engine: graph: Add err mutex

Here's a race that pops up. This is suboptimal locking, but it's not
important for now.

engine/graph/engine.go

@@ -91,6 +91,8 @@ type Engine struct {
 	paused    bool // are we paused?
 	fastPause *atomic.Bool
 	isClosing bool // are we shutting down?
+
+	errMutex *sync.Mutex // wraps the *state workerErr (one mutex for all)
 }
 
 // Init initializes the internal structures and starts this the graph running.
@@ -133,6 +135,8 @@ func (obj *Engine) Init() error {
 	obj.paused = true // start off true, so we can Resume after first Commit
 	obj.fastPause = &atomic.Bool{}
 
+	obj.errMutex = &sync.Mutex{}
+
 	obj.Exporter = &Exporter{
 		World: obj.World,
 		Debug: obj.Debug,
@@ -311,7 +315,9 @@ func (obj *Engine) Commit() error {
 						obj.Logf("%s: Exited...", v)
 					}
 				}
+				obj.errMutex.Lock()
 				obj.state[v].workerErr = err // store the error
+				obj.errMutex.Unlock()
 				// If the Rewatch metaparam is true, then this will get
 				// restarted if we do a graph cmp swap. This is why the
 				// graph cmp function runs the removes before the adds.
@@ -379,12 +385,15 @@ func (obj *Engine) Commit() error {
 		s1, ok1 := obj.state[v1]
 		s2, ok2 := obj.state[v2]
 		x1, x2 := false, false
+		// no need to have different mutexes for each state atm
+		obj.errMutex.Lock()
 		if ok1 {
 			x1 = s1.workerErr != nil && swap1
 		}
 		if ok2 {
 			x2 = s2.workerErr != nil && swap2
 		}
+		obj.errMutex.Unlock()
 
 		if x1 || x2 {
 			// We swap, even if they're the same, so that we reload!