putty-source/sftp.c

/*
 * sftp.c: SFTP generic client code.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <limits.h>

#include "misc.h"
#include "tree234.h"
#include "sftp.h"

static const char *fxp_error_message;
static int fxp_errtype;

static void fxp_internal_error(const char *msg);

/* ----------------------------------------------------------------------
 * Client-specific parts of the send- and receive-packet system.
 */

bool sftp_send(struct sftp_packet *pkt)
{
    bool ret;
    sftp_send_prepare(pkt);
    ret = sftp_senddata(pkt->data, pkt->length);
    sftp_pkt_free(pkt);
    return ret;
}

struct sftp_packet *sftp_recv(void)
{
    struct sftp_packet *pkt;
    char x[4];

    if (!sftp_recvdata(x, 4))
        return NULL;

    /* Impose _some_ upper bound on packet size. We never expect to
     * receive more than 32K of data in response to an FXP_READ,
     * because we decide how much data to ask for. FXP_READDIR and
     * pathname-returning things like FXP_REALPATH don't have an
     * explicit bound, so I suppose we just have to trust the server
     * to be sensible. */
    unsigned pktlen = GET_32BIT_MSB_FIRST(x);
    if (pktlen > (1<<20))
        return NULL;

    pkt = sftp_recv_prepare(pktlen);

    if (!sftp_recvdata(pkt->data, pkt->length)) {
        sftp_pkt_free(pkt);
        return NULL;
    }

    if (!sftp_recv_finish(pkt)) {
        sftp_pkt_free(pkt);
        return NULL;
    }

    return pkt;
}

/* ----------------------------------------------------------------------
 * Request ID allocation and temporary dispatch routines.
 */

#define REQUEST_ID_OFFSET 256

struct sftp_request {
    unsigned id;
    bool registered;
    void *userdata;
};

static int sftp_reqcmp(void *av, void *bv)
{
    struct sftp_request *a = (struct sftp_request *)av;
    struct sftp_request *b = (struct sftp_request *)bv;
    if (a->id < b->id)
        return -1;
    if (a->id > b->id)
        return +1;
    return 0;
}
static int sftp_reqfind(void *av, void *bv)
{
    unsigned *a = (unsigned *) av;
    struct sftp_request *b = (struct sftp_request *)bv;
    if (*a < b->id)
        return -1;
    if (*a > b->id)
        return +1;
    return 0;
}

static tree234 *sftp_requests;

static struct sftp_request *sftp_alloc_request(void)
{
    unsigned low, high, mid;
    int tsize;
    struct sftp_request *r;

    if (sftp_requests == NULL)
        sftp_requests = newtree234(sftp_reqcmp);

    /*
     * First-fit allocation of request IDs: always pick the lowest
     * unused one. To do this, binary-search using the counted
     * B-tree to find the largest ID which is in a contiguous
     * sequence from the beginning. (Precisely everything in that
     * sequence must have ID equal to its tree index plus
     * REQUEST_ID_OFFSET.)
     */
    tsize = count234(sftp_requests);

    low = -1;
    high = tsize;
    while (high - low > 1) {
        mid = (high + low) / 2;
        r = index234(sftp_requests, mid);
        if (r->id == mid + REQUEST_ID_OFFSET)
            low = mid;                 /* this one is fine */
        else
            high = mid;                /* this one is past it */
    }
    /*
     * Now low points to either -1, or the tree index of the
     * largest ID in the initial sequence.
     */
    {
        unsigned i = low + 1 + REQUEST_ID_OFFSET;
        assert(NULL == find234(sftp_requests, &i, sftp_reqfind));
    }

    /*
     * So the request ID we need to create is
     * low + 1 + REQUEST_ID_OFFSET.
     */
    r = snew(struct sftp_request);
    r->id = low + 1 + REQUEST_ID_OFFSET;
    r->registered = false;
    r->userdata = NULL;
    add234(sftp_requests, r);
    return r;
}

void sftp_cleanup_request(void)
{
    if (sftp_requests != NULL) {
        freetree234(sftp_requests);
        sftp_requests = NULL;
    }
}

void sftp_register(struct sftp_request *req)
{
    req->registered = true;
}

struct sftp_request *sftp_find_request(struct sftp_packet *pktin)
{
    unsigned id;
    struct sftp_request *req;

    if (!pktin) {
        fxp_internal_error("did not receive a valid SFTP packet\n");
        return NULL;
    }

    id = get_uint32(pktin);
    if (get_err(pktin)) {
        fxp_internal_error("did not receive a valid SFTP packet\n");
        return NULL;
    }

    req = find234(sftp_requests, &id, sftp_reqfind);
    if (!req || !req->registered) {
        fxp_internal_error("request ID mismatch\n");
        return NULL;
    }

    del234(sftp_requests, req);

    return req;
}

/* ----------------------------------------------------------------------
 * SFTP primitives.
 */

