以下的分析基于libvirt 3.0版本。

   libvirt是一套免费，开源的支持linux下的主流虚拟化管理工具，目前有大量的应用程序构建在libvirt之上，很多虚拟化产品的开发都是灵活调用libvirt的API接口去实现的。对于应用程序，libvirt提供一套非阻塞调用的框架。

   涉及相关的API：

   virInitialize：初始化libvirt库，主要针对多线程编程

   virEventRegisterDefaultImpl：基于poll系统调用注册默认事件实现，这是一个通用实现。

   virEventRunDefaultImpl：循环运行事件，需要在线程中单独运行该函数

   virConnectOpen：连接libvirt服务端，即libvirtd

   virConnectSetKeepAlive：设置保活周期，此函数控制客户端发送keepalive消息。

相关的实现代码如下：

if (virInitialize()< 0) {

          printf("callvirInitialize initialize libvirt fail\n");

     return 1;

}

   

/* register defaultlibvirt event implement */

if (virEventRegisterDefaultImpl() < 0) {

//必须

    printf("failed to registerdefault event implementation\n");

    return 2;

}

 

//创建线程去分发事件，必须

void*libvirt_thread_cb(void *data)

{

    (void)data;

   

    while (!isexit) {

        virEventRunDefaultImpl();

    }

    pthread_exit((void*)"libvirt pthreadwill exit!!!");

    return (void*)0;

}

 

//连接libvirt服务端，并且设计保活机制

contor =virConnectOpen(NULL);//参数为空，表示连接本地的libvirtd服务端

if (contor) {

    //第二个参数，心跳发送周期；  第三个参数，心跳参数，当超过该次数时，连接断开

    if (virConnectSetKeepAlive(contor, 10, 6)< 0) {

       printf("failed to set connkeep alive config\n");

        virConnectClose(contor);

        contor = NULL;

    }

}

通过以上的实现，后续就可以非阻塞调用libvirt其它的API接口，当libvirtd阻塞时，能够超时返回。

对于libvirtd端相关的心跳周期保存在libvirtd.conf文件中，可以修改参数，然后再重启libvirtd即可生效：

keepalive_interval = 10 //default is 5s

keepalive_count = 6 //default is 5s

相关的源码分析：

virEventRegisterDefaultImpl函数分析，源码如下：

int virEventRegisterDefaultImpl(void)

{     VIR_DEBUG("registering default event implementation");    virResetLastError();    if (virEventPollInit() < 0) {         virDispatchError(NULL);        return -1;    }    virEventRegisterImpl(        virEventPollAddHandle,        virEventPollUpdateHandle,        virEventPollRemoveHandle,        virEventPollAddTimeout,        virEventPollUpdateTimeout,        virEventPollRemoveTimeout        );    return 0;}通过源码分析，调用virEventRegisterImpl函数对全局函数赋值，为什么会这样做，下一步再分析

virConnectSetKeepAlive函数分析：

通过源码分析，最终会调用virKeepAliveStart函数去启用定时器，发送keepalive消息。

virKeepAliveStart(virKeepAlivePtr ka,

                  int interval,                  unsigned int count){     int ret = -1;    time_t delay;    int timeout;    time_t now;    virObjectLock(ka);    if (ka->timer >= 0) { //如果定时器存在，不做处理        VIR_DEBUG("Keepalive messages already enabled");        ret = 0;        goto cleanup;    }    if (interval > 0) {         if (ka->interval > 0) { //心跳周期已设置，不在设置            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",                           _("keepalive interval already set"));            goto cleanup;        }        /* Guard against overflow */        if (interval > INT_MAX / 1000) {             virReportError(VIR_ERR_INTERNAL_ERROR,                           _("keepalive interval %d too large"), interval);            goto cleanup;        }        ka->interval = interval;        ka->count = count;        ka->countToDeath = count;    }    if (ka->interval <= 0) { //心跳周期小于0，禁用keepalive        VIR_DEBUG("Keepalive messages disabled by configuration");        ret = 0;        goto cleanup;    }    PROBE(RPC_KEEPALIVE_START,          "ka=%p client=%p interval=%d count=%u",          ka, ka->client, interval, count);    now = time(NULL);    delay = now - ka->lastPacketReceived;    if (delay > ka->interval)        timeout = 0;    else        timeout = ka->interval - delay;    ka->intervalStart = now - (ka->interval - timeout);    ka->timer = virEventAddTimeout(timeout * 1000, virKeepAliveTimer,                                   ka, virObjectFreeCallback);//创建心跳定时器    if (ka->timer < 0)        goto cleanup;    /* the timer now has another reference to this object */    virObjectRef(ka);    ret = 0; cleanup:    virObjectUnlock(ka);    return ret;}继续分析virEventAddTimeout函数，源码如下：

int

virEventAddTimeout(int timeout,                   virEventTimeoutCallback cb,                   void *opaque,                   virFreeCallback ff){     if (!addTimeoutImpl)        return -1;    return addTimeoutImpl(timeout, cb, opaque, ff);}

