Add param h2_rapid_reset

Only RST frames received earlier than this duration will be considered
rapid.

bin/varnishd/http2/cache_http2_proto.c

@@ -341,6 +341,11 @@ h2_rx_rst_stream(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 		return (0);
 
 	now = VTIM_real();
+	CHECK_OBJ_NOTNULL(r2->req, REQ_MAGIC);
+	AN(r2->req->t_first);
+	if (now - r2->req->t_first > cache_param->h2_rapid_reset)
+		return (0);
+
 	d = now - h2->last_rst;
 	h2->rst_budget += cache_param->h2_rapid_reset_limit * d /
 	    cache_param->h2_rapid_reset_period;

include/tbl/params.h

@@ -1257,6 +1257,20 @@ PARAM_SIMPLE(
 	"HTTP2 maximum size of an uncompressed header list."
 )
 
+PARAM_SIMPLE(
+	/* name */	h2_rapid_reset,
+	/* typ */	timeout,
+	/* min */	"0.000",
+	/* max */	NULL,
+	/* def */	"1.000",
+	/* units */	"seconds",
+	/* descr */
+       "The upper threshold for how rapid an http/2 RST has to come for "
+       "it to be treated as suspect and subjected to the rate limits "
+       "specified by h2_rapid_reset_limit and h2_rapid_reset_period.",
+	/* flags */	EXPERIMENTAL,
+)
+
 PARAM_SIMPLE(
 	/* name */	h2_rapid_reset_limit,
 	/* typ */	uint,
@@ -1284,7 +1298,6 @@ PARAM_SIMPLE(
 	/* flags */	EXPERIMENTAL|WIZARD,
 )
 
-
 /*--------------------------------------------------------------------
  * Memory pool parameters
  */