/*
 * Deal with (and free) an FXP_STATUS packet. Return 1 if
 * SSH_FX_OK, 0 if SSH_FX_EOF, and -1 for anything else (error).
 * Also place the status into fxp_errtype.
 */
static int fxp_got_status(struct sftp_packet *pktin)
{
    static const char *const messages[] = {
        /* SSH_FX_OK. The only time we will display a _message_ for this
         * is if we were expecting something other than FXP_STATUS on
         * success, so this is actually an error message! */
        "unexpected OK response",
        "end of file",
        "no such file or directory",
        "permission denied",
        "failure",
        "bad message",
        "no connection",
        "connection lost",
        "operation unsupported",
    };

    if (pktin->type != SSH_FXP_STATUS) {
        fxp_error_message = "expected FXP_STATUS packet";
        fxp_errtype = -1;
    } else {
        fxp_errtype = get_uint32(pktin);
        if (get_err(pktin)) {
            fxp_error_message = "malformed FXP_STATUS packet";
            fxp_errtype = -1;
        } else {
            if (fxp_errtype < 0 || fxp_errtype >= lenof(messages))
                fxp_error_message = "unknown error code";
            else
                fxp_error_message = messages[fxp_errtype];
        }
    }

    if (fxp_errtype == SSH_FX_OK)
        return 1;
    else if (fxp_errtype == SSH_FX_EOF)
        return 0;
    else
        return -1;
}

static void fxp_internal_error(const char *msg)
{
    fxp_error_message = msg;
    fxp_errtype = -1;
}

const char *fxp_error(void)
{
    return fxp_error_message;
}

int fxp_error_type(void)
{
    return fxp_errtype;
}

/*
 * Perform exchange of init/version packets. Return 0 on failure.
 */
bool fxp_init(void)
{
    struct sftp_packet *pktout, *pktin;
    unsigned long remotever;

    pktout = sftp_pkt_init(SSH_FXP_INIT);
    put_uint32(pktout, SFTP_PROTO_VERSION);
    sftp_send(pktout);

    pktin = sftp_recv();
    if (!pktin) {
        fxp_internal_error("could not connect");
        return false;
    }
    if (pktin->type != SSH_FXP_VERSION) {
        fxp_internal_error("did not receive FXP_VERSION");
        sftp_pkt_free(pktin);
        return false;
    }
    remotever = get_uint32(pktin);
    if (get_err(pktin)) {
        fxp_internal_error("malformed FXP_VERSION packet");
        sftp_pkt_free(pktin);
        return false;
    }
    if (remotever > SFTP_PROTO_VERSION) {
        fxp_internal_error
            ("remote protocol is more advanced than we support");
        sftp_pkt_free(pktin);
        return false;
    }
    /*
     * In principle, this packet might also contain extension-
     * string pairs. We should work through them and look for any
     * we recognise. In practice we don't currently do so because
     * we know we don't recognise _any_.
     */
    sftp_pkt_free(pktin);

    return true;
}

/*
 * Canonify a pathname.
 */
struct sftp_request *fxp_realpath_send(const char *path)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_REALPATH);
    put_uint32(pktout, req->id);
    put_stringz(pktout, path);
    sftp_send(pktout);

    return req;
}

char *fxp_realpath_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    sfree(req);

    if (pktin->type == SSH_FXP_NAME) {
        unsigned long count;
        char *path;
        ptrlen name;

        count = get_uint32(pktin);
        if (get_err(pktin) || count != 1) {
            fxp_internal_error("REALPATH did not return name count of 1\n");
            sftp_pkt_free(pktin);
            return NULL;
        }
        name = get_string(pktin);
        if (get_err(pktin)) {
            fxp_internal_error("REALPATH returned malformed FXP_NAME\n");
            sftp_pkt_free(pktin);
            return NULL;
        }
        path = mkstr(name);
        sftp_pkt_free(pktin);
        return path;
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return NULL;
    }
}

