- Handle the new incremental-recursion mode.

[rsync/rsync.git] / io.c

/*
 * Socket and pipe I/O utilities used in rsync.
 *
 * Copyright (C) 1996-2001 Andrew Tridgell
 * Copyright (C) 1996 Paul Mackerras
 * Copyright (C) 2001, 2002 Martin Pool <mbp@samba.org>
 * Copyright (C) 2003, 2004, 2005, 2006 Wayne Davison
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street - Fifth Floor, Boston, MA 02110-1301, USA.
 */

/* Rsync provides its own multiplexing system, which is used to send
 * stderr and stdout over a single socket.
 *
 * For historical reasons this is off during the start of the
 * connection, but it's switched on quite early using
 * io_start_multiplex_out() and io_start_multiplex_in(). */

#include "rsync.h"

/** If no timeout is specified then use a 60 second select timeout */
#define SELECT_TIMEOUT 60

extern int bwlimit;
extern size_t bwlimit_writemax;
extern int io_timeout;
extern int allowed_lull;
extern int am_server;
extern int am_daemon;
extern int am_sender;
extern int am_generator;
extern int incremental;
extern int io_error;
extern int eol_nulls;
extern int flist_eof;
extern int read_batch;
extern int csum_length;
extern int checksum_seed;
extern int protocol_version;
extern int remove_source_files;
extern int preserve_hard_links;
extern char *filesfrom_host;
extern struct stats stats;
extern struct file_list *cur_flist, *first_flist;

const char phase_unknown[] = "unknown";
int ignore_timeout = 0;
int batch_fd = -1;
int done_cnt = 0;

/* Ignore an EOF error if non-zero. See whine_about_eof(). */
int kluge_around_eof = 0;

int msg_fd_in = -1;
int msg_fd_out = -1;
int sock_f_in = -1;
int sock_f_out = -1;

static int iobuf_f_in = -1;
static char *iobuf_in;
static size_t iobuf_in_siz;
static size_t iobuf_in_ndx;
static size_t iobuf_in_remaining;

static int iobuf_f_out = -1;
static char *iobuf_out;
static int iobuf_out_cnt;

int flist_forward_from = -1;

static int io_multiplexing_out;
static int io_multiplexing_in;
static time_t last_io_in;
static time_t last_io_out;
static int no_flush;

static int write_batch_monitor_in = -1;
static int write_batch_monitor_out = -1;

static int io_filesfrom_f_in = -1;
static int io_filesfrom_f_out = -1;
static char io_filesfrom_buf[2048];
static char *io_filesfrom_bp;
static char io_filesfrom_lastchar;
static int io_filesfrom_buflen;
static int defer_forwarding_messages = 0;
static int select_timeout = SELECT_TIMEOUT;
static int active_filecnt = 0;
static OFF_T active_bytecnt = 0;

static char int_byte_cnt[64] = {
        3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, /* (00 - 3F)/4 */
        3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, /* (40 - 7F)/4 */
        4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, /* (80 - BF)/4 */
        5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 8, 9, /* (C0 - FF)/4 */
};

static int readfd_unbuffered(int fd, char *buf, size_t len);
static void writefd(int fd, const char *buf, size_t len);
static void writefd_unbuffered(int fd, const char *buf, size_t len);
static void decrement_active_files(int ndx);
static void decrement_flist_in_progress(int ndx, int redo);

struct flist_ndx_item {
        struct flist_ndx_item *next;
        int ndx;
};

struct flist_ndx_list {
        struct flist_ndx_item *head, *tail;
};

static struct flist_ndx_list redo_list, hlink_list;

struct msg_list_item {
        struct msg_list_item *next;
        int len;
        char buf[1];
};

struct msg_list {
        struct msg_list_item *head, *tail;
};

static struct msg_list msg2genr, msg2sndr;

static void flist_ndx_push(struct flist_ndx_list *lp, int ndx)
{
        struct flist_ndx_item *item;

        if (!(item = new(struct flist_ndx_item)))
                out_of_memory("flist_ndx_push");
        item->next = NULL;
        item->ndx = ndx;
        if (lp->tail)
                lp->tail->next = item;
        else
                lp->head = item;
        lp->tail = item;
}

static int flist_ndx_pop(struct flist_ndx_list *lp)
{
        struct flist_ndx_item *next;
        int ndx;

        if (!lp->head)
                return -1;

        ndx = lp->head->ndx;
        next = lp->head->next;
        free(lp->head);
        lp->head = next;
        if (!next)
                lp->tail = NULL;

        return ndx;
}

static void check_timeout(void)
{
        time_t t;

        if (!io_timeout || ignore_timeout)
                return;

        if (!last_io_in) {
                last_io_in = time(NULL);
                return;
        }

        t = time(NULL);

        if (t - last_io_in >= io_timeout) {
                if (!am_server && !am_daemon) {
                        rprintf(FERROR, "io timeout after %d seconds -- exiting\n",
                                (int)(t-last_io_in));
                }
                exit_cleanup(RERR_TIMEOUT);
        }
}

/* Note the fds used for the main socket (which might really be a pipe
 * for a local transfer, but we can ignore that). */
void io_set_sock_fds(int f_in, int f_out)
{
        sock_f_in = f_in;
        sock_f_out = f_out;
}

void set_io_timeout(int secs)
{
        io_timeout = secs;

        if (!io_timeout || io_timeout > SELECT_TIMEOUT)
                select_timeout = SELECT_TIMEOUT;
        else
                select_timeout = io_timeout;

        allowed_lull = read_batch ? 0 : (io_timeout + 1) / 2;
}

/* Setup the fd used to receive MSG_* messages.  Only needed during the
 * early stages of being a local sender (up through the sending of the
 * file list) or when we're the generator (to fetch the messages from
 * the receiver). */
void set_msg_fd_in(int fd)
{
        msg_fd_in = fd;
}

/* Setup the fd used to send our MSG_* messages.  Only needed when
 * we're the receiver (to send our messages to the generator). */
void set_msg_fd_out(int fd)
{
        msg_fd_out = fd;
        set_nonblocking(msg_fd_out);
}

