Function begin_lock 

pub(crate) fn begin_lock(
    conn: &mut Smb2Conn,
    server: &Arc<ServerShared>,
    hdr: &Header2,
    buf: &[u8],
) -> AsyncStart

Begin a LOCK ([MS-SMB2] §3.3.5.14): apply unlocks, try acquisitions, and either answer immediately or (for a blocking lock) spawn a waiter and defer.