Re: mandoc mishandles tzfile(5)'s .IP \(bu "\w'\(bu 'u"

[Ingo Schwarze <schwarze@usta.de>]

Hi Paul,

Paul Eggert wrote on Sun, Oct 22, 2023 at 05:41:28PM -0700:
> On 2023-10-22 14:06, Ingo Schwarze wrote:

>> mandoc only supports ASCII strings as arguments to \w, not escape
>> sequences or formatting instructions.
 
> For the TZDB man pages mandoc need not support all that, just \(bu.

Thank you for identifying a subset of the functionality that is
both useful and feasible.  That's so much better than the TODO
item for \w i have lying around - that one is much less important
than what you suggested but horrifically difficult.

> Just to make sure we're on the same page, I reproduced the problem by 
> running the command "mandoc -man -Tascii t.5", where t.5 contains the 
> following lines:
> 
>          .TH tzfile 5
>          .SH NAME
>          .IP \(bu "\w'\(bu  'u"
>          xxx
>          .PP
>          yyy
> 
> The output should contain two spaces between the bullet's "o" and the 
> "x", but with current mandoc it contains five spaces.

Yes, that matches my understanding.

> Proposed mandoc patch attached. This isn't a perfect emulation of groff, 

In this case, that's a virtue, even though in many other cases,
compatibility with groff is indeed among the main goals.

> nor have I tested with fancy constructs,

Not a major roadblock; such testing becomes significantly easier
with the existing test suite, and by adding to it.

> but it should be good enough for tzfile(5).

And likely for some other pages, too.

Regarding the implementation:

I did not like that you did most of the work in roff_escape.c.
That function is called each and every time any parser or formatter
wants to deal with any escape sequence.  Your patch did additional
work for almost every kind of escape sequence, even though \w is
the only escape sequence needing that kind of work.

On top of that, while changing the internal API mandoc_escape(3)
is not unheard of and can be done when it is really important,
the need for an API break did not seem very urgent to me here.

Consequently, i chose to do all the work at one local place in the
internal function roff_expand().  Even though that requires iterating
the argument of every \w escape sequence twice, it causes less work
regarding the grand total because \w is a rare escape sequence,
occurring much less frequently than other sequences, and even for \w
relevant extra work is only done if the argument contains embedded
escape sequences, which will occur still less frequently.  Encountering
long, complicated, deeply nested esacpe sequences inside \w would
definitely be unusual.  Doing extra work for every parsing and every
formatting of each and every escape sequence sounds clearly worse.

Besides, roff_expand() is really the place where the logic belongs.
The function roff_escape() is a parser.  Its job is to figure out where
the various syntax elements (sequence, name, argument) begin and end
and what the class of the sequence is.  Doing calculations bases on
the content of the argument is out of scope there, and considering that
it's a parser, doing calculations related to formatting even more so.

Calculating the information needed for interpolation really belongs
into the string interpolation function roff_expand(), and given that \w
in particular is related to formatting, the normal internal ESCAPE_*
API that the formatters use for such purposes should be used here,
too.  Potentially, that might also make future refinements of the
functionality easier: the more this code already resenmbles typical
formatter code, the better.

To summarize, i committed the following patch.

Does that patch work for you as well as your own?

Yours,
  Ingo


Log Message:
-----------
Support some escape sequences, in particular character escape sequences,
inside \w arguments, and skip most other escape sequences when measuring
the output length in this way because most escape sequences contribute
little or nothing to text width: for example, consider font escapes in
terminal output.

This implementation is very rudimentary.  In particular, it assumes that
every character has the same width.  No attempt is made to detect 
double-width or zero-width Unicode characters or to take dependencies on
output devices or fonts into account.  These limitations are hard to
avoid because mandoc has to interpolate \w at the parsing stage when the
output device is not yet known.  I really do not want the content of the
syntax tree to depend on the output device.

