Implement agent interface and helper.

state/agent.go

+// launchpad.net/juju/state
+//
+// Copyright (c) 2011-2012 Canonical Ltd.
+package state
+
+import (
+        "fmt"
+        "launchpad.net/juju/go/state/presence"
+        "time"
+)
+
+const (
+        agentPingerPeriod = 1 * time.Second
+)
+
+// agent represents a juju agent.
+type agent struct {
+        st     *State
+        path   string
+        pinger *presence.Pinger
+}
+
+// Connected returns true if its entity state has an agent connected.
+func (a *agent) Connected() (bool, error) {
+        return presence.Alive(a.st.zk, a.path)
+}
+
+// WaitConnected waits until an agent has connected.
+func (a *agent) WaitConnected(timeout time.Duration) error {
+        alive, watch, err := presence.AliveW(a.st.zk, a.path)
+        if err != nil {
+                return err
+        }
+        // Quick return if already connected.
+        if alive {
+                return nil
+        }
+        // Wait for connection with timeout.
+        select {
+        case alive, ok := <-watch:
+                if !ok {
+                        return fmt.Errorf("wait connection closed")
+                }
+                if !alive {
+                        return fmt.Errorf("not connected, must not happen")
+                }
+        case <-time.After(timeout):
+                return fmt.Errorf("wait for connected agent timed out")
+        }
+        return nil
+}
+
+// Connect informs juju that an associated agent is alive.
+func (a *agent) Connect() (err error) {
+        if a.pinger != nil {
+                return fmt.Errorf("agent is already connected")
+        }
+        a.pinger, err = presence.StartPinger(a.st.zk, a.path, agentPingerPeriod)
+        return
+}
+
+// Disconnect informs juju that an associated agent stops working.
+func (a *agent) Disconnect() error {
+        if a.pinger == nil {
+                return fmt.Errorf("agent is not connected")
+        }
+        a.pinger.Kill()
+        a.pinger = nil
+        return nil
+}