feat: dialer refactor to support modular dialing/checking/proxying

pkg/pmapi/dialer_proxy.go

@@ -0,0 +1,40 @@
+package pmapi
+
+import (
+	"net"
+)
+
+// ProxyTLSDialer wraps a TLSDialer to switch to a proxy if the initial dial fails.
+type ProxyTLSDialer struct {
+	dialer TLSDialer
+
+	cm *ClientManager
+}
+
+// NewProxyTLSDialer constructs a dialer which provides a proxy-managing layer on top of an underlying dialer.
+func NewProxyTLSDialer(dialer TLSDialer, cm *ClientManager) *ProxyTLSDialer {
+	return &ProxyTLSDialer{
+		dialer: dialer,
+		cm:     cm,
+	}
+}
+
+// DialTLS dials the given network/address. If it fails, it retries using a proxy.
+func (d *ProxyTLSDialer) DialTLS(network, address string) (conn net.Conn, err error) {
+	if conn, err = d.dialer.DialTLS(network, address); err == nil {
+		return
+	}
+
+	var proxy string
+
+	if proxy, err = d.cm.switchToReachableServer(); err != nil {
+		return
+	}
+
+	_, port, err := net.SplitHostPort(address)
+	if err != nil {
+		return
+	}
+
+	return d.dialer.DialTLS(network, net.JoinHostPort(proxy, port))
+}