/* Add a message to the pending MSG_* list. */
static void msg_list_add(struct msg_list *lst, int code, const char *buf, int len)
{
        struct msg_list_item *m;
        int sz = len + 4 + sizeof m[0] - 1;

        if (!(m = (struct msg_list_item *)new_array(char, sz)))
                out_of_memory("msg_list_add");
        m->next = NULL;
        m->len = len + 4;
        SIVAL(m->buf, 0, ((code+MPLEX_BASE)<<24) | len);
        memcpy(m->buf + 4, buf, len);
        if (lst->tail)
                lst->tail->next = m;
        else
                lst->head = m;
        lst->tail = m;
}

/* Read a message from the MSG_* fd and handle it.  This is called either
 * during the early stages of being a local sender (up through the sending
 * of the file list) or when we're the generator (to fetch the messages
 * from the receiver). */
static void read_msg_fd(void)
{
        char buf[2048];
        size_t n;
        struct file_list *flist;
        int fd = msg_fd_in;
        int tag, len;

        /* Temporarily disable msg_fd_in.  This is needed to avoid looping back
         * to this routine from writefd_unbuffered(). */
        msg_fd_in = -1;

        readfd_unbuffered(fd, buf, 4);
        tag = IVAL(buf, 0);

        len = tag & 0xFFFFFF;
        tag = (tag >> 24) - MPLEX_BASE;

        switch (tag) {
        case MSG_DONE:
                if (len != 0 || !am_generator) {
                  invalid_msg:
                        rprintf(FERROR, "invalid message %d:%d [%s%s]\n",
                                tag, len, who_am_i(),
                                incremental ? "/incremental" : "");
                        exit_cleanup(RERR_STREAMIO);
                }
                done_cnt++;
                break;
        case MSG_REDO:
                if (len != 4 || !am_generator)
                        goto invalid_msg;
                readfd_unbuffered(fd, buf, 4);
                if (remove_source_files)
                        decrement_active_files(IVAL(buf,0));
                flist_ndx_push(&redo_list, IVAL(buf,0));
                if (incremental)
                        decrement_flist_in_progress(IVAL(buf,0), 1);
                break;
        case MSG_FLIST:
                if (len != 4 || !am_generator || !incremental)
                        goto invalid_msg;
                readfd_unbuffered(fd, buf, 4);
                /* Read extra file list from receiver. */
                assert(iobuf_in != NULL);
                assert(iobuf_f_in == fd);
                flist = recv_file_list(fd);
                flist->parent_ndx = IVAL(buf,0);
                break;
        case MSG_FLIST_EOF:
                if (len != 0 || !am_generator || !incremental)
                        goto invalid_msg;
                flist_eof = 1;
                break;
        case MSG_DELETED:
                if (len >= (int)sizeof buf || !am_generator)
                        goto invalid_msg;
                readfd_unbuffered(fd, buf, len);
                send_msg(MSG_DELETED, buf, len);
                break;
        case MSG_SUCCESS:
                if (len != 4 || !am_generator)
                        goto invalid_msg;
                readfd_unbuffered(fd, buf, len);
                if (remove_source_files) {
                        decrement_active_files(IVAL(buf,0));
                        send_msg(MSG_SUCCESS, buf, len);
                }
                if (preserve_hard_links)
                        flist_ndx_push(&hlink_list, IVAL(buf,0));
                if (incremental)
                        decrement_flist_in_progress(IVAL(buf,0), 0);
                break;
        case MSG_NO_SEND:
                if (len != 4 || !am_generator)
                        goto invalid_msg;
                readfd_unbuffered(fd, buf, len);
                if (incremental)
                        decrement_flist_in_progress(IVAL(buf,0), 0);
                break;
        case MSG_SOCKERR:
        case MSG_CLIENT:
                if (!am_generator)
                        goto invalid_msg;
                if (tag == MSG_SOCKERR)
                        io_end_multiplex_out();
                /* FALL THROUGH */
        case MSG_INFO:
        case MSG_ERROR:
        case MSG_LOG:
                while (len) {
                        n = len;
                        if (n >= sizeof buf)
                                n = sizeof buf - 1;
                        readfd_unbuffered(fd, buf, n);
                        rwrite((enum logcode)tag, buf, n);
                        len -= n;
                }
                break;
        default:
                rprintf(FERROR, "unknown message %d:%d [%s]\n",
                        tag, len, who_am_i());
                exit_cleanup(RERR_STREAMIO);
        }

        msg_fd_in = fd;
}

/* This is used by the generator to limit how many file transfers can
 * be active at once when --remove-source-files is specified.  Without
 * this, sender-side deletions were mostly happening at the end. */
void increment_active_files(int ndx, int itemizing, enum logcode code)
{
        /* TODO: tune these limits? */
        while (active_filecnt >= (active_bytecnt >= 128*1024 ? 10 : 50)) {
#ifdef SUPPORT_HARD_LINKS
                if (hlink_list.head)
                        check_for_finished_hlinks(itemizing, code);
#endif
                read_msg_fd();
        }

        active_filecnt++;
        active_bytecnt += F_LENGTH(cur_flist->files[ndx]);
}

static void decrement_active_files(int ndx)
{
        struct file_list *flist = flist_for_ndx(ndx);
        assert(flist != NULL);
        active_filecnt--;
        active_bytecnt -= F_LENGTH(flist->files[ndx - flist->ndx_start]);
}

static void decrement_flist_in_progress(int ndx, int redo)
{
        struct file_list *flist = cur_flist ? cur_flist : first_flist;

        while (ndx < flist->ndx_start) {
                if (flist == first_flist) {
                  invalid_ndx:
                        rprintf(FERROR,
                                "Invalid file index: %d (%d - %d) [%s]\n",
                                ndx, first_flist->ndx_start,
                                first_flist->prev->ndx_start + first_flist->prev->count - 1,
                                who_am_i());
                        exit_cleanup(RERR_PROTOCOL);
                }
                flist = flist->prev;
        }
        while (ndx >= flist->ndx_start + flist->count) {
                if (!(flist = flist->next))
                        goto invalid_ndx;
        }

        flist->in_progress--;
        if (redo)
                flist->to_redo++;
}

/* Try to push messages off the list onto the wire.  If we leave with more
 * to do, return 0.  On error, return -1.  If everything flushed, return 1.
 * This is only active in the receiver. */
static int msg2genr_flush(void)
{
        if (msg_fd_out < 0 || no_flush)
                return -1;

