Why would this not constitute a bug in readline?  Readline isn't going to be active all of the time in most applications, so why shouldn't it be readline's responsibility to install the signal handler at initialization and handle it?  If the application wants control of that signal, it can then install its own handler after readline initialization completes.  Clearly it used to work that way.  Is there some documentation from readline you can point us to that explains the reasoning behind this change in behavior?