bug#17007: 24.3.50; describe-key/function evaluates documentation function in the wrong buffer

[martin rudalics <rudalics@gmx.at>]

 > It's simpler if I give you an emacs -Q recipe, right? In your *scratch*
 > buffer:
 >
 >     (defun foo ())
 >     (defun foo-doc ()
 >       (format "Foo does nothing, and by the way your tab does `%s'"
 >             	     (key-binding "\t")))
 >     (put 'foo 'function-documentation '(foo-doc))
 >     (describe-function 'foo)
 >
 > This fails on the latest Emacs, i.e. the last line returned by the last
 > form is
 >
 >     "Foo does nothing, and by the way your tab does `forward-button'"
 >
 > Whereas it should report, as in Emacs 24.3
 >
 >     "Foo does nothing, and by the way your tab does `indent-for-tab-command'"

Confirmed.

 > The trace of functions is the following:
 >
 > 1 -> (describe-function foo)
 > | 2 -> (describe-function-1 foo)
 > | | 3 -> (documentation foo t)
 > | | 3 <- documentation: "Foo does nothing, and by the way your tab does `forward-button'"
 > | 2 <- describe-function-1: nil
 > 1 <- describe-function: "foo is a Lisp function.
 >
 > But that doesn't show how the macro `with-temp-buffer-window', expanded
 > from `with-help-window', eventually wraps the latter's BODY in a
 > `with-current-buffer' call, making its forms be evaluated in the newly
 > created *Help* buffer, instead of *scratch*.

Indeed.

 > One of those forms is the
 > call to `describe-function-1', which eventually calls `foo-doc'.
 >
 > As I explained in my last email (which got sent from the wrong address
 > but is here
 > http://lists.gnu.org/archive/html/bug-gnu-emacs/2014-03/msg00411.html)
 > it was the reversion of two lines in `with-temp-buffer-window' that
 > broke it.

Not really.  The current version of `with-temp-buffer-window' and that
released with Emacs 24.3 are the same.  It changed only intermittently
and that's what you noticed.  Or can you spot a difference now?

 >  But maybe it was fixing something else that I don't understand. Can
 > you help me understand what you were achieving with the original fix
 > and the reversion?

I first reacted to a bug report by Nicolas Richard

http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16816

and tried to make it behave as in the doc-string.  Unfortunately, this
broke things as subsequently remarked by Juanma in the same thread and
also broke code Thierry wrote as reported in

http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16882

so I decided to restore the 24.3 behavior.  I'm still not fine with that
but it's now some time since 24.3 is out and I'm a bit reluctant to
change it back again, mostly so because I don't know how many bugs such
a change might produce and how soon they'd get detected.

And I'm still too silly to understand how this (apparently?) worked with
24.3 and why it's broken now.  Any ideas?

martin