tailcfg, control/controlclient: (mapver 16) add Node.Online, MapResponse.OnlineChange

And fix PeerSeenChange bug where it was ignored unless there were
other peer changes.

Updates tailscale/corp#1574

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -38,7 +38,8 @@ import (
 //    13: 2021-03-19: client understands FilterRule.IPProto
 //    14: 2021-04-07: client understands DNSConfig.Routes and DNSConfig.Resolvers
 //    15: 2021-04-12: client treats nil MapResponse.DNSConfig as meaning unchanged
-const CurrentMapRequestVersion = 15
+//    16: 2021-04-15: client understands Node.Online, MapResponse.OnlineChange
+const CurrentMapRequestVersion = 16
 
 type StableID string
 
@@ -156,7 +157,17 @@ type Node struct {
 	DERP       string             `json:",omitempty"` // DERP-in-IP:port ("127.3.3.40:N") endpoint
 	Hostinfo   Hostinfo
 	Created    time.Time
-	LastSeen   *time.Time `json:",omitempty"`
+
+	// LastSeen is when the node was last online. It is not
+	// updated when Online is true. It is nil if the current
+	// node doesn't have permission to know, or the node
+	// has never been online.
+	LastSeen *time.Time `json:",omitempty"`
+
+	// Online is whether the node is currently connected to the
+	// coordination server.  A value of nil means unknown, or the
+	// current node doesn't have permission to know.
+	Online *bool `json:",omitempty"`
 
 	KeepAlive bool `json:",omitempty"` // open and keep open a connection to this peer
 
@@ -907,6 +918,9 @@ type MapResponse struct {
 	// the LastSeen time is now. Absent means unchanged.
 	PeerSeenChange map[NodeID]bool `json:",omitempty"`
 
+	// OnlineChange changes the value of a Peer Node.Online value.
+	OnlineChange map[NodeID]bool `json:",omitempty"`
+
 	// DNS is the same as DNSConfig.Nameservers.
 	// Only populated if MapRequest.Version < 9.
 	DNS []netaddr.IP `json:",omitempty"`
@@ -1048,6 +1062,7 @@ func (n *Node) Equal(n2 *Node) bool {
 		n.KeyExpiry.Equal(n2.KeyExpiry) &&
 		n.Machine == n2.Machine &&
 		n.DiscoKey == n2.DiscoKey &&
+		eqBoolPtr(n.Online, n2.Online) &&
 		eqCIDRs(n.Addresses, n2.Addresses) &&
 		eqCIDRs(n.AllowedIPs, n2.AllowedIPs) &&
 		eqStrings(n.Endpoints, n2.Endpoints) &&
@@ -1062,6 +1077,17 @@ func (n *Node) Equal(n2 *Node) bool {
 		n.ComputedNameWithHost == n2.ComputedNameWithHost
 }
 
+func eqBoolPtr(a, b *bool) bool {
+	if a == b { // covers nil
+		return true
+	}
+	if a == nil || b == nil {
+		return false
+	}
+	return *a == *b
+
+}
+
 func eqStrings(a, b []string) bool {
 	if len(a) != len(b) || ((a == nil) != (b == nil)) {
 		return false

tailcfg/tailcfg_clone.go

@@ -53,6 +53,10 @@ func (src *Node) Clone() *Node {
 		dst.LastSeen = new(time.Time)
 		*dst.LastSeen = *src.LastSeen
 	}
+	if dst.Online != nil {
+		dst.Online = new(bool)
+		*dst.Online = *src.Online
+	}
 	dst.Capabilities = append(src.Capabilities[:0:0], src.Capabilities...)
 	return dst
 }
@@ -76,6 +80,7 @@ var _NodeNeedsRegeneration = Node(struct {
 	Hostinfo                Hostinfo
 	Created                 time.Time
 	LastSeen                *time.Time
+	Online                  *bool
 	KeepAlive               bool
 	MachineAuthorized       bool
 	Capabilities            []string

tailcfg/tailcfg_test.go

@@ -193,7 +193,7 @@ func TestNodeEqual(t *testing.T) {
 		"ID", "StableID", "Name", "User", "Sharer",
 		"Key", "KeyExpiry", "Machine", "DiscoKey",
 		"Addresses", "AllowedIPs", "Endpoints", "DERP", "Hostinfo",
-		"Created", "LastSeen", "KeepAlive", "MachineAuthorized",
+		"Created", "LastSeen", "Online", "KeepAlive", "MachineAuthorized",
 		"Capabilities",
 		"ComputedName", "computedHostIfDifferent", "ComputedNameWithHost",
 	}