squashme: Harden test case

CCI showed that the test wasn't as bulletproof as I thought. If
the second client came in after the first had initiated the fetch,
it would not end up on the waitinglist and the VTC would time out.

Author: AlveElde

bin/varnishtest/tests/b00083.vtc

@@ -1,6 +1,7 @@
 varnishtest "req.ttl = 0s forces cache MISS with request coalescing"
 
 barrier b1 cond 3 -cyclic
+barrier b2 sock 2 -cyclic
 
 server s1 {
 	rxreq
@@ -16,19 +17,26 @@ server s1 {
 
 varnish v1 -vcl+backend {
 	import std;
+	import vtc;
 
 	sub vcl_recv {
 		if (req.http.ttl) {
 			set req.ttl = std.duration(req.http.ttl, 0s);
 		}
+
+		// Make sure both client tasks are started before the fetch is
+		// initiated to guarantee that one ends up on the waitinglist.
+		if (req.http.sync) {
+			vtc.barrier_sync("${b2_sock}");
+		}
 	}
 } -start
 
 # c1 and c2 send a request and sync on the barrier only after having received
 # headers. This ensures that the two requests are coalesced.
 
 client c1 {
-	txreq -hdr "ttl: 0s"
+	txreq -hdr "sync: yes" -hdr "ttl: 0s"
 	rxresphdrs
 	expect resp.status == 200
 	expect resp.http.version == "1"
@@ -38,7 +46,7 @@ client c1 {
 } -start
 
 client c2 {
-	txreq -hdr "ttl: 0s"
+	txreq -hdr "sync: yes" -hdr "ttl: 0s"
 	rxresphdrs
 	expect resp.status == 200
 	expect resp.http.version == "1"
@@ -69,7 +77,7 @@ client c3 {
 # c4 and c5 force a cache MISS, where the two requests are again coalesced.
 
 client c4 {
-	txreq -hdr "ttl: 0s"
+	txreq -hdr "sync: yes" -hdr "ttl: 0s"
 	rxresphdrs
 	expect resp.status == 200
 	expect resp.http.version == "2"
@@ -79,7 +87,7 @@ client c4 {
 } -start
 
 client c5 {
-	txreq -hdr "ttl: 0s"
+	txreq -hdr "sync: yes" -hdr "ttl: 0s"
 	rxresphdrs
 	expect resp.status == 200
 	expect resp.http.version == "2"