fix(sftp): return scanner from (*sftpReader).initScanner

This prevents a race condition between two calls to ReadBatch clobbering
each other.

internal/impl/sftp/input.go

@@ -209,18 +209,19 @@ func (s *sftpReader) ReadBatch(ctx context.Context) (service.MessageBatch, servi
 }
 
 func (s *sftpReader) tryReadBatch(ctx context.Context) (service.MessageBatch, service.AckFunc, error) {
-	if err := s.initScanner(ctx); err != nil {
+	scanner, err := s.initScanner(ctx)
+	if err != nil {
 		return nil, nil, err
 	}
 
-	s.stateLock.Lock()
-	defer s.stateLock.Unlock()
-
-	parts, codecAckFn, err := s.scanner.NextBatch(ctx)
+	parts, codecAckFn, err := scanner.NextBatch(ctx)
 	if err != nil {
 		if ctx.Err() != nil {
 			return nil, nil, ctx.Err()
 		}
+		s.stateLock.Lock()
+		defer s.stateLock.Unlock()
+
 		_ = s.scanner.Close(ctx)
 		s.scanner = nil
 		s.currentPath = ""
@@ -237,12 +238,12 @@ func (s *sftpReader) tryReadBatch(ctx context.Context) (service.MessageBatch, se
 	return parts, codecAckFn, nil
 }
 
-func (s *sftpReader) initScanner(ctx context.Context) error {
+func (s *sftpReader) initScanner(ctx context.Context) (codec.DeprecatedFallbackStream, error) {
 	s.stateLock.Lock()
 	scanner := s.scanner
 	s.stateLock.Unlock()
 	if scanner != nil {
-		return nil
+		return scanner, nil
 	}
 
 	var file *sftp.File
@@ -252,10 +253,10 @@ func (s *sftpReader) initScanner(ctx context.Context) error {
 		var err error
 		path, ok, err = s.pathProvider.Next(ctx)
 		if err != nil {
-			return fmt.Errorf("finding next file path: %w", err)
+			return nil, fmt.Errorf("finding next file path: %w", err)
 		}
 		if !ok {
-			return service.ErrEndOfInput
+			return nil, service.ErrEndOfInput
 		}
 
 		file, err = s.client.Open(path)
@@ -278,14 +279,14 @@ func (s *sftpReader) initScanner(ctx context.Context) error {
 		scanner, err := s.scannerCtor.Create(file, s.newCodecAckFn(path), details)
 		if err != nil {
 			_ = file.Close()
-			return fmt.Errorf("creating scanner: %w", err)
+			return nil, fmt.Errorf("creating scanner: %w", err)
 		}
 
 		s.stateLock.Lock()
 		s.scanner = scanner
 		s.currentPath = path
 		s.stateLock.Unlock()
-		return nil
+		return scanner, nil
 	}
 }
 
@@ -307,7 +308,6 @@ func (s *sftpReader) newCodecAckFn(path string) service.AckFunc {
 			}
 		}
 
-		time.Sleep(time.Millisecond * 100)
 		return nil
 	}
 }