Add a server option to fallback to RFC 7540 priorities

Add nghttp2_option_set_server_fallback_rfc7540_priorities.  If it is
set to nonzero, and server submits SETTINGS_NO_RFC7540_PRIORITIES = 1,
but it does not receive SETTINGS_NO_RFC7540_PRIORITIES from client,
server falls back to RFC 7540 priorities.  Only minimal set of
features are enabled in this fallback case.

doc/Makefile.am

@@ -69,6 +69,7 @@ APIDOCS= \
 	nghttp2_option_set_no_http_messaging.rst \
 	nghttp2_option_set_no_recv_client_magic.rst \
 	nghttp2_option_set_peer_max_concurrent_streams.rst \
+	nghttp2_option_set_server_fallback_rfc7540_priorities.rst \
 	nghttp2_option_set_user_recv_extension_type.rst \
 	nghttp2_option_set_max_outbound_ack.rst \
 	nghttp2_option_set_max_settings.rst \

lib/includes/nghttp2/nghttp2.h

@@ -2732,6 +2732,24 @@ NGHTTP2_EXTERN void nghttp2_option_set_max_outbound_ack(nghttp2_option *option,
 NGHTTP2_EXTERN void nghttp2_option_set_max_settings(nghttp2_option *option,
                                                     size_t val);
 
+/**
+ * @function
+ *
+ * This option, if set to nonzero, allows server to fallback to
+ * :rfc:`7540` priorities if SETTINGS_NO_RFC7540_PRIORITIES was not
+ * received from client, and server submitted
+ * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
+ * = 1 via `nghttp2_submit_settings()`.  Most of the advanced
+ * functionality for RFC 7540 priorities are still disabled.  This
+ * fallback only enables the minimal feature set of RFC 7540
+ * priorities to deal with priority signaling from client.
+ *
+ * Client session ignores this option.
+ */
+NGHTTP2_EXTERN void
+nghttp2_option_set_server_fallback_rfc7540_priorities(nghttp2_option *option,
+                                                      int val);
+
 /**
  * @function
  *

lib/nghttp2_option.c

@@ -130,3 +130,9 @@ void nghttp2_option_set_max_settings(nghttp2_option *option, size_t val) {
   option->opt_set_mask |= NGHTTP2_OPT_MAX_SETTINGS;
   option->max_settings = val;
 }
+
+void nghttp2_option_set_server_fallback_rfc7540_priorities(
+    nghttp2_option *option, int val) {
+  option->opt_set_mask |= NGHTTP2_OPT_SERVER_FALLBACK_RFC7540_PRIORITIES;
+  option->server_fallback_rfc7540_priorities = val;
+}

lib/nghttp2_option.h

@@ -68,6 +68,7 @@ typedef enum {
   NGHTTP2_OPT_NO_CLOSED_STREAMS = 1 << 10,
   NGHTTP2_OPT_MAX_OUTBOUND_ACK = 1 << 11,
   NGHTTP2_OPT_MAX_SETTINGS = 1 << 12,
+  NGHTTP2_OPT_SERVER_FALLBACK_RFC7540_PRIORITIES = 1 << 13,
 } nghttp2_option_flag;
 
 /**
@@ -127,6 +128,10 @@ struct nghttp2_option {
    * NGHTTP2_OPT_NO_CLOSED_STREAMS
    */
   int no_closed_streams;
+  /**
+   * NGHTTP2_OPT_SERVER_FALLBACK_RFC7540_PRIORITIES
+   */
+  int server_fallback_rfc7540_priorities;
   /**
    * NGHTTP2_OPT_USER_RECV_EXT_TYPES
    */

lib/nghttp2_session.c

@@ -144,6 +144,11 @@ static int session_detect_idle_stream(nghttp2_session *session,
   return 0;
 }
 
