lib, gapi, cli: Add a wait flag to empty and a new default

Change the default "wait" state for if you run the empty frontend when
there's already an available deploy waiting. You almost certainly want
to start running it right away.

Example:

mgmt etcd

mgmt run --hostname h1 --no-server --tmp-prefix --seeds=http://127.0.0.1:2379 empty
mgmt run --hostname h2 --no-server --tmp-prefix --seeds=http://127.0.0.1:2379 empty

mgmt deploy --no-git --seeds=http://127.0.0.1:2379 lang examples/lang/hello0.mcl

mgmt run --hostname h3 --no-server --tmp-prefix --seeds=http://127.0.0.1:2379 empty

In fact, you don't even need to start up etcd first for this to all
work.

gapi/empty/empty.go

@@ -33,6 +33,7 @@ import (
 	"fmt"
 	"sync"
 
+	cliUtil "github.com/purpleidea/mgmt/cli/util"
 	"github.com/purpleidea/mgmt/gapi"
 	"github.com/purpleidea/mgmt/pgraph"
 )
@@ -48,6 +49,17 @@ func init() {
 
 // GAPI implements the main lang GAPI interface.
 type GAPI struct {
+	// Wait should be true if we don't use any existing (stale) deploys.
+	// This means that if you start an empty GAPI, then it will immediately
+	// try to look for and run any existing deploys that have been stored in
+	// the cluster that it has connected to. If this is true, then it will
+	// only start on the next deploy. To be honest, we should probably never
+	// wait, but this was accidentally how it was initially implemented, so
+	// we'll change the default and add this in as a flag for now. We may
+	// remove this in the future unless someone has a good reason for
+	// needing it.
+	Wait bool
+
 	data        *gapi.Data
 	initialized bool
 	closeChan   chan struct{}
@@ -57,11 +69,19 @@ type GAPI struct {
 // Cli takes an *Info struct, and returns our deploy if activated, and if there
 // are any validation problems, you should return an error. If there is no
 // deploy, then you should return a nil deploy and a nil error.
-func (obj *GAPI) Cli(*gapi.Info) (*gapi.Deploy, error) {
+func (obj *GAPI) Cli(info *gapi.Info) (*gapi.Deploy, error) {
+	args, ok := info.Args.(*cliUtil.EmptyArgs)
+	if !ok {
+		// programming error
+		return nil, fmt.Errorf("could not convert to our struct")
+	}
+
 	return &gapi.Deploy{
 		Name: Name,
 		//Noop: false,
-		GAPI: &GAPI{},
+		GAPI: &GAPI{
+			Wait: args.Wait,
+		},
 	}, nil
 }