priv: Refactor kern_mknod and narrow privileges

[dragonfly.git] / sbin / dump / tape.c

/*-
 * Copyright (c) 1980, 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. 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.
 *
 * @(#)tape.c   8.4 (Berkeley) 5/1/95
 * $FreeBSD: src/sbin/dump/tape.c,v 1.12.2.3 2002/02/23 22:32:51 iedowse Exp $
 * $DragonFly: src/sbin/dump/tape.c,v 1.14 2005/08/28 04:35:12 dillon Exp $
 */

#include <sys/param.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <sys/stat.h>
#ifdef sunos
#include <sys/vnode.h>

#include <ufs/fs.h>
#include <ufs/inode.h>
#else
#include <vfs/ufs/dinode.h>
#include <vfs/ufs/fs.h>
#endif

#include <protocols/dumprestore.h>

#include <errno.h>
#include <fcntl.h>
#include <setjmp.h>
#include <signal.h>
#include <stdio.h>
#ifdef __STDC__
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#else
int     write(), read();
#endif

#include "dump.h"

static int writesize;           /* size of malloc()ed buffer for tape */
static long lastspclrec = -1;   /* tape block number of last written header */
static int trecno = 0;          /* next record to write in current block */
static long blocksthisvol;              /* number of blocks on current output file */
static const char       *nexttape;
static int tapeno = 0;  /* current tape number */

static int      atomic_read(int, void *, int);
static int      atomic_write(int, const void *, int);
#ifdef WRITEDEBUG
static void     doslave(int, int);
#else
static void     doslave(int);
#endif
static void     enslave(void);
static void     flushtape(void);
static void     killall(void);
static void     rollforward(void);

/*
 * Concurrent dump mods (Caltech) - disk block reading and tape writing
 * are exported to several slave processes.  While one slave writes the
 * tape, the others read disk blocks; they pass control of the tape in
 * a ring via signals. The parent process traverses the filesystem and
 * sends writeheader()'s and lists of daddr's to the slaves via pipes.
 * The following structure defines the instruction packets sent to slaves.
 */
struct req {
        daddr_t dblk;
        int count;
};
int reqsiz;

#define SLAVES 3                /* 1 slave writing, 1 reading, 1 for slack */
struct slave {
        int tapea;              /* header number at start of this chunk */
        int count;              /* count to next header (used for TS_TAPE */
                                /* after EOT) */
        int inode;              /* inode that we are currently dealing with */
        int fd;                 /* FD for this slave */
        int pid;                /* PID for this slave */
        int sent;               /* 1 == we've sent this slave requests */
        int firstrec;           /* record number of this block */
        char (*tblock)[TP_BSIZE]; /* buffer for data blocks */
        struct req *req;        /* buffer for requests */
} slaves[SLAVES+1];
struct slave *slp;

char    (*nextblock)[TP_BSIZE];

int master;             /* pid of master, for sending error signals */
int tenths;             /* length of tape used per block written */
static int caught;      /* have we caught the signal to proceed? */
static int ready;       /* have we reached the lock point without having */
                        /* received the SIGUSR2 signal from the prev slave? */
static jmp_buf jmpbuf;  /* where to jump to if we are ready when the */
                        /* SIGUSR2 arrives from the previous slave */

int
alloctape(void)
{
        int pgoff = getpagesize() - 1;
        char *buf;
        int i;

        writesize = ntrec * TP_BSIZE;
        reqsiz = (ntrec + 1) * sizeof(struct req);
        /*
         * CDC 92181's and 92185's make 0.8" gaps in 1600-bpi start/stop mode
         * (see DEC TU80 User's Guide).  The shorter gaps of 6250-bpi require
         * repositioning after stopping, i.e, streaming mode, where the gap is
         * variable, 0.30" to 0.45".  The gap is maximal when the tape stops.
         */
        if (blocksperfile == 0 && !unlimited)
                tenths = writesize / density +
                    (cartridge ? 16 : density == 625 ? 5 : 8);
        /*
         * Allocate tape buffer contiguous with the array of instruction
         * packets, so flushtape() can write them together with one write().
         * Align tape buffer on page boundary to speed up tape write().
         */
        for (i = 0; i <= SLAVES; i++) {
                buf = (char *)
                    malloc((unsigned)(reqsiz + writesize + pgoff + TP_BSIZE));
                if (buf == NULL)
                        return(0);
                slaves[i].tblock = (char (*)[TP_BSIZE])
                    (((long)&buf[ntrec + 1] + pgoff) &~ pgoff);
                slaves[i].req = (struct req *)slaves[i].tblock - ntrec - 1;
        }
        slp = &slaves[0];
        slp->count = 1;
        slp->tapea = 0;
        slp->firstrec = 0;
        nextblock = slp->tblock;
        return(1);
}

