gofiber/fiber · error

open file error: %w

Error message

open file error: %w

What it means

Returned in addFormFile (client/hooks.go:308) when os.Open(f.path) fails for a file added via Request.SetFiles (path-based, no reader). The client opens each file lazily at body-build time; any open error — missing file, permission denied, path too long — is wrapped here.

Source

Thrown at client/hooks.go:308

		if f.fieldName == "" {
			f.fieldName = "file" + strconv.Itoa(i+1)
		}

		if err := addFormFile(mw, f, fileBuf); err != nil {
			return err
		}
	}

	return nil
}

func addFormFile(mw *multipart.Writer, f *File, fileBuf *[]byte) error {
	// If reader is not set, open the file.
	if f.reader == nil {
		var err error
		f.reader, err = os.Open(f.path)
		if err != nil {
			return fmt.Errorf("open file error: %w", err)
		}
	}

	// Ensure the file reader is always closed after copying.
	defer f.reader.Close() //nolint:errcheck // not needed

	// Create form file and copy the content.
	w, err := mw.CreateFormFile(f.fieldName, f.name)
	if err != nil {
		return fmt.Errorf("create file error: %w", err)
	}

	if _, err := io.CopyBuffer(w, f.reader, *fileBuf); err != nil {
		return fmt.Errorf("failed to copy file data: %w", err)
	}

	return nil
}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Verify the file exists and is readable (os.Stat / os.Open) before calling SetFiles.
2. Use absolute paths or resolve relative paths against a known base directory.
3. Ensure the process user has read permission on the file.
4. If the file is transient, use SetFileReader with an in-memory/stream reader so no filesystem open is needed.

Example fix

// before — relative path that may not resolve
req.SetFiles("./uploads/owner.pdf")

// after — absolute path validated up front
abs, _ := filepath.Abs("./uploads/owner.pdf")
if _, err := os.Stat(abs); err != nil {
    return fmt.Errorf("upload file missing: %w", err)
}
req.SetFiles(abs)

// Verify a file is readable before adding it to the upload.
func readableFile(path string) error {
    f, err := os.Open(path)
    if err != nil {
        return err
    }
    f.Close()
    return nil
}

if _, err := os.Stat(path); err != nil {
    return fmt.Errorf("upload file not available: %w", err)
}
req.SetFiles(path)

When it happens

Trigger: Calling Request.SetFiles("/nonexistent/path") or with a path the process cannot read; a relative path resolved from an unexpected working directory; a file deleted between adding it and sending the request; insufficient filesystem permissions.

Common situations: Deploying with a file path that exists in dev but not in the container; relative paths that break under a different CWD; tempfile already cleaned up; permission mismatch between the app user and the file owner; path from user input that points outside allowed dirs.

Related errors

• failed to copy file data: %w
• failed to check directory: %w
• failed to create directory: %w
• failed to create file: %w
• failed to close multipart writer: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/f722538c35e2f63e.json. Report an issue: GitHub.