/*
 * Open a file.
 */
struct sftp_request *fxp_open_send(const char *path, int type,
                                   const struct fxp_attrs *attrs)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_OPEN);
    put_uint32(pktout, req->id);
    put_stringz(pktout, path);
    put_uint32(pktout, type);
    put_fxp_attrs(pktout, attrs ? *attrs : no_attrs);
    sftp_send(pktout);

    return req;
}

static struct fxp_handle *fxp_got_handle(struct sftp_packet *pktin)
{
    ptrlen id;
    struct fxp_handle *handle;

    id = get_string(pktin);
    if (get_err(pktin)) {
        fxp_internal_error("received malformed FXP_HANDLE");
        sftp_pkt_free(pktin);
        return NULL;
    }
    handle = snew(struct fxp_handle);
    handle->hstring = mkstr(id);
    handle->hlen = id.len;
    sftp_pkt_free(pktin);
    return handle;
}

struct fxp_handle *fxp_open_recv(struct sftp_packet *pktin,
                                 struct sftp_request *req)
{
    sfree(req);

    if (pktin->type == SSH_FXP_HANDLE) {
        return fxp_got_handle(pktin);
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return NULL;
    }
}

/*
 * Open a directory.
 */
struct sftp_request *fxp_opendir_send(const char *path)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_OPENDIR);
    put_uint32(pktout, req->id);
    put_stringz(pktout, path);
    sftp_send(pktout);

    return req;
}

struct fxp_handle *fxp_opendir_recv(struct sftp_packet *pktin,
                                    struct sftp_request *req)
{
    sfree(req);
    if (pktin->type == SSH_FXP_HANDLE) {
        return fxp_got_handle(pktin);
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return NULL;
    }
}

/*
 * Close a file/dir.
 */
struct sftp_request *fxp_close_send(struct fxp_handle *handle)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_CLOSE);
    put_uint32(pktout, req->id);
    put_string(pktout, handle->hstring, handle->hlen);
    sftp_send(pktout);

    sfree(handle->hstring);
    sfree(handle);

    return req;
}

bool fxp_close_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    sfree(req);
    fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return fxp_errtype == SSH_FX_OK;
}

struct sftp_request *fxp_mkdir_send(const char *path,
                                    const struct fxp_attrs *attrs)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_MKDIR);
    put_uint32(pktout, req->id);
    put_stringz(pktout, path);
    put_fxp_attrs(pktout, attrs ? *attrs : no_attrs);
    sftp_send(pktout);

    return req;
}

bool fxp_mkdir_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    int id;
    sfree(req);
    id = fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return id == 1;
}

struct sftp_request *fxp_rmdir_send(const char *path)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_RMDIR);
    put_uint32(pktout, req->id);
    put_stringz(pktout, path);
    sftp_send(pktout);

    return req;
}

bool fxp_rmdir_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    int id;
    sfree(req);
    id = fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return id == 1;
}

struct sftp_request *fxp_remove_send(const char *fname)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_REMOVE);
    put_uint32(pktout, req->id);
    put_stringz(pktout, fname);
    sftp_send(pktout);

    return req;
}

bool fxp_remove_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    int id;
    sfree(req);
    id = fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return id == 1;
}

struct sftp_request *fxp_rename_send(const char *srcfname,
                                     const char *dstfname)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_RENAME);
    put_uint32(pktout, req->id);
    put_stringz(pktout, srcfname);
    put_stringz(pktout, dstfname);
    sftp_send(pktout);

    return req;
}

bool fxp_rename_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    int id;
    sfree(req);
    id = fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return id == 1;
}

