libvirt/src/rpc/virnetclient.c

/*
 * virnetclient.c: generic network RPC client
 *
 * Copyright (C) 2006-2011 Red Hat, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307  USA
 *
 * Author: Daniel P. Berrange <berrange@redhat.com>
 */

#include <config.h>

#include <unistd.h>
#include <poll.h>
#include <signal.h>
#include <fcntl.h>

#include "virnetclient.h"
#include "virnetsocket.h"
#include "memory.h"
#include "threads.h"
#include "files.h"
#include "logging.h"
#include "util.h"
#include "virterror_internal.h"

#define VIR_FROM_THIS VIR_FROM_RPC
#define virNetError(code, ...)                                    \
    virReportErrorHelper(VIR_FROM_THIS, code, __FILE__,           \
                         __FUNCTION__, __LINE__, __VA_ARGS__)

typedef struct _virNetClientCall virNetClientCall;
typedef virNetClientCall *virNetClientCallPtr;

enum {
    VIR_NET_CLIENT_MODE_WAIT_TX,
    VIR_NET_CLIENT_MODE_WAIT_RX,
    VIR_NET_CLIENT_MODE_COMPLETE,
};

struct _virNetClientCall {
    int mode;

    virNetMessagePtr msg;
    bool expectReply;

    virCond cond;

    virNetClientCallPtr next;
};


struct _virNetClient {
    int refs;

    virMutex lock;

    virNetSocketPtr sock;

    virNetTLSSessionPtr tls;
    char *hostname;

    virNetClientProgramPtr *programs;
    size_t nprograms;

    /* For incoming message packets */
    virNetMessage msg;

#if HAVE_SASL
    virNetSASLSessionPtr sasl;
#endif

    /* Self-pipe to wakeup threads waiting in poll() */
    int wakeupSendFD;
    int wakeupReadFD;

    /* List of threads currently waiting for dispatch */
    virNetClientCallPtr waitDispatch;

    size_t nstreams;
    virNetClientStreamPtr *streams;
};


static void virNetClientLock(virNetClientPtr client)
{
    virMutexLock(&client->lock);
}


static void virNetClientUnlock(virNetClientPtr client)
{
    virMutexUnlock(&client->lock);
}


static void virNetClientIncomingEvent(virNetSocketPtr sock,
                                      int events,
                                      void *opaque);

static virNetClientPtr virNetClientNew(virNetSocketPtr sock,
                                       const char *hostname)
{
    virNetClientPtr client = NULL;
    int wakeupFD[2] = { -1, -1 };

    if (pipe2(wakeupFD, O_CLOEXEC) < 0) {
        virReportSystemError(errno, "%s",
                             _("unable to make pipe"));
        goto error;
    }

    if (VIR_ALLOC(client) < 0)
        goto no_memory;

    client->refs = 1;

    if (virMutexInit(&client->lock) < 0)
        goto error;

    client->sock = sock;
    client->wakeupReadFD = wakeupFD[0];
    client->wakeupSendFD = wakeupFD[1];
    wakeupFD[0] = wakeupFD[1] = -1;

    if (hostname &&
        !(client->hostname = strdup(hostname)))
        goto no_memory;

    /* Set up a callback to listen on the socket data */
    if (virNetSocketAddIOCallback(client->sock,
                                  VIR_EVENT_HANDLE_READABLE,
                                  virNetClientIncomingEvent,
                                  client) < 0)
        VIR_DEBUG("Failed to add event watch, disabling events");

    return client;

no_memory:
    virReportOOMError();
error:
    VIR_FORCE_CLOSE(wakeupFD[0]);
    VIR_FORCE_CLOSE(wakeupFD[1]);
    virNetClientFree(client);
    return NULL;
}


virNetClientPtr virNetClientNewUNIX(const char *path,
                                    bool spawnDaemon,
                                    const char *binary)
{
    virNetSocketPtr sock;

    if (virNetSocketNewConnectUNIX(path, spawnDaemon, binary, &sock) < 0)
        return NULL;

    return virNetClientNew(sock, NULL);
}


virNetClientPtr virNetClientNewTCP(const char *nodename,
                                   const char *service)
{
    virNetSocketPtr sock;

    if (virNetSocketNewConnectTCP(nodename, service, &sock) < 0)
        return NULL;

