Fix function signature, use default random boundary (#3422)

Fixes the function signature of `parseMultipartResponse` and uses the
default random boundary when creating a new multipart response.

mediaapi/routing/download.go

@@ -33,7 +33,6 @@ import (
 	"sync"
 	"unicode"
 
-	"github.com/google/uuid"
 	"github.com/matrix-org/dendrite/mediaapi/fileutils"
 	"github.com/matrix-org/dendrite/mediaapi/storage"
 	"github.com/matrix-org/dendrite/mediaapi/thumbnailer"
@@ -400,22 +399,16 @@ func (r *downloadRequest) respondFromLocalFile(
 }
 
 func multipartResponse(w http.ResponseWriter, r *downloadRequest, contentType string, responseFile io.Reader) (int64, error) {
-	// Update the header to be multipart/mixed; boundary=$randomBoundary
-	boundary := uuid.NewString()
-	w.Header().Set("Content-Type", "multipart/mixed; boundary="+boundary)
-
-	w.Header().Del("Content-Length") // let Go handle the content length
 	mw := multipart.NewWriter(w)
+	// Update the header to be multipart/mixed; boundary=$randomBoundary
+	w.Header().Set("Content-Type", "multipart/mixed; boundary="+mw.Boundary())
+	w.Header().Del("Content-Length") // let Go handle the content length
 	defer func() {
 		if err := mw.Close(); err != nil {
 			r.Logger.WithError(err).Error("Failed to close multipart writer")
 		}
 	}()
 
-	if err := mw.SetBoundary(boundary); err != nil {
-		return 0, fmt.Errorf("failed to set multipart boundary: %w", err)
-	}
-
 	// JSON object part
 	jsonWriter, err := mw.CreatePart(textproto.MIMEHeader{
 		"Content-Type": {"application/json"},
@@ -858,7 +851,7 @@ func (r *downloadRequest) fetchRemoteFile(
 	var reader io.Reader
 	var parseErr error
 	if isAuthed {
-		parseErr, contentLength, reader = parseMultipartResponse(r, resp, maxFileSizeBytes)
+		contentLength, reader, parseErr = parseMultipartResponse(r, resp, maxFileSizeBytes)
 	} else {
 		// The reader returned here will be limited either by the Content-Length
 		// and/or the configured maximum media size.
@@ -928,48 +921,48 @@ func (r *downloadRequest) fetchRemoteFile(
 	return types.Path(finalPath), duplicate, nil
 }
 
-func parseMultipartResponse(r *downloadRequest, resp *http.Response, maxFileSizeBytes config.FileSizeBytes) (error, int64, io.Reader) {
+func parseMultipartResponse(r *downloadRequest, resp *http.Response, maxFileSizeBytes config.FileSizeBytes) (int64, io.Reader, error) {
 	_, params, err := mime.ParseMediaType(resp.Header.Get("Content-Type"))
 	if err != nil {
-		return err, 0, nil
+		return 0, nil, err
 	}
 	if params["boundary"] == "" {
-		return fmt.Errorf("no boundary header found on media %s from %s", r.MediaMetadata.MediaID, r.MediaMetadata.Origin), 0, nil
+		return 0, nil, fmt.Errorf("no boundary header found on media %s from %s", r.MediaMetadata.MediaID, r.MediaMetadata.Origin)
 	}
 	mr := multipart.NewReader(resp.Body, params["boundary"])
 
 	// Get the first, JSON, part
 	p, err := mr.NextPart()
 	if err != nil {
-		return err, 0, nil
+		return 0, nil, err
 	}
 	defer p.Close() // nolint: errcheck
 
 	if p.Header.Get("Content-Type") != "application/json" {
-		return fmt.Errorf("first part of the response must be application/json"), 0, nil
+		return 0, nil, fmt.Errorf("first part of the response must be application/json")
 	}
 	// Try to parse media meta information
 	meta := mediaMeta{}
 	if err = json.NewDecoder(p).Decode(&meta); err != nil {
-		return err, 0, nil
+		return 0, nil, err
 	}
 	defer p.Close() // nolint: errcheck
 
 	// Get the actual media content
 	p, err = mr.NextPart()
 	if err != nil {
-		return err, 0, nil
+		return 0, nil, err
 	}
 
 	redirect := p.Header.Get("Location")
 	if redirect != "" {
-		return fmt.Errorf("Location header is not yet supported"), 0, nil
+		return 0, nil, fmt.Errorf("Location header is not yet supported")
 	}
 
 	contentLength, reader, err := r.GetContentLengthAndReader(p.Header.Get("Content-Length"), p, maxFileSizeBytes)
 	// For multipart requests, we need to get the Content-Type of the second part, which is the actual media
 	r.MediaMetadata.ContentType = types.ContentType(p.Header.Get("Content-Type"))
-	return err, contentLength, reader
+	return contentLength, reader, err
 }
 
 // contentDispositionFor returns the Content-Disposition for a given

mediaapi/routing/download_test.go

@@ -35,7 +35,7 @@ func Test_Multipart(t *testing.T) {
 	assert.NoError(t, err)
 	defer resp.Body.Close()
 	// contentLength is always 0, since there's no Content-Length header on the multipart part.
-	err, _, reader := parseMultipartResponse(r, resp, 1000)
+	_, reader, err := parseMultipartResponse(r, resp, 1000)
 	assert.NoError(t, err)
 	gotResponse, err := io.ReadAll(reader)
 	assert.NoError(t, err)