ipn/local: add the suggested exit node to the ipn bus

fixes tailscale/corp#26369

The suggested exit node is currently only calculated during a localAPI request.
For older UIs, this wasn't a bad choice - we could just fetch it on-demand when a menu
presented itself.  For newer incarnations however, this is an always-visible field
that needs to react to changes in the suggested exit node's value.

This change recalculates the suggested exit node ID on netmap updates and
broadcasts it on the IPN bus.   The localAPI version of this remains intact for the
time being.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

ipn/backend.go

@@ -83,6 +83,8 @@ const (
 	NotifyRateLimit NotifyWatchOpt = 1 << 8 // if set, rate limit spammy netmap updates to every few seconds
 
 	NotifyHealthActions NotifyWatchOpt = 1 << 9 // if set, include PrimaryActions in health.State. Otherwise append the action URL to the text
+
+	NotifyInitialSuggestedExitNode NotifyWatchOpt = 1 << 10 // if set, the first Notify message (sent immediately) will contain the current SuggestedExitNode if available
 )
 
 // Notify is a communication from a backend (e.g. tailscaled) to a frontend
@@ -98,7 +100,7 @@ type Notify struct {
 	// This field is only set in the first message when requesting
 	// NotifyInitialState. Clients must store it on their side as
 	// following notifications will not include this field.
-	SessionID string `json:",omitempty"`
+	SessionID string `json:",omitzero"`
 
 	// ErrMessage, if non-nil, contains a critical error message.
 	// For State InUseOtherUser, ErrMessage is not critical and just contains the details.
@@ -116,7 +118,7 @@ type Notify struct {
 	// user's preferred storage location.
 	//
 	// Deprecated: use LocalClient.AwaitWaitingFiles instead.
-	FilesWaiting *empty.Message `json:",omitempty"`
+	FilesWaiting *empty.Message `json:",omitzero"`
 
 	// IncomingFiles, if non-nil, specifies which files are in the
 	// process of being received. A nil IncomingFiles means this
@@ -125,22 +127,22 @@ type Notify struct {
 	// of being transferred.
 	//
 	// Deprecated: use LocalClient.AwaitWaitingFiles instead.
-	IncomingFiles []PartialFile `json:",omitempty"`
+	IncomingFiles []PartialFile `json:",omitzero"`
 
 	// OutgoingFiles, if non-nil, tracks which files are in the process of
 	// being sent via TailDrop, including files that finished, whether
 	// successful or failed. This slice is sorted by Started time, then Name.
-	OutgoingFiles []*OutgoingFile `json:",omitempty"`
+	OutgoingFiles []*OutgoingFile `json:",omitzero"`
 
 	// LocalTCPPort, if non-nil, informs the UI frontend which
 	// (non-zero) localhost TCP port it's listening on.
 	// This is currently only used by Tailscale when run in the
 	// macOS Network Extension.
-	LocalTCPPort *uint16 `json:",omitempty"`
+	LocalTCPPort *uint16 `json:",omitzero"`
 
 	// ClientVersion, if non-nil, describes whether a client version update
 	// is available.
-	ClientVersion *tailcfg.ClientVersion `json:",omitempty"`
+	ClientVersion *tailcfg.ClientVersion `json:",omitzero"`
 
 	// DriveShares tracks the full set of current DriveShares that we're
 	// publishing. Some client applications, like the MacOS and Windows clients,
@@ -153,7 +155,11 @@ type Notify struct {
 	// Health is the last-known health state of the backend. When this field is
 	// non-nil, a change in health verified, and the API client should surface
 	// any changes to the user in the UI.
-	Health *health.State `json:",omitempty"`
+	Health *health.State `json:",omitzero"`
+
+	// SuggestedExitNode, if non-nil, is the node that the backend has determined to
+	// be the best exit node for the current network conditions.
+	SuggestedExitNode *tailcfg.StableNodeID `json:",omitzero"`
 
 	// type is mirrored in xcode/IPN/Core/LocalAPI/Model/LocalAPIModel.swift
 }
@@ -194,6 +200,10 @@ func (n Notify) String() string {
 	if n.Health != nil {
 		sb.WriteString("Health{...} ")
 	}
+	if n.SuggestedExitNode != nil {
+		fmt.Fprintf(&sb, "SuggestedExitNode=%v ", *n.SuggestedExitNode)
+	}
+
 	s := sb.String()
 	return s[0:len(s)-1] + "}"
 }

ipn/ipnlocal/bus.go

@@ -156,5 +156,6 @@ func isNotableNotify(n *ipn.Notify) bool {
 		n.Health != nil ||
 		len(n.IncomingFiles) > 0 ||
 		len(n.OutgoingFiles) > 0 ||
-		n.FilesWaiting != nil
+		n.FilesWaiting != nil ||
+		n.SuggestedExitNode != nil
 }

ipn/ipnlocal/local.go

@@ -1741,6 +1741,10 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 
 		b.send(ipn.Notify{NetMap: st.NetMap})
 
