bug#47167: Multi-line comment-region with empty comment-continue

[Juri Linkov]

Tags: patch

Tried to customize 'comment-region' to use multi-line comments,
so when commenting out such region in html-mode:

line 1
line 2
line 3

instead of adding comments to each line:

<!-- line 1 -->
<!-- line 2 -->
<!-- line 3 -->

to add comments only at the beginning/end:

<!--
line 1
line 2
line 3
  -->

According to the documentation, a legitimate way to do this is
to use such customization:

(setq-local comment-style 'extra-line
            comment-continue "")

But currently it has no effect.  So needed to fix newcomment.el.
One possible fix is to return the original string from 'comment-padright'
even when the string contains only whitespace.  Maybe this would be
the right fix, but still not sure about other calls of 'comment-padright'.
So a safer fix would be to do this in the caller:

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index ea47eec4fd..a5bfb06795 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -1300,7 +1300,11 @@ comment-region-default-1
         (let ((s (comment-padleft comment-end numarg)))
           (and s (if (string-match comment-end-skip s) s
                    (comment-padright comment-end))))
-        (if multi (comment-padright comment-continue numarg))
+        (if multi
+             (or (comment-padright comment-continue numarg)
+                 ;; `comment-padright' returns nil when
+                 ;; `comment-continue' contains only whitespace
+                 (and (stringp comment-continue) comment-continue)))
         (if multi
             (comment-padleft (comment-string-reverse comment-continue) numarg))
         block