varnishtest: Tweak default h2 window parameters

This change increases the initial size and reduces the low watermark.

RFC7540 says this:

> Flow-controlled frames from the sender and WINDOW_UPDATE frames from
> the receiver are completely asynchronous with respect to each other.
> This property allows a receiver to aggressively update the window
> size kept by the sender to prevent streams from stalling.

The default parameters are very much on the low-latency aggressive
updates end of the spectrum, which increases asynchronicity at the
expense of determinism in test cases.

The tweaks made by varnishtest allows basic tests to send a few request
bodies before being bothered by window update race conditions. Test
cases that cover h2 flow control or anything else related to window
updates may reset parameters or pick other specific values. This frees
us from a bunch of barriers where the purpose of mitigating this race
was rarely even documented.

This successfully passed the following test locally:

    git grep -Fl +http2 -- '*.vtc' |
    xargs bin/varnishtest/varnishtest -i -n100 -j32

We can hope that h2 test cases will be overall more stable from now on.

Refs #3442

bin/varnishtest/tests/r02305.vtc

 varnishtest "#2305: h/2 reembark with a request body"
 
 barrier b1 cond 2
-barrier b2 sock 2
 
 server s1 {
 	rxreq
@@ -16,15 +15,9 @@ varnish v1 -cliok "param.set debug +syncvsl"
 varnish v1 -cliok "param.set debug +waitinglist"
 
 varnish v1 -vcl+backend {
-	import vtc;
 	sub vcl_recv {
 		return (hash);
 	}
-	sub vcl_deliver {
-		if (req.http.sync) {
-			vtc.barrier_sync("${b2_sock}");
-		}
-	}
 } -start
 
 client c1 {
@@ -35,11 +28,7 @@ client c1 {
 	} -start
 	stream 3 {
 		barrier b1 sync
-		txreq -req POST -hdr sync 1 -body "foo"
-		rxwinup
-		# barrier b2 is here to make HEADERS vs WINDOW_UPDATE
-		# less racy
-		barrier b2 sync
+		txreq -req POST -body "foo"
 		rxresp
 		expect resp.status == 200
 	} -run

bin/varnishtest/tests/r02679.vtc

@@ -16,6 +16,7 @@ varnish v1 -vcl+backend {
 
 varnish v1 -cliok "param.set feature +http2"
 varnish v1 -cliok "param.set h2_rx_window_low_water 65535"
+varnish v1 -cliok "param.reset h2_initial_window_size"
 
 client c1 {
 	stream 1 {

bin/varnishtest/tests/t02000.vtc

@@ -33,6 +33,7 @@ client c1 {
 } -run
 
 varnish v1 -cliok "param.set feature +http2"
+varnish v1 -cliok "param.reset h2_initial_window_size"
 
 client c1 {
 	stream 1 {
@@ -67,8 +68,10 @@ varnish v1 -expect MEMPOOL.sess1.live == 0
 
 process p1 -stop
 # shell {cat ${tmpdir}/vlog}
-shell -match {1001 H2TxHdr        c \[000006040000000000\]} \
-	{cat ${tmpdir}/vlog}
+# SETTINGS with default initial window size
+shell -match {1001 H2TxHdr        c \[000006040000000000\]} {
+	cat ${tmpdir}/vlog
+}
 
 # While we're here, test sess.xid over H2 as well
 

bin/varnishtest/tests/t02005.vtc

 varnishtest "H2 POST"
 