        no_flush++;
        while (msg2genr.head) {
                struct msg_list_item *m = msg2genr.head;
                writefd(msg_fd_out, m->buf, m->len);
                msg2genr.head = m->next;
                if (!msg2genr.head)
                        msg2genr.tail = NULL;
                free(m);
        }
        if (iobuf_out_cnt) {
                writefd_unbuffered(iobuf_f_out, iobuf_out, iobuf_out_cnt);
                iobuf_out_cnt = 0;
        }
        no_flush--;
        return 1;
}

int send_msg(enum msgcode code, const char *buf, int len)
{
        if (msg_fd_out < 0) {
                if (!defer_forwarding_messages)
                        return io_multiplex_write(code, buf, len);
                if (!io_multiplexing_out)
                        return 0;
                msg_list_add(&msg2sndr, code, buf, len);
                return 1;
        }
        msg_list_add(&msg2genr, code, buf, len);
        msg2genr_flush();
        return 1;
}

void send_msg_int(enum msgcode code, int num)
{
        char numbuf[4];
        SIVAL(numbuf, 0, num);
        send_msg(code, numbuf, 4);
}

void wait_for_receiver(void)
{
        read_msg_fd();
}

int get_redo_num(void)
{
        return flist_ndx_pop(&redo_list);
}

int get_hlink_num(void)
{
        return flist_ndx_pop(&hlink_list);
}

/**
 * When we're the receiver and we have a local --files-from list of names
 * that needs to be sent over the socket to the sender, we have to do two
 * things at the same time: send the sender a list of what files we're
 * processing and read the incoming file+info list from the sender.  We do
 * this by augmenting the read_timeout() function to copy this data.  It
 * uses the io_filesfrom_buf to read a block of data from f_in (when it is
 * ready, since it might be a pipe) and then blast it out f_out (when it
 * is ready to receive more data).
 */
void io_set_filesfrom_fds(int f_in, int f_out)
{
        io_filesfrom_f_in = f_in;
        io_filesfrom_f_out = f_out;
        io_filesfrom_bp = io_filesfrom_buf;
        io_filesfrom_lastchar = '\0';
        io_filesfrom_buflen = 0;
}

/* It's almost always an error to get an EOF when we're trying to read from the
 * network, because the protocol is (for the most part) self-terminating.
 *
 * There is one case for the receiver when it is at the end of the transfer
 * (hanging around reading any keep-alive packets that might come its way): if
 * the sender dies before the generator's kill-signal comes through, we can end
 * up here needing to loop until the kill-signal arrives.  In this situation,
 * kluge_around_eof will be < 0.
 *
 * There is another case for older protocol versions (< 24) where the module
 * listing was not terminated, so we must ignore an EOF error in that case and
 * exit.  In this situation, kluge_around_eof will be > 0. */
static void whine_about_eof(int fd)
{
        if (kluge_around_eof && fd == sock_f_in) {
                int i;
                if (kluge_around_eof > 0)
                        exit_cleanup(0);
                /* If we're still here after 10 seconds, exit with an error. */
                for (i = 10*1000/20; i--; )
                        msleep(20);
        }

        rprintf(FERROR, RSYNC_NAME ": connection unexpectedly closed "
                "(%.0f bytes received so far) [%s]\n",
                (double)stats.total_read, who_am_i());

        exit_cleanup(RERR_STREAMIO);
}

/**
 * Read from a socket with I/O timeout. return the number of bytes
 * read. If no bytes can be read then exit, never return a number <= 0.
 *
 * TODO: If the remote shell connection fails, then current versions
 * actually report an "unexpected EOF" error here.  Since it's a
 * fairly common mistake to try to use rsh when ssh is required, we
 * should trap that: if we fail to read any data at all, we should
 * give a better explanation.  We can tell whether the connection has
 * started by looking e.g. at whether the remote version is known yet.
 */
static int read_timeout(int fd, char *buf, size_t len)
{
        int n, cnt = 0;

        io_flush(NORMAL_FLUSH);

        while (cnt == 0) {
                /* until we manage to read *something* */
                fd_set r_fds, w_fds;
                struct timeval tv;
                int maxfd = fd;
                int count;

                FD_ZERO(&r_fds);
                FD_ZERO(&w_fds);
                FD_SET(fd, &r_fds);
                if (msg2genr.head) {
                        FD_SET(msg_fd_out, &w_fds);
                        if (msg_fd_out > maxfd)
                                maxfd = msg_fd_out;
                }
                if (io_filesfrom_f_out >= 0) {
                        int new_fd;
                        if (io_filesfrom_buflen == 0) {
                                if (io_filesfrom_f_in >= 0) {
                                        FD_SET(io_filesfrom_f_in, &r_fds);
                                        new_fd = io_filesfrom_f_in;
                                } else {
                                        io_filesfrom_f_out = -1;
                                        new_fd = -1;
                                }
                        } else {
                                FD_SET(io_filesfrom_f_out, &w_fds);
                                new_fd = io_filesfrom_f_out;
                        }
                        if (new_fd > maxfd)
                                maxfd = new_fd;
                }

                tv.tv_sec = select_timeout;
                tv.tv_usec = 0;

                errno = 0;

                count = select(maxfd + 1, &r_fds, &w_fds, NULL, &tv);

                if (count <= 0) {
                        if (errno == EBADF)
                                exit_cleanup(RERR_SOCKETIO);
                        check_timeout();
                        continue;
                }

                if (msg2genr.head && FD_ISSET(msg_fd_out, &w_fds))
                        msg2genr_flush();