+static int session_no_rfc7540_pri_no_fallback(nghttp2_session *session) {
+  return session->pending_no_rfc7540_priorities == 1 &&
+         !session->fallback_rfc7540_priorities;
+}
+
 static int check_ext_type_set(const uint8_t *ext_types, uint8_t type) {
   return (ext_types[type / 8] & (1 << (type & 0x7))) > 0;
 }
@@ -554,6 +559,13 @@ static int session_new(nghttp2_session **session_ptr,
         option->max_settings) {
       (*session_ptr)->max_settings = option->max_settings;
     }
+
+    if ((option->opt_set_mask &
+         NGHTTP2_OPT_SERVER_FALLBACK_RFC7540_PRIORITIES) &&
+        option->server_fallback_rfc7540_priorities) {
+      (*session_ptr)->opt_flags |=
+          NGHTTP2_OPTMASK_SERVER_FALLBACK_RFC7540_PRIORITIES;
+    }
   }
 
   rv = nghttp2_hd_deflate_init2(&(*session_ptr)->hd_deflater,
@@ -810,7 +822,8 @@ int nghttp2_session_reprioritize_stream(
   nghttp2_priority_spec pri_spec_default;
   const nghttp2_priority_spec *pri_spec = pri_spec_in;
 
-  assert(session->pending_no_rfc7540_priorities != 1);
+  assert((!session->server && session->pending_no_rfc7540_priorities != 1) ||
+         (session->server && !session_no_rfc7540_pri_no_fallback(session)));
   assert(pri_spec->stream_id != stream->stream_id);
 
   if (!nghttp2_stream_in_dep_tree(stream)) {
@@ -1296,7 +1309,7 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
         return NULL;
       }
 
-      if (session->pending_no_rfc7540_priorities == 1) {
+      if (session_no_rfc7540_pri_no_fallback(session)) {
         stream->flags |= NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES;
       }
     }
@@ -1309,7 +1322,7 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
     stream_alloc = 1;
   }
 
-  if (session->pending_no_rfc7540_priorities == 1 ||
+  if (session_no_rfc7540_pri_no_fallback(session) ||
       session->remote_settings.no_rfc7540_priorities == 1) {
     /* For client which has not received server
        SETTINGS_NO_RFC7540_PRIORITIES = 1, send a priority signal
@@ -1369,7 +1382,7 @@ nghttp2_stream *nghttp2_session_open_stream(nghttp2_session *session,
                         (int32_t)session->local_settings.initial_window_size,
                         stream_user_data, mem);
 
-    if (session->pending_no_rfc7540_priorities == 1) {
+    if (session_no_rfc7540_pri_no_fallback(session)) {
       stream->seq = session->stream_seq++;
     }
 
@@ -4440,7 +4453,7 @@ int nghttp2_session_on_priority_received(nghttp2_session *session,
   int rv;
   nghttp2_stream *stream;
 
-  assert(session->pending_no_rfc7540_priorities != 1);
+  assert(!session_no_rfc7540_pri_no_fallback(session));
 
   if (frame->hd.stream_id == 0) {
     return session_handle_invalid_connection(session, frame, NGHTTP2_ERR_PROTO,
@@ -4499,7 +4512,7 @@ static int session_process_priority_frame(nghttp2_session *session) {
   nghttp2_inbound_frame *iframe = &session->iframe;
   nghttp2_frame *frame = &iframe->frame;
 
-  assert(session->pending_no_rfc7540_priorities != 1);
+  assert(!session_no_rfc7540_pri_no_fallback(session));
 
   nghttp2_frame_unpack_priority_payload(&frame->priority, iframe->sbuf.pos);
 
@@ -4927,6 +4940,12 @@ int nghttp2_session_on_settings_received(nghttp2_session *session,
 
   if (session->remote_settings.no_rfc7540_priorities == UINT32_MAX) {
     session->remote_settings.no_rfc7540_priorities = 0;
+
+    if (session->server && session->pending_no_rfc7540_priorities &&
+        (session->opt_flags &
+         NGHTTP2_OPTMASK_SERVER_FALLBACK_RFC7540_PRIORITIES)) {
+      session->fallback_rfc7540_priorities = 1;
+    }
   }
 
   if (!noack && !session_is_closing(session)) {
@@ -6380,7 +6399,7 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session, const uint8_t *in,
               break;
             }
 
