Code maintenance: OS package for file system

- OS custom internal package for file system interaction
- Remove fileManager external dependency
- Closer API to Go's native API on the OS
- Create directories at startup
- Better testability
- Move Unsetenv to os interface

internal/storage/sync.go

@@ -3,10 +3,10 @@ package storage
 import (
 	"encoding/json"
 	"fmt"
-	"os"
 	"reflect"
 
 	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/os"
 )
 
 const (
@@ -29,14 +29,18 @@ func (s *storage) SyncServers(hardcodedServers models.AllServers, write bool) (
 	allServers models.AllServers, err error) {
 	// Eventually read file
 	var serversOnFile models.AllServers
-	_, err = s.osStat(jsonFilepath)
+	file, err := s.os.OpenFile(jsonFilepath, os.O_RDONLY, 0)
+	if err != nil && !os.IsNotExist(err) {
+		return allServers, err
+	}
 	if err == nil {
-		serversOnFile, err = s.readFromFile()
-		if err != nil {
+		var serversOnFile models.AllServers
+		decoder := json.NewDecoder(file)
+		if err := decoder.Decode(&serversOnFile); err != nil {
+			_ = file.Close()
 			return allServers, err
 		}
-	} else if !os.IsNotExist(err) {
-		return allServers, err
+		return allServers, file.Close()
 	}
 
 	// Merge data from file and hardcoded
@@ -51,24 +55,16 @@ func (s *storage) SyncServers(hardcodedServers models.AllServers, write bool) (
 	return allServers, s.FlushToFile(allServers)
 }
 
-func (s *storage) readFromFile() (servers models.AllServers, err error) {
-	bytes, err := s.readFile(jsonFilepath)
-	if err != nil {
-		return servers, err
-	}
-	if err := json.Unmarshal(bytes, &servers); err != nil {
-		return servers, err
-	}
-	return servers, nil
-}
-
 func (s *storage) FlushToFile(servers models.AllServers) error {
-	bytes, err := json.MarshalIndent(servers, "", "  ")
+	file, err := s.os.OpenFile(jsonFilepath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0644)
 	if err != nil {
-		return fmt.Errorf("cannot write to file: %w", err)
-	}
-	if err := s.writeFile(jsonFilepath, bytes, 0644); err != nil {
 		return err
 	}
-	return nil
+	encoder := json.NewEncoder(file)
+	encoder.SetIndent("", "  ")
+	if err := encoder.Encode(servers); err != nil {
+		_ = file.Close()
+		return fmt.Errorf("cannot write to file: %w", err)
+	}
+	return file.Close()
 }