1 files changed, 10 insertions, 30 deletions

diff --git a/src/net/http/server.go b/src/net/http/server.go
index ad99741177d..198102ffd21 100644
--- a/src/net/http/server.go
+++ b/src/net/http/server.go
@@ -577,37 +577,17 @@ func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
 		return io.CopyBuffer(writerOnly{w}, src, buf)
 	}
 
-	// sendfile path:
-
-	// Do not start actually writing response until src is readable.
-	// If body length is <= sniffLen, sendfile/splice path will do
-	// little anyway. This small read also satisfies sniffing the
-	// body in case Content-Type is missing.
-	nr, er := src.Read(buf[:sniffLen])
-	atEOF := errors.Is(er, io.EOF)
-	n += int64(nr)
-
-	if nr > 0 {
-		// Write the small amount read normally.
-		nw, ew := w.Write(buf[:nr])
-		if ew != nil {
-			err = ew
-		} else if nr != nw {
-			err = io.ErrShortWrite
+	// Copy the first sniffLen bytes before switching to ReadFrom.
+	// This ensures we don't start writing the response before the
+	// source is available (see golang.org/issue/5660) and provides
+	// enough bytes to perform Content-Type sniffing when required.
+	if !w.cw.wroteHeader {
+		n0, err := io.CopyBuffer(writerOnly{w}, io.LimitReader(src, sniffLen), buf)
+		n += n0
+		if err != nil || n0 < sniffLen {
+			return n, err
 		}
 	}
-	if err == nil && er != nil && !atEOF {
-		err = er
-	}
-
-	// Do not send StatusOK in the error case where nothing has been written.
-	if err == nil && !w.wroteHeader {
-		w.WriteHeader(StatusOK) // nr == 0, no error (or EOF)
-	}
-
-	if err != nil || atEOF {
-		return n, err
-	}
 
 	w.w.Flush()  // get rid of any previous writes
 	w.cw.flush() // make sure Header is written; flush data to rwc
@@ -620,7 +600,7 @@ func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
 		return n, err
 	}
 
-	n0, err := io.Copy(writerOnly{w}, src)
+	n0, err := io.CopyBuffer(writerOnly{w}, src, buf)
 	n += n0
 	return n, err
 }