Add sanity check for "systemd" config values (MR 2273)

Make sure users don't set systemd to "true" or other not allowed values
(allowed are "always", "default", "never").

Check for it:
* after loading the config
* when using 'pmbootstrap config systemd <newvalue>'

pmb/config/__init__.py

@@ -9,7 +9,7 @@ from typing import List
 #
 # Exported functions
 #
-from pmb.config.load import load
+from pmb.config.load import load, sanity_checks
 from pmb.config.save import save
 from pmb.config.merge_with_args import merge_with_args
 from pmb.config.sudo import which_sudo
@@ -149,6 +149,9 @@ defaults = {
     "log": "$WORK/log.txt",
 }
 
+allowed_values = {
+    "systemd": ["default", "always", "never"],
+}
 
 # Whether we're connected to a TTY (which allows things like e.g. printing
 # progress bars)

pmb/config/init.py

@@ -203,7 +203,7 @@ def ask_for_systemd(args, ui):
     logging.info("Based on your UI selection, 'default' will result"
                  f" in{not_str}installing systemd.")
 
-    choices = ["default", "always", "never"]
+    choices = pmb.config.allowed_values["systemd"]
     answer = pmb.helpers.cli.ask("Install systemd?",
                                  choices,
                                  args.systemd,

pmb/config/load.py

@@ -3,9 +3,30 @@
 import logging
 import configparser
 import os
+import sys
 import pmb.config
 
 
+def sanity_check(args, cfg, key, allowed, print_path):
+    value = cfg["pmbootstrap"][key]
+
+    if value in allowed:
+        return
+
+    logging.error(f"pmbootstrap.cfg: invalid value for {key}: '{value}'")
+    logging.error(f"Allowed: {', '.join(allowed)}")
+
+    if print_path:
+        logging.error(f"Fix it here and try again: {args.config}")
+
+    sys.exit(1)
+
+
+def sanity_checks(args, cfg, print_path=True):
+    for key, allowed in pmb.config.allowed_values.items():
+        sanity_check(args, cfg, key, allowed, print_path)
+
+
 def load(args):
     cfg = configparser.ConfigParser()
     if os.path.isfile(args.config):
@@ -31,4 +52,6 @@ def load(args):
                           f" {cfg['pmbootstrap'][key]}")
             del cfg["pmbootstrap"][key]
 
+    sanity_checks(args, cfg)
+
     return cfg

pmb/helpers/frontend.py

@@ -213,6 +213,7 @@ def config(args):
         pmb.config.save(args, cfg)
     elif args.value is not None:
         cfg["pmbootstrap"][args.name] = args.value
+        pmb.config.sanity_checks(args, cfg, False)
         logging.info("Config changed: " + args.name + "='" + args.value + "'")
         pmb.config.save(args, cfg)
     elif args.name: