Add dbx library: PostgreSQL cluster with master/replica routing, retry, health checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

balancer_test.go

@@ -0,0 +1,73 @@
+package dbx
+
+import (
+	"testing"
+)
+
+func TestRoundRobinBalancer_Empty(t *testing.T) {
+	b := NewRoundRobinBalancer()
+	if n := b.Next(nil); n != nil {
+		t.Errorf("expected nil for empty slice, got %v", n)
+	}
+}
+
+func TestRoundRobinBalancer_AllHealthy(t *testing.T) {
+	b := NewRoundRobinBalancer()
+	nodes := makeTestNodes("a", "b", "c")
+
+	seen := map[string]int{}
+	for range 30 {
+		n := b.Next(nodes)
+		if n == nil {
+			t.Fatal("unexpected nil")
+		}
+		seen[n.name]++
+	}
+
+	if len(seen) != 3 {
+		t.Errorf("expected 3 distinct nodes, got %d", len(seen))
+	}
+	for name, count := range seen {
+		if count != 10 {
+			t.Errorf("node %s hit %d times, expected 10", name, count)
+		}
+	}
+}
+
+func TestRoundRobinBalancer_SkipsUnhealthy(t *testing.T) {
+	b := NewRoundRobinBalancer()
+	nodes := makeTestNodes("a", "b", "c")
+	nodes[1].healthy.Store(false) // b is down
+
+	for range 20 {
+		n := b.Next(nodes)
+		if n == nil {
+			t.Fatal("unexpected nil")
+		}
+		if n.name == "b" {
+			t.Error("should not return unhealthy node b")
+		}
+	}
+}
+
+func TestRoundRobinBalancer_AllUnhealthy(t *testing.T) {
+	b := NewRoundRobinBalancer()
+	nodes := makeTestNodes("a", "b")
+	for _, n := range nodes {
+		n.healthy.Store(false)
+	}
+
+	if n := b.Next(nodes); n != nil {
+		t.Errorf("expected nil when all unhealthy, got %v", n.name)
+	}
+}
+
+func makeTestNodes(names ...string) []*Node {
+	nodes := make([]*Node, len(names))
+	for i, name := range names {
+		n := &Node{name: name}
+		n.healthy.Store(true)
+		nodes[i] = n
+	}
+	return nodes
+}