Add assertions to catch circular lists during dynamic updates

Circular lists can cause infinite loops. Besides causing problems for
users, these make automated testing impossible. The assertions added
in this commit should properly throw errors on circular lists.
However, these assertions should be disabled in any release since they
require iterating through the entire list, and therefore have the
potential to noticeably slow down the package.

ido-completing-read+.el

@@ -879,12 +879,20 @@ result."
        (nreverse filtered-collection)
      filtered-collection)))
 
+(defun ido-cr+-cyclicp (x)
+  "Returns non-nill if X is a list containing a circular reference."
+  (cl-loop
+   for tortoise on x
+   for hare on (cdr x) by #'cddr
+   thereis (eq tortoise hare)))
+
 (defun ido-cr+-update-dynamic-collection ()
   "Update the set of completions for a dynamic collection.
 
 This has no effect unless `ido-cr+-dynamic-collection' is non-nil."
   (when (and (ido-cr+-active)
              ido-cr+-dynamic-collection)
+    (cl-assert (not (ido-cr+-cyclicp ido-cur-list)))
     (let ((orig-ido-cur-list ido-cur-list))
       (condition-case-unless-debug err
           (let* ((ido-text
@@ -915,6 +923,7 @@ This has no effect unless `ido-cr+-dynamic-collection' is non-nil."
                     string ido-cr+-dynamic-collection predicate)
                    into result
                    finally return result)))
+            (cl-assert (not (ido-cr+-cyclicp new-completions)))
             ;; Put the previous first match back at the front if possible
             (when (and new-completions
                        first-match
@@ -951,7 +960,9 @@ This has no effect unless `ido-cr+-dynamic-collection' is non-nil."
          ;; the failed update
          (setq ido-cur-list orig-ido-cur-list)
          ;; Prevent any further attempts at dynamic updating
-         (setq ido-cr+-dynamic-collection nil)))))
+         (setq ido-cr+-dynamic-collection nil)
+         (cl-assert (not (ido-cr+-cyclicp ido-cur-list)))
+         (cl-assert (null ido-cr+-dynamic-collection))))))
   ;; Always cancel an active timer when this function is called.
   (when ido-cr+-dynamic-update-timer
     (cancel-timer ido-cr+-dynamic-update-timer)
@@ -965,6 +976,7 @@ This has no effect unless `ido-cr+-dynamic-collection' is non-nil."
     (when ido-cr+-dynamic-update-timer
       (cancel-timer ido-cr+-dynamic-update-timer)
       (setq ido-cr+-dynamic-update-timer nil))
+    (cl-assert (not (ido-cr+-cyclicp ido-cur-list)))
     (if (<= (length ido-matches) 1)
         ;; If we've narrowed it down to zero or one matches, update
         ;; immediately.
@@ -1163,6 +1175,18 @@ blacklist was modified."
 
 (ido-cr+-maybe-update-blacklist)
 
+;; TODO: Remove this when it's no longer necessary for debugging
+(defun ido-chop@ido-cr+-prevent-infinite-loops (items elem)
+  "This advice checks for conditions that cause infinite loops in `ido-chop'.
+
+If any of these conditions is detected, it throws an error.
+Ideally this should not be necessary, but it helps with
+debugging."
+  (cl-assert (not (ido-cr+-cyclicp items)))
+  (cl-assert (member elem items)))
+(advice-add 'ido-chop :before
+            #'ido-chop@ido-cr+-prevent-infinite-loops)
+
 (provide 'ido-completing-read+)
 
 ;;; ido-completing-read+.el ends here