vcl_vrt: Skip VCL execution if the client is gone

Once a client is reportedly gone, processing its VCL task(s) is just a
waste of resources. The execution of client-facing VCL is intercepted
and an artificial return(fail) is returned in that scenario.

Thanks to the introduction of the universal return(fail) proper error
handling and resource tear down is already in place, which makes this
change safe modulus unknown bugs. This adds a circuit breaker anywhere
in the client state machine where there is VCL execution.

A new Reset time stamp is logged to convey when a task does not complete
because the client is gone. This is a good complement to the walk away
feature and its original circuit breaker for the waiting list, but this
has not been integrated yet.

While the request is technically failed, it won't increase the vcl_fail
counter, and a new req_reset counter is incremented. This new behavior
is guarded by a new vcl_req_reset feature flag, enabled by default.

Refs #3835
Refs 61a15cb
Refs e5efc2c
Refs ba54dc9
Refs 6f50a00
Refs b881699

bin/varnishd/cache/cache_vrt_vcl.c

@@ -42,6 +42,7 @@
 #include "vbm.h"
 
 #include "cache_director.h"
+#include "cache_transport.h"
 #include "cache_vcl.h"
 #include "vcc_interface.h"
 
@@ -521,6 +522,40 @@ VRT_VCL_Allow_Discard(struct vclref **refp)
 	FREE_OBJ(ref);
 }
 
+/*--------------------------------------------------------------------
+ */
+
+static int
+req_poll(struct worker *wrk, struct req *req)
+{
+	struct req *top;
+
+	/* NB: Since a fail transition leads to vcl_synth, the request may be
+	 * short-circuited twice.
+	 */
+	if (req->req_reset) {
+		wrk->vpi->handling = VCL_RET_FAIL;
+		return (-1);
+	}
+
+	top = req->top->topreq;
+	CHECK_OBJ_NOTNULL(top, REQ_MAGIC);
+	CHECK_OBJ_NOTNULL(top->transport, TRANSPORT_MAGIC);
+
+	if (!FEATURE(FEATURE_VCL_REQ_RESET))
+		return (0);
+	if (top->transport->poll == NULL)
+		return (0);
+	if (top->transport->poll(top) >= 0)
+		return (0);
+
+	VSLb_ts_req(req, "Reset", W_TIM_real(wrk));
+	wrk->stats->req_reset++;
+	wrk->vpi->handling = VCL_RET_FAIL;
+	req->req_reset = 1;
+	return (-1);
+}
+
 /*--------------------------------------------------------------------
  * Method functions to call into VCL programs.
  *
@@ -552,6 +587,8 @@ vcl_call_method(struct worker *wrk, struct req *req, struct busyobj *bo,
 		CHECK_OBJ_NOTNULL(req->sp, SESS_MAGIC);
 		CHECK_OBJ_NOTNULL(req->vcl, VCL_MAGIC);
 		CHECK_OBJ_NOTNULL(req->top, REQTOP_MAGIC);
+		if (req_poll(wrk, req))
+			return;
 		VCL_Req2Ctx(&ctx, req);
 	}
 	assert(ctx.now != 0);

doc/sphinx/reference/vsl.rst

@@ -76,6 +76,10 @@ Resp
 Restart
 	Client request is being restarted.
 
+Reset
+        The client closed its connection or reset its stream. Request
+        processing is interrupted and considered failed.
+
 Pipe handling timestamps
 ~~~~~~~~~~~~~~~~~~~~~~~~
 

include/tbl/feature_bits.h

@@ -91,6 +91,11 @@ FEATURE_BIT(TRACE,			trace,
     "Required for tracing vcl_init / vcl_fini"
 )
 
+FEATURE_BIT(VCL_REQ_RESET,			vcl_req_reset,
+    "Stop processing client VCL once the client is gone. "
+    "When this happens MAIN.req_reset is incremented."
+)
+
 #undef FEATURE_BIT
 
 /*lint -restore */

include/tbl/params.h

@@ -1824,7 +1824,9 @@ PARAM_PRE
 PARAM_BITS(
 	/* name */	feature,
 	/* fld */	feature_bits,
-	/* def */	"+validate_headers",
+	/* def */
+	"+validate_headers,"
+	"+vcl_req_reset",
 	/* descr */
 	"Enable/Disable various minor features.\n"
 	"\tdefault\tSet default value\n"

include/tbl/req_flags.h

@@ -40,6 +40,7 @@ REQ_FLAG(is_hit,		0, 0, "")
 REQ_FLAG(waitinglist,		0, 0, "")
 REQ_FLAG(want100cont,		0, 0, "")
 REQ_FLAG(late100cont,		0, 0, "")
+REQ_FLAG(req_reset,		0, 0, "")
 #define REQ_BEREQ_FLAG(lower, vcl_r, vcl_w, doc) \
 	REQ_FLAG(lower, vcl_r, vcl_w, doc)
 #include "tbl/req_bereq_flags.h"

lib/libvsc/VSC_main.vsc

@@ -345,6 +345,13 @@
 	Number of times an HTTP/2 stream was refused because the queue was
 	too long already. See also parameter thread_queue_limit.
 
+.. varnish_vsc:: req_reset
+	:group: wrk
+	:oneliner:	Requests reset
+
+	Number of times a client left before the VCL processing of its
+	requests completed.
+
 .. varnish_vsc:: n_object
 	:type:	gauge
 	:group: wrk