feat(webconnectivity@v0.5): record late DNS replies

Part of ooni/probe#2237

internal/experiment/webconnectivity/dnsresolvers.go

@@ -249,6 +249,21 @@ func (t *DNSResolvers) lookupHostUDP(parentCtx context.Context, udpAddress strin
 
 	ol.Stop(err)
 	out <- addrs
+
+	// wait for late DNS replies
+	t.WaitGroup.Add(1)
+	go t.waitForLateReplies(parentCtx, trace)
+}
+
+// Waits for late DNS replies.
+func (t *DNSResolvers) waitForLateReplies(parentCtx context.Context, trace *measurexlite.Trace) {
+	defer t.WaitGroup.Done()
+	const lateTimeout = 500 * time.Millisecond
+	events := trace.DelayedDNSResponseWithTimeout(parentCtx, lateTimeout)
+	if length := len(events); length > 0 {
+		t.Logger.Warnf("got %d late DNS replies", length)
+	}
+	t.TestKeys.AppendDNSLateReplies(events...)
 }
 
 // Divides queries generated by Do53 in Do53-proper queries and other queries.

internal/experiment/webconnectivity/measurer.go

@@ -36,7 +36,7 @@ func (m *Measurer) ExperimentName() string {
 
 // ExperimentVersion implements model.ExperimentMeasurer.
 func (m *Measurer) ExperimentVersion() string {
-	return "0.5.0"
+	return "0.5.1"
 }
 
 // Run implements model.ExperimentMeasurer.

internal/experiment/webconnectivity/testkeys.go

@@ -30,6 +30,10 @@ type TestKeys struct {
 	// Do53 contains ancillary observations collected by Do53 resolvers.
 	Do53 *TestKeysDo53 `json:"x_do53"`
 
+	// DNSLateReplies contains late replies we didn't expect to receive from
+	// a resolver (which may raise eyebrows if they're different).
+	DNSLateReplies []*model.ArchivalDNSLookupResult `json:"x_dns_late_replies"`
+
 	// Queries contains DNS queries.
 	Queries []*model.ArchivalDNSLookupResult `json:"queries"`
 
@@ -161,6 +165,13 @@ func (tk *TestKeys) AppendNetworkEvents(v ...*model.ArchivalNetworkEvent) {
 	tk.mu.Unlock()
 }
 
+// AppendDNSLateReplies appends to DNSLateReplies.
+func (tk *TestKeys) AppendDNSLateReplies(v ...*model.ArchivalDNSLookupResult) {
+	tk.mu.Lock()
+	tk.DNSLateReplies = append(tk.DNSLateReplies, v...)
+	tk.mu.Unlock()
+}
+
 // AppendQueries appends to Queries.
 func (tk *TestKeys) AppendQueries(v ...*model.ArchivalDNSLookupResult) {
 	tk.mu.Lock()