Update context example

lectures/05-concurrency/context/cancelation/cancelation.go

@@ -35,7 +35,7 @@ func doSlowJob(ctx context.Context) error {
 	for {
 		select {
 		case <-ctx.Done():
-			return ctx.Err()
+			return context.Cause(ctx)
 		default:
 			// perform a portion of slow job
 			time.Sleep(1 * time.Second)

lectures/05-concurrency/lecture.slide

@@ -372,6 +372,10 @@ Concurrency with Shared Memory
   ctx, cancel := context.WithCancel(ctx)
   defer cancel()
 
+  // manual cancel with explicit cause
+  ctx, cancel := context.WithCancelCause(ctx)
+  defer cancel(fmt.Errorf("job not needed"))
+
   // cancel by timeout
   ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
   defer cancel()