gofiber/fiber · error

failed to copy file data

Error message

failed to copy file data: %w

What it means

addFormFile copies the file content into the multipart part via io.CopyBuffer. This wraps a failure of that copy: a read error from the file (disk I/O, read after close) or a write error from the multipart writer (body buffer/pipe failed).

Solutions

1. If using SetReader, ensure the reader stays open and readable for the whole send.
2. Inspect the wrapped cause to distinguish read (file) vs write (body) failures.
3. Retry idempotent uploads on transient I/O errors; surface permanent ones to the caller.

if err := c.Post(url, req); err != nil {
    var perr *fs.PathError
    if errors.As(err, &perr) { /* disk read failure */ }
}

When it happens

Trigger: The file is read during send and the underlying disk returns an error; the file reader was closed by another goroutine mid-copy; the body writer's write side fails (broken pipe, buffer overflow on streaming).

Common situations: Network-backed or FUSE filesystems that error mid-read; concurrent close of the file handle; uploading from a stream whose producer errored; very large files where a transient I/O fault occurs.

Related errors

• create file error
• failed to close multipart writer
• open file error
• write formdata error
• boundary generation

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/b64ae50ed31fcd75. Report an issue: GitHub.

Appendix: source

Thrown at client/hooks.go:322

	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
}

// parserResponseCookie parses the Set-Cookie headers from the response and stores them.
func parserResponseCookie(c *Client, resp *Response, req *Request) error {
	var err error
	for key, value := range resp.RawResponse.Header.Cookies() {
		cookie := fasthttp.AcquireCookie()
		if err = cookie.ParseBytes(value); err != nil {
			fasthttp.ReleaseCookie(cookie)
			break
		}
		cookie.SetKeyBytes(key)
		resp.cookie = append(resp.cookie, cookie)
	}

View on GitHub (pinned to a105acad6c)