Fix wrong tree operation to avoid cycle

https://tools.ietf.org/html/rfc7540#section-5.3.3 explains how to
transform dependency tree to avoid circular dependency.  Previously,
we wrongly always moved the dependent stream under the root stream.
The correct destination is the parent stream of the stream to
reprioritize.  This commit fixes this bug.

lib/nghttp2_session.c

@@ -772,7 +772,7 @@ int nghttp2_session_reprioritize_stream(
                    stream->stream_id));
 
     nghttp2_stream_dep_remove_subtree(dep_stream);
-    rv = nghttp2_stream_dep_add_subtree(&session->root, dep_stream);
+    rv = nghttp2_stream_dep_add_subtree(stream->dep_prev, dep_stream);
     if (rv != 0) {
       return rv;
     }

tests/nghttp2_session_test.c

@@ -4003,22 +4003,23 @@ void test_nghttp2_session_upgrade2(void) {
 void test_nghttp2_session_reprioritize_stream(void) {
   nghttp2_session *session;
   nghttp2_session_callbacks callbacks;
-  my_user_data ud;
   nghttp2_stream *stream;
   nghttp2_stream *dep_stream;
   nghttp2_priority_spec pri_spec;
+  int rv;
 
   memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
   callbacks.send_callback = block_count_send_callback;
 
-  nghttp2_session_server_new(&session, &callbacks, &ud);
+  nghttp2_session_server_new(&session, &callbacks, NULL);
 
   stream = open_recv_stream(session, 1);
 
   nghttp2_priority_spec_init(&pri_spec, 0, 10, 0);
 
-  nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
 
+  CU_ASSERT(0 == rv);
   CU_ASSERT(10 == stream->weight);
   CU_ASSERT(&session->root == stream->dep_prev);
 
@@ -4026,8 +4027,9 @@ void test_nghttp2_session_reprioritize_stream(void) {
 
   nghttp2_priority_spec_init(&pri_spec, 3, 99, 0);
 
-  nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
 
+  CU_ASSERT(0 == rv);
   CU_ASSERT(99 == stream->weight);
   CU_ASSERT(3 == stream->dep_prev->stream_id);
 
@@ -4040,16 +4042,18 @@ void test_nghttp2_session_reprioritize_stream(void) {
   /* Change weight */
   pri_spec.weight = 128;
 
-  nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
 
+  CU_ASSERT(0 == rv);
   CU_ASSERT(128 == stream->weight);
   CU_ASSERT(dep_stream == stream->dep_prev);
 
   /* Change weight again to test short-path case */
   pri_spec.weight = 100;
 
-  nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
 
+  CU_ASSERT(0 == rv);
   CU_ASSERT(100 == stream->weight);
   CU_ASSERT(dep_stream == stream->dep_prev);
   CU_ASSERT(100 == dep_stream->sum_dep_weight);
@@ -4058,8 +4062,9 @@ void test_nghttp2_session_reprioritize_stream(void) {
      root.  Then stream 3 depends on it. */
   nghttp2_priority_spec_init(&pri_spec, 1, 1, 0);
 
-  nghttp2_session_reprioritize_stream(session, dep_stream, &pri_spec);
+  rv = nghttp2_session_reprioritize_stream(session, dep_stream, &pri_spec);
 
+  CU_ASSERT(0 == rv);
   CU_ASSERT(1 == dep_stream->weight);
   CU_ASSERT(stream == dep_stream->dep_prev);
 
@@ -4069,11 +4074,76 @@ void test_nghttp2_session_reprioritize_stream(void) {
 
   nghttp2_priority_spec_init(&pri_spec, 5, 5, 0);
 
-  nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
 
+  CU_ASSERT(0 == rv);
   CU_ASSERT(NGHTTP2_DEFAULT_WEIGHT == stream->weight);
 
   nghttp2_session_del(session);
+
+  nghttp2_session_server_new(&session, &callbacks, NULL);
+
+  /* circular dependency; in case of stream which is not a direct
+     descendant of root.  Use exclusive dependency. */
+  stream = open_recv_stream(session, 1);
+  stream = open_recv_stream_with_dep(session, 3, stream);
+  stream = open_recv_stream_with_dep(session, 5, stream);
+  stream = open_recv_stream_with_dep(session, 7, stream);
+  open_recv_stream_with_dep(session, 9, stream);
+
+  nghttp2_priority_spec_init(&pri_spec, 7, 1, 1);
+
+  stream = nghttp2_session_get_stream(session, 3);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+
+  CU_ASSERT(0 == rv);
+  CU_ASSERT(7 == stream->dep_prev->stream_id);
+
+  stream = nghttp2_session_get_stream(session, 7);
+
+  CU_ASSERT(1 == stream->dep_prev->stream_id);
+
+  stream = nghttp2_session_get_stream(session, 9);
+
+  CU_ASSERT(3 == stream->dep_prev->stream_id);
+
+  stream = nghttp2_session_get_stream(session, 5);
+
+  CU_ASSERT(3 == stream->dep_prev->stream_id);
+
+  nghttp2_session_del(session);
+
+  nghttp2_session_server_new(&session, &callbacks, NULL);
+
+  /* circular dependency; in case of stream which is not a direct
+     descendant of root.  Without exclusive dependency. */
+  stream = open_recv_stream(session, 1);
+  stream = open_recv_stream_with_dep(session, 3, stream);
+  stream = open_recv_stream_with_dep(session, 5, stream);
+  stream = open_recv_stream_with_dep(session, 7, stream);
+  open_recv_stream_with_dep(session, 9, stream);
+
+  nghttp2_priority_spec_init(&pri_spec, 7, 1, 0);
+
+  stream = nghttp2_session_get_stream(session, 3);
+  rv = nghttp2_session_reprioritize_stream(session, stream, &pri_spec);
+
+  CU_ASSERT(0 == rv);
+  CU_ASSERT(7 == stream->dep_prev->stream_id);
+
+  stream = nghttp2_session_get_stream(session, 7);
+
+  CU_ASSERT(1 == stream->dep_prev->stream_id);
+
+  stream = nghttp2_session_get_stream(session, 9);
+
+  CU_ASSERT(7 == stream->dep_prev->stream_id);
+
+  stream = nghttp2_session_get_stream(session, 5);
+
+  CU_ASSERT(3 == stream->dep_prev->stream_id);
+
+  nghttp2_session_del(session);
 }
 
 void test_nghttp2_session_reprioritize_stream_with_idle_stream_dep(void) {