Add obarray-clear and use it

* lisp/obarray.el (obarray-clear): New.
* lisp/abbrev.el (clear-abbrev-table):
* lisp/vc/vc.el (vc-clear-context): Use it instead of assuming the
obarray is a vector that can be 0-filled.
* test/lisp/obarray-tests.el (obarray-clear): New test.

lisp/abbrev.el

@@ -602,8 +602,7 @@ It is nil if the abbrev has already been unexpanded.")
   "Undefine all abbrevs in abbrev table TABLE, leaving TABLE empty."
   (setq abbrevs-changed t)
   (let* ((sym (obarray-get table "")))
-    (dotimes (i (length table))
+    (obarray-clear table)
-      (aset table i 0))
     ;; Preserve the table's properties.
     (cl-assert sym)
     (let ((newsym (obarray-put table "")))

lisp/obarray.el

@@ -66,5 +66,10 @@ Return t on success, nil otherwise."
   "Call function FN on every symbol in obarray OB and return nil."
   (mapatoms fn ob))
 
+(defun obarray-clear (ob)
+  "Remove all symbols from obarray OB."
+  ;; FIXME: This doesn't change the symbols to uninterned status.
+  (fillarray ob 0))
+
 (provide 'obarray)
 ;;; obarray.el ends here

lisp/vc/vc.el

@@ -935,7 +935,7 @@ is sensitive to blank lines."
 (defun vc-clear-context ()
   "Clear all cached file properties."
   (interactive)
-  (fillarray vc-file-prop-obarray 0))
+  (obarray-clear vc-file-prop-obarray))
 
 (defmacro with-vc-properties (files form settings)
   "Execute FORM, then maybe set per-file properties for FILES.

test/lisp/obarray-tests.el

@@ -89,5 +89,15 @@
     (obarray-map collect-names table)
     (should (equal (sort syms #'string<) '("a" "b" "c")))))
 
+(ert-deftest obarray-clear ()
+  (let ((o (obarray-make)))
+    (intern "a" o)
+    (intern "b" o)
+    (intern "c" o)
+    (obarray-clear o)
+    (let ((n 0))
+      (mapatoms (lambda (_) (setq n (1+ n))) o)
+      (should (equal n 0)))))
+
 (provide 'obarray-tests)
 ;;; obarray-tests.el ends here