    return virNetClientNew(sock, nodename);
}

virNetClientPtr virNetClientNewSSH(const char *nodename,
                                   const char *service,
                                   const char *binary,
                                   const char *username,
                                   bool noTTY,
                                   const char *netcat,
                                   const char *path)
{
    virNetSocketPtr sock;

    if (virNetSocketNewConnectSSH(nodename, service, binary, username, noTTY, netcat, path, &sock) < 0)
        return NULL;

    return virNetClientNew(sock, NULL);
}

virNetClientPtr virNetClientNewExternal(const char **cmdargv)
{
    virNetSocketPtr sock;

    if (virNetSocketNewConnectExternal(cmdargv, &sock) < 0)
        return NULL;

    return virNetClientNew(sock, NULL);
}


void virNetClientRef(virNetClientPtr client)
{
    virNetClientLock(client);
    client->refs++;
    virNetClientUnlock(client);
}


void virNetClientFree(virNetClientPtr client)
{
    int i;

    if (!client)
        return;

    virNetClientLock(client);
    client->refs--;
    if (client->refs > 0) {
        virNetClientUnlock(client);
        return;
    }

    for (i = 0 ; i < client->nprograms ; i++)
        virNetClientProgramFree(client->programs[i]);
    VIR_FREE(client->programs);

    VIR_FORCE_CLOSE(client->wakeupSendFD);
    VIR_FORCE_CLOSE(client->wakeupReadFD);

    VIR_FREE(client->hostname);

    virNetSocketRemoveIOCallback(client->sock);
    virNetSocketFree(client->sock);
    virNetTLSSessionFree(client->tls);
#if HAVE_SASL
    virNetSASLSessionFree(client->sasl);
#endif
    virNetClientUnlock(client);
    virMutexDestroy(&client->lock);

    VIR_FREE(client);
}


#if HAVE_SASL
void virNetClientSetSASLSession(virNetClientPtr client,
                                virNetSASLSessionPtr sasl)
{
    virNetClientLock(client);
    client->sasl = sasl;
    virNetSASLSessionRef(sasl);
    virNetSocketSetSASLSession(client->sock, client->sasl);
    virNetClientUnlock(client);
}
#endif


int virNetClientSetTLSSession(virNetClientPtr client,
                              virNetTLSContextPtr tls)
{
    int ret;
    char buf[1];
    int len;
    struct pollfd fds[1];
#ifdef HAVE_PTHREAD_SIGMASK
    sigset_t oldmask, blockedsigs;

    sigemptyset (&blockedsigs);
    sigaddset (&blockedsigs, SIGWINCH);
    sigaddset (&blockedsigs, SIGCHLD);
    sigaddset (&blockedsigs, SIGPIPE);
#endif

    virNetClientLock(client);

    if (!(client->tls = virNetTLSSessionNew(tls,
                                            client->hostname)))
        goto error;

    virNetSocketSetTLSSession(client->sock, client->tls);

    for (;;) {
        ret = virNetTLSSessionHandshake(client->tls);

        if (ret < 0)
            goto error;
        if (ret == 0)
            break;

        fds[0].fd = virNetSocketGetFD(client->sock);
        fds[0].revents = 0;
        if (virNetTLSSessionGetHandshakeStatus(client->tls) ==
            VIR_NET_TLS_HANDSHAKE_RECVING)
            fds[0].events = POLLIN;
        else
            fds[0].events = POLLOUT;

        /* Block SIGWINCH from interrupting poll in curses programs,
         * then restore the original signal mask again immediately
         * after the call (RHBZ#567931).  Same for SIGCHLD and SIGPIPE
         * at the suggestion of Paolo Bonzini and Daniel Berrange.
         */
#ifdef HAVE_PTHREAD_SIGMASK
        ignore_value(pthread_sigmask(SIG_BLOCK, &blockedsigs, &oldmask));
#endif

    repoll:
        ret = poll(fds, ARRAY_CARDINALITY(fds), -1);
        if (ret < 0 && errno == EAGAIN)
            goto repoll;

#ifdef HAVE_PTHREAD_SIGMASK
        ignore_value(pthread_sigmask(SIG_BLOCK, &oldmask, NULL));
#endif
    }

    ret = virNetTLSContextCheckCertificate(tls, client->tls);

    if (ret < 0)
        goto error;

