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:

> below
>
>> > 2. Instead of testing whether the max-length var is nil, I'd suggest
>> > testing it with `natnump', to take care of the unexpected case where
>> > it might get assigned a non-number.
>> Yes, `natnump' is a better choice.
>
>> +    (if (or (not (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 ""))))
>
> I'd suggest the other way around.  What you have lets someone or
> some code assign a non-number and get the same slow behavior we
> want to avoid.  I'd say (and (natnump ...) (< region-len ...)).
>
> IOW, if it's not a natnump and the size is not smaller than that
> number then don't use the region.
It sounds reasonable.  Thank you.
If i don't see further comments in a few days i will push
the following patch to the master branch:

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From 7e8268b975c8385015769755e8ba1e9854d64da1 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Sat, 24 Dec 2016 11:31:53 +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            | 25 ++++++++++++++++-------
 test/lisp/ffap-tests.el | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 72 insertions(+), 7 deletions(-)
 create mode 100644 test/lisp/ffap-tests.el

diff --git a/lisp/ffap.el b/lisp/ffap.el
index 3d7cebadcf..99bb65faaf 100644
--- a/lisp/ffap.el
+++ b/lisp/ffap.el
@@ -203,6 +203,11 @@ ffap-foo-at-bar-prefix
 		 )
   :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.")
+
 \f
 ;;; Peanut Gallery (More User Variables):
 ;;
@@ -1101,8 +1106,10 @@ ffap-string-at-point
 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 @@ 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 (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.
diff --git a/test/lisp/ffap-tests.el b/test/lisp/ffap-tests.el
new file mode 100644
index 0000000000..61fa891fe7
--- /dev/null
+++ b/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
-- 
2.11.0

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In GNU Emacs 26.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.5)
 of 2016-12-23
Repository revision: eff901b8a39f42ddedf4c1db833b9071cae5962f