ffap-string-at-point: Limit max length of active region

Prevents that 'ffap-guesser' waste time checking large strings
which are likely not valid candidates (Bug#25243).
* lisp/ffap.el (ffap-max-region-length): New variable.
(ffap-string-at-point): Use it.
* test/lisp/ffap-tests.el: New test suite.
(ffap-tests-25243): Add test for this bug.

lisp/ffap.el

@@ -203,6 +203,11 @@ Sensible values are nil, \"news\", or \"mailto\"."
 		 )
   :group 'ffap)
 
+(defvar ffap-max-region-length 1024
+  "Maximum active region length.
+When the region is active and larger than this value,
+`ffap-string-at-point' returns an empty string.")
+
 
 ;;; Peanut Gallery (More User Variables):
 ;;
@@ -1101,8 +1106,10 @@ MODE (defaults to value of `major-mode') is a symbol used to look up
 string syntax parameters in `ffap-string-at-point-mode-alist'.
 If MODE is not found, we use `file' instead of MODE.
 If the region is active, return a string from the region.
-Sets the variable `ffap-string-at-point' and the variable
-`ffap-string-at-point-region'."
+Set the variable `ffap-string-at-point' and the variable
+`ffap-string-at-point-region'.
+When the region is active and larger than `ffap-max-region-length',
+return an empty string, and set `ffap-string-at-point-region' to '(1 1)."
   (let* ((args
 	  (cdr
 	   (or (assq (or mode major-mode) ffap-string-at-point-mode-alist)
@@ -1119,11 +1126,15 @@ Sets the variable `ffap-string-at-point' and the variable
 		(save-excursion
 		  (skip-chars-forward (car args))
 		  (skip-chars-backward (nth 2 args) pt)
-		  (point)))))
-    (setq ffap-string-at-point
-	  (buffer-substring-no-properties
-	   (setcar ffap-string-at-point-region beg)
-	   (setcar (cdr ffap-string-at-point-region) end)))))
+		  (point))))
+         (region-len (- (max beg end) (min beg end))))
+    (if (and (natnump ffap-max-region-length)
+             (< region-len ffap-max-region-length)) ; Bug#25243.
+        (setf ffap-string-at-point-region (list beg end)
+              ffap-string-at-point
+              (buffer-substring-no-properties beg end))
+      (setf ffap-string-at-point-region (list 1 1)
+            ffap-string-at-point ""))))
 
 (defun ffap-string-around ()
   ;; Sometimes useful to decide how to treat a string.

test/lisp/ffap-tests.el

@@ -0,0 +1,54 @@
+;;; ffap-tests.el --- Test suite for ffap.el -*- lexical-binding: t -*-
+
+;; Copyright (C) 2016 Free Software Foundation, Inc.
+
+;; Author: Tino Calancha <tino.calancha@gmail.com>
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert)
+(require 'ffap)
+
+(ert-deftest ffap-tests-25243 ()
+  "Test for http://debbugs.gnu.org/25243 ."
+  (let ((file (make-temp-file "test-Bug#25243")))
+    (unwind-protect
+        (with-temp-file file
+          (let ((str "diff --git b/lisp/ffap.el a/lisp/ffap.el
+index 3d7cebadcf..ad4b70d737 100644
+--- b/lisp/ffap.el
++++ a/lisp/ffap.el
+@@ -203,6 +203,9 @@ ffap-foo-at-bar-prefix
+"))
+            (transient-mark-mode 1)
+            (when (natnump ffap-max-region-length)
+              (insert
+               (concat
+                str
+                (make-string ffap-max-region-length #xa)
+                (format "%s ENDS HERE" file)))
+              (mark-whole-buffer)
+              (should (equal "" (ffap-string-at-point)))
+              (should (equal '(1 1) ffap-string-at-point-region)))))
+      (and (file-exists-p file) (delete-file file)))))
+
+(provide 'ffap-tests)
+
+;;; ffap-tests.el ends here