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If no tags were generated at all, unlink(2) the empty tags file as
soon as the condition can be detected and do not pass it to less(1).
This may happen for man(7) pages, for preformatted pages, and for
very simple pages like true(1).  The main benefit is that :t inside
less(1) yields the clearer diagnostic message "No tags file" rather
than the mildly confusing "No such tag in tags file": the latter
might encourage further, futile attempts to jump to other tags.

Improvement suggested by Leah Neukirchen <leah at vuxu dot org>
from The Void.

/*	$Id: tag.c,v 1.23 2019/07/19 20:27:25 schwarze Exp $ */
/*
 * Copyright (c) 2015, 2016, 2018, 2019 Ingo Schwarze <schwarze@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
#include "config.h"

#include <sys/types.h>

#include <errno.h>
#include <limits.h>
#include <signal.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "mandoc_aux.h"
#include "mandoc_ohash.h"
#include "mandoc.h"
#include "tag.h"

struct tag_entry {
	size_t	*lines;
	size_t	 maxlines;
	size_t	 nlines;
	int	 prio;
	char	 s[];
};

static	void	 tag_signal(int) __attribute__((__noreturn__));

static struct ohash	 tag_data;
static struct tag_files	 tag_files;


/*
 * Prepare for using a pager.
 * Not all pagers are capable of using a tag file,
 * but for simplicity, create it anyway.
 */
struct tag_files *
tag_init(void)
{
	struct sigaction	 sa;
	int			 ofd;

	ofd = -1;
	tag_files.tfd = -1;
	tag_files.tcpgid = -1;

	/* Clean up when dying from a signal. */

	memset(&sa, 0, sizeof(sa));
	sigfillset(&sa.sa_mask);
	sa.sa_handler = tag_signal;
	sigaction(SIGHUP, &sa, NULL);
	sigaction(SIGINT, &sa, NULL);
	sigaction(SIGTERM, &sa, NULL);

	/*
	 * POSIX requires that a process calling tcsetpgrp(3)
	 * from the background gets a SIGTTOU signal.
	 * In that case, do not stop.
	 */

	sa.sa_handler = SIG_IGN;
	sigaction(SIGTTOU, &sa, NULL);

	/* Save the original standard output for use by the pager. */

	if ((tag_files.ofd = dup(STDOUT_FILENO)) == -1) {
		mandoc_msg(MANDOCERR_DUP, 0, 0, "%s", strerror(errno));
		goto fail;
	}

	/* Create both temporary output files. */

	(void)strlcpy(tag_files.ofn, "/tmp/man.XXXXXXXXXX",
	    sizeof(tag_files.ofn));
	(void)strlcpy(tag_files.tfn, "/tmp/man.XXXXXXXXXX",
	    sizeof(tag_files.tfn));
	if ((ofd = mkstemp(tag_files.ofn)) == -1) {
		mandoc_msg(MANDOCERR_MKSTEMP, 0, 0,
		    "%s: %s", tag_files.ofn, strerror(errno));
		goto fail;
	}
	if ((tag_files.tfd = mkstemp(tag_files.tfn)) == -1) {
		mandoc_msg(MANDOCERR_MKSTEMP, 0, 0,
		    "%s: %s", tag_files.tfn, strerror(errno));
		goto fail;
	}
	if (dup2(ofd, STDOUT_FILENO) == -1) {
		mandoc_msg(MANDOCERR_DUP, 0, 0, "%s", strerror(errno));
		goto fail;
	}
	close(ofd);

	/*
	 * Set up the ohash table to collect output line numbers
	 * where various marked-up terms are documented.
	 */

	mandoc_ohash_init(&tag_data, 4, offsetof(struct tag_entry, s));
	return &tag_files;

fail:
	tag_unlink();
	if (ofd != -1)
		close(ofd);
	if (tag_files.ofd != -1)
		close(tag_files.ofd);
	if (tag_files.tfd != -1)
		close(tag_files.tfd);
	*tag_files.ofn = '\0';
	*tag_files.tfn = '\0';
	tag_files.ofd = -1;
	tag_files.tfd = -1;
	return NULL;
}

/*
 * Set the line number where a term is defined,
 * unless it is already defined at a lower priority.
 */
void
tag_put(const char *s, int prio, size_t line)
{
	struct tag_entry	*entry;
	const char		*se;
	size_t			 len;
	unsigned int		 slot;

	if (tag_files.tfd <= 0)
		return;

	if (s[0] == '\\' && (s[1] == '&' || s[1] == 'e'))
		s += 2;

