Subject: Re: bug#63535: Master branch: Error in forw_comment (syntax.c) handling of escaped LFs

From: Alan Mackenzie <[email-masked]>
Date: Mon, 22 May 2023 14:59:32 +0000

Hello, Stefan.

On Wed, May 17, 2023 at 18:01:32 -0400, Stefan Monnier wrote:
> Hi Alan,

[ .... ]

> AFAIK this is your code, so you should know better, but AFAICT
> `prev_syntax` is not updated in the loop, so it only reflects the syntax
> before the beginning of the scanned text, rather than anything near `from`.
> Are you sure this is right?

Thanks, you are correct, the patch was not good.

It turned out to be quite tricky to get working.  As well as
forw_comment, I had to amend scan_sexps_forward to make it return a
quoted state to its caller when this happens at the limit of the scan.

I think the following patch is better.  Would you please have a look at
it, in the hope I haven't made any other silly mistakes.  Thanks!

diff --git a/src/syntax.c b/src/syntax.c
index e9e04e2d638..94b2ac2b591 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -2338,13 +2338,16 @@ forw_comment (ptrdiff_t from, ptrdiff_t from_byte, ptrdiff_t stop,
 	  return 0;
 	}
       c = FETCH_CHAR_AS_MULTIBYTE (from_byte);
+      prev_syntax = syntax;
       syntax = SYNTAX_WITH_FLAGS (c);
       code = syntax & 0xff;
       if (code == Sendcomment
 	  && SYNTAX_FLAGS_COMMENT_STYLE (syntax, 0) == style
 	  && (SYNTAX_FLAGS_COMMENT_NESTED (syntax) ?
 	      (nesting > 0 && --nesting == 0) : nesting < 0)
-          && !(comment_end_can_be_escaped && char_quoted (from, from_byte)))
+          && !(comment_end_can_be_escaped
+	       && ((prev_syntax & 0xff) == Sescape
+		   || (prev_syntax & 0xff) == Scharquote)))
 	/* We have encountered a comment end of the same style
 	   as the comment sequence which began this comment
 	   section.  */
@@ -2368,7 +2371,11 @@ forw_comment (ptrdiff_t from, ptrdiff_t from_byte, ptrdiff_t stop,
           inc_both (&from, &from_byte);
           UPDATE_SYNTAX_TABLE_FORWARD (from);
           if (from == stop) continue; /* Failure */
-        }
+	  c = FETCH_CHAR_AS_MULTIBYTE (from_byte);
+	  prev_syntax = syntax;
+	  syntax = Smax;
+	  code = syntax;
+	}
       inc_both (&from, &from_byte);
       UPDATE_SYNTAX_TABLE_FORWARD (from);
 
@@ -3349,7 +3356,14 @@ do { prev_from = from;				\
 	     are invalid now.  Luckily, the `done' doesn't use them
 	     and the INC_FROM sets them to a sane value without
 	     looking at them. */
-	  if (!found) goto done;
+	  if (!found)
+	    {
+	      if ((prev_from_syntax & 0xff) == Sescape
+		  || (prev_from_syntax & 0xff) == Scharquote)
+		goto endquoted;
+	      else
+		goto done;
+	    }
 	  INC_FROM;
 	  state->incomment = 0;
 	  state->comstyle = 0;	/* reset the comment style */

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).