Avoid infloop in python

Fix bug#24905

* lisp/progmodes/python.el (python-info-docstring-p): Improve
infloop avoidance: replace (bobp) with generic test for
forward progress.
* test/lisp/progmodes/python-tests.el (python-bob-infloop-avoid): Add
test for bug#24905

1 files changed, 9 insertions, 2 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 9091289b62e..d0d4a7f766e 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -4865,12 +4865,19 @@ point's current `syntax-ppss'."
              ;; Allow up to two consecutive docstrings only.
              (>=
               2
-              (progn
+              (let (last-backward-sexp-point)
                 (while (save-excursion
                          (python-nav-backward-sexp)
                          (setq backward-sexp-point (point))
                          (and (= indentation (current-indentation))
-                              (not (bobp)) ; Prevent infloop.
+                              ;; Make sure we're always moving point.
+                              ;; If we get stuck in the same position
+                              ;; on consecutive loop iterations,
+                              ;; bail out.
+                              (prog1 (not (eql last-backward-sexp-point
+                                               backward-sexp-point))
+                                (setq last-backward-sexp-point
+                                      backward-sexp-point))
                               (looking-at-p
                                (concat "[uU]?[rR]?"
                                        (python-rx string-delimiter)))))