bug#28428: comment-search-backward with no comments

[N. Raghavendra]

I am using Emacs 26.0.50.  In the cases that I tried, when there is no
comment in the portion of the buffer between the search limit and point,
`comment-search-backward' does not move point to the search limit before
raising an error.  This behaviour contradicts its docstring.  My message
to emacs-devel about this is at
http://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00193.html

Attached are a patch that seems to fix the problem, and a file
containing a test.  Running the test without the change in the patch
fails, and passes with the change:

$ git checkout master

$ emacs -Q -batch -l lisp/newcomment.el -l /tmp/test.el \
  -f ert-run-tests-batch-and-exit
----------------------------------------------------------------------
(ert-test-failed
     ((should
       (=
        (point)
        limit))
      :form
      (= 70 41)
      :value nil))
   FAILED  1/1  test-comment-search-backward-absent
----------------------------------------------------------------------

$ git checkout test-comment-search-forward-absent

$ emacs -Q -batch -l lisp/newcomment.el -l /tmp/test.el \
  -f ert-run-tests-batch-and-exit
----------------------------------------------------------------------
Running 1 tests (2017-09-12 03:31:48+0530)
   passed  1/1  test-comment-search-backward-absent
----------------------------------------------------------------------

Raghu.

--
N. Raghavendra <raghu@hri.res.in>, http://www.retrotexts.net/
Harish-Chandra Research Institute, http://www.hri.res.in/

>From eb8e32c6fe4a3d08be8b856b3ff4a300a2e69d4b Mon Sep 17 00:00:00 2001
From: "N. Raghavendra" <raghu@hri.res.in>
Date: Tue, 12 Sep 2017 01:41:16 +0530
Subject: [PATCH] comment-search-backword: move point when there is no comment

* lisp/newcomment.el (comment-search-backward): Move point to
LIMIT when there is no comment between LIMIT and point.
---
 lisp/newcomment.el | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 8772b523..d849fc1 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -525,7 +525,9 @@ comment-search-backward
   ;; comment-set-column) and to find the comment-start string (via
   ;; comment-beginning) in indent-new-comment-line, it should be harmless.
   (if (not (re-search-backward comment-start-skip limit t))
-      (unless noerror (error "No comment"))
+      (progn
+        (goto-char limit)
+        (unless noerror (error "No comment")))
     (beginning-of-line)
     (let* ((end (match-end 0))
           (cs (comment-search-forward end t))
-- 
2.7.4

test.el
Description: application/emacs-lisp