debian 10 (4.x) and
Debian 11 (5.x). Instead of worse hacks, create a wrapper script to
start the worker with the correct arguments instead.

api-daemon/pvcapid-worker.service

@@ -9,7 +9,7 @@ Type = simple
 WorkingDirectory = /usr/share/pvc
 Environment = PYTHONUNBUFFERED=true
 Environment = PVC_CONFIG_FILE=/etc/pvc/pvcapid.yaml
-ExecStart = /usr/bin/celery --app pvcapid.flaskapi.celery worker --concurrency 1 --loglevel INFO
+ExecStart = /usr/share/pvc/pvcapid-worker.sh
 Restart = on-failure
 
 [Install]

api-daemon/pvcapid-worker.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+
+# pvcapid-worker.py - API Celery worker daemon startup stub
+# Part of the Parallel Virtual Cluster (PVC) system
+#
+#    Copyright (C) 2018-2021 Joshua M. Boniface <joshua@boniface.me>
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation, version 3.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+#
+###############################################################################
+
+CELERY_BIN="$( which celery )"
+
+# This absolute hackery is needed because Celery got the bright idea to change how their
+# app arguments work in a non-backwards-compatible way with Celery 5.
+case "$( cat /etc/debian_version )" in
+    10.*)
+        CELERY_ARGS="worker --app pvcapid.flaskapi.celery --concurrency 1 --loglevel INFO"
+    ;;
+    11.*)
+        CELERY_ARGS="--app pvcapid.flaskapi.celery worker --concurrency 1 --loglevel INFO"
+    ;;
+    *)
+        echo "Invalid Debian version found!"
+        exit 1
+    ;;
+esac
+
+${CELERY_BIN} ${CELERY_ARGS}
+exit $?