                if (io_filesfrom_f_out >= 0) {
                        if (io_filesfrom_buflen) {
                                if (FD_ISSET(io_filesfrom_f_out, &w_fds)) {
                                        int l = write(io_filesfrom_f_out,
                                                      io_filesfrom_bp,
                                                      io_filesfrom_buflen);
                                        if (l > 0) {
                                                if (!(io_filesfrom_buflen -= l))
                                                        io_filesfrom_bp = io_filesfrom_buf;
                                                else
                                                        io_filesfrom_bp += l;
                                        } else {
                                                /* XXX should we complain? */
                                                io_filesfrom_f_out = -1;
                                        }
                                }
                        } else if (io_filesfrom_f_in >= 0) {
                                if (FD_ISSET(io_filesfrom_f_in, &r_fds)) {
                                        int l = read(io_filesfrom_f_in,
                                                     io_filesfrom_buf,
                                                     sizeof io_filesfrom_buf);
                                        if (l <= 0) {
                                                /* Send end-of-file marker */
                                                io_filesfrom_buf[0] = '\0';
                                                io_filesfrom_buf[1] = '\0';
                                                io_filesfrom_buflen = io_filesfrom_lastchar? 2 : 1;
                                                io_filesfrom_f_in = -1;
                                        } else {
                                                if (!eol_nulls) {
                                                        char *s = io_filesfrom_buf + l;
                                                        /* Transform CR and/or LF into '\0' */
                                                        while (s-- > io_filesfrom_buf) {
                                                                if (*s == '\n' || *s == '\r')
                                                                        *s = '\0';
                                                        }
                                                }
                                                if (!io_filesfrom_lastchar) {
                                                        /* Last buf ended with a '\0', so don't
                                                         * let this buf start with one. */
                                                        while (l && !*io_filesfrom_bp)
                                                                io_filesfrom_bp++, l--;
                                                }
                                                if (!l)
                                                        io_filesfrom_bp = io_filesfrom_buf;
                                                else {
                                                        char *f = io_filesfrom_bp;
                                                        char *t = f;
                                                        char *eob = f + l;
                                                        /* Eliminate any multi-'\0' runs. */
                                                        while (f != eob) {
                                                                if (!(*t++ = *f++)) {
                                                                        while (f != eob && !*f)
                                                                                f++, l--;
                                                                }
                                                        }
                                                        io_filesfrom_lastchar = f[-1];
                                                }
                                                io_filesfrom_buflen = l;
                                        }
                                }
                        }
                }

                if (!FD_ISSET(fd, &r_fds))
                        continue;

                n = read(fd, buf, len);

                if (n <= 0) {
                        if (n == 0)
                                whine_about_eof(fd); /* Doesn't return. */
                        if (errno == EINTR || errno == EWOULDBLOCK
                            || errno == EAGAIN)
                                continue;

                        /* Don't write errors on a dead socket. */
                        if (fd == sock_f_in) {
                                io_end_multiplex_out();
                                rsyserr(FSOCKERR, errno, "read error");
                        } else
                                rsyserr(FERROR, errno, "read error");
                        exit_cleanup(RERR_STREAMIO);
                }

                buf += n;
                len -= n;
                cnt += n;

                if (fd == sock_f_in && io_timeout)
                        last_io_in = time(NULL);
        }

        return cnt;
}

/**
 * Read a line into the "fname" buffer (which must be at least MAXPATHLEN
 * characters long).
 */
int read_filesfrom_line(int fd, char *fname)
{
        char ch, *s, *eob = fname + MAXPATHLEN - 1;
        int cnt;
        int reading_remotely = filesfrom_host != NULL;
        int nulls = eol_nulls || reading_remotely;

  start:
        s = fname;
        while (1) {
                cnt = read(fd, &ch, 1);
                if (cnt < 0 && (errno == EWOULDBLOCK
                  || errno == EINTR || errno == EAGAIN)) {
                        struct timeval tv;
                        fd_set r_fds, e_fds;
                        FD_ZERO(&r_fds);
                        FD_SET(fd, &r_fds);
                        FD_ZERO(&e_fds);
                        FD_SET(fd, &e_fds);
                        tv.tv_sec = select_timeout;
                        tv.tv_usec = 0;
                        if (!select(fd+1, &r_fds, NULL, &e_fds, &tv))
                                check_timeout();
                        if (FD_ISSET(fd, &e_fds)) {
                                rsyserr(FINFO, errno,
                                        "select exception on fd %d", fd);
                        }
                        continue;
                }
                if (cnt != 1)
                        break;
                if (nulls? !ch : (ch == '\r' || ch == '\n')) {
                        /* Skip empty lines if reading locally. */
                        if (!reading_remotely && s == fname)
                                continue;
                        break;
                }
                if (s < eob)
                        *s++ = ch;
        }
        *s = '\0';

        /* Dump comments. */
        if (*fname == '#' || *fname == ';')
                goto start;

        return s - fname;
}

int io_start_buffering_out(int f_out)
{
        if (iobuf_out) {
                assert(f_out == iobuf_f_out);
                return 0;
        }
        if (!(iobuf_out = new_array(char, IO_BUFFER_SIZE)))
                out_of_memory("io_start_buffering_out");
        iobuf_out_cnt = 0;
        iobuf_f_out = f_out;
        return 1;
}

int io_start_buffering_in(int f_in)
{
        if (iobuf_in) {
                assert(f_in == iobuf_f_in);
                return 0;
        }
        iobuf_in_siz = 2 * IO_BUFFER_SIZE;
        if (!(iobuf_in = new_array(char, iobuf_in_siz)))
                out_of_memory("io_start_buffering_in");
        iobuf_f_in = f_in;
        return 1;
}

void io_end_buffering_in(void)
{
        if (!iobuf_in)
                return;
        free(iobuf_in);
        iobuf_in = NULL;
        iobuf_in_ndx = 0;
        iobuf_in_remaining = 0;
        iobuf_f_in = -1;
}

void io_end_buffering_out(void)
{
        if (!iobuf_out)
                return;
        io_flush(FULL_FLUSH);
        free(iobuf_out);
        iobuf_out = NULL;
        iobuf_f_out = -1;
}

void maybe_flush_socket(void)
{
        if (iobuf_out && iobuf_out_cnt && time(NULL) - last_io_out >= 5)
                io_flush(NORMAL_FLUSH);
}

void maybe_send_keepalive(void)
{
        if (time(NULL) - last_io_out >= allowed_lull) {
                if (!iobuf_out || !iobuf_out_cnt) {
                        if (protocol_version < 29)
                                return; /* there's nothing we can do */
                        if (protocol_version >= 30)
                                send_msg(MSG_NOOP, "", 0);
                        else {
                                write_int(sock_f_out, cur_flist->count);
                                write_shortint(sock_f_out, ITEM_IS_NEW);
                        }
                }
                if (iobuf_out)
                        io_flush(NORMAL_FLUSH);
        }
}

void start_flist_forward(int f_in)
{
        assert(iobuf_out != NULL);
        assert(iobuf_f_out == msg_fd_out);
        flist_forward_from = f_in;
}

void stop_flist_forward()
{
        io_flush(NORMAL_FLUSH);
        flist_forward_from = -1;
}

