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less uses a correct raceless signal method of indicating signal events in
a volatile sig_atomic_t variable, and then processing events in the mainloop.
But only one variable was used for 3 signals, with |= bit operations which
are signal interruptable!  Rewrite the code to use 3 independent variables
and cleanup how the mainloop observes indications.
ok schwarze

/*
 * Copyright (C) 1984-2012  Mark Nudelman
 * Modified for use with illumos by Garrett D'Amore.
 * Copyright 2014 Garrett D'Amore <garrett@damore.org>
 *
 * You may distribute under the terms of either the GNU General Public
 * License or the Less License, as specified in the README file.
 *
 * For more information, see the README file.
 */

/*
 * Primitives for displaying the file on the screen,
 * scrolling either forward or backward.
 */

#include "less.h"
#include "position.h"

int screen_trashed;
int squished;
int no_back_scroll = 0;
int forw_prompt;

extern int top_scroll;
extern int quiet;
extern int sc_width, sc_height;
extern int plusoption;
extern int forw_scroll;
extern int back_scroll;
extern int ignore_eoi;
extern int clear_bg;
extern int final_attr;
extern int oldbot;
extern char *tagoption;

/*
 * Sound the bell to indicate user is trying to move past end of file.
 */
static void
eof_bell(void)
{
	if (quiet == NOT_QUIET)
		ring_bell();
	else
		vbell();
}

/*
 * Check to see if the end of file is currently displayed.
 */
int
eof_displayed(void)
{
	off_t pos;

	if (ignore_eoi)
		return (0);

	if (ch_length() == -1)
		/*
		 * If the file length is not known,
		 * we can't possibly be displaying EOF.
		 */
		return (0);

	/*
	 * If the bottom line is empty, we are at EOF.
	 * If the bottom line ends at the file length,
	 * we must be just at EOF.
	 */
	pos = position(BOTTOM_PLUS_ONE);
	return (pos == -1 || pos == ch_length());
}

/*
 * Check to see if the entire file is currently displayed.
 */
int
entire_file_displayed(void)
{
	off_t pos;

	/* Make sure last line of file is displayed. */
	if (!eof_displayed())
		return (0);

	/* Make sure first line of file is displayed. */
	pos = position(0);
	return (pos == -1 || pos == 0);
}

/*
 * If the screen is "squished", repaint it.
 * "Squished" means the first displayed line is not at the top
 * of the screen; this can happen when we display a short file
 * for the first time.
 */
void
squish_check(void)
{
	if (!squished)
		return;
	squished = 0;
	repaint();
}

/*
 * Display n lines, scrolling forward,
 * starting at position pos in the input file.
 * "force" means display the n lines even if we hit end of file.
 * "only_last" means display only the last screenful if n > screen size.
 * "nblank" is the number of blank lines to draw before the first
 *   real line.  If nblank > 0, the pos must be -1.
 *   The first real line after the blanks will start at ch_zero().
 */
void
forw(int n, off_t pos, int force, int only_last, int nblank)
{
	int nlines = 0;
	int do_repaint;
	static int first_time = 1;

	squish_check();

	/*
	 * do_repaint tells us not to display anything till the end,
	 * then just repaint the entire screen.
	 * We repaint if we are supposed to display only the last
	 * screenful and the request is for more than a screenful.
	 * Also if the request exceeds the forward scroll limit
	 * (but not if the request is for exactly a screenful, since
	 * repainting itself involves scrolling forward a screenful).
	 */
	do_repaint = (only_last && n > sc_height-1) ||
	    (forw_scroll >= 0 && n > forw_scroll && n != sc_height-1);

	if (!do_repaint) {
		if (top_scroll && n >= sc_height - 1 && pos != ch_length()) {
			/*
			 * Start a new screen.
			 * {{ This is not really desirable if we happen
			 *    to hit eof in the middle of this screen,
			 *    but we don't yet know if that will happen. }}
			 */
			pos_clear();
			add_forw_pos(pos);
			force = 1;
			do_clear();
			home();
		}

		if (pos != position(BOTTOM_PLUS_ONE) || empty_screen()) {
			/*
			 * This is not contiguous with what is
			 * currently displayed.  Clear the screen image
			 * (position table) and start a new screen.
			 */
			pos_clear();
			add_forw_pos(pos);
			force = 1;
			if (top_scroll) {
				do_clear();
				home();
			} else if (!first_time) {
				putstr("...skipping...\n");
			}
		}
	}

