Add nghttp2_session_request_allowed() API function

This function is useful for the client application to know that there
is a chance that request can be sent.  If this function returns 0,
there is zero chance to make a request.

This commit also set error_code passed to
nghttp2_on_stream_close_callback to NGHTTP2_REFUSED_STREAM if request
is not sent.

doc/Makefile.am

@@ -105,6 +105,7 @@ APIDOCS= \
 	nghttp2_session_mem_recv.rst \
 	nghttp2_session_mem_send.rst \
 	nghttp2_session_recv.rst \
+	nghttp2_session_request_allowed.rst \
 	nghttp2_session_resume_data.rst \
 	nghttp2_session_send.rst \
 	nghttp2_session_server_new.rst \

lib/includes/nghttp2/nghttp2.h

@@ -3479,6 +3479,23 @@ NGHTTP2_EXTERN int nghttp2_submit_goaway(nghttp2_session *session,
 NGHTTP2_EXTERN int32_t
     nghttp2_session_get_last_proc_stream_id(nghttp2_session *session);
 
+/**
+ * @function
+ *
+ * Returns nonzero if new request can be sent from local endpoint.
+ *
+ * This function return 0 if request is not allowed for this session.
+ * There are several reasons why request is not allowed.  Some of the
+ * reasons are: session is server; stream ID has been spent; GOAWAY
+ * has been sent or received.
+ *
+ * The application can call `nghttp2_submit_request()` without
+ * consulting this function.  In that case, `nghttp2_submit_request()`
+ * may return error.  Or, request is failed to sent, and
+ * :type:`nghttp2_on_stream_close_callback` is called.
+ */
+NGHTTP2_EXTERN int nghttp2_session_request_allowed(nghttp2_session *session);
+
 /**
  * @function
  *

lib/nghttp2_session.c

@@ -1322,6 +1322,12 @@ static int session_predicate_for_stream_send(nghttp2_session *session,
   return 0;
 }
 
+int nghttp2_session_request_allowed(nghttp2_session *session) {
+  return !session->server && session->next_stream_id <= INT32_MAX &&
+         (session->goaway_flags &
+          (NGHTTP2_GOAWAY_TERM_ON_SEND | NGHTTP2_GOAWAY_RECV)) == 0;
+}
+
 /*
  * This function checks request HEADERS frame, which opens stream, can
  * be sent at this time.
@@ -2685,6 +2691,10 @@ static ssize_t nghttp2_session_mem_send_internal(nghttp2_session *session,
             opened_stream_id = item->frame.hd.stream_id;
             if (item->aux_data.headers.canceled) {
               error_code = item->aux_data.headers.error_code;
+            } else {
+              /* Set error_code to REFUSED_STREAM so that application
+                 can send request again. */
+              error_code = NGHTTP2_REFUSED_STREAM;
             }
           }
           break;

src/h2load_http2_session.cc

@@ -210,6 +210,10 @@ void Http2Session::on_connect() {
 }
 
 int Http2Session::submit_request(RequestStat *req_stat) {
+  if (nghttp2_session_request_allowed(session_) == 0) {
+    return -1;
+  }
+
   auto config = client_->worker->config;
   auto &nva = config->nva[client_->reqidx++];