Reintroduce priority adjustment for DATA frame

This mechanism existed but was deleted. We bring it back in order to
prevent lower priority streams from starving.

lib/nghttp2_outbound_item.h

@@ -48,7 +48,12 @@ typedef struct {
   nghttp2_frame_category frame_cat;
   void *frame;
   void *aux_data;
-  int pri;
+  /* The priority used in priority comparion */
+  int32_t pri;
+  /* The initial priority */
+  int32_t inipri;
+  /* The amount of priority decrement in next time */
+  uint32_t pri_decay;
   int64_t seq;
 } nghttp2_outbound_item;
 

lib/nghttp2_session.c

@@ -424,7 +424,8 @@ static int outbound_item_update_pri
       return 0;
     }
   }
-  item->pri = stream->pri;
+  /* We only update initial priority */
+  item->inipri = stream->pri;
   return 1;
 }
 
@@ -442,10 +443,12 @@ void nghttp2_session_reprioritize_stream
     return;
   }
   stream->pri = pri;
+  /* For submitted frames, we only update initial priority, so the
+     structure of the queue will remain unchanged. */
   nghttp2_pq_update(&session->ob_pq, update_stream_pri, stream);
   nghttp2_pq_update(&session->ob_ss_pq, update_stream_pri, stream);
   if(stream->deferred_data) {
-    stream->deferred_data->pri = pri;
+    stream->deferred_data->inipri = pri;
   }
   if(session->aob.item) {
     outbound_item_update_pri(session->aob.item, stream);
@@ -471,6 +474,7 @@ int nghttp2_session_add_frame(nghttp2_session *session,
   item->seq = session->next_seq++;
   /* Set priority to the default value at the moment. */
   item->pri = NGHTTP2_PRI_DEFAULT;
+  item->pri_decay = 1;
   if(frame_cat == NGHTTP2_CAT_CTRL) {
     nghttp2_frame *frame = (nghttp2_frame*)abs_frame;
     nghttp2_stream *stream = NULL;
@@ -546,6 +550,7 @@ int nghttp2_session_add_frame(nghttp2_session *session,
     free(item);
     return r;
   }
+  item->inipri = item->pri;
   return 0;
 }
 
@@ -1385,6 +1390,23 @@ nghttp2_outbound_item* nghttp2_session_pop_next_ob_item
   }
 }
 
+/*
+ * Adjust priority of the |item|. In order to prevent the low priority
+ * streams from starving, lower the priority of the |item| by
+ * item->pri_decay. If the resulting priority exceeds
+ * NGHTTP2_PRI_DEFAULT, back to the original priority.
+ */
+static void adjust_pri(nghttp2_outbound_item *item)
+{
+  if(item->pri >= (int32_t)(NGHTTP2_PRI_LOWEST - (item->pri_decay - 1))) {
+    item->pri = item->inipri;
+    item->pri_decay = 1;
+    return;
+  }
+  item->pri += (int32_t)(item->pri_decay - 1);
+  item->pri_decay <<= 1;
+}
+
 /*
  * Called after a frame is sent.
  *
@@ -1563,6 +1585,7 @@ static int nghttp2_session_after_frame_sent(nghttp2_session *session)
     } else {
       nghttp2_outbound_item* next_item;
       next_item = nghttp2_session_get_next_ob_item(session);
+      adjust_pri(session->aob.item);
       /* If priority of this stream is higher or equal to other stream
          waiting at the top of the queue, we continue to send this
          data. */

tests/nghttp2_session_test.c

@@ -1869,13 +1869,13 @@ void test_nghttp2_session_reprioritize_stream(void)
   nghttp2_session_reprioritize_stream(session, stream, 120);
 
   CU_ASSERT(session->aob.item != NULL);
-  CU_ASSERT(120 == session->aob.item->pri);
+  CU_ASSERT(120 == session->aob.item->inipri);
   CU_ASSERT(120 == stream->pri);
   CU_ASSERT(5000 == nghttp2_session_get_stream(session, 1)->pri);
   item = nghttp2_session_get_next_ob_item(session);
-  CU_ASSERT(120 == item->pri);
+  CU_ASSERT(5000 == item->inipri);
   CU_ASSERT(NGHTTP2_HEADERS == OB_CTRL_TYPE(item));
-  CU_ASSERT(3 == OB_CTRL(item)->hd.stream_id);
+  CU_ASSERT(1 == OB_CTRL(item)->hd.stream_id);
 
   nghttp2_session_del(session);