HTTP/2: limited number of PRIORITY frames.

Fixed excessive CPU usage caused by a peer that continuously shuffles
priority of streams.  Fix is to limit the number of PRIORITY frames.

src/http/v2/ngx_http_v2.c

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 88e2bb9..e55f9ba 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -273,6 +273,7 @@
     h2scf = ngx_http_get_module_srv_conf(hc->conf_ctx, ngx_http_v2_module);
 
     h2c->concurrent_pushes = h2scf->concurrent_pushes;
+    h2c->priority_limit = h2scf->concurrent_streams;
 
     h2c->pool = ngx_create_pool(h2scf->pool_size, h2c->connection->log);
     if (h2c->pool == NULL) {
@@ -1804,6 +1805,13 @@
         return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_SIZE_ERROR);
     }
 
+    if (--h2c->priority_limit == 0) {
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "client sent too many PRIORITY frames");
+
+        return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_ENHANCE_YOUR_CALM);
+    }
+
     if (end - pos < NGX_HTTP_V2_PRIORITY_SIZE) {
         return ngx_http_v2_state_save(h2c, pos, end,
                                       ngx_http_v2_state_priority);
@@ -3120,6 +3128,8 @@
         h2c->processing++;
     }
 
+    h2c->priority_limit += h2scf->concurrent_streams;
+
     return stream;
 }
 

src/http/v2/ngx_http_v2.h

diff --git a/src/http/v2/ngx_http_v2.h b/src/http/v2/ngx_http_v2.h
index 715b7d3..69d55d1 100644
--- a/src/http/v2/ngx_http_v2.h
+++ b/src/http/v2/ngx_http_v2.h
@@ -122,6 +122,7 @@
     ngx_uint_t                       processing;
     ngx_uint_t                       frames;
     ngx_uint_t                       idle;
+    ngx_uint_t                       priority_limit;
 
     ngx_uint_t                       pushing;
     ngx_uint_t                       concurrent_pushes;