/**
 * Continue trying to read len bytes - don't return until len has been
 * read.
 **/
static void read_loop(int fd, char *buf, size_t len)
{
        while (len) {
                int n = read_timeout(fd, buf, len);

                buf += n;
                len -= n;
        }
}

/**
 * Read from the file descriptor handling multiplexing - return number
 * of bytes read.
 *
 * Never returns <= 0.
 */
static int readfd_unbuffered(int fd, char *buf, size_t len)
{
        size_t msg_bytes;
        int tag, cnt = 0;
        char line[BIGPATHBUFLEN];

        if (!iobuf_in || fd != iobuf_f_in)
                return read_timeout(fd, buf, len);

        if (!io_multiplexing_in && iobuf_in_remaining == 0) {
                iobuf_in_remaining = read_timeout(fd, iobuf_in, iobuf_in_siz);
                iobuf_in_ndx = 0;
        }

        while (cnt == 0) {
                if (iobuf_in_remaining) {
                        len = MIN(len, iobuf_in_remaining);
                        memcpy(buf, iobuf_in + iobuf_in_ndx, len);
                        iobuf_in_ndx += len;
                        iobuf_in_remaining -= len;
                        cnt = len;
                        break;
                }

                read_loop(fd, line, 4);
                tag = IVAL(line, 0);

                msg_bytes = tag & 0xFFFFFF;
                tag = (tag >> 24) - MPLEX_BASE;

                switch (tag) {
                case MSG_DATA:
                        if (msg_bytes > iobuf_in_siz) {
                                if (!(iobuf_in = realloc_array(iobuf_in, char,
                                                               msg_bytes)))
                                        out_of_memory("readfd_unbuffered");
                                iobuf_in_siz = msg_bytes;
                        }
                        read_loop(fd, iobuf_in, msg_bytes);
                        iobuf_in_remaining = msg_bytes;
                        iobuf_in_ndx = 0;
                        break;
                case MSG_NOOP:
                        if (am_sender)
                                maybe_send_keepalive();
                        break;
                case MSG_IO_ERROR:
                        if (msg_bytes != 4)
                                goto invalid_msg;
                        read_loop(fd, line, msg_bytes);
                        io_error |= IVAL(line, 0);
                        break;
                case MSG_DELETED:
                        if (msg_bytes >= sizeof line)
                                goto overflow;
                        read_loop(fd, line, msg_bytes);
                        /* A directory name was sent with the trailing null */
                        if (msg_bytes > 0 && !line[msg_bytes-1])
                                log_delete(line, S_IFDIR);
                        else {
                                line[msg_bytes] = '\0';
                                log_delete(line, S_IFREG);
                        }
                        break;
                case MSG_SUCCESS:
                        if (msg_bytes != 4) {
                          invalid_msg:
                                rprintf(FERROR, "invalid multi-message %d:%ld [%s]\n",
                                        tag, (long)msg_bytes, who_am_i());
                                exit_cleanup(RERR_STREAMIO);
                        }
                        read_loop(fd, line, msg_bytes);
                        successful_send(IVAL(line, 0));
                        break;
                case MSG_NO_SEND:
                        if (msg_bytes != 4)
                                goto invalid_msg;
                        read_loop(fd, line, msg_bytes);
                        send_msg_int(MSG_NO_SEND, IVAL(line, 0));
                        break;
                case MSG_INFO:
                case MSG_ERROR:
                        if (msg_bytes >= sizeof line) {
                            overflow:
                                rprintf(FERROR,
                                        "multiplexing overflow %d:%ld [%s]\n",
                                        tag, (long)msg_bytes, who_am_i());
                                exit_cleanup(RERR_STREAMIO);
                        }
                        read_loop(fd, line, msg_bytes);
                        rwrite((enum logcode)tag, line, msg_bytes);
                        break;
                default:
                        rprintf(FERROR, "unexpected tag %d [%s]\n",
                                tag, who_am_i());
                        exit_cleanup(RERR_STREAMIO);
                }
        }

        if (iobuf_in_remaining == 0)
                io_flush(NORMAL_FLUSH);

        return cnt;
}

/**
 * Do a buffered read from @p fd.  Don't return until all @p n bytes
 * have been read.  If all @p n can't be read then exit with an
 * error.
 **/
static void readfd(int fd, char *buffer, size_t N)
{
        int  cnt;
        size_t total = 0;

        while (total < N) {
                cnt = readfd_unbuffered(fd, buffer + total, N-total);
                total += cnt;
        }

        if (fd == write_batch_monitor_in) {
                if ((size_t)write(batch_fd, buffer, total) != total)
                        exit_cleanup(RERR_FILEIO);
        }

        if (fd == flist_forward_from)
                writefd(iobuf_f_out, buffer, total);

        if (fd == sock_f_in)
                stats.total_read += total;
}

unsigned short read_shortint(int f)
{
        char b[2];
        readfd(f, b, 2);
        return (UVAL(b, 1) << 8) + UVAL(b, 0);
}

int32 read_int(int f)
{
        char b[4];
        int32 num;

        readfd(f, b, 4);
        num = IVAL(b, 0);
#if SIZEOF_INT32 > 4
        if (num & (int32)0x80000000)
                num |= ~(int32)0xffffffff;
#endif
        return num;
}

int64 read_longint(int f)
{
        int64 num;
        char b[9];

        if (protocol_version < 30) {
                num = read_int(f);

                if ((int32)num != (int32)0xffffffff)
                        return num;

