Maint: internal/storage rework

- No more global variables
- Inject merged servers to configuration package
- Fix #566: configuration parsing to use persisted servers.json
- Move server data files from `internal/constants` to `internal/storage`

internal/storage/flush.go

@@ -1,7 +1,38 @@
 package storage
 
-import "github.com/qdm12/gluetun/internal/models"
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
 
-func (s *storage) FlushToFile(allServers models.AllServers) error {
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+var _ Flusher = (*Storage)(nil)
+
+type Flusher interface {
+	FlushToFile(allServers models.AllServers) error
+}
+
+func (s *Storage) FlushToFile(allServers models.AllServers) error {
 	return flushToFile(s.filepath, allServers)
 }
+
+func flushToFile(path string, servers models.AllServers) error {
+	dirPath := filepath.Dir(path)
+	if err := os.MkdirAll(dirPath, 0644); err != nil {
+		return err
+	}
+
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0644)
+	if err != nil {
+		return err
+	}
+	encoder := json.NewEncoder(file)
+	encoder.SetIndent("", "  ")
+	if err := encoder.Encode(servers); err != nil {
+		_ = file.Close()
+		return err
+	}
+	return file.Close()
+}