void
writerec(const void *dp, int isspcl)
{

        slp->req[trecno].dblk = (daddr_t)0;
        slp->req[trecno].count = 1;
        bcopy(dp, *(nextblock)++, sizeof (union u_spcl));
        if (isspcl)
                lastspclrec = spcl.c_tapea;
        trecno++;
        spcl.c_tapea++;
        if (trecno >= ntrec)
                flushtape();
}

void
dumpblock(daddr_t blkno, int size)
{
        int avail, tpblks, dblkno;

        dblkno = fsbtodb(sblock, blkno);
        tpblks = size >> tp_bshift;
        while ((avail = MIN(tpblks, ntrec - trecno)) > 0) {
                slp->req[trecno].dblk = dblkno;
                slp->req[trecno].count = avail;
                trecno += avail;
                spcl.c_tapea += avail;
                if (trecno >= ntrec)
                        flushtape();
                dblkno += avail << (tp_bshift - dev_bshift);
                tpblks -= avail;
        }
}

int     nogripe = 0;

static void
tperror(int signo __unused)
{

        if (pipeout) {
                msg("write error on %s\n", tape);
                quit("Cannot recover\n");
                /* NOTREACHED */
        }
        msg("write error %ld blocks into volume %d\n", blocksthisvol, tapeno);
        broadcast("DUMP WRITE ERROR!\n");
        if (!query("Do you want to restart?"))
                dumpabort(0);
        msg("Closing this volume.  Prepare to restart with new media;\n");
        msg("this dump volume will be rewritten.\n");
        killall();
        nogripe = 1;
        close_rewind();
        Exit(X_REWRITE);
}

static void
sigpipe(int signo __unused)
{

        quit("Broken pipe\n");
}

static void
flushtape(void)
{
        int i, blks, got;
        long lastfirstrec;

        int siz = (char *)nextblock - (char *)slp->req;

        slp->req[trecno].count = 0;                     /* Sentinel */

        if (atomic_write(slp->fd, slp->req, siz) != siz)
                quit("error writing command pipe: %s\n", strerror(errno));
        slp->sent = 1; /* we sent a request, read the response later */

        lastfirstrec = slp->firstrec;

        if (++slp >= &slaves[SLAVES])
                slp = &slaves[0];

        /* Read results back from next slave */
        if (slp->sent) {
                if (atomic_read(slp->fd, &got, sizeof got)
                    != sizeof got) {
                        perror("  DUMP: error reading command pipe in master");
                        dumpabort(0);
                }
                slp->sent = 0;

                /* Check for end of tape */
                if (got < writesize) {
                        msg("End of tape detected\n");

                        /*
                         * Drain the results, don't care what the values were.
                         * If we read them here then trewind won't...
                         */
                        for (i = 0; i < SLAVES; i++) {
                                if (slaves[i].sent) {
                                        if (atomic_read(slaves[i].fd,
                                            &got, sizeof got)
                                            != sizeof got) {
                                                perror("  DUMP: error reading command pipe in master");
                                                dumpabort(0);
                                        }
                                        slaves[i].sent = 0;
                                }
                        }