#if SIZEOF_INT64 < 8
                rprintf(FERROR, "Integer overflow: attempted 64-bit offset\n");
                exit_cleanup(RERR_UNSUPPORTED);
#else
                readfd(f, b, 8);
                num = IVAL(b,0) | (((int64)IVAL(b,4))<<32);
#endif
        } else {
                int cnt;
                readfd(f, b, 3);
                cnt = int_byte_cnt[CVAL(b, 0) / 4];
#if SIZEOF_INT64 < 8
                if (cnt > 5 || (cnt == 5 && (CVAL(b,0)&0x3F || CVAL(b,1)&0x80))) {
                        rprintf(FERROR, "Integer overflow: attempted 64-bit offset\n");
                        exit_cleanup(RERR_UNSUPPORTED);
                }
#endif
                if (cnt > 3)
                        readfd(f, b + 3, cnt - 3);
                switch (cnt) {
                case 3:
                        num = NVAL3(b, 0);
                        break;
                case 4:
                        num = NVAL4(b, 0x80);
                        break;
                case 5:
                        num = NVAL5(b, 0xC0);
                        break;
#if SIZEOF_INT64 >= 8
                case 6:
                        num = NVAL6(b, 0xE0);
                        break;
                case 7:
                        num = NVAL7(b, 0xF0);
                        break;
                case 8:
                        num = NVAL8(b, 0xF8);
                        break;
                case 9:
                        num = NVAL8(b+1, 0);
                        break;
#endif
                default:
                        exit_cleanup(RERR_PROTOCOL); /* impossible... */
                }
        }

        return num;
}

void read_buf(int f, char *buf, size_t len)
{
        readfd(f,buf,len);
}

void read_sbuf(int f, char *buf, size_t len)
{
        readfd(f, buf, len);
        buf[len] = '\0';
}

uchar read_byte(int f)
{
        uchar c;
        readfd(f, (char *)&c, 1);
        return c;
}

int read_vstring(int f, char *buf, int bufsize)
{
        int len = read_byte(f);

        if (len & 0x80)
                len = (len & ~0x80) * 0x100 + read_byte(f);

        if (len >= bufsize) {
                rprintf(FERROR, "over-long vstring received (%d > %d)\n",
                        len, bufsize - 1);
                return -1;
        }

        if (len)
                readfd(f, buf, len);
        buf[len] = '\0';
        return len;
}

/* Populate a sum_struct with values from the socket.  This is
 * called by both the sender and the receiver. */
void read_sum_head(int f, struct sum_struct *sum)
{
        sum->count = read_int(f);
        if (sum->count < 0) {
                rprintf(FERROR, "Invalid checksum count %ld [%s]\n",
                        (long)sum->count, who_am_i());
                exit_cleanup(RERR_PROTOCOL);
        }
        sum->blength = read_int(f);
        if (sum->blength < 0 || sum->blength > MAX_BLOCK_SIZE) {
                rprintf(FERROR, "Invalid block length %ld [%s]\n",
                        (long)sum->blength, who_am_i());
                exit_cleanup(RERR_PROTOCOL);
        }
        sum->s2length = protocol_version < 27 ? csum_length : (int)read_int(f);
        if (sum->s2length < 0 || sum->s2length > MD4_SUM_LENGTH) {
                rprintf(FERROR, "Invalid checksum length %d [%s]\n",
                        sum->s2length, who_am_i());
                exit_cleanup(RERR_PROTOCOL);
        }
        sum->remainder = read_int(f);
        if (sum->remainder < 0 || sum->remainder > sum->blength) {
                rprintf(FERROR, "Invalid remainder length %ld [%s]\n",
                        (long)sum->remainder, who_am_i());
                exit_cleanup(RERR_PROTOCOL);
        }
}

/* Send the values from a sum_struct over the socket.  Set sum to
 * NULL if there are no checksums to send.  This is called by both
 * the generator and the sender. */
void write_sum_head(int f, struct sum_struct *sum)
{
        static struct sum_struct null_sum;

        if (sum == NULL)
                sum = &null_sum;

        write_int(f, sum->count);
        write_int(f, sum->blength);
        if (protocol_version >= 27)
                write_int(f, sum->s2length);
        write_int(f, sum->remainder);
}

/**
 * Sleep after writing to limit I/O bandwidth usage.
 *
 * @todo Rather than sleeping after each write, it might be better to
 * use some kind of averaging.  The current algorithm seems to always
 * use a bit less bandwidth than specified, because it doesn't make up
 * for slow periods.  But arguably this is a feature.  In addition, we
 * ought to take the time used to write the data into account.
 *
 * During some phases of big transfers (file FOO is uptodate) this is
 * called with a small bytes_written every time.  As the kernel has to
 * round small waits up to guarantee that we actually wait at least the
 * requested number of microseconds, this can become grossly inaccurate.
 * We therefore keep track of the bytes we've written over time and only
 * sleep when the accumulated delay is at least 1 tenth of a second.
 **/
static void sleep_for_bwlimit(int bytes_written)
{
        static struct timeval prior_tv;
        static long total_written = 0;
        struct timeval tv, start_tv;
        long elapsed_usec, sleep_usec;

#define ONE_SEC 1000000L /* # of microseconds in a second */

        if (!bwlimit_writemax)
                return;

        total_written += bytes_written;

        gettimeofday(&start_tv, NULL);
        if (prior_tv.tv_sec) {
                elapsed_usec = (start_tv.tv_sec - prior_tv.tv_sec) * ONE_SEC
                             + (start_tv.tv_usec - prior_tv.tv_usec);
                total_written -= elapsed_usec * bwlimit / (ONE_SEC/1024);
                if (total_written < 0)
                        total_written = 0;
        }

        sleep_usec = total_written * (ONE_SEC/1024) / bwlimit;
        if (sleep_usec < ONE_SEC / 10) {
                prior_tv = start_tv;
                return;
        }

        tv.tv_sec  = sleep_usec / ONE_SEC;
        tv.tv_usec = sleep_usec % ONE_SEC;
        select(0, NULL, NULL, NULL, &tv);

        gettimeofday(&prior_tv, NULL);
        elapsed_usec = (prior_tv.tv_sec - start_tv.tv_sec) * ONE_SEC
                     + (prior_tv.tv_usec - start_tv.tv_usec);
        total_written = (sleep_usec - elapsed_usec) * bwlimit / (ONE_SEC/1024);
}

