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

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

config.go

@@ -0,0 +1,88 @@
+package dbx
+
+import "time"
+
+// Config is the top-level configuration for a Cluster.
+type Config struct {
+	Master      NodeConfig
+	Replicas    []NodeConfig
+	Retry       RetryConfig
+	Logger      Logger
+	Metrics     *MetricsHook
+	HealthCheck HealthCheckConfig
+}
+
+// NodeConfig describes a single database node.
+type NodeConfig struct {
+	Name string // human-readable name for logs/metrics, e.g. "master", "replica-1"
+	DSN  string
+	Pool PoolConfig
+}
+
+// PoolConfig controls pgxpool.Pool parameters.
+type PoolConfig struct {
+	MaxConns          int32
+	MinConns          int32
+	MaxConnLifetime   time.Duration
+	MaxConnIdleTime   time.Duration
+	HealthCheckPeriod time.Duration
+}
+
+// RetryConfig controls retry behaviour.
+type RetryConfig struct {
+	MaxAttempts     int                // default: 3
+	BaseDelay       time.Duration      // default: 50ms
+	MaxDelay        time.Duration      // default: 1s
+	RetryableErrors func(error) bool   // optional custom classifier
+}
+
+// HealthCheckConfig controls the background health checker.
+type HealthCheckConfig struct {
+	Interval time.Duration // default: 5s
+	Timeout  time.Duration // default: 2s
+}
+
+// defaults fills zero-valued fields with sensible defaults.
+func (c *Config) defaults() {
+	if c.Logger == nil {
+		c.Logger = nopLogger{}
+	}
+	if c.Retry.MaxAttempts <= 0 {
+		c.Retry.MaxAttempts = 3
+	}
+	if c.Retry.BaseDelay <= 0 {
+		c.Retry.BaseDelay = 50 * time.Millisecond
+	}
+	if c.Retry.MaxDelay <= 0 {
+		c.Retry.MaxDelay = time.Second
+	}
+	if c.HealthCheck.Interval <= 0 {
+		c.HealthCheck.Interval = 5 * time.Second
+	}
+	if c.HealthCheck.Timeout <= 0 {
+		c.HealthCheck.Timeout = 2 * time.Second
+	}
+	if c.Master.Name == "" {
+		c.Master.Name = "master"
+	}
+	for i := range c.Replicas {
+		if c.Replicas[i].Name == "" {
+			c.Replicas[i].Name = "replica-" + itoa(i+1)
+		}
+	}
+}
+
+// itoa is a minimal int-to-string without importing strconv.
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	buf := [20]byte{}
+	i := len(buf)
+	for n > 0 {
+		i--
+		buf[i] = byte('0' + n%10)
+		n /= 10
+	}
+	return string(buf[i:])
+}