allow protection from invalid config values (#19460)

we have had numerous reports on some config
values not having default values, causing
features misbehaving and not having default
values set properly.

This PR tries to address all these concerns
once and for all.

Each new sub-system that gets added

- must check for invalid keys
- must have default values set
- must not "return err" when being saved into
  a global state() instead collate as part of
  other subsystem errors allow other sub-systems
  to independently initialize.

internal/config/heal/heal.go

@@ -157,11 +157,14 @@ func LookupConfig(kvs config.KVS) (cfg Config, err error) {
 	if err = config.CheckValidKeys(config.HealSubSys, kvs, DefaultKVS); err != nil {
 		return cfg, err
 	}
-	cfg.Bitrot = env.Get(EnvBitrot, kvs.GetWithDefault(Bitrot, DefaultKVS))
-	_, err = parseBitrotConfig(cfg.Bitrot)
-	if err != nil {
+
+	bitrot := env.Get(EnvBitrot, kvs.GetWithDefault(Bitrot, DefaultKVS))
+	if _, err = parseBitrotConfig(bitrot); err != nil {
 		return cfg, fmt.Errorf("'heal:bitrotscan' value invalid: %w", err)
 	}
+
+	cfg.Bitrot = bitrot
+
 	cfg.Sleep, err = time.ParseDuration(env.Get(EnvSleep, kvs.GetWithDefault(Sleep, DefaultKVS)))
 	if err != nil {
 		return cfg, fmt.Errorf("'heal:max_sleep' value invalid: %w", err)