（HTTP）通过将io.Reader从os.Open传递到http.Post（HTTP）来发布本地文件时，Content-Length为零

Question

heya everyone: Got a short question, I would like to post(http) a file. I figured, since os.Open returns an io.Reader and http.Post takes an io.Reader I do not have read the file into memory in a separate step and can just pass around the Reader. However, the Content-Length will be set to zero- which kinda makes sense but is not what I need.

file, _ := os.Open("some file")
req, _ := http.NewRequest("POST", "some url", file)
dump, _ := httputil.DumpRequestOut(req, false)
fmt.Println(string(dump))

My question: Do i have to read the file into memory (with ioutil.ReadFile or some such) and make a new reader or is there a way to pass the reader from the file directly to the Post request without the "Reading" step?

I guess I could set the Content-Length by getting it via file.Stat, but I was wondering if there is a more elegant way of doing this?

Answer 1

Short answer is: you are absolutely right about giving the os.File to http.Request and about using os.File.Stat() to get the file size and setting it on the http.Request headers. Personally I've found it to be the simplest way around.

i.e.

file, _ := os.Open("some file")
info, _ := file.Stat()
req, _ := http.NewRequest("POST", "http://bla.com", file)
req.ContentLength = info.Size()
dump, _ := httputil.DumpRequestOut(req, false)
fmt.Println(string(dump))

Also note that as per http.Request.Write() documentation (I quote):

If Body is present, Content-Length is <= 0 and TransferEncoding hasn't been set to "identity", Write adds "Transfer-Encoding: chunked" to the header. Body is closed after it is sent.

So in that case the client itself would do the (arguably) most sensible thing to do for you.