reverseproxy: skip unavailable upstream with a single weighted_round_robin weight (#8320)

With fewer than two weights, WeightedRoundRobinSelection returned the
first upstream without checking that it is available.

modules/caddyhttp/reverseproxy/selectionpolicies.go

@@ -146,6 +146,9 @@ func (r *WeightedRoundRobinSelection) Select(pool UpstreamPool, _ *http.Request,
 		return nil
 	}
 	if len(r.Weights) < 2 {
+		if !pool[0].Available() {
+			return nil
+		}
 		return pool[0]
 	}
 	var index, totalWeight int

modules/caddyhttp/reverseproxy/selectionpolicies_test.go

@@ -148,6 +148,24 @@ func TestWeightedRoundRobinPolicyFewerWeightsThanUpstreams(t *testing.T) {
 	}
 }
 
+func TestWeightedRoundRobinPolicySingleWeight(t *testing.T) {
+	pool := testPool()
+	wrrPolicy := WeightedRoundRobinSelection{
+		Weights:     []int{1},
+		totalWeight: 1,
+	}
+	req, _ := http.NewRequest("GET", "/", nil)
+
+	if h := wrrPolicy.Select(pool, req, nil); h != pool[0] {
+		t.Errorf("got %v, want the only weighted host %v", h, pool[0])
+	}
+
+	pool[0].setHealthy(false)
+	if h := wrrPolicy.Select(pool, req, nil); h != nil {
+		t.Errorf("got %v, want nil when the only weighted host is unavailable", h)
+	}
+}
+
 func TestWeightedRoundRobinSelection_Validate(t *testing.T) {
 	tests := []struct {
 		name    string