Moved the hack for triggering sendfile in bufio.ReadFrom from FSHandler to Response.Write, so all response body streams may benefit from sendfile optimization

fshandler.go

@@ -1,7 +1,6 @@
 package fasthttp
 
 import (
-	"bufio"
 	"bytes"
 	"errors"
 	"fmt"
@@ -124,6 +123,7 @@ func (ff *fsFile) smallFileReader() io.Reader {
 	return r
 }
 
+// files bigger than this size are sent with sendfile
 const maxSmallFileSize = 2 * 4096
 
 func (ff *fsFile) isBig() bool {
@@ -194,15 +194,8 @@ func (r *bigFileReader) Read(p []byte) (int, error) {
 }
 
 func (r *bigFileReader) WriteTo(w io.Writer) (int64, error) {
-	// fast path
 	if rf, ok := w.(io.ReaderFrom); ok {
-		// This is a hack for triggering sendfile path in bufio.Writer:
-		// the buffer must be empty before calling ReadFrom.
-		if bw, ok := w.(*bufio.Writer); ok && bw.Buffered() > 0 {
-			if err := bw.Flush(); err != nil {
-				return 0, err
-			}
-		}
+		// fast path. Senfile must be triggered
 		return rf.ReadFrom(r.f)
 	}
 

http.go

@@ -452,6 +452,13 @@ func (resp *Response) Write(w *bufio.Writer) error {
 			if err = resp.Header.Write(w); err != nil {
 				return err
 			}
+			if contentLength > maxSmallFileSize {
+				// w buffer must be empty for triggering
+				// sendfile path in bufio.Writer.
+				if err = w.Flush(); err != nil {
+					return err
+				}
+			}
 			if err = writeBodyFixedSize(w, resp.bodyStream, contentLength); err != nil {
 				return err
 			}