Feature requested by Paul <Eggert at cs dot ucla dot edu>, who also
submitted a patch, but i chose to commit this very different patch
with almost the same functionality.
His input was still very valuable because complete support for \w is
out of the question, and consequently, the main task is identifying
subsets of the feature that are needed for real-world manual pages 
and can be supported without uprooting the whole forest.

Modified Files:
--------------
    mandoc:
        roff.7
        roff.c
    mandoc/regress/roff/esc:
        w.in
        w.out_ascii
        w.out_lint

Revision Data
-------------
Index: roff.7
===================================================================
RCS file: /home/cvs/mandoc/mandoc/roff.7,v
retrieving revision 1.120
retrieving revision 1.121
diff -Lroff.7 -Lroff.7 -u -p -r1.120 -r1.121
--- roff.7
+++ roff.7
@@ -1,6 +1,6 @@
 .\" $Id$
 .\"
-.\" Copyright (c) 2010-2019, 2022 Ingo Schwarze <schwarze@openbsd.org>
+.\" Copyright (c) 2010-2019, 2022-2023 Ingo Schwarze <schwarze@openbsd.org>
 .\" Copyright (c) 2010, 2011, 2012 Kristaps Dzonsons <kristaps@bsd.lv>
 .\"
 .\" Permission to use, copy, modify, and distribute this software for any
@@ -2224,7 +2224,8 @@ The
 .Xr mandoc 1
 implementation assumes that after expansion of user-defined strings, the
 .Ar string
-only contains normal characters, no escape sequences, and that each
+only contains normal characters, characters expressed as escape sequences,
+and zero-width escape sequences, and that each
 character has a width of 24 basic units.
 .It Ic \eX\(aq Ns Ar string Ns Ic \(aq
 Output
Index: roff.c
===================================================================
RCS file: /home/cvs/mandoc/mandoc/roff.c,v
retrieving revision 1.398
retrieving revision 1.399
diff -Lroff.c -Lroff.c -u -p -r1.398 -r1.399
--- roff.c
+++ roff.c
@@ -1,6 +1,6 @@
 /* $Id$ */
 /*
- * Copyright (c) 2010-2015, 2017-2022 Ingo Schwarze <schwarze@openbsd.org>
+ * Copyright (c) 2010-2015, 2017-2023 Ingo Schwarze <schwarze@openbsd.org>
  * Copyright (c) 2008-2012, 2014 Kristaps Dzonsons <kristaps@bsd.lv>
  *
  * Permission to use, copy, modify, and distribute this software for any
@@ -1362,6 +1362,7 @@ roff_expand(struct roff *r, struct buf *
 	const char	*res;		/* the string to be pasted */
 	const char	*src;		/* source for copying */
 	char		*dst;		/* destination for copying */
+	enum mandoc_esc	 subtype;	/* return value from roff_escape */
 	int		 iesc;		/* index of leading escape char */
 	int		 inam;		/* index of the escape name */
 	int		 iarg;		/* index beginning the argument */
@@ -1551,8 +1552,34 @@ roff_expand(struct roff *r, struct buf *
 			res = ubuf;
 			break;
 		case 'w':
-			(void)snprintf(ubuf, sizeof(ubuf),
-			    "%d", (iendarg - iarg) * 24);
+			rsz = 0;
+			subtype = ESCAPE_UNDEF;
+			while (iarg < iendarg) {
+				asz = subtype == ESCAPE_SKIPCHAR ? 0 : 1;
+				if (buf->buf[iarg] != '\\') {
+					rsz += asz;
+					iarg++;
+					continue;
+				}
+				switch ((subtype = roff_escape(buf->buf, 0,
+				    iarg, NULL, NULL, NULL, NULL, &iarg))) {
+				case ESCAPE_SPECIAL:
+				case ESCAPE_NUMBERED:
+				case ESCAPE_UNICODE:
+				case ESCAPE_OVERSTRIKE:
+				case ESCAPE_UNDEF:
+					break;
+				case ESCAPE_DEVICE:
+					asz *= 8;
+					break;
+				case ESCAPE_EXPAND:
+					abort();
+				default:
+					continue;
+				}
+				rsz += asz;
+			}
+			(void)snprintf(ubuf, sizeof(ubuf), "%d", rsz * 24);
 			res = ubuf;
 			break;
 		default:
Index: w.out_ascii
===================================================================
RCS file: /home/cvs/mandoc/mandoc/regress/roff/esc/w.out_ascii,v
retrieving revision 1.3
retrieving revision 1.4
diff -Lregress/roff/esc/w.out_ascii -Lregress/roff/esc/w.out_ascii -u -p -r1.3 -r1.4
--- regress/roff/esc/w.out_ascii
+++ regress/roff/esc/w.out_ascii
@@ -8,6 +8,13 @@ D\bDE\bES\bSC\bCR\bRI\bIP\bPT\bTI\bIO\bON\bN
      character: 24
      blank: 24
      text: 96
+     special: 24
+     numbered: 24
+     Unicode: 24
+     overstrike: 24
+     undefined: 24
+     zero-width: 0
+     skipchar: 48
 
