Protect against an infloop in python-mode

There appears to be an edge case caused by using `syntax-ppss' in a
narrowed buffer during JIT lock inside of Python triple-quote strings.
Unfortunately it is impossible to reproduce without manually
destroying the syntactic information in the Python buffer, but it has
been observed in practice.  In that case it can happen that the syntax
caches get sufficiently out of whack so that there appear to be
overlapping strings in the buffer.  As Python has no nested strings,
this situation is impossible and leads to an infloop in
`python-nav-end-of-statement'.  Protect against this by checking
whether the search for the end of the current string makes progress.

* python.el (python-nav-end-of-statement): Protect against infloop.
* progmodes/python-tests.el
(python-tests--python-nav-end-of-statement--infloop): Add unit test.

1 files changed, 13 insertions, 3 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 228a4484616..2697f1a3107 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -1491,10 +1491,18 @@ Optional argument NOEND is internal and makes the logic to not
 jump to the end of line when moving forward searching for the end
 of the statement."
   (interactive "^")
-  (let (string-start bs-pos)
+  (let (string-start bs-pos (last-string-end 0))
     (while (and (or noend (goto-char (line-end-position)))
                 (not (eobp))
                 (cond ((setq string-start (python-syntax-context 'string))
+                       ;; The assertion can only fail if syntax table
+                       ;; text properties and the `syntax-ppss' cache
+                       ;; are somehow out of whack.  This has been
+                       ;; observed when using `syntax-ppss' during
+                       ;; narrowing.
+                       (cl-assert (> string-start last-string-end)
+                                  :show-args
+                                  "Overlapping strings detected")
                        (goto-char string-start)
                        (if (python-syntax-context 'paren)
                            ;; Ended up inside a paren, roll again.
@@ -1504,8 +1512,10 @@ of the statement."
                          (goto-char (+ (point)
                                        (python-syntax-count-quotes
                                         (char-after (point)) (point))))
-                         (or (re-search-forward (rx (syntax string-delimiter)) nil t)
-                             (goto-char (point-max)))))
+                         (setq last-string-end
+                               (or (re-search-forward
+                                    (rx (syntax string-delimiter)) nil t)
+                                   (goto-char (point-max))))))
                       ((python-syntax-context 'paren)
                        ;; The statement won't end before we've escaped
                        ;; at least one level of parenthesis.