/*
 * Retrieve the attributes of a file. We have fxp_stat which works
 * on filenames, and fxp_fstat which works on open file handles.
 */
struct sftp_request *fxp_stat_send(const char *fname)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_STAT);
    put_uint32(pktout, req->id);
    put_stringz(pktout, fname);
    sftp_send(pktout);

    return req;
}

static bool fxp_got_attrs(struct sftp_packet *pktin, struct fxp_attrs *attrs)
{
    get_fxp_attrs(pktin, attrs);
    if (get_err(pktin)) {
        fxp_internal_error("malformed SSH_FXP_ATTRS packet");
        sftp_pkt_free(pktin);
        return false;
    }
    sftp_pkt_free(pktin);
    return true;
}

bool fxp_stat_recv(struct sftp_packet *pktin, struct sftp_request *req,
                  struct fxp_attrs *attrs)
{
    sfree(req);
    if (pktin->type == SSH_FXP_ATTRS) {
        return fxp_got_attrs(pktin, attrs);
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return false;
    }
}

struct sftp_request *fxp_fstat_send(struct fxp_handle *handle)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_FSTAT);
    put_uint32(pktout, req->id);
    put_string(pktout, handle->hstring, handle->hlen);
    sftp_send(pktout);

    return req;
}

bool fxp_fstat_recv(struct sftp_packet *pktin, struct sftp_request *req,
                    struct fxp_attrs *attrs)
{
    sfree(req);
    if (pktin->type == SSH_FXP_ATTRS) {
        return fxp_got_attrs(pktin, attrs);
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return false;
    }
}

/*
 * Set the attributes of a file.
 */
struct sftp_request *fxp_setstat_send(const char *fname,
                                      struct fxp_attrs attrs)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_SETSTAT);
    put_uint32(pktout, req->id);
    put_stringz(pktout, fname);
    put_fxp_attrs(pktout, attrs);
    sftp_send(pktout);

    return req;
}

bool fxp_setstat_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    int id;
    sfree(req);
    id = fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return id == 1;
}

struct sftp_request *fxp_fsetstat_send(struct fxp_handle *handle,
                                       struct fxp_attrs attrs)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_FSETSTAT);
    put_uint32(pktout, req->id);
    put_string(pktout, handle->hstring, handle->hlen);
    put_fxp_attrs(pktout, attrs);
    sftp_send(pktout);

    return req;
}

bool fxp_fsetstat_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    int id;
    sfree(req);
    id = fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return id == 1;
}

/*
 * Read from a file. Returns the number of bytes read, or -1 on an
 * error, or possibly 0 if EOF. (I'm not entirely sure whether it
 * will return 0 on EOF, or return -1 and store SSH_FX_EOF in the
 * error indicator. It might even depend on the SFTP server.)
 */
struct sftp_request *fxp_read_send(struct fxp_handle *handle,
                                   uint64_t offset, int len)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_READ);
    put_uint32(pktout, req->id);
    put_string(pktout, handle->hstring, handle->hlen);
    put_uint64(pktout, offset);
    put_uint32(pktout, len);
    sftp_send(pktout);

    return req;
}

int fxp_read_recv(struct sftp_packet *pktin, struct sftp_request *req,
                  char *buffer, int len)
{
    sfree(req);
    if (pktin->type == SSH_FXP_DATA) {
        ptrlen data;

        data = get_string(pktin);
        if (get_err(pktin)) {
            fxp_internal_error("READ returned malformed SSH_FXP_DATA packet");
            sftp_pkt_free(pktin);
            return -1;
        }

        if (data.len > len) {
            fxp_internal_error("READ returned more bytes than requested");
            sftp_pkt_free(pktin);
            return -1;
        }

        memcpy(buffer, data.ptr, data.len);
        sftp_pkt_free(pktin);
        return data.len;
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return -1;
    }
}

/*
 * Read from a directory.
 */
struct sftp_request *fxp_readdir_send(struct fxp_handle *handle)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_READDIR);
    put_uint32(pktout, req->id);
    put_string(pktout, handle->hstring, handle->hlen);
    sftp_send(pktout);

    return req;
}

struct fxp_names *fxp_readdir_recv(struct sftp_packet *pktin,
                                   struct sftp_request *req)
{
    sfree(req);
    if (pktin->type == SSH_FXP_NAME) {
        struct fxp_names *ret;
        unsigned long i;

        i = get_uint32(pktin);

        /*
         * Sanity-check the number of names. Minimum is obviously
         * zero. Maximum is the remaining space in the packet
         * divided by the very minimum length of a name, which is
         * 12 bytes (4 for an empty filename, 4 for an empty
         * longname, 4 for a set of attribute flags indicating that
         * no other attributes are supplied).
         */
        if (get_err(pktin) || i > get_avail(pktin) / 12) {
            fxp_internal_error("malformed FXP_NAME packet");
            sftp_pkt_free(pktin);
            return NULL;
        }

