engine: graph: Avoid race on fast pause variable

This code is basically unused, but let's keep it in for now in case we
eventually replace it with some contextual ctx code instead.

engine/graph/actions.go

@@ -376,7 +376,7 @@ func (obj *Engine) Process(ctx context.Context, vertex pgraph.Vertex) error {
 			// so that the graph doesn't go on running forever until
 			// it's completely done. This is an optional feature and
 			// we can select it via ^C on user exit or via the GAPI.
-			if obj.fastPause {
+			if obj.fastPause.Load() {
 				obj.Logf("%s: fast pausing, poke skipped", res)
 				continue
 			}

engine/graph/engine.go

@@ -37,6 +37,7 @@ import (
 	"os"
 	"path"
 	"sync"
+	"sync/atomic"
 
 	"github.com/purpleidea/mgmt/converger"
 	"github.com/purpleidea/mgmt/engine"
@@ -88,7 +89,7 @@ type Engine struct {
 	wg *sync.WaitGroup // wg for the whole engine (only used for close)
 
 	paused    bool // are we paused?
-	fastPause bool
+	fastPause *atomic.Bool
 	isClosing bool // are we shutting down?
 }
 
@@ -130,6 +131,7 @@ func (obj *Engine) Init() error {
 	obj.wg = &sync.WaitGroup{}
 
 	obj.paused = true // start off true, so we can Resume after first Commit
+	obj.fastPause = &atomic.Bool{}
 
 	obj.Exporter = &Exporter{
 		World: obj.World,
@@ -502,7 +504,7 @@ func (obj *Engine) Resume() error {
 // poke. In general this is only called when you're trying to hurry up the exit.
 // XXX: Not implemented
 func (obj *Engine) SetFastPause() {
-	obj.fastPause = true
+	obj.fastPause.Store(true)
 }
 
 // Pause the active, running graph.
@@ -515,7 +517,7 @@ func (obj *Engine) Pause(fastPause bool) error {
 		return fmt.Errorf("already paused")
 	}
 
-	obj.fastPause = fastPause
+	obj.fastPause.Store(fastPause)
 	topoSort, _ := obj.graph.TopologicalSort()
 	for _, vertex := range topoSort { // squeeze out the events...
 		// The Event is sent to an unbuffered channel, so this event is
@@ -528,7 +530,7 @@ func (obj *Engine) Pause(fastPause bool) error {
 	obj.paused = true
 
 	// we are now completely paused...
-	obj.fastPause = false // reset
+	obj.fastPause.Store(false) // reset
 	return nil
 }