Adjust priority of outbound DATA frames so that it does not starve others

lib/spdylay_outbound_item.h

@@ -50,6 +50,17 @@ typedef struct {
   spdylay_frame_category frame_cat;
   void *frame;
   void *aux_data;
+  /* Initial priority of this item */
+  int inipri;
+  /* The current priority of this item. Initially, pri equals to
+     inipri. The item is chosen from the queue based on pri and
+     seq. For control frames, they consist of just 1 frame and pri
+     does not change. For DATA frame, they could split up to several
+     frames. After sending a frame, the pri is increased by 1. If it
+     becomes more than lowest priority, then it returns back to inipri
+     and do the same sequence again and again. By doing this, the
+     higher priority long DATA frames don't starve the lower
+     prioritized streams. */
   int pri;
   int64_t seq;
 } spdylay_outbound_item;

lib/spdylay_session.c

@@ -444,6 +444,7 @@ int spdylay_session_add_frame(spdylay_session *session,
     free(item);
     return r;
   }
+  item->inipri = item->pri;
   return 0;
 }
 
@@ -1246,6 +1247,20 @@ spdylay_outbound_item* spdylay_session_pop_next_ob_item
   }
 }
 
+/*
+ * Adjust priority of item so that the higher priority long DATA
+ * frames don't starve lower priority streams.
+ */
+static void spdylay_outbound_item_adjust_pri(spdylay_session *session,
+                                             spdylay_outbound_item *item)
+{
+  if(item->pri > spdylay_session_get_pri_lowest(session)) {
+    item->pri = item->inipri;
+  } else {
+    ++item->pri;
+  }
+}
+
 /*
  * Called after a frame is sent.
  *
@@ -1400,6 +1415,7 @@ static int spdylay_session_after_frame_sent(spdylay_session *session)
     } else {
       spdylay_outbound_item* next_item;
       next_item = spdylay_session_get_next_ob_item(session);
+      spdylay_outbound_item_adjust_pri(session, 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. */