Add API to change stream priority without sending PRIORITY frame

The added API is nghttp2_session_change_stream_priority().  This
provides the same functionality to re-prioritize stream when PRIORITY
frame.  is received, but we do it without PRIORITY frame.  This could
be useful for server to change pushed stream's priority silently.

doc/Makefile.am

@@ -105,6 +105,7 @@ APIDOCS= \
 	nghttp2_session_mem_recv.rst \
 	nghttp2_session_mem_send.rst \
 	nghttp2_session_recv.rst \
+	nghttp2_session_change_stream_priority.rst \
 	nghttp2_session_check_request_allowed.rst \
 	nghttp2_session_check_server_session.rst \
 	nghttp2_session_resume_data.rst \

lib/includes/nghttp2/nghttp2.h

@@ -2853,6 +2853,40 @@ NGHTTP2_EXTERN int nghttp2_session_consume_stream(nghttp2_session *session,
                                                   int32_t stream_id,
                                                   size_t size);
 
+/**
+ * @function
+ *
+ * Changes priority of existing stream denoted by |stream_id|.  The
+ * new priority specification is |pri_spec|.
+ *
+ * The priority is changed silently and instantly, and no PRIORITY
+ * frame will be sent to notify the peer of this change.  This
+ * function may be useful for server to change the priority of pushed
+ * stream.
+ *
+ * If |session| is initialized as server, and ``pri_spec->stream_id``
+ * points to the idle stream, the idle stream is created if it does
+ * not exist.  The created idle stream will depend on root stream
+ * (stream 0) with weight 16.
+ *
+ * If stream denoted by ``pri_spec->stream_id`` is not found, we use
+ * default priority instead of given |pri_spec|.  That is make stream
+ * depend on root stream with weight 16.
+ *
+ * This function returns 0 if it succeeds, or one of the following
+ * negative error codes:
+ *
+ * :enum:`NGHTTP2_ERR_NOMEM`
+ *     Out of memory.
+ * :enum:`NGHTTP2_ERR_INVALID_ARGUMENT`
+ *     Attempted to depend on itself; no stream exist for the given
+ *     |stream_id|.
+ */
+NGHTTP2_EXTERN int
+nghttp2_session_change_stream_priority(nghttp2_session *session,
+                                       int32_t stream_id,
+                                       const nghttp2_priority_spec *pri_spec);
+
 /**
  * @function
  *

lib/nghttp2_priority_spec.c

@@ -42,3 +42,11 @@ int nghttp2_priority_spec_check_default(const nghttp2_priority_spec *pri_spec) {
   return pri_spec->stream_id == 0 &&
          pri_spec->weight == NGHTTP2_DEFAULT_WEIGHT && pri_spec->exclusive == 0;
 }
+
+void nghttp2_priority_spec_normalize_weight(nghttp2_priority_spec *pri_spec) {
+  if (pri_spec->weight < NGHTTP2_MIN_WEIGHT) {
+    pri_spec->weight = NGHTTP2_MIN_WEIGHT;
+  } else if (pri_spec->weight > NGHTTP2_MAX_WEIGHT) {
+    pri_spec->weight = NGHTTP2_MAX_WEIGHT;
+  }
+}

lib/nghttp2_priority_spec.h

@@ -31,4 +31,12 @@
 
 #include <nghttp2/nghttp2.h>
 
+/*
+ * This function normalizes pri_spec->weight if it is out of range.
+ * If pri_spec->weight is less than NGHTTP2_MIN_WEIGHT, it is set to
+ * NGHTTP2_MIN_WEIGHT.  If pri_spec->weight is larger than
+ * NGHTTP2_MAX_WEIGHT, it is set to NGHTTP2_MAX_WEIGHT.
+ */
+void nghttp2_priority_spec_normalize_weight(nghttp2_priority_spec *pri_spec);
+
 #endif /* NGHTTP2_PRIORITY_SPEC_H */

lib/nghttp2_session.c

@@ -6771,3 +6771,24 @@ nghttp2_stream *nghttp2_session_get_root_stream(nghttp2_session *session) {
 int nghttp2_session_check_server_session(nghttp2_session *session) {
   return session->server;
 }
+
+int nghttp2_session_change_stream_priority(
+    nghttp2_session *session, int32_t stream_id,
+    const nghttp2_priority_spec *pri_spec) {
+  nghttp2_stream *stream;
+  nghttp2_priority_spec pri_spec_copy;
+
+  if (stream_id == pri_spec->stream_id) {
+    return NGHTTP2_ERR_INVALID_ARGUMENT;
+  }
+
+  stream = nghttp2_session_get_stream_raw(session, stream_id);
+  if (!stream) {
+    return NGHTTP2_ERR_INVALID_ARGUMENT;
+  }
+
+  pri_spec_copy = *pri_spec;
+  nghttp2_priority_spec_normalize_weight(&pri_spec_copy);
+
+  return nghttp2_session_reprioritize_stream(session, stream, &pri_spec_copy);
+}

lib/nghttp2_submit.c

@@ -117,14 +117,6 @@ fail2:
   return rv;
 }
 
-static void adjust_priority_spec_weight(nghttp2_priority_spec *pri_spec) {
-  if (pri_spec->weight < NGHTTP2_MIN_WEIGHT) {
-    pri_spec->weight = NGHTTP2_MIN_WEIGHT;
-  } else if (pri_spec->weight > NGHTTP2_MAX_WEIGHT) {
-    pri_spec->weight = NGHTTP2_MAX_WEIGHT;
-  }
-}
-
 static int32_t submit_headers_shared_nva(nghttp2_session *session,
                                          uint8_t flags, int32_t stream_id,
                                          const nghttp2_priority_spec *pri_spec,
@@ -141,7 +133,7 @@ static int32_t submit_headers_shared_nva(nghttp2_session *session,
 
   if (pri_spec) {
     copy_pri_spec = *pri_spec;
-    adjust_priority_spec_weight(&copy_pri_spec);
+    nghttp2_priority_spec_normalize_weight(&copy_pri_spec);
   } else {
     nghttp2_priority_spec_default_init(&copy_pri_spec);
   }
@@ -206,7 +198,7 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags _U_,
 
   copy_pri_spec = *pri_spec;
 
-  adjust_priority_spec_weight(&copy_pri_spec);
+  nghttp2_priority_spec_normalize_weight(&copy_pri_spec);
 
   item = nghttp2_mem_malloc(mem, sizeof(nghttp2_outbound_item));