/* Write len bytes to the file descriptor fd, looping as necessary to get
 * the job done and also (in certain circumstances) reading any data on
 * msg_fd_in to avoid deadlock.
 *
 * This function underlies the multiplexing system.  The body of the
 * application never calls this function directly. */
static void writefd_unbuffered(int fd, const char *buf, size_t len)
{
        size_t n, total = 0;
        fd_set w_fds, r_fds, e_fds;
        int maxfd, count, cnt, using_r_fds;
        int defer_save = defer_forwarding_messages;
        struct timeval tv;

        no_flush++;

        while (total < len) {
                FD_ZERO(&w_fds);
                FD_SET(fd, &w_fds);
                FD_ZERO(&e_fds);
                FD_SET(fd, &e_fds);
                maxfd = fd;

                if (msg_fd_in >= 0) {
                        FD_ZERO(&r_fds);
                        FD_SET(msg_fd_in, &r_fds);
                        if (msg_fd_in > maxfd)
                                maxfd = msg_fd_in;
                        using_r_fds = 1;
                } else
                        using_r_fds = 0;

                tv.tv_sec = select_timeout;
                tv.tv_usec = 0;

                errno = 0;
                count = select(maxfd + 1, using_r_fds ? &r_fds : NULL,
                               &w_fds, &e_fds, &tv);

                if (count <= 0) {
                        if (count < 0 && errno == EBADF)
                                exit_cleanup(RERR_SOCKETIO);
                        check_timeout();
                        continue;
                }

                if (FD_ISSET(fd, &e_fds)) {
                        rsyserr(FINFO, errno,
                                "select exception on fd %d", fd);
                }

                if (using_r_fds && FD_ISSET(msg_fd_in, &r_fds))
                        read_msg_fd();

                if (!FD_ISSET(fd, &w_fds))
                        continue;

                n = len - total;
                if (bwlimit_writemax && n > bwlimit_writemax)
                        n = bwlimit_writemax;
                cnt = write(fd, buf + total, n);

                if (cnt <= 0) {
                        if (cnt < 0) {
                                if (errno == EINTR)
                                        continue;
                                if (errno == EWOULDBLOCK || errno == EAGAIN) {
                                        msleep(1);
                                        continue;
                                }
                        }

                        /* Don't try to write errors back across the stream. */
                        if (fd == sock_f_out)
                                io_end_multiplex_out();
                        rsyserr(FERROR, errno,
                                "writefd_unbuffered failed to write %ld bytes [%s]",
                                (long)len, who_am_i());
                        /* If the other side is sending us error messages, try
                         * to grab any messages they sent before they died. */
                        while (fd == sock_f_out && io_multiplexing_in) {
                                set_io_timeout(30);
                                ignore_timeout = 0;
                                readfd_unbuffered(sock_f_in, io_filesfrom_buf,
                                                  sizeof io_filesfrom_buf);
                        }
                        exit_cleanup(RERR_STREAMIO);
                }

                total += cnt;
                defer_forwarding_messages = 1;

                if (fd == sock_f_out) {
                        if (io_timeout || am_generator)
                                last_io_out = time(NULL);
                        sleep_for_bwlimit(cnt);
                }
        }

        defer_forwarding_messages = defer_save;
        no_flush--;
}

static void msg2sndr_flush(void)
{
        if (defer_forwarding_messages)
                return;

        while (msg2sndr.head && io_multiplexing_out) {
                struct msg_list_item *m = msg2sndr.head;
                if (!(msg2sndr.head = m->next))
                        msg2sndr.tail = NULL;
                stats.total_written += m->len;
                defer_forwarding_messages = 1;
                writefd_unbuffered(sock_f_out, m->buf, m->len);
                defer_forwarding_messages = 0;
                free(m);
        }
}

/**
 * Write an message to a multiplexed stream. If this fails then rsync
 * exits.
 **/
static void mplex_write(enum msgcode code, const char *buf, size_t len)
{
        char buffer[1024];
        size_t n = len;

        SIVAL(buffer, 0, ((MPLEX_BASE + (int)code)<<24) + len);

        if (n > sizeof buffer - 4)
                n = 0;
        else
                memcpy(buffer + 4, buf, n);

        writefd_unbuffered(sock_f_out, buffer, n+4);

        len -= n;
        buf += n;

        if (len) {
                defer_forwarding_messages = 1;
                writefd_unbuffered(sock_f_out, buf, len);
                defer_forwarding_messages = 0;
                msg2sndr_flush();
        }
}

void io_flush(UNUSED(int flush_it_all))
{
        msg2genr_flush();
        msg2sndr_flush();

        if (!iobuf_out_cnt || no_flush)
                return;

        if (io_multiplexing_out)
                mplex_write(MSG_DATA, iobuf_out, iobuf_out_cnt);
        else
                writefd_unbuffered(iobuf_f_out, iobuf_out, iobuf_out_cnt);
        iobuf_out_cnt = 0;
}

static void writefd(int fd, const char *buf, size_t len)
{
        if (fd == sock_f_out)
                stats.total_written += len;

        if (fd == write_batch_monitor_out) {
                if ((size_t)write(batch_fd, buf, len) != len)
                        exit_cleanup(RERR_FILEIO);
        }

        if (!iobuf_out || fd != iobuf_f_out) {
                writefd_unbuffered(fd, buf, len);
                return;
        }

        while (len) {
                int n = MIN((int)len, IO_BUFFER_SIZE - iobuf_out_cnt);
                if (n > 0) {
                        memcpy(iobuf_out+iobuf_out_cnt, buf, n);
                        buf += n;
                        len -= n;
                        iobuf_out_cnt += n;
                }

                if (iobuf_out_cnt == IO_BUFFER_SIZE)
                        io_flush(NORMAL_FLUSH);
        }
}

void write_shortint(int f, unsigned short x)
{
        char b[2];
        b[0] = (char)x;
        b[1] = (char)(x >> 8);
        writefd(f, b, 2);
}

void write_int(int f, int32 x)
{
        char b[4];
        SIVAL(b, 0, x);
        writefd(f, b, 4);
}

/*
 * Note: int64 may actually be a 32-bit type if ./configure couldn't find any
 * 64-bit types on this platform.
 */
void write_longint(int f, int64 x)
{
        char b[12];

