Take into account remainder due to integer division when calculating cycle

lib/nghttp2_stream.c

@@ -80,6 +80,7 @@ void nghttp2_stream_init(nghttp2_stream *stream, int32_t stream_id,
   stream->queued = 0;
   stream->descendant_last_cycle = 0;
   stream->cycle = 0;
+  stream->pending_penalty = 0;
   stream->descendant_next_seq = 0;
   stream->seq = 0;
   stream->last_writelen = 0;
@@ -115,9 +116,14 @@ static int stream_subtree_active(nghttp2_stream *stream) {
 /*
  * Returns next cycle for |stream|.
  */
-static uint64_t stream_next_cycle(nghttp2_stream *stream, uint64_t last_cycle) {
-  return last_cycle +
-         stream->last_writelen * NGHTTP2_MAX_WEIGHT / (uint32_t)stream->weight;
+static void stream_next_cycle(nghttp2_stream *stream, uint64_t last_cycle) {
+  size_t penalty;
+
+  penalty =
+      stream->last_writelen * NGHTTP2_MAX_WEIGHT + stream->pending_penalty;
+
+  stream->cycle = last_cycle + penalty / (uint32_t)stream->weight;
+  stream->pending_penalty = penalty % (uint32_t)stream->weight;
 }
 
 static int stream_obq_push(nghttp2_stream *dep_stream, nghttp2_stream *stream) {
@@ -125,7 +131,6 @@ static int stream_obq_push(nghttp2_stream *dep_stream, nghttp2_stream *stream) {
 
   for (; dep_stream && !stream->queued;
        stream = dep_stream, dep_stream = dep_stream->dep_prev) {
-    stream->cycle =
     stream_next_cycle(stream, dep_stream->descendant_last_cycle);
     stream->seq = dep_stream->descendant_next_seq++;
 
@@ -169,6 +174,7 @@ static void stream_obq_remove(nghttp2_stream *stream) {
 
     stream->queued = 0;
     stream->cycle = 0;
+    stream->pending_penalty = 0;
     stream->descendant_last_cycle = 0;
     stream->last_writelen = 0;
 
@@ -209,7 +215,6 @@ void nghttp2_stream_reschedule(nghttp2_stream *stream) {
   for (; dep_stream; stream = dep_stream, dep_stream = dep_stream->dep_prev) {
     nghttp2_pq_remove(&dep_stream->obq, &stream->pq_entry);
 
-    stream->cycle =
     stream_next_cycle(stream, dep_stream->descendant_last_cycle);
     stream->seq = dep_stream->descendant_next_seq++;
 
@@ -225,8 +230,8 @@ void nghttp2_stream_reschedule(nghttp2_stream *stream) {
 void nghttp2_stream_change_weight(nghttp2_stream *stream, int32_t weight) {
   nghttp2_stream *dep_stream;
   uint64_t last_cycle;
-  uint64_t cycle;
   int32_t old_weight;
+  size_t wlen_penalty;
 
   if (stream->weight == weight) {
     return;
@@ -247,20 +252,28 @@ void nghttp2_stream_change_weight(nghttp2_stream *stream, int32_t weight) {
     return;
   }
 
-  last_cycle =
-      stream->cycle -
-      stream->last_writelen * NGHTTP2_MAX_WEIGHT / (uint32_t)old_weight;
+  nghttp2_pq_remove(&dep_stream->obq, &stream->pq_entry);
 
-  cycle = stream_next_cycle(stream, last_cycle);
+  wlen_penalty = stream->last_writelen * NGHTTP2_MAX_WEIGHT;
 
-  if (cycle < dep_stream->descendant_last_cycle) {
-    cycle = dep_stream->descendant_last_cycle;
-  }
+  /* Compute old stream->pending_penalty we used to calculate
+     stream->cycle */
+  stream->pending_penalty = (stream->pending_penalty + (uint32_t)old_weight -
+                             (wlen_penalty % (uint32_t)old_weight)) %
+                            (uint32_t)old_weight;
 
-  nghttp2_pq_remove(&dep_stream->obq, &stream->pq_entry);
+  last_cycle = stream->cycle -
+               (wlen_penalty + stream->pending_penalty) / (uint32_t)old_weight;
 
-  stream->cycle = cycle;
-  stream->seq = dep_stream->descendant_next_seq++;
+  /* Now we have old stream->pending_penalty and new stream->weight in
+     place */
+  stream_next_cycle(stream, last_cycle);
+
+  if (stream->cycle < dep_stream->descendant_last_cycle) {
+    stream->cycle = dep_stream->descendant_last_cycle;
+  }
+
+  /* Continue to use same stream->seq */
 
   nghttp2_pq_push(&dep_stream->obq, &stream->pq_entry);
 

lib/nghttp2_stream.h

@@ -197,6 +197,8 @@ struct nghttp2_stream {
   int32_t local_window_size;
   /* weight of this stream */
   int32_t weight;
+  /* This is unpaid penalty (offset) when calculating cycle. */
+  uint32_t pending_penalty;
   /* sum of weight of direct descendants */
   int32_t sum_dep_weight;
   nghttp2_stream_state state;