Fully support explicit tagging of .Sh and .Ss.

This fixes the offset of two lines in terminal output
and this improves HTML output by putting the id= attribute
and <a> element into the respective <h1> or <h2> element rather
than writing an additional <mark> element.

To that end, introduce node flags NODE_ID (to make the node a link
target, for example by writing an HTML id= attribute or by calling
tag_put()) and NODE_HREF (to make the node a link source, used only
in HTML output, used only to write an <a class="permalink"> element).

In particular:
* In the validator, generalize the concept of the "next node"
such that it also works before .Sh and .Ss.
* If the first argument of .Tg is empty, don't forget to complain
if there are additional arguments, which will be ignored.
* In the terminal formatter, support writing of explicit tags
for all kinds of nodes, not just for .Tg.
* In deroff(), allow nodes to have an explicit string representation
even when they aren't text nodes.  Use this for explicitly tagged
section headers.  Suprisingly, this is sufficient to make HTML
output work, without explicit code changes in the HTML formatter.
* In syntax tree output, display NODE_ID and NODE_HREF.

10 files changed, 141 insertions, 18 deletions

diff --git a/mdoc_term.c b/mdoc_term.c
index 776f1130..35e05809 100644
--- a/mdoc_term.c
+++ b/mdoc_term.c
@@ -117,7 +117,6 @@ static	int	  termp_pp_pre(DECL_ARGS);
 static	int	  termp_ss_pre(DECL_ARGS);
 static	int	  termp_sy_pre(DECL_ARGS);
 static	int	  termp_tag_pre(DECL_ARGS);
-static	int	  termp_tg_pre(DECL_ARGS);
 static	int	  termp_under_pre(DECL_ARGS);
 static	int	  termp_vt_pre(DECL_ARGS);
 static	int	  termp_xr_pre(DECL_ARGS);
@@ -244,7 +243,7 @@ static const struct mdoc_term_act mdoc_term_acts[MDOC_MAX - MDOC_Dd] = {
 	{ NULL, termp____post }, /* %Q */
 	{ NULL, termp____post }, /* %U */
 	{ NULL, NULL }, /* Ta */
-	{ termp_tg_pre, NULL }, /* Tg */
+	{ termp_skip_pre, NULL }, /* Tg */
 };
 
 static	int	 fn_prio = TAG_STRONG;
@@ -341,6 +340,10 @@ print_mdoc_node(DECL_ARGS)
 	memset(&npair, 0, sizeof(struct termpair));
 	npair.ppair = pair;
 
+	if (n->flags & NODE_ID)
+		tag_put(n->string == NULL ? n->child->string : n->string,
+		    TAG_MANUAL, p->line);
+
 	/*
 	 * Keeps only work until the end of a line.  If a keep was
 	 * invoked in a prior line, revert it to PREKEEP.
@@ -2066,13 +2069,6 @@ termp_tag_pre(DECL_ARGS)
 }
 
 static int
-termp_tg_pre(DECL_ARGS)
-{
-	tag_put(n->child->string, TAG_MANUAL, p->line);
-	return 0;
-}
-
-static int
 termp_abort_pre(DECL_ARGS)
 {
 	abort();
diff --git a/mdoc_validate.c b/mdoc_validate.c
index 6ab715d3..c8e261f9 100644
--- a/mdoc_validate.c
+++ b/mdoc_validate.c
@@ -1094,21 +1094,32 @@ post_st(POST_ARGS)
 static void
 post_tg(POST_ARGS)
 {
-	struct roff_node	*n, *nch;
+	struct roff_node	*n, *nch, *nn;
 	size_t			len;
 
