/*      $NetBSD: forward.c,v 1.34 2024/01/14 17:37:32 christos Exp $    */

/*-
 * Copyright (c) 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Edward Sze-Tyan Wang.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#ifndef lint
#if 0
static char sccsid[] = "@(#)forward.c   8.1 (Berkeley) 6/6/93";
#endif
__RCSID("$NetBSD: forward.c,v 1.34 2024/01/14 17:37:32 christos Exp $");
#endif /* not lint */

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/event.h>

#include <limits.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "extern.h"

static int rlines(FILE *, off_t, struct stat *);

/* defines for inner loop actions */
#define USE_SLEEP       0
#define USE_KQUEUE      1
#define ADD_EVENTS      2

/*
 * forward -- display the file, from an offset, forward.
 *
 * There are eight separate cases for this -- regular and non-regular
 * files, by bytes or lines and from the beginning or end of the file.
 *
 * FBYTES       byte offset from the beginning of the file
 *      REG     seek
 *      NOREG   read, counting bytes
 *
 * FLINES       line offset from the beginning of the file
 *      REG     read, counting lines
 *      NOREG   read, counting lines
 *
 * RBYTES       byte offset from the end of the file
 *      REG     seek
 *      NOREG   cyclically read characters into a wrap-around buffer
 *
 * RLINES
 *      REG     mmap the file and step back until reach the correct offset.
 *      NOREG   cyclically read lines into a wrap-around array of buffers
 */
void
forward(FILE *fp, enum STYLE style, off_t off, struct stat *sbp)
{
        int ch, n;
        int kq=-1, action=USE_SLEEP;
        struct stat statbuf;
        struct kevent ev[2];

        switch(style) {
        case FBYTES:
                if (off == 0)
                        break;
                if (S_ISREG(sbp->st_mode) && sbp->st_size > 0) {
                        if (sbp->st_size < off)
                                off = sbp->st_size;
                        if (fseeko(fp, off, SEEK_SET) == -1) {
                                ierr();
                                return;
                        }
                } else while (off--)
                        if ((ch = getc(fp)) == EOF) {
                                if (ferror(fp)) {
                                        ierr();
                                        return;
                                }
                                break;
                        }
                break;
        case FLINES:
                if (off == 0)
                        break;
                for (;;) {
                        if ((ch = getc(fp)) == EOF) {
                                if (ferror(fp)) {
                                        ierr();
                                        return;
                                }
                                break;
                        }
                        if (ch == '\n' && !--off)
                                break;
                }
                break;
        case RBYTES:
                if (S_ISREG(sbp->st_mode) && sbp->st_size > 0) {
                        if (sbp->st_size >= off &&
                            fseeko(fp, -off, SEEK_END) == -1) {
                                ierr();
                                return;
                        }
                } else if (off == 0) {
                        while (getc(fp) != EOF);
                        if (ferror(fp)) {
                                ierr();
                                return;
                        }
                } else {
                        if (displaybytes(fp, off))
                                return;
                }
                break;
        case RLINES:
                if (S_ISREG(sbp->st_mode) && sbp->st_size > 0) {
                        if (!off) {
                                if (fseek(fp, 0L, SEEK_END) == -1) {
                                        ierr();
                                        return;
                                }
                        } else {
                                if (rlines(fp, off, sbp))
                                        return;
                        }
                } else if (off == 0) {
                        while (getc(fp) != EOF);
                        if (ferror(fp)) {
                                ierr();
                                return;
                        }
                } else {
                        if (displaylines(fp, off))
                                return;
                }
                break;
        default:
                break;
        }

        if (fflag) {
                kq = kqueue();
                if (kq < 0)
                        xerr(1, "kqueue");
                action = ADD_EVENTS;
        }

        for (;;) {
                while ((ch = getc(fp)) != EOF)  {
                        if (putchar(ch) == EOF)
                                oerr();
                }
                if (ferror(fp)) {
                        ierr();
                        return;
                }
                (void)fflush(stdout);
                if (!fflag)
                        break;

                clearerr(fp);

                switch (action) {
                case ADD_EVENTS:
                        n = 0;