    /* At this point, the server is verifying _our_ certificate, IP address,
     * etc.  If we make the grade, it will send us a '\1' byte.
     */

    fds[0].fd = virNetSocketGetFD(client->sock);
    fds[0].revents = 0;
    fds[0].events = POLLIN;

#ifdef HAVE_PTHREAD_SIGMASK
    /* Block SIGWINCH from interrupting poll in curses programs */
    ignore_value(pthread_sigmask(SIG_BLOCK, &blockedsigs, &oldmask));
#endif

    repoll2:
    ret = poll(fds, ARRAY_CARDINALITY(fds), -1);
    if (ret < 0 && errno == EAGAIN)
        goto repoll2;

#ifdef HAVE_PTHREAD_SIGMASK
    ignore_value(pthread_sigmask(SIG_BLOCK, &oldmask, NULL));
#endif

    len = virNetTLSSessionRead(client->tls, buf, 1);
    if (len < 0) {
        virReportSystemError(errno, "%s",
                             _("Unable to read TLS confirmation"));
        goto error;
    }
    if (len != 1 || buf[0] != '\1') {
        virNetError(VIR_ERR_RPC, "%s",
                    _("server verification (of our certificate or IP "
                      "address) failed"));
        goto error;
    }

    virNetClientUnlock(client);
    return 0;

error:
    virNetTLSSessionFree(client->tls);
    client->tls = NULL;
    virNetClientUnlock(client);
    return -1;
}

bool virNetClientIsEncrypted(virNetClientPtr client)
{
    bool ret = false;
    virNetClientLock(client);
    if (client->tls)
        ret = true;
#if HAVE_SASL
    if (client->sasl)
        ret = true;
#endif
    virNetClientUnlock(client);
    return ret;
}


int virNetClientAddProgram(virNetClientPtr client,
                           virNetClientProgramPtr prog)
{
    virNetClientLock(client);

    if (VIR_EXPAND_N(client->programs, client->nprograms, 1) < 0)
        goto no_memory;

    client->programs[client->nprograms-1] = prog;
    virNetClientProgramRef(prog);

    virNetClientUnlock(client);
    return 0;

no_memory:
    virReportOOMError();
    virNetClientUnlock(client);
    return -1;
}


int virNetClientAddStream(virNetClientPtr client,
                          virNetClientStreamPtr st)
{
    virNetClientLock(client);

    if (VIR_EXPAND_N(client->streams, client->nstreams, 1) < 0)
        goto no_memory;

    client->streams[client->nstreams-1] = st;
    virNetClientStreamRef(st);

    virNetClientUnlock(client);
    return 0;

no_memory:
    virReportOOMError();
    virNetClientUnlock(client);
    return -1;
}


void virNetClientRemoveStream(virNetClientPtr client,
                              virNetClientStreamPtr st)
{
    virNetClientLock(client);
    size_t i;
    for (i = 0 ; i < client->nstreams ; i++) {
        if (client->streams[i] == st)
            break;
    }
    if (i == client->nstreams)
        goto cleanup;

    if (client->nstreams > 1) {
        memmove(client->streams + i,
                client->streams + i + 1,
                sizeof(*client->streams) *
                (client->nstreams - (i + 1)));
        VIR_SHRINK_N(client->streams, client->nstreams, 1);
    } else {
        VIR_FREE(client->streams);
        client->nstreams = 0;
    }
    virNetClientStreamFree(st);

cleanup:
    virNetClientUnlock(client);
}


const char *virNetClientLocalAddrString(virNetClientPtr client)
{
    return virNetSocketLocalAddrString(client->sock);
}

const char *virNetClientRemoteAddrString(virNetClientPtr client)
{
    return virNetSocketRemoteAddrString(client->sock);
}

int virNetClientGetTLSKeySize(virNetClientPtr client)
{
    int ret = 0;
    virNetClientLock(client);
    if (client->tls)
        ret = virNetTLSSessionGetKeySize(client->tls);
    virNetClientUnlock(client);
    return ret;
}

static int
virNetClientCallDispatchReply(virNetClientPtr client)
{
    virNetClientCallPtr thecall;

