fix memory leak in gif thumbnail encoding

When using an io.Pipe, it must be ensured that the pipe is drained. If
it is not, all references required for the goroutine will not be freed
by the garbage collector, as an active reference still exists (the
abandoned goroutine the pipe is written to.

To fix this, write to a non-blocking buffer. This may increase the RAM
usage in the short run, but can be fully garbage collected, even if not
read from. Since the size of the buffer is at most the size of the
thumbnail and it being freed quickly, this should post a significantly
smaller burden on the server.

Signed-off-by: Moritz Poldrack <git@moritz.sh>
Fixes: 4378a4e ("Avoid use of buffers throughout thumbnailing")
Fixes: t2bot#561

thumbnailing/i/gif.go

@@ -1,7 +1,9 @@
 package i
 
 import (
+	"bytes"
 	"errors"
+	"fmt"
 	"image"
 	"image/draw"
 	"image/gif"
@@ -76,19 +78,15 @@ func (d gifGenerator) GenerateThumbnail(b io.Reader, contentType string, width i
 			}
 
 			// The thumbnailer decided that it shouldn't thumbnail, so encode it ourselves
-			pr, pw := io.Pipe()
-			go func(pw *io.PipeWriter, p *image.Paletted) {
-				err = u.Encode(ctx, pw, p)
-				if err != nil {
-					_ = pw.CloseWithError(errors.New("gif: error encoding still frame thumbnail: " + err.Error()))
-				} else {
-					_ = pw.Close()
-				}
-			}(pw, targetImg)
+			buf := bytes.NewBuffer(make([]byte, 0, 128*1024))
+			err = u.Encode(ctx, buf, targetImg)
+			if err != nil {
+				return nil, fmt.Errorf("gif: error encoding static thumbnail: %w", err)
+			}
 			return &m.Thumbnail{
 				Animated:    false,
 				ContentType: "image/png",
-				Reader:      pr,
+				Reader:      io.NopCloser(buf),
 			}, nil
 		}
 
@@ -110,20 +108,16 @@ func (d gifGenerator) GenerateThumbnail(b io.Reader, contentType string, width i
 	g.Config.Width = g.Image[0].Bounds().Max.X
 	g.Config.Height = g.Image[0].Bounds().Max.Y
 
-	pr, pw := io.Pipe()
-	go func(pw *io.PipeWriter, g *gif.GIF) {
-		err = gif.EncodeAll(pw, g)
-		if err != nil {
-			_ = pw.CloseWithError(errors.New("gif: error encoding final thumbnail: " + err.Error()))
-		} else {
-			_ = pw.Close()
-		}
-	}(pw, g)
+	buf := bytes.NewBuffer(make([]byte, 0, 512*1024))
+	err = gif.EncodeAll(buf, g)
+	if err != nil {
+		return nil, fmt.Errorf("gif: error encoding animated thumbnail: %w", err)
+	}
 
 	return &m.Thumbnail{
 		ContentType: "image/gif",
 		Animated:    true,
-		Reader:      pr,
+		Reader:      io.NopCloser(buf),
 	}, nil
 }