reverseproxy: Add mention of which half a copyBuffer err comes from (#5472)

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/modules/caddyhttp/reverseproxy/streaming.go b/modules/caddyhttp/reverseproxy/streaming.go
index 1db107a..1f5387e 100644
--- a/modules/caddyhttp/reverseproxy/streaming.go
+++ b/modules/caddyhttp/reverseproxy/streaming.go
@@ -20,6 +20,7 @@ package reverseproxy
 
 import (
 	"context"
+	"fmt"
 	"io"
 	weakrand "math/rand"
 	"mime"
@@ -215,7 +216,7 @@ func (h Handler) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, er
 				written += int64(nw)
 			}
 			if werr != nil {
-				return written, werr
+				return written, fmt.Errorf("writing: %w", werr)
 			}
 			if nr != nw {
 				return written, io.ErrShortWrite
@@ -223,9 +224,9 @@ func (h Handler) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, er
 		}
 		if rerr != nil {
 			if rerr == io.EOF {
-				rerr = nil
+				return written, nil
 			}
-			return written, rerr
+			return written, fmt.Errorf("reading: %w", rerr)
 		}
 	}
 }