reverseproxy: Allow `0` as weights for `weighted_round_robin` (#6681)

* Allow 0 as weights

Change positive to non-negative

* reverseproxy: allow 0 as weighted round robin value

* test: add more wrr select test

---------

Co-authored-by: peanutduck <[email protected]>

2 files changed, 65 insertions, 6 deletions

diff --git a/modules/caddyhttp/reverseproxy/selectionpolicies.go b/modules/caddyhttp/reverseproxy/selectionpolicies.go
index 293ff75e2..fcf7f90f6 100644
--- a/modules/caddyhttp/reverseproxy/selectionpolicies.go
+++ b/modules/caddyhttp/reverseproxy/selectionpolicies.go
@@ -111,8 +111,8 @@ func (r *WeightedRoundRobinSelection) UnmarshalCaddyfile(d *caddyfile.Dispenser)
 		if err != nil {
 			return d.Errf("invalid weight value '%s': %v", weight, err)
 		}
-		if weightInt < 1 {
-			return d.Errf("invalid weight value '%s': weight should be non-zero and positive", weight)
+		if weightInt < 0 {
+			return d.Errf("invalid weight value '%s': weight should be non-negative", weight)
 		}
 		r.Weights = append(r.Weights, weightInt)
 	}
@@ -136,8 +136,15 @@ func (r *WeightedRoundRobinSelection) Select(pool UpstreamPool, _ *http.Request,
 		return pool[0]
 	}
 	var index, totalWeight int
+	var weights []int
+
+	for _, w := range r.Weights {
+		if w > 0 {
+			weights = append(weights, w)
+		}
+	}
 	currentWeight := int(atomic.AddUint32(&r.index, 1)) % r.totalWeight
-	for i, weight := range r.Weights {
+	for i, weight := range weights {
 		totalWeight += weight
 		if currentWeight < totalWeight {
 			index = i
@@ -145,9 +152,9 @@ func (r *WeightedRoundRobinSelection) Select(pool UpstreamPool, _ *http.Request,
 		}
 	}
 
-	upstreams := make([]*Upstream, 0, len(r.Weights))
-	for _, upstream := range pool {
-		if !upstream.Available() {
+	upstreams := make([]*Upstream, 0, len(weights))
+	for i, upstream := range pool {
+		if !upstream.Available() || r.Weights[i] == 0 {
 			continue
 		}
 		upstreams = append(upstreams, upstream)
diff --git a/modules/caddyhttp/reverseproxy/selectionpolicies_test.go b/modules/caddyhttp/reverseproxy/selectionpolicies_test.go
index a4701ce86..580abbdde 100644
--- a/modules/caddyhttp/reverseproxy/selectionpolicies_test.go
+++ b/modules/caddyhttp/reverseproxy/selectionpolicies_test.go
@@ -131,6 +131,58 @@ func TestWeightedRoundRobinPolicy(t *testing.T) {
 	}
 }
 
+func TestWeightedRoundRobinPolicyWithZeroWeight(t *testing.T) {
+	pool := testPool()
+	wrrPolicy := WeightedRoundRobinSelection{
+		Weights:     []int{0, 2, 1},
+		totalWeight: 3,
+	}
+	req, _ := http.NewRequest("GET", "/", nil)
+
+	h := wrrPolicy.Select(pool, req, nil)
+	if h != pool[1] {
+		t.Error("Expected first weighted round robin host to be second host in the pool.")
+	}
+
+	h = wrrPolicy.Select(pool, req, nil)
+	if h != pool[2] {
+		t.Error("Expected second weighted round robin host to be third host in the pool.")
+	}
+
+	h = wrrPolicy.Select(pool, req, nil)
+	if h != pool[1] {
+		t.Error("Expected third weighted round robin host to be second host in the pool.")
+	}
+
+	// mark second host as down
+	pool[1].setHealthy(false)
+	h = wrrPolicy.Select(pool, req, nil)
+	if h != pool[2] {
+		t.Error("Expect select next available host.")
+	}
+
+	h = wrrPolicy.Select(pool, req, nil)
+	if h != pool[2] {
+		t.Error("Expect select only available host.")
+	}
+	// mark second host as up
+	pool[1].setHealthy(true)
+
+	h = wrrPolicy.Select(pool, req, nil)
+	if h != pool[1] {
+		t.Error("Expect select first host on availability.")
+	}
+
+	// test next select in full cycle
+	expected := []*Upstream{pool[1], pool[2], pool[1], pool[1], pool[2], pool[1]}
+	for i, want := range expected {
+		got := wrrPolicy.Select(pool, req, nil)
+		if want != got {
+			t.Errorf("Selection %d: got host[%s], want host[%s]", i+1, got, want)
+		}
+	}
+}
+
 func TestLeastConnPolicy(t *testing.T) {
 	pool := testPool()
 	lcPolicy := LeastConnSelection{}