                        memset(ev, 0, sizeof(ev));
                        if (fflag == 2 && fp != stdin) {
                                EV_SET(&ev[n], fileno(fp), EVFILT_VNODE,
                                    EV_ADD | EV_ENABLE | EV_CLEAR,
                                    NOTE_DELETE | NOTE_RENAME, 0, 0);
                                n++;
                        }
                        EV_SET(&ev[n], fileno(fp), EVFILT_READ,
                            EV_ADD | EV_ENABLE, 0, 0, 0);
                        n++;

                        if (kevent(kq, ev, n, NULL, 0, NULL) == -1) {
                                close(kq);
                                kq = -1;
                                action = USE_SLEEP;
                        } else {
                                action = USE_KQUEUE;
                        }
                        break;

                case USE_KQUEUE:
                        if (kevent(kq, NULL, 0, ev, 1, NULL) == -1)
                                xerr(1, "kevent");

                        if (ev[0].filter == EVFILT_VNODE) {
                                /* file was rotated, wait until it reappears */
                                action = USE_SLEEP;
                        } else if (ev[0].data < 0) {
                                /* file shrank, reposition to end */
                                if (fseek(fp, 0L, SEEK_END) == -1) {
                                        ierr();
                                        return;
                                }
                        }
                        break;

                case USE_SLEEP:
                        /*
                         * We pause for one second after displaying any data
                         * that has accumulated since we read the file.
                         */
                        (void) sleep(1);

                        if (fflag == 2 && fp != stdin &&
                            stat(fname, &statbuf) != -1) {
                                if (statbuf.st_ino != sbp->st_ino ||
                                    statbuf.st_dev != sbp->st_dev ||
                                    statbuf.st_rdev != sbp->st_rdev ||
                                    statbuf.st_nlink == 0) {
                                        fp = freopen(fname, "r", fp);
                                        if (fp == NULL) {
                                                ierr();
                                                goto out;
                                        }
                                        *sbp = statbuf;
                                        if (kq != -1)
                                                action = ADD_EVENTS;
                                } else if (kq != -1)
                                        action = USE_KQUEUE;
                        }
                        break;
                }
        }
out:
        if (fflag && kq != -1)
                close(kq);
}

/*
 * rlines -- display the last offset lines of the file.
 *
 * Non-zero return means than a (non-fatal) error occurred.
 */
static int
rlines(FILE *fp, off_t off, struct stat *sbp)
{
        off_t file_size;
        off_t file_remaining;
        char *p = NULL;
        char *start = NULL;
        off_t mmap_size;
        off_t mmap_offset;
        off_t mmap_remaining = 0;

#define MMAP_MAXSIZE  (10 * 1024 * 1024)

        if (!(file_size = sbp->st_size))
                return 0;
        file_remaining = file_size;

        if (file_remaining > MMAP_MAXSIZE) {
                mmap_size = MMAP_MAXSIZE;
                mmap_offset = file_remaining - MMAP_MAXSIZE;
        } else {
                mmap_size = file_remaining;
                mmap_offset = 0;
        }

        while (off) {
                start = mmap(NULL, (size_t)mmap_size, PROT_READ,
                             MAP_FILE|MAP_SHARED, fileno(fp), mmap_offset);
                if (start == MAP_FAILED) {
                        xerr(0, "%s", fname);
                        return 1;
                }

                mmap_remaining = mmap_size;
                /* Last char is special, ignore whether newline or not. */
                for (p = start + mmap_remaining - 1 ; --mmap_remaining ; )
                        if (*--p == '\n' && !--off) {
                                ++p;
                                break;
                        }

                file_remaining -= mmap_size - mmap_remaining;

                if (off == 0)
                        break;

                if (file_remaining == 0)
                        break;

                if (munmap(start, mmap_size)) {
                        xerr(0, "%s", fname);
                        return 1;
                }

                if (mmap_offset >= MMAP_MAXSIZE) {
                        mmap_offset -= MMAP_MAXSIZE;
                } else {
                        mmap_offset = 0;
                        mmap_size = file_remaining;
                }
        }

        /*
         * Output the (perhaps partial) data in this mmap'd block.
         */
        WR(p, mmap_size - mmap_remaining);
        file_remaining += mmap_size - mmap_remaining;
        if (munmap(start, mmap_size)) {
                xerr(0, "%s", fname);
                return 1;
        }

        /*
         * Set the file pointer to reflect the length displayed.
         * This will cause the caller to redisplay the data if/when
         * needed.
         */
        if (fseeko(fp, file_remaining, SEEK_SET) == -1) {
                ierr();
                return 1;
        }
        return 0;
}