use SO_RCVBUFFORCE to force receive buffer increase on Linux (#3804)

* Add ability to force change the receive buffer size using SO_RCVBUFFORCE in Linux

* Fix imports

* Update test

* Add sys_conn_helper_not_linux

* Rename file

* ignore the error on SetReadBuffer

* also run unit tests as root

---------

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

sys_conn_helper_linux.go

@@ -2,7 +2,13 @@
 
 package quic
 
-import "golang.org/x/sys/unix"
+import (
+	"errors"
+	"fmt"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
 
 const msgTypeIPTOS = unix.IP_TOS
 
@@ -17,3 +23,23 @@ const (
 )
 
 const batchSize = 8 // needs to smaller than MaxUint8 (otherwise the type of oobConn.readPos has to be changed)
+
+func forceSetReceiveBuffer(c interface{}, bytes int) error {
+	conn, ok := c.(interface {
+		SyscallConn() (syscall.RawConn, error)
+	})
+	if !ok {
+		return errors.New("doesn't have a SyscallConn")
+	}
+	rawConn, err := conn.SyscallConn()
+	if err != nil {
+		return fmt.Errorf("couldn't get syscall.RawConn: %w", err)
+	}
+	var serr error
+	if err := rawConn.Control(func(fd uintptr) {
+		serr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_RCVBUFFORCE, bytes)
+	}); err != nil {
+		return err
+	}
+	return serr
+}