bug#33959: 26.1.90; python.el font-lock buffer wreaks havoc when eldoc is enabled

[Carlos Pita]

Simply open the font lock buffer *Python-font-lock* while keeping an
inferior mode python buffer at its side. Now start typing, wait a couple
of seconds and type another character, play like this for a while and
you will find out that many new lines are added to the buffer and that
the current line is replicated between those empty lines. This is bad by
itself, but the symptom that made me realize of the problem is that
unclosed strings pair with their replicas in other lines and hence get
quite a random fontification.

Easier: enter the font lock buffer and scroll to the top. Maybe write
something. Sooner than later the buffer will scroll down by
itself. Delete everything, add a character, wait a moment, newlines
appear below the character.

I started disabling minor modes one by one until I found out that the
culprit was eldoc. I haven't investigated further but I believe that
just disabling eldoc there would be fine, don't you?

I will submit a patch in a moment.

Btw, I've submitted a patch for another font lock buffer related bug [1]
a time ago and I might be wrong but I got the impression that python.el
is rather forgotten these days. I wouldn't mind becoming a maintainer at
all, or helping with maintenance, if necessary.

[1] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=32390

Best regards
--
Carlos

---------------------------------------

In GNU Emacs 26.1.90 (build 4, x86_64-pc-linux-gnu, GTK+ Version 3.24.1)
 of 2018-12-05 built on archlinux
Repository revision: c7897c27861fd8b2690f40e77402edced6aa0ceb
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
Recent messages:
delete-backward-char: Beginning of buffer
Winner mode disabled
delete-backward-char: Beginning of buffer
Xclip mode disabled
Invalid byte code in 
/home/carlos/local/stow/emacs-26/share/emacs/26.1.90/lisp/emacs-lisp/cl-extra.elc
Show-Paren mode disabled
Ido-Ubiquitous mode disabled
Invalid byte code in 
/home/carlos/local/stow/emacs-26/share/emacs/26.1.90/lisp/emacs-lisp/cl-extra.elc
Eldoc mode disabled in current buffer
delete-backward-char: Text is read-only

Configured using:
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Major mode: Help

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