    /* Ok, definitely got an RPC reply now find
       out who's been waiting for it */
    thecall = client->waitDispatch;
    while (thecall &&
           !(thecall->msg->header.prog == client->msg.header.prog &&
             thecall->msg->header.vers == client->msg.header.vers &&
             thecall->msg->header.serial == client->msg.header.serial))
        thecall = thecall->next;

    if (!thecall) {
        virNetError(VIR_ERR_RPC,
                    _("no call waiting for reply with prog %d vers %d serial %d"),
                    client->msg.header.prog, client->msg.header.vers, client->msg.header.serial);
        return -1;
    }

    memcpy(thecall->msg->buffer, client->msg.buffer, sizeof(client->msg.buffer));
    memcpy(&thecall->msg->header, &client->msg.header, sizeof(client->msg.header));
    thecall->msg->bufferLength = client->msg.bufferLength;
    thecall->msg->bufferOffset = client->msg.bufferOffset;

    thecall->mode = VIR_NET_CLIENT_MODE_COMPLETE;

    return 0;
}

static int virNetClientCallDispatchMessage(virNetClientPtr client)
{
    size_t i;
    virNetClientProgramPtr prog = NULL;

    for (i = 0 ; i < client->nprograms ; i++) {
        if (virNetClientProgramMatches(client->programs[i],
                                       &client->msg)) {
            prog = client->programs[i];
            break;
        }
    }
    if (!prog) {
        VIR_DEBUG("No program found for event with prog=%d vers=%d",
                  client->msg.header.prog, client->msg.header.vers);
        return -1;
    }

    virNetClientProgramDispatch(prog, client, &client->msg);

    return 0;
}

static int virNetClientCallDispatchStream(virNetClientPtr client)
{
    size_t i;
    virNetClientStreamPtr st = NULL;
    virNetClientCallPtr thecall;

    /* First identify what stream this packet is directed at */
    for (i = 0 ; i < client->nstreams ; i++) {
        if (virNetClientStreamMatches(client->streams[i],
                                      &client->msg)) {
            st = client->streams[i];
            break;
        }
    }
    if (!st) {
        VIR_DEBUG("No stream found for packet with prog=%d vers=%d serial=%u proc=%u",
                  client->msg.header.prog, client->msg.header.vers,
                  client->msg.header.serial, client->msg.header.proc);
        return -1;
    }

    /* Finish/Abort are synchronous, so also see if there's an
     * (optional) call waiting for this stream packet */
    thecall = client->waitDispatch;
    while (thecall &&
           !(thecall->msg->header.prog == client->msg.header.prog &&
             thecall->msg->header.vers == client->msg.header.vers &&
             thecall->msg->header.serial == client->msg.header.serial))
        thecall = thecall->next;

    VIR_DEBUG("Found call %p", thecall);

    /* Status is either
     *   - REMOTE_OK - no payload for streams
     *   - REMOTE_ERROR - followed by a remote_error struct
     *   - REMOTE_CONTINUE - followed by a raw data packet
     */
    switch (client->msg.header.status) {
    case VIR_NET_CONTINUE: {
        if (virNetClientStreamQueuePacket(st, &client->msg) < 0)
            return -1;

        if (thecall && thecall->expectReply) {
            VIR_DEBUG("Got sync data packet completion");
            thecall->mode = VIR_NET_CLIENT_MODE_COMPLETE;
        }
        return 0;
    }

    case VIR_NET_OK:
        if (thecall && thecall->expectReply) {
            VIR_DEBUG("Got a synchronous confirm");
            thecall->mode = VIR_NET_CLIENT_MODE_COMPLETE;
        } else {
            VIR_DEBUG("Got unexpected async stream finish confirmation");
            return -1;
        }
        return 0;

    case VIR_NET_ERROR:
        /* No call, so queue the error against the stream */
        if (virNetClientStreamSetError(st, &client->msg) < 0)
            return -1;

        if (thecall && thecall->expectReply) {
            VIR_DEBUG("Got a synchronous error");
            /* Raise error now, so that this call will see it immediately */
            virNetClientStreamRaiseError(st);
            thecall->mode = VIR_NET_CLIENT_MODE_COMPLETE;
        }
        return 0;

    default:
        VIR_WARN("Stream with unexpected serial=%d, proc=%d, status=%d",
                 client->msg.header.serial, client->msg.header.proc,
                 client->msg.header.status);
        return -1;
    }

    return 0;
}


static int
virNetClientCallDispatch(virNetClientPtr client)
{
    if (virNetMessageDecodeHeader(&client->msg) < 0)
        return -1;