+		// The error here is unimportant as is the result.  This will recalculate the suggested exit node
+		// cache the value and push any changes to the IPN bus.
+		b.SuggestExitNode()
+
 		// Check and update the exit node if needed, now that we have a new netmap.
 		//
 		// This must happen after the netmap change is sent via [ipn.Notify],
@@ -2037,7 +2041,13 @@ func (b *LocalBackend) UpdateNetmapDelta(muts []netmap.NodeMutation) (handled bo
 		}
 	}
 
+	if cn.NetMap() != nil && mutationsAreWorthyOfRecalculatingSuggestedExitNode(muts, cn, b.lastSuggestedExitNode) {
+		// Recompute the suggested exit node
+		b.suggestExitNodeLocked()
+	}
+
 	if cn.NetMap() != nil && mutationsAreWorthyOfTellingIPNBus(muts) {
+
 		nm := cn.netMapWithPeers()
 		notify = &ipn.Notify{NetMap: nm}
 	} else if testenv.InTest() {
@@ -2049,6 +2059,41 @@ func (b *LocalBackend) UpdateNetmapDelta(muts []netmap.NodeMutation) (handled bo
 	return true
 }
 
+// mustationsAreWorthyOfRecalculatingSuggestedExitNode reports whether any mutation type in muts is
+// worthy of recalculating the suggested exit node.
+func mutationsAreWorthyOfRecalculatingSuggestedExitNode(muts []netmap.NodeMutation, cn *nodeBackend, sid tailcfg.StableNodeID) bool {
+	for _, m := range muts {
+		n, ok := cn.NodeByID(m.NodeIDBeingMutated())
+		if !ok {
+			// The node being mutated is not in the netmap.
+			continue
+		}
+
+		// The previously suggested exit node itself is being mutated.
+		if sid != "" && n.StableID() == sid {
+			return true
+		}
+
+		allowed := n.AllowedIPs().AsSlice()
+		isExitNode := slices.Contains(allowed, tsaddr.AllIPv4()) || slices.Contains(allowed, tsaddr.AllIPv6())
+		// The node being mutated is not an exit node.  We don't care about it - unless
+		// it was our previously suggested exit node which we catch above.
+		if !isExitNode {
+			continue
+		}
+
+		// Some exit node is being mutated.  We care about it if it's online
+		// or offline state has changed.  We *might* eventually care about it for other reasons
+		// but for the sake of finding a "better" suggested exit node, this is probably
+		// sufficient.
+		switch m.(type) {
+		case netmap.NodeMutationOnline:
+			return true
+		}
+	}
+	return false
+}
+
 // mutationsAreWorthyOfTellingIPNBus reports whether any mutation type in muts is
 // worthy of spamming the IPN bus (the Windows & Mac GUIs, basically) to tell them
 // about the update.
@@ -3068,7 +3113,7 @@ func (b *LocalBackend) WatchNotificationsAs(ctx context.Context, actor ipnauth.A
 
 	b.mu.Lock()
 
-	const initialBits = ipn.NotifyInitialState | ipn.NotifyInitialPrefs | ipn.NotifyInitialNetMap | ipn.NotifyInitialDriveShares
+	const initialBits = ipn.NotifyInitialState | ipn.NotifyInitialPrefs | ipn.NotifyInitialNetMap | ipn.NotifyInitialDriveShares | ipn.NotifyInitialSuggestedExitNode
 	if mask&initialBits != 0 {
 		cn := b.currentNode()
 		ini = &ipn.Notify{Version: version.Long()}
@@ -3091,6 +3136,11 @@ func (b *LocalBackend) WatchNotificationsAs(ctx context.Context, actor ipnauth.A
 		if mask&ipn.NotifyInitialHealthState != 0 {
 			ini.Health = b.HealthTracker().CurrentState()
 		}
+		if mask&ipn.NotifyInitialSuggestedExitNode != 0 {
+			if en, err := b.SuggestExitNode(); err != nil {
+				ini.SuggestedExitNode = &en.ID
+			}
+		}
 	}
 
 	ctx, cancel := context.WithCancel(ctx)
@@ -7706,7 +7756,12 @@ func (b *LocalBackend) suggestExitNodeLocked() (response apitype.ExitNodeSuggest
 	if err != nil {
 		return res, err
 	}
+	if prevSuggestion != res.ID {
+		// Notify the clients via the IPN bus if the exit node suggestion has changed.
+		b.sendToLocked(ipn.Notify{SuggestedExitNode: &res.ID}, allClients)
+	}
 	b.lastSuggestedExitNode = res.ID
+
 	return res, err
 }