ert-with-function-mocked: New macro

* lisp/emacs-lisp/ert-x.el (ert-with-function-mocked): New macro which
allows evaluating code while particular function is replaced with
a mock.  The original definition of said function is restored once the
macro finishes.

1 files changed, 40 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/ert-x.el b/lisp/emacs-lisp/ert-x.el
index 2a2418fa7d2..eb10c845d3f 100644
--- a/lisp/emacs-lisp/ert-x.el
+++ b/lisp/emacs-lisp/ert-x.el
@@ -285,6 +285,46 @@ BUFFER defaults to current buffer.  Does not modify BUFFER."
             (kill-buffer clone)))))))
 
 
+(defmacro ert-with-function-mocked (name mock &rest body)
+  "Mocks function NAME with MOCK and run BODY.
+
+Once BODY finishes (be it normally by returning a value or
+abnormally by throwing or signalling), the old definition of
+function NAME is restored.
+
+BODY may further change the mock with `fset'.
+
+If MOCK is nil, the function NAME is mocked with a function
+`ert-fail'ing when called.
+
+For example:
+
+    ;; Regular use, function is mocked inside the BODY:
+    (should (eq 2 (+ 1 1)))
+    (ert-with-function-mocked ((+ (lambda (a b) (- a b))))
+      (should (eq 0 (+ 1 1))))
+    (should (eq 2 (+ 1 1)))
+
+    ;; Macro correctly recovers from a throw or signal:
+    (should
+      (catch 'done
+        (ert-with-function-mocked ((+ (lambda (a b) (- a b))))
+          (should (eq 0 (+ 1 1))))
+          (throw 'done t)))
+    (should (eq 2 (+ 1 1)))
+"
+  (declare (indent 2))
+  (let ((old-var (make-symbol "old-var"))
+        (mock-var (make-symbol "mock-var")))
+    `(let ((,old-var (symbol-function (quote ,name))) (,mock-var ,mock))
+       (fset (quote ,name)
+             (or ,mock-var (lambda (&rest _)
+                             (ert-fail (concat "`" ,(symbol-name name)
+                                               "' unexpectedly called.")))))
+       (unwind-protect
+           (progn ,@body)
+         (fset (quote ,name) ,old-var)))))
+
 (provide 'ert-x)
 
 ;;; ert-x.el ends here