bug#47657: python-shell font-lock with multi-line input: runaway fontification buffer length

[JD Smith <jdtsmith@gmail.com>]

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> On Apr 13, 2021, at 4:05 AM, Lars Ingebrigtsen <larsi@gnus.org> wrote:
> 
> JD Smith <jdtsmith@gmail.com> writes:
> 
>> Do you have a step-by-step recipe, starting from "emacs -Q", to
>> reproduce this problem?
>> 
>> 1 emacs -Q
>> 2 M-x run-python
>> 3 C-x 5 b “ *Python-font-lock*”
>> 4 In inferior python shell: type any line.
>> 5 C-c SPC (`comint-accumulate’) to continue.
>> 6 Type another line.  Notice the first line is repeated. 
>> 7 Repeat steps 5 & 6 several times. 
> 
> You didn't say what Emacs version you're using -- I tested this in Emacs
> 28, and was unable to reproduce this.  The " *Python-font-lock*" buffer
> never contains more than a single non-blank line using this recipe.
> (The "any line" I used to test was "5”.)

Mac port, v27.2, python-mode v0.26.1.  I have a hard time understanding how this would not reproduce, as (buffer-substring-no-properties  prompt-end (point-max)) clearly takes all of the text (multi-lines included) from the prompt onward.  I took a look here <https://github.com/emacs-mirror/emacs/blob/b064ddd3f600ed28e62b09d556ecced5f80d9883/lisp/progmodes/python.el#L2710> (python-mode v0.27.1) and the input is gathered in the same manner in the post-command-hook.  The “repeated” lines in #6 are in the *Python-font-lock* buffer, btw. 

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