golang template upload file how to validate the file is empty

Question

I am a beginner with go-lang. I met a problem when I uploaded a file with html template. I google a lot but not solved.

<input  type="file" name="myfile"/>

Use func (*Request) FormFile get the file.

file, header, err := req.FormFile("receipt")

But how to validate the file whether it is empty from server side? I know I can read request.Body to find whether myfile is empty.

Is there a better way to implement it?

score 3 · Accepted Answer · edited May 23 '17 at 11:54

I don't think it's possible to know the size of the file until you read it. See this answer:

To read the file content is the only reliable way. Having said that, if the content-lenght is present and is too big, to close the connection would be a reasonable thing to do.

So I guess you'll have to read a part of the content into a small temporary buffer and look at the size.

If you want to validate, whether the user even sent a file, you can check against http.ErrMissingFile:

    file, header, err := r.FormFile("f")
    switch err {
    case nil:
        // do nothing
    case http.ErrMissingFile:
        log.Println("no file")
    default:
        log.Println(err)
    }

score 1 · Answer 2 · answered Jul 04 '16 at 02:23

No, you can't find the length of a file without reading it. (You can try to use Content-Length header, but you have to know that it can be modified and thus not reliable).

Here is how you can get the file size:

file, handler, err := r.FormFile("receipt") // r is *http.Request
var buff bytes.Buffer
fileSize, err := buff.ReadFrom(file)
fmt.Println(fileSize) // this will return you a file size.

score 0 · Answer 3 · answered Apr 22 '18 at 07:52

You can read the file size from the header variable (in your example), which is a FileHeader type, returned by FormValue, and contains the size of the file:

file, header, err := req.FormFile("receipt")
if err != nil || header.Size == 0 {
    // file was not sent
} else {
    // process file
}

Though I'm not sure how reliable is this data in terms of security (I guess an attacker could fake the headers sent to the server maliciously).

golang template upload file how to validate the file is empty

3 Answers3