        /*
         * Ensure the implicit multiplication in the snewn() call
         * doesn't suffer integer overflow and cause us to malloc
         * too little space.
         */
        if (i > INT_MAX / sizeof(struct fxp_name)) {
            fxp_internal_error("unreasonably large FXP_NAME packet");
            sftp_pkt_free(pktin);
            return NULL;
        }

        ret = snew(struct fxp_names);
        ret->nnames = i;
        ret->names = snewn(ret->nnames, struct fxp_name);
        for (i = 0; i < (unsigned long)ret->nnames; i++) {
            ret->names[i].filename = mkstr(get_string(pktin));
            ret->names[i].longname = mkstr(get_string(pktin));
            get_fxp_attrs(pktin, &ret->names[i].attrs);
        }

        if (get_err(pktin)) {
            fxp_internal_error("malformed FXP_NAME packet");
            for (i = 0; i < (unsigned long)ret->nnames; i++) {
                sfree(ret->names[i].filename);
                sfree(ret->names[i].longname);
            }
            sfree(ret->names);
            sfree(ret);
            sfree(pktin);
            return NULL;
        }
        sftp_pkt_free(pktin);
        return ret;
    } else {
        fxp_got_status(pktin);
        sftp_pkt_free(pktin);
        return NULL;
    }
}

/*
 * Write to a file. Returns 0 on error, 1 on OK.
 */
struct sftp_request *fxp_write_send(struct fxp_handle *handle,
                                    void *buffer, uint64_t offset, int len)
{
    struct sftp_request *req = sftp_alloc_request();
    struct sftp_packet *pktout;

    pktout = sftp_pkt_init(SSH_FXP_WRITE);
    put_uint32(pktout, req->id);
    put_string(pktout, handle->hstring, handle->hlen);
    put_uint64(pktout, offset);
    put_string(pktout, buffer, len);
    sftp_send(pktout);

    return req;
}

bool fxp_write_recv(struct sftp_packet *pktin, struct sftp_request *req)
{
    sfree(req);
    fxp_got_status(pktin);
    sftp_pkt_free(pktin);
    return fxp_errtype == SSH_FX_OK;
}

/*
 * Free up an fxp_names structure.
 */
void fxp_free_names(struct fxp_names *names)
{
    int i;

    for (i = 0; i < names->nnames; i++) {
        sfree(names->names[i].filename);
        sfree(names->names[i].longname);
    }
    sfree(names->names);
    sfree(names);
}

/*
 * Duplicate an fxp_name structure.
 */
struct fxp_name *fxp_dup_name(struct fxp_name *name)
{
    struct fxp_name *ret;
    ret = snew(struct fxp_name);
    ret->filename = dupstr(name->filename);
    ret->longname = dupstr(name->longname);
    ret->attrs = name->attrs;          /* structure copy */
    return ret;
}

/*
 * Free up an fxp_name structure.
 */
void fxp_free_name(struct fxp_name *name)
{
    sfree(name->filename);
    sfree(name->longname);
    sfree(name);
}

/*
 * Store user data in an sftp_request structure.
 */
void *fxp_get_userdata(struct sftp_request *req)
{
    return req->userdata;
}

void fxp_set_userdata(struct sftp_request *req, void *data)
{
    req->userdata = data;
}

/*
 * A wrapper to go round fxp_read_* and fxp_write_*, which manages
 * the queueing of multiple read/write requests.
 */

struct req {
    char *buffer;
    int len, retlen, complete;
    uint64_t offset;
    struct req *next, *prev;
};

struct fxp_xfer {
    uint64_t offset, furthestdata, filesize;
    int req_totalsize, req_maxsize;
    bool eof, err;
    struct fxp_handle *fh;
    struct req *head, *tail;
};

static struct fxp_xfer *xfer_init(struct fxp_handle *fh, uint64_t offset)
{
    struct fxp_xfer *xfer = snew(struct fxp_xfer);

    xfer->fh = fh;
    xfer->offset = offset;
    xfer->head = xfer->tail = NULL;
    xfer->req_totalsize = 0;
    xfer->req_maxsize = 1048576;
    xfer->err = false;
    xfer->filesize = UINT64_MAX;
    xfer->furthestdata = 0;