	/*
	 * Skip whitespace and whatever follows it,
	 * and if there is any, downgrade the priority.
	 */

	len = strcspn(s, " \t");
	if (len == 0)
		return;

	se = s + len;
	if (*se != '\0')
		prio = INT_MAX;

	slot = ohash_qlookupi(&tag_data, s, &se);
	entry = ohash_find(&tag_data, slot);

	if (entry == NULL) {

		/* Build a new entry. */

		entry = mandoc_malloc(sizeof(*entry) + len + 1);
		memcpy(entry->s, s, len);
		entry->s[len] = '\0';
		entry->lines = NULL;
		entry->maxlines = entry->nlines = 0;
		ohash_insert(&tag_data, slot, entry);

	} else {

		/*
		 * Lower priority numbers take precedence,
		 * but 0 is special.
		 * A tag with priority 0 is only used
		 * if the tag occurs exactly once.
		 */

		if (prio == 0) {
			if (entry->prio == 0)
				entry->prio = -1;
			return;
		}

		/* A better entry is already present, ignore the new one. */

		if (entry->prio > 0 && entry->prio < prio)
			return;

		/* The existing entry is worse, clear it. */

		if (entry->prio < 1 || entry->prio > prio)
			entry->nlines = 0;
	}

	/* Remember the new line. */

	if (entry->maxlines == entry->nlines) {
		entry->maxlines += 4;
		entry->lines = mandoc_reallocarray(entry->lines,
		    entry->maxlines, sizeof(*entry->lines));
	}
	entry->lines[entry->nlines++] = line;
	entry->prio = prio;
}

/*
 * Write out the tags file using the previously collected
 * information and clear the ohash table while going along.
 */
void
tag_write(void)
{
	FILE			*stream;
	struct tag_entry	*entry;
	size_t			 i;
	unsigned int		 slot;
	int			 empty;

	if (tag_files.tfd <= 0)
		return;
	if (tag_files.tagname != NULL && ohash_find(&tag_data,
            ohash_qlookup(&tag_data, tag_files.tagname)) == NULL) {
		mandoc_msg(MANDOCERR_TAG, 0, 0, "%s", tag_files.tagname);
		tag_files.tagname = NULL;
	}
	if ((stream = fdopen(tag_files.tfd, "w")) == NULL)
		mandoc_msg(MANDOCERR_FDOPEN, 0, 0, "%s", strerror(errno));
	empty = 1;
	entry = ohash_first(&tag_data, &slot);
	while (entry != NULL) {
		if (stream != NULL && entry->prio >= 0) {
			for (i = 0; i < entry->nlines; i++) {
				fprintf(stream, "%s %s %zu\n",
				    entry->s, tag_files.ofn, entry->lines[i]);
				empty = 0;
			}
		}
		free(entry->lines);
		free(entry);
		entry = ohash_next(&tag_data, &slot);
	}
	ohash_delete(&tag_data);
	if (stream != NULL)
		fclose(stream);
	else
		close(tag_files.tfd);
	tag_files.tfd = -1;
	if (empty) {
		unlink(tag_files.tfn);
		*tag_files.tfn = '\0';
	}
}

void
tag_unlink(void)
{
	pid_t	 tc_pgid;

	if (tag_files.tcpgid != -1) {
		tc_pgid = tcgetpgrp(tag_files.ofd);
		if (tc_pgid == tag_files.pager_pid ||
		    tc_pgid == getpgid(0) ||
		    getpgid(tc_pgid) == -1)
			(void)tcsetpgrp(tag_files.ofd, tag_files.tcpgid);
	}
	if (*tag_files.ofn != '\0')
		unlink(tag_files.ofn);
	if (*tag_files.tfn != '\0')
		unlink(tag_files.tfn);
}

static void
tag_signal(int signum)
{
	struct sigaction	 sa;

	tag_unlink();
	memset(&sa, 0, sizeof(sa));
	sigemptyset(&sa.sa_mask);
	sa.sa_handler = SIG_DFL;
	sigaction(signum, &sa, NULL);
	kill(getpid(), signum);
	/* NOTREACHED */
	_exit(1);
}