feature/sdnotify: move util/systemd to a modular feature

Updates #12614

Change-Id: I08e714c83b455df7f538cc99cafe940db936b480
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -102,7 +102,6 @@ import (
 	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/policyclient"
 	"tailscale.com/util/syspolicy/ptype"
-	"tailscale.com/util/systemd"
 	"tailscale.com/util/testenv"
 	"tailscale.com/util/usermetric"
 	"tailscale.com/version"
@@ -5488,7 +5487,7 @@ func (b *LocalBackend) enterStateLockedOnEntry(newState ipn.State, unlock unlock
 
 	switch newState {
 	case ipn.NeedsLogin:
-		systemd.Status("Needs login: %s", authURL)
+		feature.SystemdStatus("Needs login: %s", authURL)
 		// always block updates on NeedsLogin even if seamless renewal is enabled,
 		// to prevent calls to authReconfig from reconfiguring the engine when our
 		// key has expired and we're waiting to authenticate to use the new key.
@@ -5503,7 +5502,7 @@ func (b *LocalBackend) enterStateLockedOnEntry(newState ipn.State, unlock unlock
 		}
 
 		if newState == ipn.Stopped && authURL == "" {
-			systemd.Status("Stopped; run 'tailscale up' to log in")
+			feature.SystemdStatus("Stopped; run 'tailscale up' to log in")
 		}
 	case ipn.Starting, ipn.NeedsMachineAuth:
 		b.authReconfig()
@@ -5515,7 +5514,7 @@ func (b *LocalBackend) enterStateLockedOnEntry(newState ipn.State, unlock unlock
 		for _, p := range addrs.All() {
 			addrStrs = append(addrStrs, p.Addr().String())
 		}
-		systemd.Status("Connected; %s; %s", activeLogin, strings.Join(addrStrs, " "))
+		feature.SystemdStatus("Connected; %s; %s", activeLogin, strings.Join(addrStrs, " "))
 	default:
 		b.logf("[unexpected] unknown newState %#v", newState)
 	}

ipn/ipnserver/server.go

@@ -23,6 +23,7 @@ import (
 
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/envknob"
+	"tailscale.com/feature"
 	"tailscale.com/ipn/ipnauth"
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/localapi"
@@ -32,7 +33,6 @@ import (
 	"tailscale.com/util/eventbus"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
-	"tailscale.com/util/systemd"
 	"tailscale.com/util/testenv"
 )
 
@@ -513,7 +513,9 @@ func (s *Server) Run(ctx context.Context, ln net.Listener) error {
 		ln.Close()
 	}()
 
-	systemd.Ready()
+	if ready, ok := feature.HookSystemdReady.GetOk(); ok {
+		ready()
+	}
 
 	hs := &http.Server{
 		Handler:     http.HandlerFunc(s.serveHTTP),