all: dns refactor, add Proxied and PerDomain flags from control (#615)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/dns/manager.go

@@ -0,0 +1,94 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dns
+
+import (
+	"time"
+
+	"tailscale.com/types/logger"
+)
+
+// reconfigTimeout is the time interval within which Manager.{Up,Down} should complete.
+//
+// This is particularly useful because certain conditions can cause indefinite hangs
+// (such as improper dbus auth followed by contextless dbus.Object.Call).
+// Such operations should be wrapped in a timeout context.
+const reconfigTimeout = time.Second
+
+type managerImpl interface {
+	// Up updates system DNS settings to match the given configuration.
+	Up(Config) error
+	// Down undoes the effects of Up.
+	// It is idempotent and performs no action if Up has never been called.
+	Down() error
+}
+
+// Manager manages system DNS settings.
+type Manager struct {
+	logf logger.Logf
+
+	impl managerImpl
+
+	config  Config
+	mconfig ManagerConfig
+}
+
+// NewManagers created a new manager from the given config.
+func NewManager(mconfig ManagerConfig) *Manager {
+	mconfig.Logf = logger.WithPrefix(mconfig.Logf, "dns: ")
+	m := &Manager{
+		logf: mconfig.Logf,
+		impl: newManager(mconfig),
+
+		config:  Config{PerDomain: mconfig.PerDomain},
+		mconfig: mconfig,
+	}
+
+	m.logf("using %T", m.impl)
+	return m
+}
+
+func (m *Manager) Set(config Config) error {
+	if config.Equal(m.config) {
+		return nil
+	}
+
+	m.logf("Set: %+v", config)
+
+	if len(config.Nameservers) == 0 {
+		err := m.impl.Down()
+		// If we save the config, we will not retry next time. Only do this on success.
+		if err == nil {
+			m.config = config
+		}
+		return err
+	}
+
+	// Switching to and from per-domain mode may require a change of manager.
+	if config.PerDomain != m.config.PerDomain {
+		if err := m.impl.Down(); err != nil {
+			return err
+		}
+		m.mconfig.PerDomain = config.PerDomain
+		m.impl = newManager(m.mconfig)
+		m.logf("switched to %T", m.impl)
+	}
+
+	err := m.impl.Up(config)
+	// If we save the config, we will not retry next time. Only do this on success.
+	if err == nil {
+		m.config = config
+	}
+
+	return err
+}
+
+func (m *Manager) Up() error {
+	return m.impl.Up(m.config)
+}
+
+func (m *Manager) Down() error {
+	return m.impl.Down()
+}