make sure two consecutive Server calls return the same list of servers

app/proxy/proxy.go

@@ -138,15 +138,9 @@ func (h *Http) Run(ctx context.Context) error {
 		gzipHandler(h.GzEnabled),         // gzip response
 	)
 
+	// no FQDNs defined, use the list of discovered servers
 	if len(h.SSLConfig.FQDNs) == 0 && h.SSLConfig.SSLMode == SSLAuto {
-		// discovery async and may happen not right away. Try to get servers for some time
-		for i := 0; i < 100; i++ {
-			h.SSLConfig.FQDNs = h.Servers() // fill all discovered if nothing defined
-			if len(h.SSLConfig.FQDNs) > 0 {
-				break
-			}
-			time.Sleep(50 * time.Millisecond)
-		}
+		h.SSLConfig.FQDNs = h.discoveredServers(ctx, 50*time.Millisecond)
 	}
 
 	switch h.SSLConfig.SSLMode {
@@ -423,3 +417,26 @@ func (h *Http) setXRealIP(r *http.Request) {
 	}
 	r.Header.Add("X-Real-IP", ip)
 }
+
+// discoveredServers gets the list of servers discovered by providers.
+// The underlying discovery is async and may happen not right away.
+// We should try to get servers for some time and make sure we have the complete list of servers
+// by checking if the number of servers has not changed between two calls.
+func (h *Http) discoveredServers(ctx context.Context, interval time.Duration) (servers []string) {
+	discoveredServers := 0
+
+	for i := 0; i < 100; i++ {
+		select {
+		case <-ctx.Done():
+			return nil
+		default:
+		}
+		servers = h.Servers() // fill all discovered if nothing defined
+		if len(servers) > 0 && len(servers) == discoveredServers {
+			break
+		}
+		discoveredServers = len(servers)
+		time.Sleep(interval)
+	}
+	return servers
+}

app/proxy/proxy_test.go

@@ -890,3 +890,27 @@ func TestHttp_matchHandler(t *testing.T) {
 		})
 	}
 }
+
+func TestHttp_discoveredServers(t *testing.T) {
+
+	calls := 0
+	m := &MatcherMock{ServersFunc: func() []string {
+		defer func() { calls++ }()
+		switch calls {
+		case 0, 1, 2, 3, 4:
+			return []string{}
+		case 5:
+			return []string{"s1", "s2"}
+		case 6, 7:
+			return []string{"s1", "s2", "s3"}
+		default:
+			t.Fatalf("shoudn't be called %d times", calls)
+			return nil
+		}
+	}}
+
+	h := Http{Matcher: m}
+
+	res := h.discoveredServers(context.Background(), time.Millisecond)
+	assert.Equal(t, []string{"s1", "s2", "s3"}, res)
+}