                        close_rewind();
                        rollforward();
                        return;
                }
        }

        blks = 0;
        if (spcl.c_type != TS_END) {
                for (i = 0; i < spcl.c_count; i++)
                        if (spcl.c_addr[i] != 0)
                                blks++;
        }
        slp->count = lastspclrec + blks + 1 - spcl.c_tapea;
        slp->tapea = spcl.c_tapea;
        slp->firstrec = lastfirstrec + ntrec;
        slp->inode = curino;
        nextblock = slp->tblock;
        trecno = 0;
        asize += tenths;
        blockswritten += ntrec;
        blocksthisvol += ntrec;
        if (!pipeout && !unlimited && (blocksperfile ?
            (blocksthisvol >= blocksperfile) : (asize > tsize))) {
                close_rewind();
                startnewtape(0);
        }
        timeest();
}

void
trewind(void)
{
        struct stat sb;
        int f;
        int got;

        for (f = 0; f < SLAVES; f++) {
                /*
                 * Drain the results, but unlike EOT we DO (or should) care
                 * what the return values were, since if we detect EOT after
                 * we think we've written the last blocks to the tape anyway,
                 * we have to replay those blocks with rollforward.
                 *
                 * fixme: punt for now.
                 */
                if (slaves[f].sent) {
                        if (atomic_read(slaves[f].fd, &got, sizeof got)
                            != sizeof got) {
                                perror("  DUMP: error reading command pipe in master");
                                dumpabort(0);
                        }
                        slaves[f].sent = 0;
                        if (got != writesize) {
                                msg("EOT detected in last 2 tape records!\n");
                                msg("Use a longer tape, decrease the size estimate\n");
                                quit("or use no size estimate at all.\n");
                        }
                }
                close(slaves[f].fd);
        }
        while (wait(NULL) >= 0) /* wait for any signals from slaves */
                /* void */;

        if (pipeout)
                return;

        msg("Closing %s\n", tape);

#ifdef RDUMP
        if (host) {
                rmtclose();
                while (rmtopen(tape, 0) < 0)
                        sleep(10);
                rmtclose();
                return;
        }
#endif
        if (fstat(tapefd, &sb) == 0 && S_ISFIFO(sb.st_mode)) {
                close(tapefd);
                return;
        }
        close(tapefd);
        while ((f = open(tape, 0)) < 0)
                sleep (10);
        close(f);
}

void
close_rewind(void)
{
        time_t tstart_changevol, tend_changevol;

        trewind();
        if (nexttape)
                return;
        time((time_t *)&(tstart_changevol));
        if (!nogripe) {
                msg("Change Volumes: Mount volume #%d\n", tapeno+1);
                broadcast("CHANGE DUMP VOLUMES!\a\a\n");
        }
        while (!query("Is the new volume mounted and ready to go?"))
                if (query("Do you want to abort?")) {
                        dumpabort(0);
                        /*NOTREACHED*/
                }
        time((time_t *)&(tend_changevol));
        if ((tstart_changevol != (time_t)-1) && (tend_changevol != (time_t)-1))
                tstart_writing += (tend_changevol - tstart_changevol);
}

void
rollforward(void)
{
        struct req *p, *q, *prev;
        struct slave *tslp;
        int i, size, savedtapea, got;
        union u_spcl *ntb, *otb;
        tslp = &slaves[SLAVES];
        ntb = (union u_spcl *)tslp->tblock[1];

        /*
         * Each of the N slaves should have requests that need to
         * be replayed on the next tape.  Use the extra slave buffers
         * (slaves[SLAVES]) to construct request lists to be sent to
         * each slave in turn.
         */
        for (i = 0; i < SLAVES; i++) {
                q = &tslp->req[1];
                otb = (union u_spcl *)slp->tblock;

                /*
                 * For each request in the current slave, copy it to tslp.
                 */

                prev = NULL;
                for (p = slp->req; p->count > 0; p += p->count) {
                        *q = *p;
                        if (p->dblk == 0)
                                *ntb++ = *otb++; /* copy the datablock also */
                        prev = q;
                        q += q->count;
                }
                if (prev == NULL)
                        quit("rollforward: protocol botch");
                if (prev->dblk != 0)
                        prev->count -= 1;
                else
                        ntb--;
                q -= 1;
                q->count = 0;
                q = &tslp->req[0];
                if (i == 0) {
                        q->dblk = 0;
                        q->count = 1;
                        trecno = 0;
                        nextblock = tslp->tblock;
                        savedtapea = spcl.c_tapea;
                        spcl.c_tapea = slp->tapea;
                        startnewtape(0);
                        spcl.c_tapea = savedtapea;
                        lastspclrec = savedtapea - 1;
                }
                size = (char *)ntb - (char *)q;
                if (atomic_write(slp->fd, q, size) != size) {
                        perror("  DUMP: error writing command pipe");
                        dumpabort(0);
                }
                slp->sent = 1;
                if (++slp >= &slaves[SLAVES])
                        slp = &slaves[0];

