Make ibuffer filters idempotent

* lisp/ibuf-ext.el (ibuffer-push-filter): Check if the
filter is already in effect (Bug#28826).
* lisp/ibuf-macs.el (define-ibuffer-filter):
Report to user if the filter was already enabled.

2 files changed, 15 insertions, 8 deletions

diff --git a/lisp/ibuf-ext.el b/lisp/ibuf-ext.el
index 686bc392b60..2c841fa7e0a 100644
--- a/lisp/ibuf-ext.el
+++ b/lisp/ibuf-ext.el
@@ -1033,8 +1033,11 @@ group definitions by setting `ibuffer-filter-groups' to nil."
       (ibuffer-jump-to-buffer (buffer-name buf)))))
 
 (defun ibuffer-push-filter (filter-specification)
-  "Add FILTER-SPECIFICATION to `ibuffer-filtering-qualifiers'."
-  (push filter-specification ibuffer-filtering-qualifiers))
+  "Add FILTER-SPECIFICATION to `ibuffer-filtering-qualifiers'.
+If FILTER-SPECIFICATION is already in the list then return nil.  Otherwise,
+return the updated list."
+  (unless (member filter-specification ibuffer-filtering-qualifiers)
+    (push filter-specification ibuffer-filtering-qualifiers)))
 
 ;;;###autoload
 (defun ibuffer-decompose-filter ()
diff --git a/lisp/ibuf-macs.el b/lisp/ibuf-macs.el
index e0c91e20edd..c30067f2f58 100644
--- a/lisp/ibuf-macs.el
+++ b/lisp/ibuf-macs.el
@@ -301,12 +301,16 @@ bound to the current value of the filter.
        (defun ,fn-name (qualifier)
          ,(or documentation "This filter is not documented.")
          (interactive (list ,reader))
-         (ibuffer-push-filter (cons ',name qualifier))
-         (message "%s"
-                  (format ,(concat (format "Filter by %s added: " description)
-                                   " %s")
-                          qualifier))
-         (ibuffer-update nil t))
+         (if (null (ibuffer-push-filter (cons ',name qualifier)))
+             (message "%s"
+                      (format ,(concat (format "Filter by %s already applied: " description)
+                                       " %s")
+                              qualifier))
+           (message "%s"
+                    (format ,(concat (format "Filter by %s added: " description)
+                                     " %s")
+                            qualifier))
+           (ibuffer-update nil t)))
        (push (list ',name ,description
                    (lambda (buf qualifier)
                      (condition-case nil