agent: notify systemd after JoinLAN (#2121)

This patch adds support for notifying systemd via the
NOTIFY_SOCKET by sending 'READY=1' to the socket after
a successful JoinLAN.

Fixes #2121

agent/agent.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/hashicorp/consul/agent/consul"
 	"github.com/hashicorp/consul/agent/consul/structs"
+	"github.com/hashicorp/consul/agent/systemd"
 	"github.com/hashicorp/consul/api"
 	"github.com/hashicorp/consul/ipaddr"
 	"github.com/hashicorp/consul/lib"
@@ -71,6 +72,11 @@ type delegate interface {
 	Stats() map[string]map[string]string
 }
 
+// notifier is called after a successful JoinLAN.
+type notifier interface {
+	Notify(string) error
+}
+
 // The agent is the long running process that is run on every machine.
 // It exposes an RPC interface that is used by the CLI to control the
 // agent. The agent runs the query interfaces like HTTP, DNS, and RPC.
@@ -141,6 +147,9 @@ type Agent struct {
 	shutdownCh   chan struct{}
 	shutdownLock sync.Mutex
 
+	// joinLANNotifier is called after a successful JoinLAN.
+	joinLANNotifier notifier
+
 	// retryJoinCh transports errors from the retry join
 	// attempts.
 	retryJoinCh chan error
@@ -198,6 +207,7 @@ func New(c *Config) (*Agent, error) {
 		checkDockers:    make(map[types.CheckID]*CheckDocker),
 		eventCh:         make(chan serf.UserEvent, 1024),
 		eventBuf:        make([]*UserEvent, 256),
+		joinLANNotifier: &systemd.Notifier{},
 		reloadCh:        make(chan chan error),
 		retryJoinCh:     make(chan error),
 		shutdownCh:      make(chan struct{}),
@@ -1216,6 +1226,11 @@ func (a *Agent) JoinLAN(addrs []string) (n int, err error) {
 	a.logger.Printf("[INFO] agent: (LAN) joining: %v", addrs)
 	n, err = a.delegate.JoinLAN(addrs)
 	a.logger.Printf("[INFO] agent: (LAN) joined: %d Err: %v", n, err)
+	if err == nil && a.joinLANNotifier != nil {
+		if notifErr := a.joinLANNotifier.Notify(systemd.Ready); notifErr != nil {
+			a.logger.Printf("[DEBUG] agent: systemd notify failed: ", notifErr)
+		}
+	}
 	return
 }
 

agent/agent_endpoint_test.go

@@ -447,6 +447,38 @@ func TestAgent_Join_ACLDeny(t *testing.T) {
 	})
 }
 
+type mockNotifier struct{ s string }
+
+func (n *mockNotifier) Notify(state string) error {
+	n.s = state
+	return nil
+}
+
+func TestAgent_JoinLANNotify(t *testing.T) {
+	t.Parallel()
+	a1 := NewTestAgent(t.Name(), nil)
+	defer a1.Shutdown()
+
+	cfg2 := TestConfig()
+	cfg2.Server = false
+	cfg2.Bootstrap = false
+	a2 := NewTestAgent(t.Name(), cfg2)
+	defer a2.Shutdown()
+
+	notif := &mockNotifier{}
+	a1.joinLANNotifier = notif
+
+	addr := fmt.Sprintf("127.0.0.1:%d", a2.Config.Ports.SerfLan)
+	_, err := a1.JoinLAN([]string{addr})
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	if got, want := notif.s, "READY=1"; got != want {
+		t.Fatalf("got joinLAN notification %q want %q", got, want)
+	}
+}
+
 func TestAgent_Leave(t *testing.T) {
 	t.Parallel()
 	a1 := NewTestAgent(t.Name(), nil)

agent/systemd/notify.go

@@ -0,0 +1,42 @@
+package systemd
+
+import (
+	"errors"
+	"net"
+	"os"
+)
+
+const (
+	// magic values for systemd
+	// from https://www.freedesktop.org/software/systemd/man/sd_notify.html#Description
+
+	Ready     = "READY=1"
+	Reloading = "RELOADING=1"
+	Stopping  = "STOPPING=1"
+)
+
+var NotifyNoSocket = errors.New("No socket")
+
+// Notifier provides a method to send a message to systemd.
+type Notifier struct{}
+
+// Notify sends a message to the init daemon. It is common to ignore the error.
+func (n *Notifier) Notify(state string) error {
+	addr := &net.UnixAddr{
+		Name: os.Getenv("NOTIFY_SOCKET"),
+		Net:  "unixgram",
+	}
+
+	if addr.Name == "" {
+		return NotifyNoSocket
+	}
+
+	conn, err := net.DialUnix(addr.Net, nil, addr)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	_, err = conn.Write([]byte(state))
+	return err
+}

website/source/docs/agent/basics.html.md

@@ -74,6 +74,11 @@ There are several important messages that [`consul agent`](/docs/commands/agent.
   Consul agents in a cluster. Not all Consul agents in a cluster have to
   use the same port, but this address **MUST** be reachable by all other nodes.
 
+When running under `systemd` on Linux, Consul notifies systemd by sending
+`READY=1` to the `$NOTIFY_SOCKET` when a LAN join has completed. For
+this either the `join` or `retry_join` option has to be set and the
+service definition file has to have `Type=notify` set.
+
 ## Stopping an Agent
 
 An agent can be stopped in two ways: gracefully or forcefully. To gracefully