Fix error when exporting body only of an empty buffer

* lisp/ox.el (org-export-filter-apply-functions): Return empty string
  instead of nil when a filter returns the empty string.

* testing/lisp/test-ox.el (test-org-export/export-scope): Add test.
(test-org-export/filter-apply-functions): New test.

Reported-by: "Stefan-W. Hahn" <stefan.hahn@s-hahn.de>
<http://permalink.gmane.org/gmane.emacs.orgmode/110371>

lisp/ox.el

@@ -2472,20 +2472,24 @@ channel, as a plist.  It must return a string or nil.")
 (defun org-export-filter-apply-functions (filters value info)
   "Call every function in FILTERS.
 
-Functions are called with arguments VALUE, current export
-back-end's name and INFO.  A function returning a nil value will
-be skipped.  If it returns the empty string, the process ends and
-VALUE is ignored.
+Functions are called with three arguments: a value, the export
+back-end name and the communication channel.  First function in
+FILTERS is called with VALUE as its first argument.  Second
+function in FILTERS is called with the previous result as its
+value, etc.
+
+Functions returning nil are skipped.  Any function returning the
+empty string ends the process, which returns the empty string.
 
 Call is done in a LIFO fashion, to be sure that developer
 specified filters, if any, are called first."
-  (catch 'exit
+  (catch :exit
     (let* ((backend (plist-get info :back-end))
 	   (backend-name (and backend (org-export-backend-name backend))))
       (dolist (filter filters value)
 	(let ((result (funcall filter value backend-name info)))
-	  (cond ((not result) value)
-		((equal value "") (throw 'exit nil))
+	  (cond ((not result))
+		((equal result "") (throw :exit ""))
 		(t (setq value result))))))))
 
 (defun org-export-install-filters (info)

testing/lisp/test-ox.el

@@ -931,15 +931,30 @@ Text"
 	    (end-of-line)
 	    (org-export-as (org-test-default-backend)))))
   ;; Body only.
-  (let ((backend (org-test-default-backend)))
-    (setf (org-export-backend-transcoders backend)
-	  (cons '(template . (lambda (body i)
-			       (format "BEGIN\n%sEND" body)))
-		(org-export-backend-transcoders backend)))
-    (org-test-with-temp-text "Text"
-      (should (equal (org-export-as backend nil nil 'body-only)
-		     "Text\n"))
-      (should (equal (org-export-as backend) "BEGIN\nText\nEND")))))
+  (should
+   (equal "Text\n"
+	  (org-test-with-temp-text "Text"
+	    (org-export-as
+	     (org-export-create-backend
+	      :transcoders
+	      '((template . (lambda (b _i) (format "BEGIN\n%sEND" b)))
+		(section . (lambda (_s c _i) c))
+		(paragraph . (lambda (_p c _i) c))))
+	     nil nil 'body-only))))
+  (should
+   (equal "BEGIN\nText\nEND"
+	  (org-test-with-temp-text "Text"
+	    (org-export-as
+	     (org-export-create-backend
+	      :transcoders
+	      '((template . (lambda (b _i) (format "BEGIN\n%sEND" b)))
+		(section . (lambda (_s c _i) c))
+		(paragraph . (lambda (_p c _i) c))))))))
+  ;; Pathological case: Body only on an empty buffer is expected to
+  ;; return an empty string, not nil.
+  (should
+   (org-test-with-temp-text ""
+     (org-export-as (org-test-default-backend) nil nil t))))
 
 (ert-deftest test-org-export/output-file-name ()
   "Test `org-export-output-file-name' specifications."
@@ -1815,6 +1830,43 @@ Para2"
 		(section . (lambda (section contents info) contents))))
 	     info)))))
 
+
+;;; Filters
+
+(ert-deftest test-org-export/filter-apply-functions ()
+  "Test `org-export-filter-apply-functions' specifications."
+  ;; Functions are applied in order and return values are reduced.
+  (should
+   (equal "210"
+	  (org-export-filter-apply-functions
+	   (list (lambda (value &rest _) (concat "1" value))
+		 (lambda (value &rest _) (concat "2" value)))
+	   "0" nil)))
+  ;; Functions returning nil are skipped.
+  (should
+   (equal "20"
+	  (org-export-filter-apply-functions
+	   (list #'ignore (lambda (value &rest _) (concat "2" value)))
+	   "0" nil)))
+  ;; If all functions are skipped, return the initial value.
+  (should
+   (equal "0"
+	  (org-export-filter-apply-functions (list #'ignore) "0" nil)))
+  ;; If any function returns the empty string, final value is the
+  ;; empty string.
+  (should
+   (equal ""
+	  (org-export-filter-apply-functions
+	   (list (lambda (value &rest _) "")
+		 (lambda (value &rest _) (concat "2" value)))
+	   "0" nil)))
+  ;; Any function returning the empty string short-circuits the
+  ;; process.
+  (should
+   (org-export-filter-apply-functions
+    (list (lambda (value &rest _) "")
+	  (lambda (value &rest _) (error "This shouldn't happen")))
+    "0" nil)))
 
 
 ;;; Footnotes