    VIR_DEBUG("Incoming message prog %d vers %d proc %d type %d status %d serial %d",
              client->msg.header.prog, client->msg.header.vers,
              client->msg.header.proc, client->msg.header.type,
              client->msg.header.status, client->msg.header.serial);

    switch (client->msg.header.type) {
    case VIR_NET_REPLY: /* Normal RPC replies */
        return virNetClientCallDispatchReply(client);

    case VIR_NET_MESSAGE: /* Async notifications */
        return virNetClientCallDispatchMessage(client);

    case VIR_NET_STREAM: /* Stream protocol */
        return virNetClientCallDispatchStream(client);

    default:
        virNetError(VIR_ERR_RPC,
                    _("got unexpected RPC call prog %d vers %d proc %d type %d"),
                    client->msg.header.prog, client->msg.header.vers,
                    client->msg.header.proc, client->msg.header.type);
        return -1;
    }
}


static ssize_t
virNetClientIOWriteMessage(virNetClientPtr client,
                           virNetClientCallPtr thecall)
{
    ssize_t ret;

    ret = virNetSocketWrite(client->sock,
                            thecall->msg->buffer + thecall->msg->bufferOffset,
                            thecall->msg->bufferLength - thecall->msg->bufferOffset);
    if (ret <= 0)
        return ret;

    thecall->msg->bufferOffset += ret;

    if (thecall->msg->bufferOffset == thecall->msg->bufferLength) {
        thecall->msg->bufferOffset = thecall->msg->bufferLength = 0;
        if (thecall->expectReply)
            thecall->mode = VIR_NET_CLIENT_MODE_WAIT_RX;
        else
            thecall->mode = VIR_NET_CLIENT_MODE_COMPLETE;
    }

    return ret;
}


static ssize_t
virNetClientIOHandleOutput(virNetClientPtr client)
{
    virNetClientCallPtr thecall = client->waitDispatch;

    while (thecall &&
           thecall->mode != VIR_NET_CLIENT_MODE_WAIT_TX)
        thecall = thecall->next;

    if (!thecall)
        return -1; /* Shouldn't happen, but you never know... */

    while (thecall) {
        ssize_t ret = virNetClientIOWriteMessage(client, thecall);
        if (ret < 0)
            return ret;

        if (thecall->mode == VIR_NET_CLIENT_MODE_WAIT_TX)
            return 0; /* Blocking write, to back to event loop */

        thecall = thecall->next;
    }

    return 0; /* No more calls to send, all done */
}

static ssize_t
virNetClientIOReadMessage(virNetClientPtr client)
{
    size_t wantData;
    ssize_t ret;

    /* Start by reading length word */
    if (client->msg.bufferLength == 0)
        client->msg.bufferLength = 4;

    wantData = client->msg.bufferLength - client->msg.bufferOffset;

    ret = virNetSocketRead(client->sock,
                           client->msg.buffer + client->msg.bufferOffset,
                           wantData);
    if (ret <= 0)
        return ret;

    client->msg.bufferOffset += ret;

    return ret;
}


static ssize_t
virNetClientIOHandleInput(virNetClientPtr client)
{
    /* Read as much data as is available, until we get
     * EAGAIN
     */
    for (;;) {
        ssize_t ret = virNetClientIOReadMessage(client);

        if (ret < 0)
            return -1;
        if (ret == 0)
            return 0;  /* Blocking on read */

        /* Check for completion of our goal */
        if (client->msg.bufferOffset == client->msg.bufferLength) {
            if (client->msg.bufferOffset == 4) {
                ret = virNetMessageDecodeLength(&client->msg);
                if (ret < 0)
                    return -1;

                /*
                 * We'll carry on around the loop to immediately
                 * process the message body, because it has probably
                 * already arrived. Worst case, we'll get EAGAIN on
                 * next iteration.
                 */
            } else {
                ret = virNetClientCallDispatch(client);
                client->msg.bufferOffset = client->msg.bufferLength = 0;
                /*
                 * We've completed one call, but we don't want to
                 * spin around the loop forever if there are many
                 * incoming async events, or replies for other
                 * thread's RPC calls. We want to get out & let
                 * any other thread take over as soon as we've
                 * got our reply. When SASL is active though, we
                 * may have read more data off the wire than we
                 * initially wanted & cached it in memory. In this
                 * case, poll() would not detect that there is more
                 * ready todo.
                 *
                 * So if SASL is active *and* some SASL data is
                 * already cached, then we'll process that now,
                 * before returning.
                 */
                if (ret == 0 &&
                    virNetSocketHasCachedData(client->sock))
                    continue;
                return ret;
            }
        }
    }
}