+	/* Find the next node. */
 	n = mdoc->last;
+	for (nn = n; nn != NULL; nn = nn->parent) {
+		if (nn->next != NULL) {
+			nn = nn->next;
+			break;
+		}
+	}
+
+	/* Add the default argument, if needed. */
 	nch = n->child;
-	if (nch == NULL && n->next != NULL &&
-	    n->next->child->type == ROFFT_TEXT) {
+	if (nch == NULL && nn != NULL && nn->child->type == ROFFT_TEXT) {
 		mdoc->next = ROFF_NEXT_CHILD;
 		roff_word_alloc(mdoc, n->line, n->pos, n->next->child->string);
 		nch = mdoc->last;
 		nch->flags |= NODE_NOSRC;
 		mdoc->last = n;
 	}
-	if (nch == NULL || *nch->string == '\0') {
+
+	/* Validate the first argument. */
+	if (nch == NULL || *nch->string == '\0')
 		mandoc_msg(MANDOCERR_MACRO_EMPTY, n->line, n->pos, "Tg");
+	if (nch == NULL) {
 		roff_node_delete(mdoc, n);
 		return;
 	}
@@ -1116,14 +1127,42 @@ post_tg(POST_ARGS)
 	if (nch->string[len] != '\0')
 		mandoc_msg(MANDOCERR_TG_SPC, nch->line, nch->pos + len + 1,
 		    "Tg %s", nch->string);
+
+	/* Keep only the first argument. */
 	if (nch->next != NULL) {
 		mandoc_msg(MANDOCERR_ARG_EXCESS, nch->next->line,
 		    nch->next->pos, "Tg ... %s", nch->next->string);
 		while (nch->next != NULL)
 			roff_node_delete(mdoc, nch->next);
 	}
-	if (nch->string[len] != '\0')
+
+	/* Drop the macro if the first argument is invalid. */
+	if (len == 0 || nch->string[len] != '\0') {
 		roff_node_delete(mdoc, n);
+		return;
+	}
+
+	/* By default, write a <mark> element. */
+	n->flags |= NODE_ID;
+	if (nn == NULL)
+		return;
+
+	/* Explicit tagging of specific macros. */
+	switch (nn->tok) {
+	case MDOC_Sh:
+	case MDOC_Ss:
+		if (nn->head->flags & NODE_ID || nn->head->child == NULL)
+			break;
+		n->flags |= NODE_NOPRT;
+		nn->head->flags |= NODE_ID | NODE_HREF;
+		assert(nn->head->string == NULL);
+		nn->head->string = mandoc_strdup(nch->string);
+		break;
+	default:
+		break;
+	}
+	if (n->flags & NODE_NOPRT)
+		n->flags &= ~NODE_ID;
 }
 
 static void
diff --git a/regress/mdoc/Sh/Makefile b/regress/mdoc/Sh/Makefile
index 213664ec..4b37bb2a 100644
--- a/regress/mdoc/Sh/Makefile
+++ b/regress/mdoc/Sh/Makefile
@@ -1,11 +1,11 @@
-# $OpenBSD: Makefile,v 1.12 2020/02/27 01:25:58 schwarze Exp $
+# $OpenBSD: Makefile,v 1.13 2020/02/27 21:38:27 schwarze Exp $
 
 REGRESS_TARGETS	 = badNAME before empty emptyNAME first nohead order
 REGRESS_TARGETS	+= orderNAME paragraph parbefore parborder punctNAME
-REGRESS_TARGETS	+= subbefore transp
+REGRESS_TARGETS	+= subbefore tag transp
 LINT_TARGETS	 = badNAME before empty emptyNAME first nohead order
 LINT_TARGETS	+= orderNAME parbefore parborder punctNAME subbefore
-HTML_TARGETS	 = paragraph
+HTML_TARGETS	 = paragraph tag
 
