Open parts files one at a time

When completing a file-based upload, open the parts files one at a time
and write them to the upload, closing each file before opening the next
one.

This is preferrable to opening them all at once and closing all files at
the end, because it consumes less file descriptors.

Updates #10660

lib/events/filesessions/filestream.go

@@ -124,30 +124,26 @@ func (h *Handler) CompleteUpload(ctx context.Context, upload events.StreamUpload
 		}
 	}()
 
-	files := make([]*os.File, 0, len(parts))
-	readers := make([]io.Reader, 0, len(parts))
-
-	defer func() {
-		for i := 0; i < len(files); i++ {
-			if err := files[i].Close(); err != nil {
-				h.WithError(err).Errorf("Failed to close file %q.", files[i].Name())
-			}
+	writePartToFile := func(path string) error {
+		file, err := os.Open(path)
+		if err != nil {
+			return err
 		}
-	}()
+		defer func() {
+			if err := file.Close(); err != nil {
+				h.WithError(err).Errorf("failed to close file %q", path)
+			}
+		}()
+
+		_, err = io.Copy(f, file)
+		return err
+	}
 
 	for _, part := range parts {
 		partPath := h.partPath(upload, part.Number)
-		file, err := os.Open(partPath)
-		if err != nil {
-			return trace.ConvertSystemError(err)
+		if err := writePartToFile(partPath); err != nil {
+			return trace.Wrap(err)
 		}
-		files = append(files, file)
-		readers = append(readers, file)
-	}
-
-	_, err = io.Copy(f, io.MultiReader(readers...))
-	if err != nil {
-		return trace.Wrap(err)
 	}
 
 	err = h.Config.OnBeforeComplete(ctx, upload)