/*
 * Process all calls pending dispatch/receive until we
 * get a reply to our own call. Then quit and pass the buck
 * to someone else.
 */
static int virNetClientIOEventLoop(virNetClientPtr client,
                                   virNetClientCallPtr thiscall)
{
    struct pollfd fds[2];
    int ret;

    fds[0].fd = virNetSocketGetFD(client->sock);
    fds[1].fd = client->wakeupReadFD;

    for (;;) {
        virNetClientCallPtr tmp = client->waitDispatch;
        virNetClientCallPtr prev;
        char ignore;
#ifdef HAVE_PTHREAD_SIGMASK
        sigset_t oldmask, blockedsigs;
#endif
        int timeout = -1;

        /* If we have existing SASL decoded data we
         * don't want to sleep in the poll(), just
         * check if any other FDs are also ready
         */
        if (virNetSocketHasCachedData(client->sock))
            timeout = 0;

        fds[0].events = fds[0].revents = 0;
        fds[1].events = fds[1].revents = 0;

        fds[1].events = POLLIN;
        while (tmp) {
            if (tmp->mode == VIR_NET_CLIENT_MODE_WAIT_RX)
                fds[0].events |= POLLIN;
            if (tmp->mode == VIR_NET_CLIENT_MODE_WAIT_TX)
                fds[0].events |= POLLOUT;

            tmp = tmp->next;
        }

        /* We have to be prepared to receive stream data
         * regardless of whether any of the calls waiting
         * for dispatch are for streams.
         */
        if (client->nstreams)
            fds[0].events |= POLLIN;

        /* Release lock while poll'ing so other threads
         * can stuff themselves on the queue */
        virNetClientUnlock(client);

        /* Block SIGWINCH from interrupting poll in curses programs,
         * then restore the original signal mask again immediately
         * after the call (RHBZ#567931).  Same for SIGCHLD and SIGPIPE
         * at the suggestion of Paolo Bonzini and Daniel Berrange.
         */
#ifdef HAVE_PTHREAD_SIGMASK
        sigemptyset (&blockedsigs);
        sigaddset (&blockedsigs, SIGWINCH);
        sigaddset (&blockedsigs, SIGCHLD);
        sigaddset (&blockedsigs, SIGPIPE);
        ignore_value(pthread_sigmask(SIG_BLOCK, &blockedsigs, &oldmask));
#endif

    repoll:
        ret = poll(fds, ARRAY_CARDINALITY(fds), timeout);
        if (ret < 0 && errno == EAGAIN)
            goto repoll;

#ifdef HAVE_PTHREAD_SIGMASK
        ignore_value(pthread_sigmask(SIG_SETMASK, &oldmask, NULL));
#endif

        virNetClientLock(client);

        /* If we have existing SASL decoded data, pretend
         * the socket became readable so we consume it
         */
        if (virNetSocketHasCachedData(client->sock))
            fds[0].revents |= POLLIN;

        if (fds[1].revents) {
            VIR_DEBUG("Woken up from poll by other thread");
            if (saferead(client->wakeupReadFD, &ignore, sizeof(ignore)) != sizeof(ignore)) {
                virReportSystemError(errno, "%s",
                                     _("read on wakeup fd failed"));
                goto error;
            }
        }

        if (ret < 0) {
            if (errno == EWOULDBLOCK)
                continue;
            virReportSystemError(errno,
                                 "%s", _("poll on socket failed"));
            goto error;
        }

        if (fds[0].revents & POLLOUT) {
            if (virNetClientIOHandleOutput(client) < 0)
                goto error;
        }

        if (fds[0].revents & POLLIN) {
            if (virNetClientIOHandleInput(client) < 0)
                goto error;
        }

