lib/rest: fix multipart uploads stopping on context cancel

Before this change when the context was cancelled (due to
--max-duration for example) this could deadlock when uploading
multipart uploads.

This change fixes the problem by introducing another go routine to
monitor the context and close the pipe with an error when the context
errors.

backend/pcloud/pcloud.go

@@ -1137,7 +1137,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	// opts.Body=0), so upload it as a multipart form POST with
 	// Content-Length set.
 	if size == 0 {
-		formReader, contentType, overhead, err := rest.MultipartUpload(in, opts.Parameters, "content", leaf)
+		formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, opts.Parameters, "content", leaf)
 		if err != nil {
 			return errors.Wrap(err, "failed to make multipart upload for 0 length file")
 		}

backend/seafile/webapi.go

@@ -682,7 +682,7 @@ func (f *Fs) upload(ctx context.Context, in io.Reader, uploadLink, filePath stri
 		"need_idx_progress": {"true"},
 		"replace":           {"1"},
 	}
-	formReader, contentType, _, err := rest.MultipartUpload(in, parameters, "file", f.opt.Enc.FromStandardName(filename))
+	formReader, contentType, _, err := rest.MultipartUpload(ctx, in, parameters, "file", f.opt.Enc.FromStandardName(filename))
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to make multipart upload")
 	}

backend/zoho/zoho.go

@@ -647,7 +647,7 @@ func (f *Fs) upload(ctx context.Context, name string, parent string, size int64,
 	params.Set("filename", name)
 	params.Set("parent_id", parent)
 	params.Set("override-name-exist", strconv.FormatBool(true))
-	formReader, contentType, overhead, err := rest.MultipartUpload(in, nil, "content", name)
+	formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, nil, "content", name)
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to make multipart upload")
 	}

fs/operations/rc_test.go

@@ -450,6 +450,7 @@ func TestRcFsInfo(t *testing.T) {
 func TestUploadFile(t *testing.T) {
 	r, call := rcNewRun(t, "operations/uploadfile")
 	defer r.Finalise()
+	ctx := context.Background()
 
 	testFileName := "test.txt"
 	testFileContent := "Hello World"
@@ -460,7 +461,7 @@ func TestUploadFile(t *testing.T) {
 	currentFile, err := os.Open(path.Join(r.LocalName, testFileName))
 	require.NoError(t, err)
 
-	formReader, contentType, _, err := rest.MultipartUpload(currentFile, url.Values{}, "file", testFileName)
+	formReader, contentType, _, err := rest.MultipartUpload(ctx, currentFile, url.Values{}, "file", testFileName)
 	require.NoError(t, err)
 
 	httpReq := httptest.NewRequest("POST", "/", formReader)
@@ -482,7 +483,7 @@ func TestUploadFile(t *testing.T) {
 	currentFile, err = os.Open(path.Join(r.LocalName, testFileName))
 	require.NoError(t, err)
 
-	formReader, contentType, _, err = rest.MultipartUpload(currentFile, url.Values{}, "file", testFileName)
+	formReader, contentType, _, err = rest.MultipartUpload(ctx, currentFile, url.Values{}, "file", testFileName)
 	require.NoError(t, err)
 
 	httpReq = httptest.NewRequest("POST", "/", formReader)

lib/rest/rest.go

@@ -308,7 +308,7 @@ func (api *Client) Call(ctx context.Context, opts *Opts) (resp *http.Response, e
 // the int64 returned is the overhead in addition to the file contents, in case Content-Length is required
 //
 // NB This doesn't allow setting the content type of the attachment
-func MultipartUpload(in io.Reader, params url.Values, contentName, fileName string) (io.ReadCloser, string, int64, error) {
+func MultipartUpload(ctx context.Context, in io.Reader, params url.Values, contentName, fileName string) (io.ReadCloser, string, int64, error) {
 	bodyReader, bodyWriter := io.Pipe()
 	writer := multipart.NewWriter(bodyWriter)
 	contentType := writer.FormDataContentType()
@@ -343,8 +343,21 @@ func MultipartUpload(in io.Reader, params url.Values, contentName, fileName stri
 
 	multipartLength := int64(buf.Len())
 
+	// Make sure we close the pipe writer to release the reader on context cancel
+	quit := make(chan struct{})
+	go func() {
+		select {
+		case <-quit:
+			break
+		case <-ctx.Done():
+			_ = bodyWriter.CloseWithError(ctx.Err())
+		}
+	}()
+
 	// Pump the data in the background
 	go func() {
+		defer close(quit)
+
 		var err error
 
 		for key, vals := range params {
@@ -452,7 +465,7 @@ func (api *Client) callCodec(ctx context.Context, opts *Opts, request interface{
 		opts = opts.Copy()
 
 		var overhead int64
-		opts.Body, opts.ContentType, overhead, err = MultipartUpload(opts.Body, params, opts.MultipartContentName, opts.MultipartFileName)
+		opts.Body, opts.ContentType, overhead, err = MultipartUpload(ctx, opts.Body, params, opts.MultipartContentName, opts.MultipartFileName)
 		if err != nil {
 			return nil, err
 		}