    return xfer;
}

bool xfer_done(struct fxp_xfer *xfer)
{
    /*
     * We're finished if we've seen EOF _and_ there are no
     * outstanding requests.
     */
    return (xfer->eof || xfer->err) && !xfer->head;
}

void xfer_download_queue(struct fxp_xfer *xfer)
{
    while (xfer->req_totalsize < xfer->req_maxsize &&
           !xfer->eof && !xfer->err) {
        /*
         * Queue a new read request.
         */
        struct req *rr;
        struct sftp_request *req;

        rr = snew(struct req);
        rr->offset = xfer->offset;
        rr->complete = 0;
        if (xfer->tail) {
            xfer->tail->next = rr;
            rr->prev = xfer->tail;
        } else {
            xfer->head = rr;
            rr->prev = NULL;
        }
        xfer->tail = rr;
        rr->next = NULL;

        rr->len = 32768;
        rr->buffer = snewn(rr->len, char);
        sftp_register(req = fxp_read_send(xfer->fh, rr->offset, rr->len));
        fxp_set_userdata(req, rr);

        xfer->offset += rr->len;
        xfer->req_totalsize += rr->len;

#ifdef DEBUG_DOWNLOAD
        printf("queueing read request %p at %"PRIu64"\n", rr, rr->offset);
#endif
    }
}

struct fxp_xfer *xfer_download_init(struct fxp_handle *fh, uint64_t offset)
{
    struct fxp_xfer *xfer = xfer_init(fh, offset);

    xfer->eof = false;
    xfer_download_queue(xfer);

    return xfer;
}

/*
 * Returns INT_MIN to indicate that it didn't even get as far as
 * fxp_read_recv and hence has not freed pktin.
 */
int xfer_download_gotpkt(struct fxp_xfer *xfer, struct sftp_packet *pktin)
{
    struct sftp_request *rreq;
    struct req *rr;

    rreq = sftp_find_request(pktin);
    if (!rreq)
        return INT_MIN;            /* this packet doesn't even make sense */
    rr = (struct req *)fxp_get_userdata(rreq);
    if (!rr) {
        fxp_internal_error("request ID is not part of the current download");
        return INT_MIN;                /* this packet isn't ours */
    }
    rr->retlen = fxp_read_recv(pktin, rreq, rr->buffer, rr->len);
#ifdef DEBUG_DOWNLOAD
    printf("read request %p has returned [%d]\n", rr, rr->retlen);
#endif

    if ((rr->retlen < 0 && fxp_error_type()==SSH_FX_EOF) || rr->retlen == 0) {
        xfer->eof = true;
        rr->retlen = 0;
        rr->complete = -1;
#ifdef DEBUG_DOWNLOAD
        printf("setting eof\n");
#endif
    } else if (rr->retlen < 0) {
        /* some error other than EOF; signal it back to caller */
        xfer_set_error(xfer);
        rr->complete = -1;
        return -1;
    }

    rr->complete = 1;

    /*
     * Special case: if we have received fewer bytes than we
     * actually read, we should do something. For the moment I'll
     * just throw an ersatz FXP error to signal this; the SFTP
     * draft I've got says that it can't happen except on special
     * files, in which case seeking probably has very little
     * meaning and so queueing an additional read request to fill
     * up the gap sounds like the wrong answer. I'm not sure what I
     * should be doing here - if it _was_ a special file, I suspect
     * I simply shouldn't have been queueing multiple requests in
     * the first place...
     */
    if (rr->retlen > 0 && xfer->furthestdata < rr->offset) {
        xfer->furthestdata = rr->offset;
#ifdef DEBUG_DOWNLOAD
        printf("setting furthestdata = %"PRIu64"\n", xfer->furthestdata);
#endif
    }

    if (rr->retlen < rr->len) {
        uint64_t filesize = rr->offset + (rr->retlen < 0 ? 0 : rr->retlen);
#ifdef DEBUG_DOWNLOAD
        printf("short block! trying filesize = %"PRIu64"\n", filesize);
#endif
        if (xfer->filesize > filesize) {
            xfer->filesize = filesize;
#ifdef DEBUG_DOWNLOAD
            printf("actually changing filesize\n");
#endif
        }
    }

    if (xfer->furthestdata > xfer->filesize) {
        fxp_error_message = "received a short buffer from FXP_READ, but not"
            " at EOF";
        fxp_errtype = -1;
        xfer_set_error(xfer);
        return -1;
    }

    return 1;
}

void xfer_set_error(struct fxp_xfer *xfer)
{
    xfer->err = true;
}

bool xfer_download_data(struct fxp_xfer *xfer, void **buf, int *len)
{
    void *retbuf = NULL;
    int retlen = 0;