        /* Iterate through waiting threads and if
         * any are complete then tell 'em to wakeup
         */
        tmp = client->waitDispatch;
        prev = NULL;
        while (tmp) {
            if (tmp != thiscall &&
                tmp->mode == VIR_NET_CLIENT_MODE_COMPLETE) {
                /* Take them out of the list */
                if (prev)
                    prev->next = tmp->next;
                else
                    client->waitDispatch = tmp->next;

                /* And wake them up....
                 * ...they won't actually wakeup until
                 * we release our mutex a short while
                 * later...
                 */
                VIR_DEBUG("Waking up sleep %p %p", tmp, client->waitDispatch);
                virCondSignal(&tmp->cond);
            } else {
                prev = tmp;
            }
            tmp = tmp->next;
        }

        /* Now see if *we* are done */
        if (thiscall->mode == VIR_NET_CLIENT_MODE_COMPLETE) {
            /* We're at head of the list already, so
             * remove us
             */
            client->waitDispatch = thiscall->next;
            VIR_DEBUG("Giving up the buck %p %p", thiscall, client->waitDispatch);
            /* See if someone else is still waiting
             * and if so, then pass the buck ! */
            if (client->waitDispatch) {
                VIR_DEBUG("Passing the buck to %p", client->waitDispatch);
                virCondSignal(&client->waitDispatch->cond);
            }
            return 0;
        }


        if (fds[0].revents & (POLLHUP | POLLERR)) {
            virNetError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("received hangup / error event on socket"));
            goto error;
        }
    }


error:
    client->waitDispatch = thiscall->next;
    VIR_DEBUG("Giving up the buck due to I/O error %p %p", thiscall, client->waitDispatch);
    /* See if someone else is still waiting
     * and if so, then pass the buck ! */
    if (client->waitDispatch) {
        VIR_DEBUG("Passing the buck to %p", client->waitDispatch);
        virCondSignal(&client->waitDispatch->cond);
    }
    return -1;
}


/*
 * This function sends a message to remote server and awaits a reply
 *
 * NB. This does not free the args structure (not desirable, since you
 * often want this allocated on the stack or else it contains strings
 * which come from the user).  It does however free any intermediate
 * results, eg. the error structure if there is one.
 *
 * NB(2). Make sure to memset (&ret, 0, sizeof ret) before calling,
 * else Bad Things will happen in the XDR code.
 *
 * NB(3) You must have the client lock before calling this
 *
 * NB(4) This is very complicated. Multiple threads are allowed to
 * use the client for RPC at the same time. Obviously only one of
 * them can. So if someone's using the socket, other threads are put
 * to sleep on condition variables. The existing thread may completely
 * send & receive their RPC call/reply while they're asleep. Or it
 * may only get around to dealing with sending the call. Or it may
 * get around to neither. So upon waking up from slumber, the other
 * thread may or may not have more work todo.
 *
 * We call this dance  'passing the buck'
 *
 *      http://en.wikipedia.org/wiki/Passing_the_buck
 *
 *   "Buck passing or passing the buck is the action of transferring
 *    responsibility or blame unto another person. It is also used as
 *    a strategy in power politics when the actions of one country/
 *    nation are blamed on another, providing an opportunity for war."
 *
 * NB(5) Don't Panic!
 */
static int virNetClientIO(virNetClientPtr client,
                          virNetClientCallPtr thiscall)
{
    int rv = -1;

    VIR_DEBUG("program=%u version=%u serial=%u proc=%d type=%d length=%zu dispatch=%p",
              thiscall->msg->header.prog,
              thiscall->msg->header.vers,
              thiscall->msg->header.serial,
              thiscall->msg->header.proc,
              thiscall->msg->header.type,
              thiscall->msg->bufferLength,
              client->waitDispatch);

    /* Check to see if another thread is dispatching */
    if (client->waitDispatch) {
        /* Stick ourselves on the end of the wait queue */
        virNetClientCallPtr tmp = client->waitDispatch;
        char ignore = 1;
        while (tmp && tmp->next)
            tmp = tmp->next;
        if (tmp)
            tmp->next = thiscall;
        else
            client->waitDispatch = thiscall;

        /* Force other thread to wakeup from poll */
        if (safewrite(client->wakeupSendFD, &ignore, sizeof(ignore)) != sizeof(ignore)) {
            if (tmp)
                tmp->next = NULL;
            else
                client->waitDispatch = NULL;
            virReportSystemError(errno, "%s",
                                 _("failed to wake up polling thread"));
            return -1;
        }

