Fix python-shell font-lock cleanup for unclosed quotes (Bug#32390)

The problem originating this report was:

-------------
In [15]: "
  File "<ipython-input-15-3b7a06bb1102>", line 1
    "
     ^
SyntaxError: EOL while scanning string literal

In [16]:   string face still here"
-------------

This happens because
python-shell-font-lock-comint-output-filter-function is called twice,
first for the error output and then for the "In [16]: " part.  The
first time python-shell-comint-end-of-output-p returns nil since
we're *not* at the end of an input prompt.  The second time it returns
0 since we're at the end of *just* an input prompt.  So we don't call
python-shell-font-lock-cleanup-buffer either time.

The current code is relying in a very weak rule: it considers "just an
input prompt" to be a continuation prompt.  Another unreliable aspect
of the current rule is that sometimes

    (python-shell-comint-end-of-output-p (ansi-color-filter-apply output))

returns 1 and not 0 for continuation prompts.  In short, the rule does
a very poor job identifying continuations.

* lisp/progmodes/python.el (python-shell-font-lock-cleanup-buffer):
Don't check for empty OUTPUT since python-shell-comint-end-of-output-p
returns nil for that anyway.  Don't check for
python-shell-comint-end-of-output-p returning a specific number
because it's unreliable, just check for any non-nil.  Identify
continuation prompts by looking for "...".

1 files changed, 6 insertions, 8 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index ae5aff351c0..b168b62c291 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -2600,14 +2600,12 @@ goes wrong and syntax highlighting in the shell gets messed up."
 
 (defun python-shell-font-lock-comint-output-filter-function (output)
   "Clean up the font-lock buffer after any OUTPUT."
-  (if (and (not (string= "" output))
-           ;; Is end of output and is not just a prompt.
-           (not (member
-                 (python-shell-comint-end-of-output-p
-                  (ansi-color-filter-apply output))
-                 '(nil 0))))
-      ;; If output is other than an input prompt then "real" output has
-      ;; been received and the font-lock buffer must be cleaned up.
+  (if (let ((output (ansi-color-filter-apply output)))
+        (and (python-shell-comint-end-of-output-p output)
+             ;; Assume "..." represents a continuation prompt.
+             (not (string-match "\\.\\.\\." output))))
+      ;; If output ends with an initial (not continuation) input prompt
+      ;; then the font-lock buffer must be cleaned up.
       (python-shell-font-lock-cleanup-buffer)
     ;; Otherwise just add a newline.
     (python-shell-font-lock-with-font-lock-buffer