                q->count = 1;

                if (prev->dblk != 0) {
                        /*
                         * If the last one was a disk block, make the
                         * first of this one be the last bit of that disk
                         * block...
                         */
                        q->dblk = prev->dblk +
                                prev->count * (TP_BSIZE / DEV_BSIZE);
                        ntb = (union u_spcl *)tslp->tblock;
                } else {
                        /*
                         * It wasn't a disk block.  Copy the data to its
                         * new location in the buffer.
                         */
                        q->dblk = 0;
                        *((union u_spcl *)tslp->tblock) = *ntb;
                        ntb = (union u_spcl *)tslp->tblock[1];
                }
        }
        slp->req[0] = *q;
        nextblock = slp->tblock;
        if (q->dblk == 0)
                nextblock++;
        trecno = 1;

        /*
         * Clear the first slaves' response.  One hopes that it
         * worked ok, otherwise the tape is much too short!
         */
        if (slp->sent) {
                if (atomic_read(slp->fd, &got, sizeof got)
                    != sizeof got) {
                        perror("  DUMP: error reading command pipe in master");
                        dumpabort(0);
                }
                slp->sent = 0;

                if (got != writesize) {
                        quit("EOT detected at start of the tape!\n");
                }
        }
}

/*
 * We implement taking and restoring checkpoints on the tape level.
 * When each tape is opened, a new process is created by forking; this
 * saves all of the necessary context in the parent.  The child
 * continues the dump; the parent waits around, saving the context.
 * If the child returns X_REWRITE, then it had problems writing that tape;
 * this causes the parent to fork again, duplicating the context, and
 * everything continues as if nothing had happened.
 */
void
startnewtape(int top)
{
        int     parentpid;
        int     childpid;
        int     status;
        int     wait_pid;
        char    *p;
#ifdef sunos
        void    (*interrupt_save)();
#else
        sig_t   interrupt_save;
#endif

        interrupt_save = signal(SIGINT, SIG_IGN);
        parentpid = getpid();

restore_check_point:
        signal(SIGINT, interrupt_save);
        /*
         *      All signals are inherited...
         */
        setproctitle(NULL);     /* Restore the proctitle. */
        childpid = fork();
        if (childpid < 0) {
                msg("Context save fork fails in parent %d\n", parentpid);
                Exit(X_ABORT);
        }
        if (childpid != 0) {
                /*
                 *      PARENT:
                 *      save the context by waiting
                 *      until the child doing all of the work returns.
                 *      don't catch the interrupt
                 */
                signal(SIGINT, SIG_IGN);
#ifdef TDEBUG
                msg("Tape: %d; parent process: %d child process %d\n",
                        tapeno+1, parentpid, childpid);
#endif /* TDEBUG */
                while ((wait_pid = wait(&status)) != childpid)
                        msg("Parent %d waiting for child %d has another child %d return\n",
                                parentpid, childpid, wait_pid);
                if (status & 0xFF) {
                        msg("Child %d returns LOB status %o\n",
                                childpid, status&0xFF);
                }
                status = (status >> 8) & 0xFF;
#ifdef TDEBUG
                switch(status) {
                        case X_FINOK:
                                msg("Child %d finishes X_FINOK\n", childpid);
                                break;
                        case X_ABORT:
                                msg("Child %d finishes X_ABORT\n", childpid);
                                break;
                        case X_REWRITE:
                                msg("Child %d finishes X_REWRITE\n", childpid);
                                break;
                        default:
                                msg("Child %d finishes unknown %d\n",
                                        childpid, status);
                                break;
                }
#endif /* TDEBUG */
                switch(status) {
                        case X_FINOK:
                                Exit(X_FINOK);
                        case X_ABORT:
                                Exit(X_ABORT);
                        case X_REWRITE:
                                goto restore_check_point;
                        default:
                                msg("Bad return code from dump: %d\n", status);
                                Exit(X_ABORT);
                }
                /*NOTREACHED*/
        } else {        /* we are the child; just continue */
#ifdef TDEBUG
                sleep(4);       /* allow time for parent's message to get out */
                msg("Child on Tape %d has parent %d, my pid = %d\n",
                        tapeno+1, parentpid, getpid());
#endif /* TDEBUG */
                /*
                 * If we have a name like "/dev/rmt0,/dev/rmt1",
                 * use the name before the comma first, and save
                 * the remaining names for subsequent volumes.
                 */
                tapeno++;               /* current tape sequence */
                if (nexttape || strchr(tape, ',')) {
                        if (nexttape && *nexttape)
                                tape = nexttape;
                        if ((p = strchr(tape, ',')) != NULL) {
                                *p = '\0';
                                nexttape = p + 1;
                        } else
                                nexttape = NULL;
                        msg("Dumping volume %d on %s\n", tapeno, tape);
                }
#ifdef RDUMP
                while ((tapefd = (host ? rmtopen(tape, 2) :
                        pipeout ? 1 : open(tape, O_WRONLY|O_CREAT, 0666))) < 0)
#else
                while ((tapefd = (pipeout ? 1 :
                                  open(tape, O_WRONLY|O_CREAT, 0666))) < 0)
#endif
                    {
                        msg("Cannot open output \"%s\".\n", tape);
                        if (!query("Do you want to retry the open?"))
                                dumpabort(0);
                }