-barrier b1 cond 2
-
 barrier b2 sock 2
 barrier b3 sock 2
 
@@ -9,7 +7,6 @@ server s1 {
 	rxreq
 	expect req.http.content-length == 7
 	expect req.http.transfer-encoding == <undef>
-	barrier b1 sync
 	txresp -hdr "Content-Type: text/plain" -body response
 
 	rxreq
@@ -30,7 +27,7 @@ varnish v1 -cliok "param.set debug +syncvsl"
 
 logexpect l1 -v v1 -g raw {
 	expect	* 1001 ReqAcct	"80 7 87 106 8 114"
-	expect	* 1000 ReqAcct	"45 8 53 72 22 94"
+	expect	* 1000 ReqAcct	"45 8 53 54 20 74"
 } -start
 
 client c1 {
@@ -38,14 +35,8 @@ client c1 {
 		txping
 		rxping
 	} -run
-	stream 0 {
-		rxwinup
-	} -start
 	stream 1 {
 		txreq -req POST -hdr content-type text/plain -hdr content-length 7 -body request
-
-		rxwinup
-		barrier b1 sync
 		# First, HTTP checks
 		rxresp
 		expect resp.http.content-Type == "text/plain"
@@ -53,21 +44,18 @@ client c1 {
 		# Then, payload checks
 		expect resp.body == response
 	} -run
-	stream 0 -wait
 } -run
 
-client c1 {
+client c2 {
 	stream 0 {
 		barrier b2 sync
 		delay 1
 		barrier b3 sync
-		rxwinup
 	} -start
 	stream 1 {
 		txreq -url "/a" -req POST -nostrend
 		txdata -datalen 100
 		rxresp
-		rxwinup
 		expect resp.status == 503
 	} -run
 	stream 3 {

bin/varnishtest/tests/t02006.vtc

 varnishtest "H2 POST w/ 100 Continue"
 
-barrier b1 cond 2
-
 server s1 {
 	rxreq
 	expect req.http.content-length == <undef>
 	expect req.http.transfer-encoding == chunked
 	expect req.proto == HTTP/1.1
-	barrier b1 sync
 	txresp -hdr "Content-Type: text/plain" -body response
 } -start
 
@@ -31,8 +28,6 @@ client c1 {
 		txdata \
 			-data request
 
-		rxwinup
-		barrier b1 sync
 		rxresp
 		expect resp.status == 200
 		expect resp.http.content-Type == "text/plain"

bin/varnishtest/tests/t02007.vtc

 varnishtest "H2 Huge response headers"
 
-barrier b1 sock 2
-
 server s1 {
 	rxreq
 	expect req.proto == HTTP/1.1
@@ -25,14 +23,7 @@ varnish v1 -cliok "param.set feature +http2"
 varnish v1 -cliok "param.set debug +syncvsl"
 varnish v1 -cliok "param.set debug +h2_nocheck"
 
-varnish v1 -vcl+backend {
-	import vtc;
-	sub vcl_deliver {
-		if (req.url == "/1") {
-			vtc.barrier_sync("${b1_sock}");
-		}
-	}
-} -start
+varnish v1 -vcl+backend {} -start
 
 client c1 {
 	stream 0 {
@@ -61,8 +52,6 @@ client c1 {
 		txdata \
 			-data request
 
-		rxwinup
-		barrier b1 sync
 		rxresp
 		expect resp.status == 200
 		expect resp.http.content-Type == "text/plain"

bin/varnishtest/tests/t02014.vtc

@@ -17,6 +17,7 @@ varnish v1 -vcl+backend {
 
 varnish v1 -cliok "param.set debug +syncvsl"
 varnish v1 -cliok "param.set feature +http2"
+varnish v1 -cliok "param.reset h2_initial_window_size"
 
 client c1 {
 	stream 0 {

bin/varnishtest/vtc_varnish.c

@@ -426,6 +426,8 @@ varnish_launch(struct varnish *v)
 	VSB_cat(vsb, " -p thread_pool_min=10");
 	VSB_cat(vsb, " -p debug=+vtc_mode");
 	VSB_cat(vsb, " -p vsl_mask=+Debug");
+	VSB_cat(vsb, " -p h2_initial_window_size=1m");
+	VSB_cat(vsb, " -p h2_rx_window_low_water=64k");
 	if (!v->has_a_arg) {
 		VSB_printf(vsb, " -a '%s'", "127.0.0.1:0");
 		if (v->proto != NULL)