feat(config): allow invalid server filters (#2419)

- Disallow setting a server filter when there is no choice available
- Allow setting an invalid server filter when there is at least one choice available
- Log at warn level when an invalid server filter is set
- Fix #2337

internal/configuration/settings/settings.go

@@ -36,7 +36,8 @@ type Storage interface {
 // Validate validates all the settings and returns an error
 // if one of them is not valid.
 // TODO v4 remove pointer for receiver (because of Surfshark).
-func (s *Settings) Validate(storage Storage, ipv6Supported bool) (err error) {
+func (s *Settings) Validate(storage Storage, ipv6Supported bool,
+	warner Warner) (err error) {
 	nameToValidation := map[string]func() error{
 		"control server":  s.ControlServer.validate,
 		"dns":             s.DNS.validate,
@@ -51,7 +52,7 @@ func (s *Settings) Validate(storage Storage, ipv6Supported bool) (err error) {
 		"version":         s.Version.validate,
 		// Pprof validation done in pprof constructor
 		"VPN": func() error {
-			return s.VPN.Validate(storage, ipv6Supported)
+			return s.VPN.Validate(storage, ipv6Supported, warner)
 		},
 	}
 
@@ -84,7 +85,7 @@ func (s *Settings) copy() (copied Settings) {
 }
 
 func (s *Settings) OverrideWith(other Settings,
-	storage Storage, ipv6Supported bool) (err error) {
+	storage Storage, ipv6Supported bool, warner Warner) (err error) {
 	patchedSettings := s.copy()
 	patchedSettings.ControlServer.overrideWith(other.ControlServer)
 	patchedSettings.DNS.overrideWith(other.DNS)
@@ -99,7 +100,7 @@ func (s *Settings) OverrideWith(other Settings,
 	patchedSettings.Version.overrideWith(other.Version)
 	patchedSettings.VPN.OverrideWith(other.VPN)
 	patchedSettings.Pprof.OverrideWith(other.Pprof)
-	err = patchedSettings.Validate(storage, ipv6Supported)
+	err = patchedSettings.Validate(storage, ipv6Supported, warner)
 	if err != nil {
 		return err
 	}