ipn/ipnext, feature/routecheck: subscribe routecheck to the IPN bus

This patch adds a new ipnext.NotifyWatcher interface that exposes
ipn.LocalBackend.WatchNotifications so that extensions inside
tailscaled can subscribe to the IPN bus, much like how the GUI
clients subscribe to it through the Local API.

This interface is used by the new feature/routecheck.RouterTracker to
watch for changes in the peer map that affect routers. RouterTracker
uses dead reckoning to incrementally maintain the set of routers. We
do this to avoid looping over the peer map repeatedly. See #17366.

RouterTracker supports two hooks:

- OnNetMapAvailable signals that the initial netmap has been received,
  so that the routecheck.Client can wake up goroutines that are
  waiting for it.

- OnRoutersChange signals that the set of routers has changed, so that
  the routecheck.Client can decide to probe a subset of the routers
  instead of all of them. Currently, this optimization hasn’t been
  implemented yet.

Updates #17366
Updates #20062
Updates tailscale/corp#33033

Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/ipnext/ipnext.go

@@ -6,6 +6,7 @@
 package ipnext
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"iter"
@@ -226,6 +227,33 @@ type SafeBackend interface {
 	TailscaleVarRoot() string
 }
 
+// NotifyWatcher is a subset of [tailscale.com/ipn/ipnlocal.LocalBackend]
+// for extensions that subscribe to the IPN notification bus from within tailscaled.
+//
+// Unlike [SafeBackend], its methods acquire LocalBackend’s internal mutex
+// and must not be called from extension hooks,
+// instead call them from a goroutine started by [Extension.Init].
+type NotifyWatcher interface {
+	// WatchNotifications subscribes to the ipn.Notify message bus notification
+	// messages.
+	//
+	// WatchNotifications blocks until ctx is done.
+	//
+	// The provided onWatchAdded, if non-nil, will be called once the watcher
+	// is installed.
+	//
+	// The provided fn will be called for each notification. It will only be
+	// called with non-nil pointers. The caller must not modify roNotify. If
+	// fn returns false, the watch also stops.
+	//
+	// Failure to consume many notifications in a row will result in one final
+	// notification with ErrMessage set, followed by the watch closing, unless mask
+	// includes ipn.NotifyInProcessNoDisconnect. Watchers using
+	// NotifyInProcessNoDisconnect must not call back into LocalBackend from fn or
+	// wait on work that might call back into LocalBackend.
+	WatchNotifications(ctx context.Context, mask ipn.NotifyWatchOpt, onWatchAdded func(), fn func(roNotify *ipn.Notify) (keepGoing bool))
+}
+
 // ExtensionServices provides access to the [Host]'s extension management services,
 // such as fetching active extensions.
 type ExtensionServices interface {