-            if (session->pending_no_rfc7540_priorities != 1 ||
+            if (!session_no_rfc7540_pri_no_fallback(session) ||
                 iframe->payloadleft > sizeof(iframe->raw_sbuf)) {
               busy = 1;
               iframe->state = NGHTTP2_IB_IGN_PAYLOAD;
@@ -6498,7 +6517,7 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session, const uint8_t *in,
 
         break;
       case NGHTTP2_PRIORITY:
-        if (session->pending_no_rfc7540_priorities != 1 &&
+        if (!session_no_rfc7540_pri_no_fallback(session) &&
             session->remote_settings.no_rfc7540_priorities != 1) {
           rv = session_process_priority_frame(session);
           if (nghttp2_is_fatal(rv)) {

lib/nghttp2_session.h

@@ -52,7 +52,8 @@ typedef enum {
   NGHTTP2_OPTMASK_NO_RECV_CLIENT_MAGIC = 1 << 1,
   NGHTTP2_OPTMASK_NO_HTTP_MESSAGING = 1 << 2,
   NGHTTP2_OPTMASK_NO_AUTO_PING_ACK = 1 << 3,
-  NGHTTP2_OPTMASK_NO_CLOSED_STREAMS = 1 << 4
+  NGHTTP2_OPTMASK_NO_CLOSED_STREAMS = 1 << 4,
+  NGHTTP2_OPTMASK_SERVER_FALLBACK_RFC7540_PRIORITIES = 1 << 5
 } nghttp2_optmask;
 
 /*
@@ -340,6 +341,8 @@ struct nghttp2_session {
   /* Unacked local SETTINGS_NO_RFC7540_PRIORITIES value, which is
      effective before it is acknowledged. */
   uint8_t pending_no_rfc7540_priorities;
+  /* Turn on fallback to RFC 7540 priorities; for server use only. */
+  uint8_t fallback_rfc7540_priorities;
   /* Nonzero if the session is server side. */
   uint8_t server;
   /* Flags indicating GOAWAY is sent and/or received. The flags are

tests/main.c

@@ -339,6 +339,8 @@ int main(void) {
                    test_nghttp2_session_set_stream_user_data) ||
       !CU_add_test(pSuite, "session_no_rfc7540_priorities",
                    test_nghttp2_session_no_rfc7540_priorities) ||
+      !CU_add_test(pSuite, "session_server_fallback_rfc7540_priorities",
+                   test_nghttp2_session_server_fallback_rfc7540_priorities) ||
       !CU_add_test(pSuite, "http_mandatory_headers",
                    test_nghttp2_http_mandatory_headers) ||
       !CU_add_test(pSuite, "http_content_length",

tests/nghttp2_session_test.c

@@ -11637,6 +11637,196 @@ void test_nghttp2_session_no_rfc7540_priorities(void) {
   nghttp2_session_del(session);
 }
 