#if SIZEOF_INT64 < 8
        if (x < 0 || x > 0x7FFFFFFF) {
                rprintf(FERROR, "Integer overflow: attempted 64-bit offset\n");
                exit_cleanup(RERR_UNSUPPORTED);
        }
#endif

        if (protocol_version < 30) {
                char * const s = b+4;
                SIVAL(s, 0, x);
#if SIZEOF_INT64 < 8
                writefd(f, s, 4);
#else
                if (x <= 0x7FFFFFFF && x >= 0) {
                        writefd(f, s, 4);
                        return;
                }

                memset(b, 0xFF, 4);
                SIVAL(s, 4, x >> 32);
                writefd(f, b, 12);
        } else if (x < 0) {
                goto all_bits;
#endif
        } else if (x < ((int32)1<<(3*8-1))) {
                b[0] = (char)(x >> 16);
                b[1] = (char)(x >> 8);
                b[2] = (char)x;
                writefd(f, b, 3);
        } else if (x < ((int64)1<<(4*8-2))) {
                b[0] = (char)((x >> 24) | 0x80);
                b[1] = (char)(x >> 16);
                b[2] = (char)(x >> 8);
                b[3] = (char)x;
                writefd(f, b, 4);
#if SIZEOF_INT64 < 8
        } else {
                b[0] = 0xC0;
                b[1] = (char)(x >> 24);
                b[2] = (char)(x >> 16);
                b[3] = (char)(x >> 8);
                b[4] = (char)x;
                writefd(f, b, 5);
        }
#else
        } else if (x < ((int64)1<<(5*8-3))) {
                b[0] = (char)((x >> 32) | 0xC0);
                b[1] = (char)(x >> 24);
                b[2] = (char)(x >> 16);
                b[3] = (char)(x >> 8);
                b[4] = (char)x;
                writefd(f, b, 5);
        } else if (x < ((int64)1<<(6*8-4))) {
                b[0] = (char)((x >> 40) | 0xE0);
                b[1] = (char)(x >> 32);
                b[2] = (char)(x >> 24);
                b[3] = (char)(x >> 16);
                b[4] = (char)(x >> 8);
                b[5] = (char)x;
                writefd(f, b, 6);
        } else if (x < ((int64)1<<(7*8-5))) {
                b[0] = (char)((x >> 48) | 0xF0);
                b[1] = (char)(x >> 40);
                b[2] = (char)(x >> 32);
                b[3] = (char)(x >> 24);
                b[4] = (char)(x >> 16);
                b[5] = (char)(x >> 8);
                b[6] = (char)x;
                writefd(f, b, 7);
        } else if (x < ((int64)1<<(8*8-6))) {
                b[0] = (char)((x >> 56) | 0xF8);
                b[1] = (char)(x >> 48);
                b[2] = (char)(x >> 40);
                b[3] = (char)(x >> 32);
                b[4] = (char)(x >> 24);
                b[5] = (char)(x >> 16);
                b[6] = (char)(x >> 8);
                b[7] = (char)x;
                writefd(f, b, 8);
        } else {
          all_bits:
                b[0] = (char)0xFC;
                b[1] = (char)(x >> 56);
                b[2] = (char)(x >> 48);
                b[3] = (char)(x >> 40);
                b[4] = (char)(x >> 32);
                b[5] = (char)(x >> 24);
                b[6] = (char)(x >> 16);
                b[7] = (char)(x >> 8);
                b[8] = (char)x;
                writefd(f, b, 9);
        }
#endif
}

void write_buf(int f, const char *buf, size_t len)
{
        writefd(f,buf,len);
}

/** Write a string to the connection */
void write_sbuf(int f, const char *buf)
{
        writefd(f, buf, strlen(buf));
}

void write_byte(int f, uchar c)
{
        writefd(f, (char *)&c, 1);
}

void write_vstring(int f, const char *str, int len)
{
        uchar lenbuf[3], *lb = lenbuf;

        if (len > 0x7F) {
                if (len > 0x7FFF) {
                        rprintf(FERROR,
                                "attempting to send over-long vstring (%d > %d)\n",
                                len, 0x7FFF);
                        exit_cleanup(RERR_PROTOCOL);
                }
                *lb++ = len / 0x100 + 0x80;
        }
        *lb = len;

        writefd(f, (char*)lenbuf, lb - lenbuf + 1);
        if (len)
                writefd(f, str, len);
}

/**
 * Read a line of up to @p maxlen characters into @p buf (not counting
 * the trailing null).  Strips the (required) trailing newline and all
 * carriage returns.
 *
 * @return 1 for success; 0 for I/O error or truncation.
 **/
int read_line(int f, char *buf, size_t maxlen)
{
        while (maxlen) {
                buf[0] = 0;
                read_buf(f, buf, 1);
                if (buf[0] == 0)
                        return 0;
                if (buf[0] == '\n')
                        break;
                if (buf[0] != '\r') {
                        buf++;
                        maxlen--;
                }
        }
        *buf = '\0';
        return maxlen > 0;
}

void io_printf(int fd, const char *format, ...)
{
        va_list ap;
        char buf[BIGPATHBUFLEN];
        int len;

        va_start(ap, format);
        len = vsnprintf(buf, sizeof buf, format, ap);
        va_end(ap);

        if (len < 0)
                exit_cleanup(RERR_STREAMIO);

        if (len > (int)sizeof buf) {
                rprintf(FERROR, "io_printf() was too long for the buffer.\n");
                exit_cleanup(RERR_STREAMIO);
        }

        write_sbuf(fd, buf);
}

/** Setup for multiplexing a MSG_* stream with the data stream. */
void io_start_multiplex_out(void)
{
        io_flush(NORMAL_FLUSH);
        io_start_buffering_out(sock_f_out);
        io_multiplexing_out = 1;
}

/** Setup for multiplexing a MSG_* stream with the data stream. */
void io_start_multiplex_in(void)
{
        io_flush(NORMAL_FLUSH);
        io_start_buffering_in(sock_f_in);
        io_multiplexing_in = 1;
}

/** Write an message to the multiplexed data stream. */
int io_multiplex_write(enum msgcode code, const char *buf, size_t len)
{
        if (!io_multiplexing_out)
                return 0;
        io_flush(NORMAL_FLUSH);
        stats.total_written += (len+4);
        mplex_write(code, buf, len);
        return 1;
}

void io_end_multiplex_in(void)
{
        io_multiplexing_in = 0;
        io_end_buffering_in();
}

/** Stop output multiplexing. */
void io_end_multiplex_out(void)
{
        io_multiplexing_out = 0;
        io_end_buffering_out();
}

void start_write_batch(int fd)
{
        /* Some communication has already taken place, but we don't
         * enable batch writing until here so that we can write a
         * canonical record of the communication even though the
         * actual communication so far depends on whether a daemon
         * is involved. */
        write_int(batch_fd, protocol_version);
        write_int(batch_fd, checksum_seed);

        if (am_sender)
                write_batch_monitor_out = fd;
        else
                write_batch_monitor_in = fd;
}

void stop_write_batch(void)
{
        write_batch_monitor_out = -1;
        write_batch_monitor_in = -1;
}