When receiving a file from a form upload, it is sometimes required to be able to grab the individual file content length, rather than limiting the size of the entire request body.

However, when using http.Request.FormFile() you are given a multipart.File, which is implemented as either an io.SectionReader or os.File depending on whether the file was larger than the defined max multipart memory size. The SectionReader is wrapped in a little struct sectionReadCloser for a no-op close.

After looking at and implementing this size check. I was wishing to have a simpler method to implement checking the size, where right now to check, you can upgrade the sectionReadCloser into a "sizer". When it's a os.File, you could either Stat() and check the size or seek to the end, grab the number of bytes read, seek to the start, and then proceed.

Example:

func uploadHandler(w http.ResponseWriter, r *http.Request) {
	if err := r.ParseMultipartForm(); err != nil {
		http.Error(w, http.StatusText(http.InternalServerError()), http.InternalServerError())
		return
	}

	file, _, err := r.FormFile("fileupload")

	type sizer interface {
		Size() int64
	}

	var size int64
	sz, ok := file.(sizer)
	if !ok {
		// we likely have a os.File here
		// we can Stat()
		//   - FileInfo has Size()
		// alternatively
		//   - size, err = Seek(0, 2) (bytes to end)
		//   - _, err = Seek(0, 0)
	} else {
		size = sz.Size()
	}

	// use the size for setting Content-Length when forwarding upload to a object store
	// use it for logging time/size ratio to determine upload connection
	// etc...
}

If possible, adding Size() directly to the multipart.File interface would be great, but in some conversation on Gopher Slack, it was pointed out this would violate the Go 1 Compat. Since this is the case, I'd be open to other ideas here, but at the very least I would like to propose wrapping the os.File that is returned in a private struct similar to sectionReadCloser to add a Size() method to make multipart.File a single interface upgrade to get the file size.