+void test_nghttp2_session_server_fallback_rfc7540_priorities(void) {
+  nghttp2_session *session;
+  nghttp2_option *option;
+  nghttp2_session_callbacks callbacks;
+  nghttp2_frame frame;
+  nghttp2_bufs bufs;
+  nghttp2_buf *buf;
+  ssize_t rv;
+  nghttp2_settings_entry iv;
+  nghttp2_mem *mem;
+  nghttp2_hd_deflater deflater;
+  nghttp2_nv *nva;
+  size_t nvlen;
+  nghttp2_priority_spec pri_spec;
+  nghttp2_stream *anchor_stream, *stream;
+  my_user_data ud;
+  nghttp2_ext_priority_update priority_update;
+  static const uint8_t field_value[] = "u=0";
+
+  mem = nghttp2_mem_default();
+  frame_pack_bufs_init(&bufs);
+
+  memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
+  callbacks.send_callback = null_send_callback;
+  callbacks.on_frame_recv_callback = on_frame_recv_callback;
+
+  nghttp2_option_new(&option);
+  nghttp2_option_set_server_fallback_rfc7540_priorities(option, 1);
+
+  iv.settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv.value = 1;
+
+  /* Server falls back to RFC 7540 priorities. */
+  nghttp2_session_server_new2(&session, &callbacks, &ud, option);
+
+  rv = nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, &iv, 1);
+
+  CU_ASSERT(0 == rv);
+
+  rv = nghttp2_session_send(session);
+
+  CU_ASSERT(0 == rv);
+
+  nghttp2_frame_settings_init(&frame.settings, NGHTTP2_FLAG_NONE, NULL, 0);
+  rv = nghttp2_frame_pack_settings(&bufs, &frame.settings);
+
+  CU_ASSERT(0 == rv);
+
+  nghttp2_frame_settings_free(&frame.settings, mem);
+
+  buf = &bufs.head->buf;
+  rv = nghttp2_session_mem_recv(session, buf->pos, nghttp2_buf_len(buf));
+
+  CU_ASSERT((ssize_t)nghttp2_buf_len(buf) == rv);
+  CU_ASSERT(1 == session->fallback_rfc7540_priorities);
+
+  nghttp2_hd_deflate_init(&deflater, mem);
+
+  nvlen = ARRLEN(reqnv);
+  nghttp2_nv_array_copy(&nva, reqnv, nvlen, mem);
+  nghttp2_priority_spec_init(&pri_spec, 3, 111, 1);
+  nghttp2_frame_headers_init(&frame.headers,
+                             NGHTTP2_FLAG_END_HEADERS | NGHTTP2_FLAG_PRIORITY,
+                             1, NGHTTP2_HCAT_HEADERS, &pri_spec, nva, nvlen);
+  nghttp2_bufs_reset(&bufs);
+  rv = nghttp2_frame_pack_headers(&bufs, &frame.headers, &deflater);
+
+  CU_ASSERT(0 == rv);
+
+  nghttp2_frame_headers_free(&frame.headers, mem);
+
+  buf = &bufs.head->buf;
+  rv = nghttp2_session_mem_recv(session, buf->pos, nghttp2_buf_len(buf));
+
+  CU_ASSERT((ssize_t)nghttp2_buf_len(buf) == rv);
+
+  anchor_stream = nghttp2_session_get_stream_raw(session, 3);
+
+  CU_ASSERT(NGHTTP2_STREAM_IDLE == anchor_stream->state);
+  CU_ASSERT(
+      !(anchor_stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES));
+  CU_ASSERT(&session->root == anchor_stream->dep_prev);
+
+  stream = nghttp2_session_get_stream(session, 1);
+
+  CU_ASSERT(NGHTTP2_STREAM_OPENING == stream->state);
+  CU_ASSERT(!(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES));
+  CU_ASSERT(anchor_stream == stream->dep_prev);
+
+  /* Make sure that PRIORITY frame updates stream priority. */
+  nghttp2_priority_spec_init(&pri_spec, 5, 1, 0);
+  nghttp2_frame_priority_init(&frame.priority, 1, &pri_spec);
+  nghttp2_bufs_reset(&bufs);
+  rv = nghttp2_frame_pack_priority(&bufs, &frame.priority);
+
+  CU_ASSERT(0 == rv);
+
+  nghttp2_frame_priority_free(&frame.priority);
+
+  buf = &bufs.head->buf;
+  rv = nghttp2_session_mem_recv(session, buf->pos, nghttp2_buf_len(buf));
+
+  CU_ASSERT((ssize_t)nghttp2_buf_len(buf) == rv);
+
+  anchor_stream = nghttp2_session_get_stream_raw(session, 5);
+
