access logger support sharing the same file, tests added for concurrent logging

internal/net/http/accesslog/file_logger.go

@@ -13,10 +13,26 @@ type File struct {
 	sync.Mutex
 }
 
+var (
+	openedFiles   = make(map[string]AccessLogIO)
+	openedFilesMu sync.Mutex
+)
+
 func NewFileAccessLogger(parent task.Parent, cfg *Config) (*AccessLogger, error) {
-	f, err := os.OpenFile(cfg.Path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
-	if err != nil {
-		return nil, fmt.Errorf("access log open error: %w", err)
+	openedFilesMu.Lock()
+
+	var io AccessLogIO
+	if opened, ok := openedFiles[cfg.Path]; ok {
+		io = opened
+	} else {
+		f, err := os.OpenFile(cfg.Path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
+		if err != nil {
+			return nil, fmt.Errorf("access log open error: %w", err)
+		}
+		io = &File{File: f}
+		openedFiles[cfg.Path] = io
 	}
-	return NewAccessLogger(parent, &File{File: f}, cfg), nil
+
+	openedFilesMu.Unlock()
+	return NewAccessLogger(parent, io, cfg), nil
 }