upgrader: new command and upgrade package

upgrade/upgrade.go

+package upgrade
+import (
+        "launchpad.net/gnuflag"
+        "sync"
+        "os"
+        "errors"
+        "fmt"
+)
+
+var upgradedFlag = gnuflag.Bool("upgraded", false, "process is started by the upgrader process")
+
+var (
+        mutex sync.Mutex
+        started bool
+        startError error
+)
+
+// Start signals to the upgrader that the command has started.  If it
+// returns with an error, the command should abort and exit; otherwise
+// it should assume normal operation.
+func Start() error {
+        if !*upgradedFlag {
+                return nil
+        }
+        mutex.Lock()
+        defer mutex.Unlock()
+        if !started {
+                WriteMsg(os.Stdout, "started")
+                _, startError = ReadMsg(os.Stdin, "run")
+                started = true
+        }
+        return startError
+}
+
+// Upgrade requests to the upgrader that the current command be upgraded
+// by running the given executable command.  If the command succeeds in
+// starting, the shutdown function will be called to shut down all
+// operations.  If it succeeds, Upgrade will return no error, and the
+// caller should exit the program to allow the new command to take over
+// operations.
+func Upgrade(shutdown func() error, cmd string, args ...string) error {
+        if !*upgradedFlag {
+                return errors.New("cannot upgrade when not started from the upgrader")
+        }
+        mutex.Lock()
+        defer mutex.Unlock()
+        if !started {
+                return errors.New("upgrade called before start")
+        }
+        if startError != nil {
+                return fmt.Errorf("start encountered an error so cannot upgrade: %v", startError)
+        }
+        m := make([]string, 2 + len(args))
+        m[0] = "upgrade"
+        m[1] = cmd
+        copy(m[2:], args)
+        WriteMsg(os.Stdout, m...)
+        _, err := ReadMsg(os.Stdin, "upgraded")
+        if err != nil {
+                return err
+        }
+        err = shutdown()
+        if err != nil {
+                WriteMsg(os.Stdout, "error", err.Error())
+                return fmt.Errorf("shutdown failed: %v", err)
+        }
+        WriteMsg(os.Stdout, "shutdown")
+        // TODO os.Exit(0) ?
+        return nil
+}