feat: add ability to set custom multipart boundary value (#820)

middleware.go

@@ -429,6 +429,13 @@ func handleMultipart(c *Client, r *Request) error {
 	r.bodyBuf = acquireBuffer()
 	w := multipart.NewWriter(r.bodyBuf)
 
+	// Set boundary if not set by user
+	if r.multipartBoundary != "" {
+		if err := w.SetBoundary(r.multipartBoundary); err != nil {
+			return err
+		}
+	}
+
 	for k, v := range c.FormData {
 		for _, iv := range v {
 			if err := w.WriteField(k, iv); err != nil {

request.go

@@ -69,6 +69,7 @@ type Request struct {
 	bodyBuf             *bytes.Buffer
 	clientTrace         *clientTrace
 	log                 Logger
+	multipartBoundary   string
 	multipartFiles      []*File
 	multipartFields     []*MultipartField
 	retryConditions     []RetryConditionFunc
@@ -458,6 +459,15 @@ func (r *Request) SetMultipartFields(fields ...*MultipartField) *Request {
 	return r
 }
 
+// SetMultipartBoundary method sets the custom multipart boundary for the multipart request.
+// Typically, the `mime/multipart` package generates a random multipart boundary, if not provided.
+//
+// Since v2.15.0
+func (r *Request) SetMultipartBoundary(boundary string) *Request {
+	r.multipartBoundary = boundary
+	return r
+}
+
 // SetContentLength method sets the HTTP header `Content-Length` value for current request.
 // By default Resty won't set `Content-Length`. Also you have an option to enable for every
 // request.

request_test.go

@@ -1034,6 +1034,29 @@ func TestMultiPartMultipartFields(t *testing.T) {
 	assertEqual(t, true, strings.Contains(responseStr, "upload-file-2.json"))
 }
 
+func TestMultiPartCustomBoundary(t *testing.T) {
+	ts := createFormPostServer(t)
+	defer ts.Close()
+	defer cleanupFiles(".testdata/upload")
+
+	_, err := dclr().
+		SetMultipartFormData(map[string]string{"first_name": "Jeevanandam", "last_name": "M", "zip_code": "00001"}).
+		SetMultipartBoundary(`"my-custom-boundary"`).
+		SetBasicAuth("myuser", "mypass").
+		Post(ts.URL + "/profile")
+
+	assertEqual(t, "mime: invalid boundary character", err.Error())
+
+	resp, err := dclr().
+		SetMultipartFormData(map[string]string{"first_name": "Jeevanandam", "last_name": "M", "zip_code": "00001"}).
+		SetMultipartBoundary("my-custom-boundary").
+		Post(ts.URL + "/profile")
+
+	assertError(t, err)
+	assertEqual(t, http.StatusOK, resp.StatusCode())
+	assertEqual(t, "Success", resp.String())
+}
+
 func TestGetWithCookie(t *testing.T) {
 	ts := createGetServer(t)
 	defer ts.Close()