                enslave();  /* Share open tape file descriptor with slaves */
                signal(SIGINFO, infosch);

                asize = 0;
                blocksthisvol = 0;
                if (top)
                        newtape++;              /* new tape signal */
                spcl.c_count = slp->count;
                /*
                 * measure firstrec in TP_BSIZE units since restore doesn't
                 * know the correct ntrec value...
                 */
                spcl.c_firstrec = slp->firstrec;
                spcl.c_volume++;
                spcl.c_type = TS_TAPE;
                spcl.c_flags |= DR_NEWHEADER;
                writeheader((ufs1_ino_t)slp->inode);
                spcl.c_flags &=~ DR_NEWHEADER;
                if (tapeno > 1)
                        msg("Volume %d begins with blocks from inode %d\n",
                                tapeno, slp->inode);
        }
}

void
dumpabort(int signo __unused)
{

        if (master != 0 && master != getpid())
                /* Signals master to call dumpabort */
                kill(master, SIGTERM);
        else {
                killall();
                msg("The ENTIRE dump is aborted.\n");
        }
#ifdef RDUMP
        rmtclose();
#endif
        Exit(X_ABORT);
}

void
Exit(int status)
{

#ifdef TDEBUG
        msg("pid = %d exits with status %d\n", getpid(), status);
#endif /* TDEBUG */
        exit(status);
}

/*
 * proceed - handler for SIGUSR2, used to synchronize IO between the slaves.
 */
static void
proceed(int signo __unused)
{

        if (ready)
                longjmp(jmpbuf, 1);
        caught++;
}

static void
enslave(void)
{
        int cmd[2];
        int i, j;

        master = getpid();

        signal(SIGTERM, dumpabort);  /* Slave sends SIGTERM on dumpabort() */
        signal(SIGPIPE, sigpipe);
        signal(SIGUSR1, tperror);    /* Slave sends SIGUSR1 on tape errors */
        signal(SIGUSR2, proceed);    /* Slave sends SIGUSR2 to next slave */

        for (i = 0; i < SLAVES; i++) {
                if (i == slp - &slaves[0]) {
                        caught = 1;
                } else {
                        caught = 0;
                }

                if (socketpair(AF_UNIX, SOCK_STREAM, 0, cmd) < 0 ||
                    (slaves[i].pid = fork()) < 0)
                        quit("too many slaves, %d (recompile smaller): %s\n",
                            i, strerror(errno));

                slaves[i].fd = cmd[1];
                slaves[i].sent = 0;
                if (slaves[i].pid == 0) {           /* Slave starts up here */
                        for (j = 0; j <= i; j++)
                                close(slaves[j].fd);
                        signal(SIGINT, SIG_IGN);    /* Master handles this */
#ifdef WRITEDEBUG
                        doslave(cmd[0], i);
#else
                        doslave(cmd[0]);
#endif
                        Exit(X_FINOK);
                }
        }

        for (i = 0; i < SLAVES; i++)
                atomic_write(slaves[i].fd, &slaves[(i + 1) % SLAVES].pid,
                    sizeof slaves[0].pid);

        master = 0;
}

void
killall(void)
{
        int i;

        for (i = 0; i < SLAVES; i++)
                if (slaves[i].pid > 0) {
                        kill(slaves[i].pid, SIGKILL);
                        slaves[i].sent = 0;
                }
}

