如何使用os.Open（）的返回值作为http.Post（）的第三个参数并设置Content-Length？

The third parameter of http.Post() allows io.Reader and that means the return value of os.Open() should work. But the below code gets unexpected result, in other words, it won't set Content-Length properly. Perhaps File type doesn't implement something. Is there any proper way to set Content-Length with *File?

package main

import (
    "bytes"
    "io/ioutil"
    "log"
    "net/http"
    "net/http/httptest"
    "os"
)

var sample = []byte(`hello`)

func main() {
    ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        log.Println(r.Header)
        if int(r.ContentLength) != len(sample) {
            log.Fatal("Unexpected Content-Length:", r.ContentLength)
        }
        w.Header().Set("Content-Type", "application/json")
        w.Write([]byte(`{}`))
    }))
    defer ts.Close()

    file, err := ioutil.TempFile(os.TempDir(), "")
    if err != nil {
        log.Fatal(err)
    }
    defer os.Remove(file.Name())
    file.Write(sample)

    // This works
    buf, err := ioutil.ReadFile(file.Name())
    if err != nil {
        log.Fatal(err)
    }
    _, err = http.Post(ts.URL, "application/octet-stream", bytes.NewBuffer(buf))
    if err != nil {
        log.Fatal(err)
    }

    // This looks fine in my opinion, though it doesn't set Content-Length
    f, err := os.Open(file.Name())
    if err != nil {
        log.Fatal(err)
    }
    _, err = http.Post(ts.URL, "application/octet-stream", f)
    if err != nil {
        log.Fatal(err)
    }
}

Output:

2009/11/10 23:00:00 map[Content-Type:[application/octet-stream] Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1] Content-Length:[5]]
2009/11/10 23:00:00 map[Content-Type:[application/octet-stream] Accept-Encoding:[gzip] User-Agent:[Go-http-client/1.1]]
2009/11/10 23:00:00 Unexpected Content-Length:-1

https://play.golang.org/p/hJLN2H9Y9p

If you look at source for NewRequest you can see that contentLength is handled specially for specific input types, and the file reader isn't one of them. You'll have to manually set the Content-Length header if that's important [chunked should also work fine, unless you're sending to an old server impl].

If you want to add a add the Content-Length, you need to stat the file to get the size. The ContentLength isn't calculated automatically because an os.File may not have a useful size.

f, err := os.Open(file.Name())
if err != nil {
    log.Fatal(err)
}

req, err := http.NewRequest("POST", ts.URL, f)
if err != nil {
    log.Fatal(err)
}

stat, err := f.Stat()
if err != nil {
    log.Fatal(err)
}

req.ContentLength = stat.Size()
req.Header.Set("Content-Type", "application/octet-stream")

resp, err = http.Do(req)
...