Having issues with multipart.NewWriter using io.pipe - go

Question

I'm having issues with requests being sent randomly empty. It doesn't always happen, but sometimes out of the blue, it will not send any of the multipart fields. I thought it might have to do with the upload server, so I created a local upload server to print out the request that's being sent, and it comes out empty.
I have added error checks everywhere, but no errors are being returned.
I tried to run the code with -race, but no race condition has been reported.
Edit: Update the code to use CloseWithError()
package main
import (
"fmt"
"io"
"log"
"mime/multipart"
"net/http"
"os"
)
var (
upload_url string = "https://upload.imagekit.io/api/v1/files/upload"
file_name string = "favicon-516140983.ico"
api_secret_key string = "PRIVATE_KEY"
)
func UploadMultipartFile(client *http.Client, uri, key, path string) (*http.Response, error) {
body, writer := io.Pipe()
req, err := http.NewRequest(http.MethodPost, uri, body)
if err != nil {
log.Println(err)
return nil, err
}
mwriter := multipart.NewWriter(writer)
req.Header.Add("Content-Type", mwriter.FormDataContentType())
req.SetBasicAuth(api_secret_key, "")
go func() {
var err error
defer func() {
if err != nil {
writer.CloseWithError(err)
} else {
writer.Close()
}
}()
var file *os.File
file, err = os.Open(path)
if err != nil {
return
}
defer file.Close()
if err = mwriter.WriteField("fileName", file_name); err != nil {
return
}
var w io.Writer
w, err = mwriter.CreateFormFile("file", path)
if err != nil {
return
}
var written int64
if written, err = io.Copy(w, file); err != nil {
err = fmt.Errorf("error copying %s (%d bytes written): %v", path, written, err)
return
}
if err = mwriter.Close(); err != nil {
return
}
}()
resp, err := client.Do(req)
if err != nil {
return nil, err
}
return resp, nil
}
func main() {
path, _ := os.Getwd()
path += "/" + file_name
client := &http.Client{}
resp, err := UploadMultipartFile(client, upload_url, "file", path)
if err != nil {
log.Println(err)
} else {
fmt.Println(resp.StatusCode)
fmt.Println(resp.Header)
_, err := io.Copy(os.Stdout, resp.Body)
if err != nil {
log.Fatal(err)
}
resp.Body.Close()
}
}