bug#25243: 26.0.50; ffap-guesser very slow w/ region active in large diff files

[Tino Calancha <tino.calancha@gmail.com>]

Drew Adams <drew.adams@oracle.com> writes:

> Amusing backstory:
Thank you very much for adding more info to the report!

> Wrt the proposed patch:
>
> 1. It is `ffap-string-at-point' that picks up the active
>    region text as a string and removes its properties.
>    Since that is what is slow, I think it is in that
>    function that preventing this from happening should
>    occur.  The patch tries to control this only in
>    `ffap-file-at-point', and it does so _after_ calling
>    `ffap-string-at-point', which is too late (AFAICS).
>
>    I think that `ffap-string-at-point' should control
>    this.  It should not try to pick up a file name from
>    a 14 KB diff buffer.
Agreed.  See updated patch below.

> 2. I'm not sure that a user option is really what's called
>    for here.  I'd suggest a defvar, but only because Emacs
>    Dev does not really like programs to bind option values
>    (I have little problem with that, myself), and that is
>    the main place where I expect the value to be changed:
>    programmatically.
Agreed.  It's good to follow an uniform style throughout core packages.
I've updated the patch to use a defvar instead.

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From 46507ea7de34bd54154b9ecb05e607104ef99d67 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Thu, 22 Dec 2016 13:14:22 +0900
Subject: [PATCH] ffap-string-at-point: Limit max length of active region

Do not spend 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            | 17 +++++++++++-----
 test/lisp/ffap-tests.el | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+), 5 deletions(-)
 create mode 100644 test/lisp/ffap-tests.el

diff --git a/lisp/ffap.el b/lisp/ffap.el
index 3d7cebadcf..ad4b70d737 100644
--- a/lisp/ffap.el
+++ b/lisp/ffap.el
@@ -203,6 +203,9 @@ ffap-foo-at-bar-prefix
 		 )
   :group 'ffap)
 
+(defvar ffap-max-region-length 1024
+  "Maximum allowed region length in `ffap-string-at-point'.")
+
 \f
 ;;; Peanut Gallery (More User Variables):
 ;;
@@ -1119,11 +1122,15 @@ ffap-string-at-point
 		(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 (or (null 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.
diff --git a/test/lisp/ffap-tests.el b/test/lisp/ffap-tests.el
new file mode 100644
index 0000000000..ca6a1f4d78
--- /dev/null
+++ b/test/lisp/ffap-tests.el
@@ -0,0 +1,53 @@
+;;; 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)
+            (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
-- 
2.11.0

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In GNU Emacs 26.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.4)
 of 2016-12-22
Repository revision: de0671096706bf7404cddce277b918c8d769f17b