nits

syncer/syncer.go

@@ -555,7 +555,14 @@ func (s *Syncer) syncRange(
 	s.concurrency = startingConcurrency
 	s.goalConcurrency = s.concurrency
 
-	// Spawn syncing goroutines
+	// We create a separate derivative context here instead of
+	// replacing the provided ctx because the context returned
+	// by errgroup.WithContext is canceled as soon as Wait returns.
+	// If this canceled context is passed to a handler or helper,
+	// it can have unintended consequences (some functions
+	// return immediately if the context is canceled).
+	//
+	// Source: https://godoc.org/golang.org/x/sync/errgroup
 	g, pipelineCtx := errgroup.WithContext(ctx)
 	g.Go(func() error {
 		return s.addBlockIndices(pipelineCtx, blockIndices, s.nextIndex, endIndex)