[Bug-readline] Readline echoing on non-terminal input

[Ken Hornstein]

I've recently added readline to one of the applications that I work
on.  This went fine.  But I started getting reports from other
developers that the application was failing the test suite, but it
didn't do that for me.

What we eventually figure out was this:

- The test suite failure was because our test suite did this:

  echo "some input" | application

  and now as part of the output we would get back "some input" in the
  output.  This wasn't expected, so the test suite checks failed.

- It worked for me because I was using readline 5, but the other developers
  were using readline 6.

- What I eventually tracked it down to was the code in rl_prep_terminal
  in rltty.c.  The code tries to check the settings of the input
  terminal but if it fails with certain errors then it always turns
  on echoing.  The key difference is that the call to check the
  terminal settings fails with EINVAL, and that wasn't checked for
  in readline 5 but it is checked for in readline 6.

So I guess my questions are:

- Is echoing supposed to be turned on when the input is not a tty?
  I would have thought "no", but it seems like a concious decision
  was made here to turn it on.

- Is there a way to get consistent echoing behavior when using readline 5
  and readline 6?  I can live with one behavior (although I'd prefer
  to have it turned off to match the non-readline test case) as
  long as it's the same between readline versions.  It seems like
  the right APIs aren't exposed to make it work.

--Ken