    A\bAr\brg\bgu\bum\bme\ben\bnt\bt d\bde\bel\bli\bim\bmi\bit\bte\ber\brs\bs
      unsupported \r: 24u
@@ -27,4 +34,4 @@ D\bDE\bES\bSC\bCR\bRI\bIP\bPT\bTI\bIO\bON\bN
      overstrike: 24u
      unterminated: 72
 
-OpenBSD                          June 8, 2022                          OpenBSD
+OpenBSD                        October 23, 2023                        OpenBSD
Index: w.out_lint
===================================================================
RCS file: /home/cvs/mandoc/mandoc/regress/roff/esc/w.out_lint,v
retrieving revision 1.7
retrieving revision 1.8
diff -Lregress/roff/esc/w.out_lint -Lregress/roff/esc/w.out_lint -u -p -r1.7 -r1.8
--- regress/roff/esc/w.out_lint
+++ regress/roff/esc/w.out_lint
@@ -1,4 +1,5 @@
-mandoc: w.in:17:20: UNSUPP: unsupported escape sequence: \r
-mandoc: w.in:17:23: UNSUPP: unsupported escape sequence: \r
-mandoc: w.in:23:16: WARNING: undefined escape, printing literally: \G
-mandoc: w.in:51:15: ERROR: incomplete escape sequence: \w'foo
+mandoc: w.in:25:15: WARNING: undefined escape, printing literally: \G
+mandoc: w.in:31:20: UNSUPP: unsupported escape sequence: \r
+mandoc: w.in:31:23: UNSUPP: unsupported escape sequence: \r
+mandoc: w.in:37:16: WARNING: undefined escape, printing literally: \G
+mandoc: w.in:65:15: ERROR: incomplete escape sequence: \w'foo
Index: w.in
===================================================================
RCS file: /home/cvs/mandoc/mandoc/regress/roff/esc/w.in,v
retrieving revision 1.3
retrieving revision 1.4
diff -Lregress/roff/esc/w.in -Lregress/roff/esc/w.in -u -p -r1.3 -r1.4
--- regress/roff/esc/w.in
+++ regress/roff/esc/w.in
@@ -1,4 +1,4 @@
-.\" $OpenBSD: w.in,v 1.4 2022/06/08 13:08:00 schwarze Exp $
+.\" $OpenBSD: w.in,v 1.5 2023/10/23 20:07:19 schwarze Exp $
 .Dd $Mdocdate$
 .Dt ESC-W 1
 .Os
@@ -13,6 +13,20 @@ character: \w'n'
 blank: \w' '
 .br
 text: \w'text'
+.br
+special: \w'\(bu'
+.br
+numbered: \w'\N'100''
+.br
+Unicode: \w'\[u2013]'
+.br
+overstrike: \w'\o'ab''
+.br
+undefined: \w'\G'
+.br
+zero-width: \w'\fB\&\fP'
+.br
+skipchar: \w'a\zb\z\(buc'
 .Ss Argument delimiters
 unsupported \er: \w\rM\ru
 .br
--
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