From ce5e6383244242adef27e98e9e87d1bcf03d6864 Mon Sep 17 00:00:00 2001
From: Zac Bergquist <zac.bergquist@goteleport.com>
Date: Wed, 2 Mar 2022 09:43:54 -0700
Subject: [PATCH] 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 | 34 ++++++++++++---------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/lib/events/filesessions/filestream.go b/lib/events/filesessions/filestream.go
index bbfb3d746c82a..4a5baf0ad4b12 100644
--- a/lib/events/filesessions/filestream.go
+++ b/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)