+  CU_ASSERT(NGHTTP2_STREAM_IDLE == anchor_stream->state);
+  CU_ASSERT(&session->root == anchor_stream->dep_prev);
+  CU_ASSERT(anchor_stream == stream->dep_prev);
+
+  /* Make sure that PRIORITY_UPDATE frame is ignored. */
+  frame.ext.payload = &priority_update;
+  nghttp2_frame_priority_update_init(&frame.ext, 1, (uint8_t *)field_value,
+                                     sizeof(field_value) - 1);
+  nghttp2_bufs_reset(&bufs);
+  nghttp2_frame_pack_priority_update(&bufs, &frame.ext);
+
+  ud.frame_recv_cb_called = 0;
+  buf = &bufs.head->buf;
+  rv = nghttp2_session_mem_recv(session, buf->pos, nghttp2_buf_len(buf));
+
+  CU_ASSERT((ssize_t)nghttp2_buf_len(buf) == rv);
+  CU_ASSERT(0 == ud.frame_recv_cb_called);
+  CU_ASSERT(NGHTTP2_EXTPRI_DEFAULT_URGENCY == stream->extpri);
+
+  nghttp2_hd_deflate_free(&deflater);
+  nghttp2_session_del(session);
+
+  /* Server does not fallback to RFC 7540 priorities. */
+  nghttp2_session_server_new2(&session, &callbacks, &ud, option);
+
+  rv = nghttp2_submit_settings(session, NGHTTP2_FLAG_NONE, &iv, 1);
+
+  CU_ASSERT(0 == rv);
+
+  rv = nghttp2_session_send(session);
+
+  CU_ASSERT(0 == rv);
+
+  iv.settings_id = NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES;
+  iv.value = 0;
+
+  nghttp2_frame_settings_init(&frame.settings, NGHTTP2_FLAG_NONE,
+                              dup_iv(&iv, 1), 1);
+  nghttp2_bufs_reset(&bufs);
+  rv = nghttp2_frame_pack_settings(&bufs, &frame.settings);
+
+  CU_ASSERT(0 == rv);
+
+  nghttp2_frame_settings_free(&frame.settings, mem);
+
+  buf = &bufs.head->buf;
+  rv = nghttp2_session_mem_recv(session, buf->pos, nghttp2_buf_len(buf));
+
+  CU_ASSERT((ssize_t)nghttp2_buf_len(buf) == rv);
+  CU_ASSERT(0 == session->fallback_rfc7540_priorities);
+
+  nghttp2_hd_deflate_init(&deflater, mem);
+
+  nvlen = ARRLEN(reqnv);
+  nghttp2_nv_array_copy(&nva, reqnv, nvlen, mem);
+  nghttp2_priority_spec_init(&pri_spec, 3, 111, 1);
+  nghttp2_frame_headers_init(&frame.headers,
+                             NGHTTP2_FLAG_END_HEADERS | NGHTTP2_FLAG_PRIORITY,
+                             1, NGHTTP2_HCAT_HEADERS, &pri_spec, nva, nvlen);
+  nghttp2_bufs_reset(&bufs);
+  rv = nghttp2_frame_pack_headers(&bufs, &frame.headers, &deflater);
+
+  CU_ASSERT(0 == rv);
+
+  nghttp2_frame_headers_free(&frame.headers, mem);
+
+  buf = &bufs.head->buf;
+  rv = nghttp2_session_mem_recv(session, buf->pos, nghttp2_buf_len(buf));
+
+  CU_ASSERT((ssize_t)nghttp2_buf_len(buf) == rv);
+  CU_ASSERT(NULL == nghttp2_session_get_stream_raw(session, 3));
+
+  stream = nghttp2_session_get_stream(session, 1);
+
+  CU_ASSERT(NGHTTP2_STREAM_OPENING == stream->state);
+  CU_ASSERT(stream->flags & NGHTTP2_STREAM_FLAG_NO_RFC7540_PRIORITIES);
+
+  nghttp2_hd_deflate_free(&deflater);
+  nghttp2_session_del(session);
+
+  nghttp2_option_del(option);
+  nghttp2_bufs_free(&bufs);
+}
+
 static void check_nghttp2_http_recv_headers_fail(
     nghttp2_session *session, nghttp2_hd_deflater *deflater, int32_t stream_id,
     int stream_state, const nghttp2_nv *nva, size_t nvlen) {

tests/nghttp2_session_test.h

@@ -166,6 +166,7 @@ void test_nghttp2_session_pause_data(void);
 void test_nghttp2_session_no_closed_streams(void);
 void test_nghttp2_session_set_stream_user_data(void);
 void test_nghttp2_session_no_rfc7540_priorities(void);
+void test_nghttp2_session_server_fallback_rfc7540_priorities(void);
 void test_nghttp2_http_mandatory_headers(void);
 void test_nghttp2_http_content_length(void);
 void test_nghttp2_http_content_length_mismatch(void);