lib/config: Error on empty folder path (fixes #5853) (#5854)

lib/config/config.go

@@ -10,6 +10,7 @@ package config
 import (
 	"encoding/json"
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -91,6 +92,12 @@ var (
 	}
 )
 
+var (
+	errFolderIDEmpty     = errors.New("folder has empty ID")
+	errFolderIDDuplicate = errors.New("folder has duplicate ID")
+	errFolderPathEmpty   = errors.New("folder has empty path")
+)
+
 func New(myID protocol.DeviceID) Configuration {
 	var cfg Configuration
 	cfg.Version = CurrentVersion
@@ -273,12 +280,17 @@ func (cfg *Configuration) clean() error {
 		folder.prepare()
 
 		if folder.ID == "" {
-			return fmt.Errorf("folder with empty ID in configuration")
+			return errFolderIDEmpty
+		}
+
+		if folder.Path == "" {
+			return fmt.Errorf("folder %q: %v", folder.ID, errFolderPathEmpty)
 		}
 
 		if _, ok := existingFolders[folder.ID]; ok {
-			return fmt.Errorf("duplicate folder ID %q in configuration", folder.ID)
+			return fmt.Errorf("folder %q: %v", folder.ID, errFolderIDDuplicate)
 		}
+
 		existingFolders[folder.ID] = folder
 	}