Allow NGHTTP2_ERR_PAUSE from nghttp2_data_source_read_callback

lib/includes/nghttp2/nghttp2.h

@@ -860,8 +860,13 @@ typedef enum {
  * achieved by returning :enum:`NGHTTP2_ERR_DEFERRED` without reading
  * any data in this invocation.  The library removes DATA frame from
  * the outgoing queue temporarily.  To move back deferred DATA frame
- * to outgoing queue, call `nghttp2_session_resume_data()`.  In case
- * of error, there are 2 choices. Returning
+ * to outgoing queue, call `nghttp2_session_resume_data()`.
+ *
+ * If the application just wants to return from
+ * `nghttp2_session_send()` or `nghttp2_session_mem_send()` without
+ * sending anything, return :enum:`NGHTTP2_ERR_PAUSE`.
+ *
+ * In case of error, there are 2 choices. Returning
  * :enum:`NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` will close the stream
  * by issuing RST_STREAM with :enum:`NGHTTP2_INTERNAL_ERROR`.  If a
  * different error code is desirable, use

lib/nghttp2_session.c

@@ -2263,6 +2263,9 @@ static int session_prep_frame(nghttp2_session *session,
     rv = nghttp2_session_pack_data(session, &session->aob.framebufs,
                                    next_readmax, frame, &item->aux_data.data,
                                    stream);
+    if (rv == NGHTTP2_ERR_PAUSE) {
+      return rv;
+    }
     if (rv == NGHTTP2_ERR_DEFERRED) {
       rv = nghttp2_stream_defer_item(stream, NGHTTP2_STREAM_FLAG_DEFERRED_USER);
 
@@ -2918,6 +2921,9 @@ static ssize_t nghttp2_session_mem_send_internal(nghttp2_session *session,
       }
 
       rv = session_prep_frame(session, item);
+      if (rv == NGHTTP2_ERR_PAUSE) {
+        return 0;
+      }
       if (rv == NGHTTP2_ERR_DEFERRED) {
         DEBUGF(fprintf(stderr, "send: frame transmission deferred\n"));
         break;
@@ -7020,7 +7026,8 @@ int nghttp2_session_pack_data(nghttp2_session *session, nghttp2_bufs *bufs,
       &aux_data->data_prd.source, session->user_data);
 
   if (payloadlen == NGHTTP2_ERR_DEFERRED ||
-      payloadlen == NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE) {
+      payloadlen == NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE ||
+      payloadlen == NGHTTP2_ERR_PAUSE) {
     DEBUGF(fprintf(stderr, "send: DATA postponed due to %s\n",
                    nghttp2_strerror((int)payloadlen)));
 

tests/main.c

@@ -310,6 +310,8 @@ int main(int argc _U_, char *argv[] _U_) {
                    test_nghttp2_session_cancel_from_before_frame_send) ||
       !CU_add_test(pSuite, "session_removed_closed_stream",
                    test_nghttp2_session_removed_closed_stream) ||
+      !CU_add_test(pSuite, "session_pause_data",
+                   test_nghttp2_session_pause_data) ||
       !CU_add_test(pSuite, "http_mandatory_headers",
                    test_nghttp2_http_mandatory_headers) ||
       !CU_add_test(pSuite, "http_content_length",

tests/nghttp2_session_test.c

@@ -77,6 +77,7 @@ typedef struct {
   size_t padlen;
   int begin_frame_cb_called;
   nghttp2_buf scratchbuf;
+  size_t data_source_read_cb_paused;
 } my_user_data;
 
 static const nghttp2_nv reqnv[] = {
@@ -9934,6 +9935,53 @@ void test_nghttp2_session_removed_closed_stream(void) {
   nghttp2_bufs_free(&bufs);
 }
 
+static ssize_t pause_once_data_source_read_callback(
+    nghttp2_session *session, int32_t stream_id, uint8_t *buf, size_t len,
+    uint32_t *data_flags, nghttp2_data_source *source, void *user_data) {
+  my_user_data *ud = user_data;
+  if (ud->data_source_read_cb_paused == 0) {
+    ++ud->data_source_read_cb_paused;
+    return NGHTTP2_ERR_PAUSE;
+  }
+
+  return fixed_length_data_source_read_callback(session, stream_id, buf, len,
+                                                data_flags, source, user_data);
+}
+
+void test_nghttp2_session_pause_data(void) {
+  nghttp2_session *session;
+  nghttp2_session_callbacks callbacks;
+  nghttp2_data_provider data_prd;
+  my_user_data ud;
+
+  memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
+  callbacks.send_callback = null_send_callback;
+  callbacks.on_frame_send_callback = on_frame_send_callback;
+
+  data_prd.read_callback = pause_once_data_source_read_callback;
+  ud.data_source_length = NGHTTP2_DATA_PAYLOADLEN;
+
+  nghttp2_session_server_new(&session, &callbacks, &ud);
+
+  open_recv_stream(session, 1);
+
+  CU_ASSERT(
+      0 == nghttp2_submit_data(session, NGHTTP2_FLAG_END_STREAM, 1, &data_prd));
+
+  ud.frame_send_cb_called = 0;
+  ud.data_source_read_cb_paused = 0;
+
+  CU_ASSERT(0 == nghttp2_session_send(session));
+  CU_ASSERT(0 == ud.frame_send_cb_called);
+  CU_ASSERT(NULL == session->aob.item);
+  CU_ASSERT(0 == nghttp2_session_send(session));
+  CU_ASSERT(1 == ud.frame_send_cb_called);
+  CU_ASSERT(NGHTTP2_DATA == ud.sent_frame_type);
+  CU_ASSERT(NULL == nghttp2_session_get_next_ob_item(session));
+
+  nghttp2_session_del(session);
+}
+
 static void check_nghttp2_http_recv_headers_fail(
     nghttp2_session *session, nghttp2_hd_deflater *deflater, int32_t stream_id,
     int stream_state, const nghttp2_nv *nva, size_t nvlen) {

tests/nghttp2_session_test.h

@@ -153,6 +153,7 @@ void test_nghttp2_session_repeated_priority_submission(void);
 void test_nghttp2_session_set_local_window_size(void);
 void test_nghttp2_session_cancel_from_before_frame_send(void);
 void test_nghttp2_session_removed_closed_stream(void);
+void test_nghttp2_session_pause_data(void);
 void test_nghttp2_http_mandatory_headers(void);
 void test_nghttp2_http_content_length(void);
 void test_nghttp2_http_content_length_mismatch(void);