        VIR_DEBUG("Going to sleep %p %p", client->waitDispatch, thiscall);
        /* Go to sleep while other thread is working... */
        if (virCondWait(&thiscall->cond, &client->lock) < 0) {
            if (client->waitDispatch == thiscall) {
                client->waitDispatch = thiscall->next;
            } else {
                tmp = client->waitDispatch;
                while (tmp && tmp->next &&
                       tmp->next != thiscall) {
                    tmp = tmp->next;
                }
                if (tmp && tmp->next == thiscall)
                    tmp->next = thiscall->next;
            }
            virNetError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("failed to wait on condition"));
            return -1;
        }

        VIR_DEBUG("Wokeup from sleep %p %p", client->waitDispatch, thiscall);
        /* Two reasons we can be woken up
         *  1. Other thread has got our reply ready for us
         *  2. Other thread is all done, and it is our turn to
         *     be the dispatcher to finish waiting for
         *     our reply
         */
        if (thiscall->mode == VIR_NET_CLIENT_MODE_COMPLETE) {
            rv = 0;
            /*
             * We avoided catching the buck and our reply is ready !
             * We've already had 'thiscall' removed from the list
             * so just need to (maybe) handle errors & free it
             */
            goto cleanup;
        }

        /* Grr, someone passed the buck onto us ... */

    } else {
        /* We're first to catch the buck */
        client->waitDispatch = thiscall;
    }

    VIR_DEBUG("We have the buck %p %p", client->waitDispatch, thiscall);
    /*
     * The buck stops here!
     *
     * At this point we're about to own the dispatch
     * process...
     */

    /*
     * Avoid needless wake-ups of the event loop in the
     * case where this call is being made from a different
     * thread than the event loop. These wake-ups would
     * cause the event loop thread to be blocked on the
     * mutex for the duration of the call
     */
    virNetSocketUpdateIOCallback(client->sock, 0);

    virResetLastError();
    rv = virNetClientIOEventLoop(client, thiscall);

    virNetSocketUpdateIOCallback(client->sock, VIR_EVENT_HANDLE_READABLE);

    if (rv == 0 &&
        virGetLastError())
        rv = -1;

cleanup:
    VIR_DEBUG("All done with our call %p %p %d", client->waitDispatch, thiscall, rv);
    return rv;
}


void virNetClientIncomingEvent(virNetSocketPtr sock,
                               int events,
                               void *opaque)
{
    virNetClientPtr client = opaque;

    virNetClientLock(client);

    /* This should be impossible, but it doesn't hurt to check */
    if (client->waitDispatch)
        goto done;

    VIR_DEBUG("Event fired %p %d", sock, events);

    if (events & (VIR_EVENT_HANDLE_HANGUP | VIR_EVENT_HANDLE_ERROR)) {
        VIR_DEBUG("%s : VIR_EVENT_HANDLE_HANGUP or "
                  "VIR_EVENT_HANDLE_ERROR encountered", __FUNCTION__);
        virNetSocketRemoveIOCallback(sock);
        goto done;
    }

    if (virNetClientIOHandleInput(client) < 0)
        VIR_DEBUG("Something went wrong during async message processing");

done:
    virNetClientUnlock(client);
}


int virNetClientSend(virNetClientPtr client,
                     virNetMessagePtr msg,
                     bool expectReply)
{
    virNetClientCallPtr call;
    int ret = -1;

    if (VIR_ALLOC(call) < 0) {
        virReportOOMError();
        return -1;
    }

    virNetClientLock(client);

    if (virCondInit(&call->cond) < 0) {
        virNetError(VIR_ERR_INTERNAL_ERROR, "%s",
                    _("cannot initialize condition variable"));
        goto cleanup;
    }

    if (msg->bufferLength)
        call->mode = VIR_NET_CLIENT_MODE_WAIT_TX;
    else
        call->mode = VIR_NET_CLIENT_MODE_WAIT_RX;
    call->msg = msg;
    call->expectReply = expectReply;

    ret = virNetClientIO(client, call);

cleanup:
    ignore_value(virCondDestroy(&call->cond));
    VIR_FREE(call);
    virNetClientUnlock(client);
    return ret;
}