Let sync return early if an error occurs (#235)

The previous approach would proceed to execute all requests prior to
returning the first error. This is solved with `errgroup.WithContext`
that cancels the context if a routine returns an error.

libs/sync/sync.go

@@ -118,8 +118,6 @@ func (s *Sync) notifyComplete(ctx context.Context, d diff) {
 }
 
 func (s *Sync) RunOnce(ctx context.Context) error {
-	applyDiff := syncCallback(ctx, s)
-
 	// tradeoff: doing portable monitoring only due to macOS max descriptor manual ulimit setting requirement
 	// https://github.com/gorakhargosh/watchdog/blob/master/src/watchdog/observers/kqueue.py#L394-L418
 	all, err := s.fileSet.All()
@@ -139,7 +137,7 @@ func (s *Sync) RunOnce(ctx context.Context) error {
 		return nil
 	}
 
-	err = applyDiff(change)
+	err = s.applyDiff(ctx, change)
 	if err != nil {
 		return err
 	}

libs/sync/watchdog.go

@@ -6,53 +6,63 @@ import (
 	"golang.org/x/sync/errgroup"
 )
 
-// See https://docs.databricks.com/resources/limits.html#limits-api-rate-limits for per api
-// rate limits
+// Maximum number of concurrent requests during sync.
 const MaxRequestsInFlight = 20
 
-func syncCallback(ctx context.Context, s *Sync) func(localDiff diff) error {
-	return func(d diff) error {
-		// Abstraction over wait groups which allows you to get the errors
-		// returned in goroutines
-		var g errgroup.Group
+// Perform a DELETE of the specified remote path.
+func (s *Sync) applyDelete(ctx context.Context, group *errgroup.Group, remoteName string) {
+	// Return early if the context has already been cancelled.
+	select {
+	case <-ctx.Done():
+		return
+	default:
+		// Proceed.
+	}
 
-		// Allow MaxRequestLimit maxiumum concurrent api calls
-		g.SetLimit(MaxRequestsInFlight)
-
-		for _, remoteName := range d.delete {
-			// Copy of remoteName created to make this safe for concurrent use.
-			// directly using remoteName can cause race conditions since the loop
-			// might iterate over to the next remoteName before the go routine function
-			// is evaluated
-			remoteNameCopy := remoteName
-			g.Go(func() error {
-				s.notifyProgress(ctx, EventActionDelete, remoteNameCopy, 0.0)
-				err := s.repoFiles.DeleteFile(ctx, remoteNameCopy)
-				if err != nil {
-					return err
-				}
-				s.notifyProgress(ctx, EventActionDelete, remoteNameCopy, 1.0)
-				return nil
-			})
-		}
-		for _, localRelativePath := range d.put {
-			// Copy of localName created to make this safe for concurrent use.
-			localRelativePathCopy := localRelativePath
-			g.Go(func() error {
-				s.notifyProgress(ctx, EventActionPut, localRelativePathCopy, 0.0)
-				err := s.repoFiles.PutFile(ctx, localRelativePathCopy)
-				if err != nil {
-					return err
-				}
-				s.notifyProgress(ctx, EventActionPut, localRelativePathCopy, 1.0)
-				return nil
-			})
-		}
-		// wait for goroutines to finish and return first non-nil error return
-		// if any
-		if err := g.Wait(); err != nil {
+	group.Go(func() error {
+		s.notifyProgress(ctx, EventActionDelete, remoteName, 0.0)
+		err := s.repoFiles.DeleteFile(ctx, remoteName)
+		if err != nil {
 			return err
 		}
+		s.notifyProgress(ctx, EventActionDelete, remoteName, 1.0)
 		return nil
-	}
+	})
+}
+
+// Perform a PUT of the specified local path.
+func (s *Sync) applyPut(ctx context.Context, group *errgroup.Group, localName string) {
+	// Return early if the context has already been cancelled.
+	select {
+	case <-ctx.Done():
+		return
+	default:
+		// Proceed.
+	}
+
+	group.Go(func() error {
+		s.notifyProgress(ctx, EventActionPut, localName, 0.0)
+		err := s.repoFiles.PutFile(ctx, localName)
+		if err != nil {
+			return err
+		}
+		s.notifyProgress(ctx, EventActionPut, localName, 1.0)
+		return nil
+	})
+}
+
+func (s *Sync) applyDiff(ctx context.Context, d diff) error {
+	group, ctx := errgroup.WithContext(ctx)
+	group.SetLimit(MaxRequestsInFlight)
+
+	for _, remoteName := range d.delete {
+		s.applyDelete(ctx, group, remoteName)
+	}
+
+	for _, localName := range d.put {
+		s.applyPut(ctx, group, localName)
+	}
+
+	// Wait for goroutines to finish and return first non-nil error return if any.
+	return group.Wait()
 }