 # groff-1.22.3 defects:
 # - .Pp before .Sh NAME causes a blank line before the header line
diff --git a/regress/mdoc/Sh/tag.in b/regress/mdoc/Sh/tag.in
new file mode 100644
index 00000000..c204fc6b
--- /dev/null
+++ b/regress/mdoc/Sh/tag.in
@@ -0,0 +1,21 @@
+.\" $OpenBSD: tag.in,v 1.1 2020/02/27 21:38:27 schwarze Exp $
+.Dd $Mdocdate$
+.Dt SH-TAG 1
+.Os
+.Sh NAME
+.Nm Sh-tag
+.Nd tagging section headers
+.Sh DESCRIPTION
+Text in the description.
+.Ss Subsection
+BEGINTEST
+.Pp
+Text in the subsection.
+.Tg examples
+.Sh EXAMPLES
+Text introducing examples.
+.Tg example
+.Ss Subsection
+Example text.
+.Pp
+ENDTEST
diff --git a/regress/mdoc/Sh/tag.out_ascii b/regress/mdoc/Sh/tag.out_ascii
new file mode 100644
index 00000000..df0ab050
--- /dev/null
+++ b/regress/mdoc/Sh/tag.out_ascii
@@ -0,0 +1,22 @@
+SH-TAG(1)                   General Commands Manual                  SH-TAG(1)
+
+NNAAMMEE
+     SShh--ttaagg - tagging section headers
+
+DDEESSCCRRIIPPTTIIOONN
+     Text in the description.
+
+   SSuubbsseeccttiioonn
+     BEGINTEST
+
+     Text in the subsection.
+
+EEXXAAMMPPLLEESS
+     Text introducing examples.
+
+   SSuubbsseeccttiioonn
+     Example text.
+
+     ENDTEST
+
+OpenBSD                        February 27, 2020                       OpenBSD
diff --git a/regress/mdoc/Sh/tag.out_html b/regress/mdoc/Sh/tag.out_html
new file mode 100644
index 00000000..9722aa8c
--- /dev/null
+++ b/regress/mdoc/Sh/tag.out_html
@@ -0,0 +1,9 @@
+<p class="Pp">Text in the subsection.</p>
+</section>
+</section>
+<section class="Sh">
+<h1 class="Sh" id="examples"><a class="permalink" href="#examples">EXAMPLES</a></h1>
+<p class="Pp">Text introducing examples.</p>
+<section class="Ss">
+<h2 class="Ss" id="example"><a class="permalink" href="#example">Subsection</a></h2>
+<p class="Pp">Example text.</p>
diff --git a/regress/mdoc/Sh/tag.out_markdown b/regress/mdoc/Sh/tag.out_markdown
new file mode 100644
index 00000000..2813f8f0
--- /dev/null
+++ b/regress/mdoc/Sh/tag.out_markdown
@@ -0,0 +1,27 @@
+SH-TAG(1) - General Commands Manual
+
+# NAME
+
+**Sh-tag** - tagging section headers
+
+# DESCRIPTION
+
+Text in the description.
+
+## Subsection
+
+BEGINTEST
+
+Text in the subsection.
+
+# EXAMPLES
+
+Text introducing examples.
+
+## Subsection
+
+Example text.
+
+ENDTEST
+
+OpenBSD - February 27, 2020
diff --git a/roff.c b/roff.c
index 55172fad..d1f364d4 100644
--- a/roff.c
+++ b/roff.c
@@ -1173,7 +1173,7 @@ deroff(char **dest, const struct roff_node *n)
 	char	*cp;
 	size_t	 sz;
 
-	if (n->type != ROFFT_TEXT) {
+	if (n->string == NULL) {
 		for (n = n->child; n != NULL; n = n->next)
 			deroff(dest, n);
 		return;
diff --git a/roff.h b/roff.h
index f9aa35de..37d90b4e 100644
--- a/roff.h
+++ b/roff.h
@@ -522,6 +522,8 @@ struct	roff_node {
 #define	NODE_NOFILL	 (1 << 8)  /* Fill mode switched off. */
 #define	NODE_NOSRC	 (1 << 9)  /* Generated node, not in input file. */
 #define	NODE_NOPRT	 (1 << 10) /* Shall not print anything. */
+#define	NODE_ID		 (1 << 11) /* Target for deep linking. */
+#define	NODE_HREF	 (1 << 12) /* Link to another place in this page. */
 	int		  prev_font; /* Before entering this node. */
 	int		  aux;     /* Decoded node data, type-dependent. */
 	enum roff_tok	  tok;     /* Request or macro ID. */
diff --git a/tree.c b/tree.c
index 1a6d4dd7..5352b868 100644
--- a/tree.c
+++ b/tree.c
@@ -199,6 +199,13 @@ print_mdoc(const struct roff_node *n, int indent)
 			putchar(')');
 		if (n->flags & NODE_EOS)
 			putchar('.');
+		if (n->flags & NODE_ID) {
+			printf(" ID");
+			if (n->string != NULL)
+				printf("=%s", n->string);
+		}
+		if (n->flags & NODE_HREF)
+			printf(" HREF");
 		if (n->flags & NODE_BROKEN)
 			printf(" BROKEN");
 		if (n->flags & NODE_NOFILL)