	while (--n >= 0) {
		/*
		 * Read the next line of input.
		 */
		if (nblank > 0) {
			/*
			 * Still drawing blanks; don't get a line
			 * from the file yet.
			 * If this is the last blank line, get ready to
			 * read a line starting at ch_zero() next time.
			 */
			if (--nblank == 0)
				pos = ch_zero();
		} else {
			/*
			 * Get the next line from the file.
			 */
			pos = forw_line(pos);
			if (pos == -1) {
				/*
				 * End of file: stop here unless the top line
				 * is still empty, or "force" is true.
				 * Even if force is true, stop when the last
				 * line in the file reaches the top of screen.
				 */
				if (!force && position(TOP) != -1)
					break;
				if (!empty_lines(0, 0) &&
				    !empty_lines(1, 1) &&
				    empty_lines(2, sc_height-1))
					break;
			}
		}
		/*
		 * Add the position of the next line to the position table.
		 * Display the current line on the screen.
		 */
		add_forw_pos(pos);
		nlines++;
		if (do_repaint)
			continue;
		/*
		 * If this is the first screen displayed and
		 * we hit an early EOF (i.e. before the requested
		 * number of lines), we "squish" the display down
		 * at the bottom of the screen.
		 * But don't do this if a + option or a -t option
		 * was given.  These options can cause us to
		 * start the display after the beginning of the file,
		 * and it is not appropriate to squish in that case.
		 */
		if (first_time && pos == -1 && !top_scroll &&
		    tagoption == NULL && !plusoption) {
			squished = 1;
			continue;
		}
		put_line();
		forw_prompt = 1;
	}

	if (nlines == 0)
		eof_bell();
	else if (do_repaint)
		repaint();
	first_time = 0;
	(void) currline(BOTTOM);
}

/*
 * Display n lines, scrolling backward.
 */
void
back(int n, off_t pos, int force, int only_last)
{
	int nlines = 0;
	int do_repaint;

	squish_check();
	do_repaint = (n > get_back_scroll() || (only_last && n > sc_height-1));
	while (--n >= 0) {
		/*
		 * Get the previous line of input.
		 */
		pos = back_line(pos);
		if (pos == -1) {
			/*
			 * Beginning of file: stop here unless "force" is true.
			 */
			if (!force)
				break;
		}
		/*
		 * Add the position of the previous line to the position table.
		 * Display the line on the screen.
		 */
		add_back_pos(pos);
		nlines++;
		if (!do_repaint) {
			home();
			add_line();
			put_line();
		}
	}

	if (nlines == 0)
		eof_bell();
	else if (do_repaint)
		repaint();
	else if (!oldbot)
		lower_left();
	(void) currline(BOTTOM);
}

/*
 * Display n more lines, forward.
 * Start just after the line currently displayed at the bottom of the screen.
 */
void
forward(int n, int force, int only_last)
{
	off_t pos;

	if (get_quit_at_eof() && eof_displayed() &&
	    !(ch_getflags() & CH_HELPFILE)) {
		/*
		 * If the -e flag is set and we're trying to go
		 * forward from end-of-file, go on to the next file.
		 */
		if (edit_next(1))
			quit(QUIT_OK);
		return;
	}

	pos = position(BOTTOM_PLUS_ONE);
	if (pos == -1 && (!force || empty_lines(2, sc_height-1))) {
		if (ignore_eoi) {
			/*
			 * ignore_eoi is to support A_F_FOREVER.
			 * Back up until there is a line at the bottom
			 * of the screen.
			 */
			if (empty_screen()) {
				pos = ch_zero();
			} else {
				do {
					back(1, position(TOP), 1, 0);
					pos = position(BOTTOM_PLUS_ONE);
				} while (pos == -1);
			}
		} else {
			eof_bell();
			return;
		}
	}
	forw(n, pos, force, only_last, 0);
}

/*
 * Display n more lines, backward.
 * Start just before the line currently displayed at the top of the screen.
 */
void
backward(int n, int force, int only_last)
{
	off_t pos;

	pos = position(TOP);
	if (pos == -1 && (!force || position(BOTTOM) == 0)) {
		eof_bell();
		return;
	}
	back(n, pos, force, only_last);
}

/*
 * Get the backwards scroll limit.
 * Must call this function instead of just using the value of
 * back_scroll, because the default case depends on sc_height and
 * top_scroll, as well as back_scroll.
 */
int
get_back_scroll(void)
{
	if (no_back_scroll)
		return (0);
	if (back_scroll >= 0)
		return (back_scroll);
	if (top_scroll)
		return (sc_height - 2);
	return (10000); /* infinity */
}