    /*
     * Discard anything at the head of the rr queue with complete <
     * 0; return the first thing with complete > 0.
     */
    while (xfer->head && xfer->head->complete && !retbuf) {
        struct req *rr = xfer->head;

        if (rr->complete > 0) {
            retbuf = rr->buffer;
            retlen = rr->retlen;
#ifdef DEBUG_DOWNLOAD
            printf("handing back data from read request %p\n", rr);
#endif
        }
#ifdef DEBUG_DOWNLOAD
        else
            printf("skipping failed read request %p\n", rr);
#endif

        xfer->head = xfer->head->next;
        if (xfer->head)
            xfer->head->prev = NULL;
        else
            xfer->tail = NULL;
        xfer->req_totalsize -= rr->len;
        sfree(rr);
    }

    if (retbuf) {
        *buf = retbuf;
        *len = retlen;
        return true;
    } else
        return false;
}

struct fxp_xfer *xfer_upload_init(struct fxp_handle *fh, uint64_t offset)
{
    struct fxp_xfer *xfer = xfer_init(fh, offset);

    /*
     * We set `eof' to 1 because this will cause xfer_done() to
     * return true iff there are no outstanding requests. During an
     * upload, our caller will be responsible for working out
     * whether all the data has been sent, so all it needs to know
     * from us is whether the outstanding requests have been
     * handled once that's done.
     */
    xfer->eof = true;

    return xfer;
}

bool xfer_upload_ready(struct fxp_xfer *xfer)
{
    return sftp_sendbuffer() == 0;
}

void xfer_upload_data(struct fxp_xfer *xfer, char *buffer, int len)
{
    struct req *rr;
    struct sftp_request *req;

    rr = snew(struct req);
    rr->offset = xfer->offset;
    rr->complete = 0;
    if (xfer->tail) {
        xfer->tail->next = rr;
        rr->prev = xfer->tail;
    } else {
        xfer->head = rr;
        rr->prev = NULL;
    }
    xfer->tail = rr;
    rr->next = NULL;

    rr->len = len;
    rr->buffer = NULL;
    sftp_register(req = fxp_write_send(xfer->fh, buffer, rr->offset, len));
    fxp_set_userdata(req, rr);

    xfer->offset += rr->len;
    xfer->req_totalsize += rr->len;

#ifdef DEBUG_UPLOAD
    printf("queueing write request %p at %"PRIu64" [len %d]\n",
           rr, rr->offset, len);
#endif
}

/*
 * Returns INT_MIN to indicate that it didn't even get as far as
 * fxp_write_recv and hence has not freed pktin.
 */
int xfer_upload_gotpkt(struct fxp_xfer *xfer, struct sftp_packet *pktin)
{
    struct sftp_request *rreq;
    struct req *rr, *prev, *next;
    bool ret;

    rreq = sftp_find_request(pktin);
    if (!rreq)
        return INT_MIN;            /* this packet doesn't even make sense */
    rr = (struct req *)fxp_get_userdata(rreq);
    if (!rr) {
        fxp_internal_error("request ID is not part of the current upload");
        return INT_MIN;                /* this packet isn't ours */
    }
    ret = fxp_write_recv(pktin, rreq);
#ifdef DEBUG_UPLOAD
    printf("write request %p has returned [%d]\n", rr, ret ? 1 : 0);
#endif

    /*
     * Remove this one from the queue.
     */
    prev = rr->prev;
    next = rr->next;
    if (prev)
        prev->next = next;
    else
        xfer->head = next;
    if (next)
        next->prev = prev;
    else
        xfer->tail = prev;
    xfer->req_totalsize -= rr->len;
    sfree(rr);

    if (!ret)
        return -1;

    return 1;
}

void xfer_cleanup(struct fxp_xfer *xfer)
{
    struct req *rr;
    while (xfer->head) {
        rr = xfer->head;
        xfer->head = xfer->head->next;
        sfree(rr->buffer);
        sfree(rr);
    }
    sfree(xfer);
}