/*
 * Synchronization - each process has a lockfile, and shares file
 * descriptors to the following process's lockfile.  When our write
 * completes, we release our lock on the following process's lock-
 * file, allowing the following process to lock it and proceed. We
 * get the lock back for the next cycle by swapping descriptors.
 */
static void
#ifdef WRITEDEBUG
doslave(int cmd, int slave_number)
#else
doslave(int cmd)
#endif
{
        int nread;
        int nextslave, size, wrote, eot_count;

        /*
         * Need our own seek pointer.
         */
        close(diskfd);
        if ((diskfd = open(disk, O_RDONLY)) < 0)
                quit("slave couldn't reopen disk: %s\n", strerror(errno));

        /*
         * Need the pid of the next slave in the loop...
         */
        if ((nread = atomic_read(cmd, &nextslave, sizeof nextslave))
            != sizeof nextslave) {
                quit("master/slave protocol botched - didn't get pid of next slave.\n");
        }

        /*
         * Get list of blocks to dump, read the blocks into tape buffer
         */
        while ((nread = atomic_read(cmd, slp->req, reqsiz)) == reqsiz) {
                struct req *p = slp->req;

                for (trecno = 0; trecno < ntrec;
                     trecno += p->count, p += p->count) {
                        if (p->dblk) {
                                bread(p->dblk, slp->tblock[trecno],
                                        p->count * TP_BSIZE);
                        } else {
                                if (p->count != 1 ||
                                    atomic_read(cmd, slp->tblock[trecno],
                                    TP_BSIZE) != TP_BSIZE)
                                       quit("master/slave protocol botched.\n");
                        }
                }
                if (setjmp(jmpbuf) == 0) {
                        ready = 1;
                        if (!caught)
                                pause();
                }
                ready = 0;
                caught = 0;

                /* Try to write the data... */
                eot_count = 0;
                size = 0;
                wrote = 0;

                while (eot_count < 10 && size < writesize) {
#ifdef RDUMP
                        if (host)
                                wrote = rmtwrite(slp->tblock[0]+size,
                                    writesize-size);
                        else
#endif
                                wrote = write(tapefd, slp->tblock[0]+size,
                                    writesize-size);
#ifdef WRITEDEBUG
                        printf("slave %d wrote %d\n", slave_number, wrote);
#endif
                        if (wrote < 0)
                                break;
                        if (wrote == 0)
                                eot_count++;
                        size += wrote;
                }

#ifdef WRITEDEBUG
                if (size != writesize)
                 printf("slave %d only wrote %d out of %d bytes and gave up.\n",
                     slave_number, size, writesize);
#endif

                /*
                 * Handle ENOSPC as an EOT condition.
                 */
                if (wrote < 0 && errno == ENOSPC) {
                        wrote = 0;
                        eot_count++;
                }

                if (eot_count > 0)
                        size = 0;

                if (wrote < 0) {
                        kill(master, SIGUSR1);
                        for (;;)
                                sigpause(0);
                } else {
                        /*
                         * pass size of write back to master
                         * (for EOT handling)
                         */
                        atomic_write(cmd, &size, sizeof size);
                }

                /*
                 * If partial write, don't want next slave to go.
                 * Also jolts him awake.
                 */
                kill(nextslave, SIGUSR2);
        }
        if (nread != 0)
                quit("error reading command pipe: %s\n", strerror(errno));
}

static int
atomic_read(int fd, void *buf, int count)
{
        int got, need = count;

        while ((got = read(fd, buf, need)) > 0 && (need -= got) > 0)
                buf = (uint8_t *)buf + got;
        return (got < 0 ? got : count - need);
}

static int
atomic_write(int fd, const void *buf, int count)
{
        int got, need = count;

        while ((got = write(fd, buf, need)) > 0 && (need -= got) > 0)
                buf = (const uint8_t *)buf + got;
        return (got < 0 ? got : count - need);
}