There are a number of tutorials about posting files using http.Request<\/code> in go, but almost invariably they start like this:
file, err := os.Open(path)
if err != nil {
return nil, err
}
fileContents, err := ioutil.ReadAll(file)
If you need to set Content-Length
, it can be done manually. The following snippet is an example of uploading file and extra parameters as a stream (the code based on Buffer-less Multipart POST in Golang )
//NOTE: for simplicity, error check is omitted
func uploadLargeFile(uri, filePath string, chunkSize int, params map[string]string) {
//open file and retrieve info
file, _ := os.Open(filePath)
fi, _ := file.Stat()
defer file.Close()
//buffer for storing multipart data
byteBuf := &bytes.Buffer{}
//part: parameters
mpWriter := multipart.NewWriter(byteBuf)
for key, value := range params {
_ = mpWriter.WriteField(key, value)
}
//part: file
mpWriter.CreateFormFile("file", fi.Name())
contentType := mpWriter.FormDataContentType()
nmulti := byteBuf.Len()
multi := make([]byte, nmulti)
_, _ = byteBuf.Read(multi)
//part: latest boundary
//when multipart closed, latest boundary is added
mpWriter.Close()
nboundary := byteBuf.Len()
lastBoundary := make([]byte, nboundary)
_, _ = byteBuf.Read(lastBoundary)
//calculate content length
totalSize := int64(nmulti) + fi.Size() + int64(nboundary)
log.Printf("Content length = %v byte(s)\n", totalSize)
//use pipe to pass request
rd, wr := io.Pipe()
defer rd.Close()
go func() {
defer wr.Close()
//write multipart
_, _ = wr.Write(multi)
//write file
buf := make([]byte, chunkSize)
for {
n, err := file.Read(buf)
if err != nil {
break
}
_, _ = wr.Write(buf[:n])
}
//write boundary
_, _ = wr.Write(lastBoundary)
}()
//construct request with rd
req, _ := http.NewRequest("POST", uri, rd)
req.Header.Set("Content-Type", contentType)
req.ContentLength = totalSize
//process request
client := &http.Client{}
resp, err := client.Do(req)
if err != nil {
log.Fatal(err)
} else {
log.Println(resp.StatusCode)
log.Println(resp.Header)
body := &bytes.Buffer{}
_, _ = body.ReadFrom(resp.Body)
resp.Body.Close()
log.Println(body)
}
}
Turns out you can actually pass the *File
(or any stream-like) object straight into NewRequest
.
Notice the caveat however, that NewRequest (as shown here: https://golang.org/src/net/http/request.go?s=21674:21746#L695 ) won't actually set the ContentLength
unless the stream is explicitly one of:
Since *File
isn't one of these, the request will be sent without a content length unless you manually set it, which may cause some servers to discard the body of the incoming request, resulting in a body of ''
on the server when it appears to have been correctly sent from the go side.
If the request must have a Content-Length
header (most file hosts reject upload requests without it), and you want to upload the file as a stream (without loading all to memory), standard library won't help you, and you have to calculate it yourself.
Here's a minimal working example (without error checks) that uses io.MultiReader
to connect os.File
with other fields while keeping a tab on the request size.
It supports regular fields (with string content) and file fields, and calculates the total request body size. It's easy to extend it with other value types by simply adding a new case
branch.
import (
"crypto/rand"
"fmt"
"io"
"io/fs"
"mime"
"path/filepath"
"strings"
)
type multipartPayload struct {
headers map[string]string
body io.Reader
size int64
}
func randomBoundary() string {
var buf [8]byte
_, err := io.ReadFull(rand.Reader, buf[:])
if err != nil {
panic(err)
}
return fmt.Sprintf("%x", buf[:])
}
// Multipart request has the following structure:
// POST /upload HTTP/1.1
// Other-Headers: ...
// Content-Type: multipart/form-data; boundary=$boundary
// \r\n
// --$boundary\r\n 👈 request body starts here
// Content-Disposition: form-data; name="field1"\r\n
// Content-Type: text/plain; charset=utf-8\r\n
// Content-Length: 4\r\n
// \r\n
// $content\r\n
// --$boundary\r\n
// Content-Disposition: form-data; name="field2"\r\n
// ...
// --$boundary--\r\n
func prepareMultipartPayload(fields map[string]interface{}) (*multipartPayload, error) {
boundary := randomBoundary()
headers := make(map[string]string)
totalSize := 0
headers["Content-Type"] = fmt.Sprintf("multipart/form-data; boundary=%s", boundary)
parts := make([]io.Reader, 0)
CRLF := "\r\n"
fieldBoundary := "--" + boundary + CRLF
for k, v := range fields {
parts = append(parts, strings.NewReader(fieldBoundary))
totalSize += len(fieldBoundary)
if v == nil {
continue
}
switch v.(type) {
case string:
header := fmt.Sprintf(`Content-Disposition: form-data; name="%s"`, k)
parts = append(
parts,
strings.NewReader(header+CRLF+CRLF),
strings.NewReader(v.(string)),
strings.NewReader(CRLF),
)
totalSize += len(header) + 2*len(CRLF) + len(v.(string)) + len(CRLF)
continue
case fs.File:
stat, _ := v.(fs.File).Stat()
contentType := mime.TypeByExtension(filepath.Ext(stat.Name()))
header := strings.Join([]string{
fmt.Sprintf(`Content-Disposition: form-data; name="%s"; filename="%s"`, k, stat.Name()),
fmt.Sprintf(`Content-Type: %s`, contentType),
fmt.Sprintf(`Content-Length: %d`, stat.Size()),
}, CRLF)
parts = append(
parts,
strings.NewReader(header+CRLF+CRLF),
v.(fs.File),
strings.NewReader(CRLF),
)
totalSize += len(header) + 2*len(CRLF) + int(stat.Size()) + len(CRLF)
continue
}
}
finishBoundary := "--" + boundary + "--" + CRLF
parts = append(parts, strings.NewReader(finishBoundary))
totalSize += len(finishBoundary)
headers["Content-Length"] = fmt.Sprintf("%d", totalSize)
return &multipartPayload{headers, io.MultiReader(parts...), int64(totalSize)}, nil
}
then prepare the request, set the content length and send it:
file, err := os.Open("/path/to/file.ext")
if err != nil {
return nil, err
}
defer file.Close()
up, err := prepareMultipartPayload(map[string]interface{}{
"a_string": "field",
"another_field": "yep",
"file": file, // you can have multiple file fields
})
r, _ := http.NewRequest("POST", "https://example.com/upload", up.body)
for k, v := range up.headers {
r.Header.Set(k, v)
}
r.ContentLength = up.size
c := http.Client{}
res, err := c.Do(r)
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