该函数实现很简单，调用全局的函数去设置定时器，addTimeoutImpl是全局的函数接口，这个函数的赋值是应用程式调用virEventRegisterDefaultImpl函数去设置的，这是为什么需要调用virEventRegisterDefaultImpl函数的原因。

如果没有提供以上的设置，为什么会阻塞，阻塞在哪个地方，通过gdb的调用，发现阻塞在virNetClientIOEventLoop函数中的poll系统调用上。堆栈如下：

(gdb) bt

#0  virNetClientIOEventLoop (client=0x2026d30, thiscall=0x1dc57a0) at ../../../src/rpc/virnetclient.c:1595#1  0x00007fd5dc8368a3 in virNetClientIO (client=0x2026d30, thiscall=0x1dc57a0) at ../../../src/rpc/virnetclient.c:1950#2  0x00007fd5dc8370b7 in virNetClientSendInternal (client=0x2026d30, msg=0x2026c60, expectReply=true, nonBlock=false) at ../../../src/rpc/virnetclient.c:2122#3  0x00007fd5dc837141 in virNetClientSendWithReply (client=0x2026d30, msg=0x2026c60) at ../../../src/rpc/virnetclient.c:2150#4  0x00007fd5dc838048 in virNetClientProgramCall (prog=0x2027150, client=0x2026d30, serial=8, proc=212, noutfds=0, outfds=0x0, ninfds=0x0, infds=0x0, args_filter=0x7fd5dc82ce9f <xdr_remote_domain_get_state_args>,     args=0x7fffd92ffba0, ret_filter=0x7fd5dc82cf19 <xdr_remote_domain_get_state_ret>, ret=0x7fffd92ffb80) at ../../../src/rpc/virnetclientprogram.c:329#5  0x00007fd5dc819442 in callFull (conn=0x1a24380, priv=0x1a77660, flags=0, fdin=0x0, fdinlen=0, fdout=0x0, fdoutlen=0x0, proc_nr=212, args_filter=0x7fd5dc82ce9f <xdr_remote_domain_get_state_args>,     args=0x7fffd92ffba0 "0T\002\002", ret_filter=0x7fd5dc82cf19 <xdr_remote_domain_get_state_ret>, ret=0x7fffd92ffb80 "") at ../../../src/remote/remote_driver.c:6637#6  0x00007fd5dc819515 in call (conn=0x1a24380, priv=0x1a77660, flags=0, proc_nr=212, args_filter=0x7fd5dc82ce9f <xdr_remote_domain_get_state_args>, args=0x7fffd92ffba0 "0T\002\002",     ret_filter=0x7fd5dc82cf19 <xdr_remote_domain_get_state_ret>, ret=0x7fffd92ffb80 "") at ../../../src/remote/remote_driver.c:6659#7  0x00007fd5dc7fd77b in remoteDomainGetState (domain=0x1dc8f60, state=0x7fffd92ffcdc, reason=0x0, flags=0) at ../../../src/remote/remote_driver.c:2458#8  0x00007fd5dc7b5b2f in virDomainGetState (domain=0x1dc8f60, state=0x7fffd92ffcdc, reason=0x0, flags=0) at ../../../src/libvirt-domain.c:2495

virNetClientIOEventLoop函数分析：

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 (;;) {         /* If we are non-blocking, then we don't want to sleep in poll() */        if (thiscall->nonBlock)            timeout = 0;        /* Limit timeout so that we can send keepalive request in time */        if (timeout == -1)            timeout = virKeepAliveTimeout(client->keepalive);//返回-1，导致poll阻塞        fds[0].events = fds[0].revents = 0;        fds[1].events = fds[1].revents = 0;        fds[1].events = POLLIN;        /* Calculate poll events for calls */        virNetClientCallMatchPredicate(client->waitDispatch,                                       virNetClientIOEventLoopPollEvents,                                       &fds[0]);        if (client->nstreams)            fds[0].events |= POLLIN;    repoll:        ret = poll(fds, ARRAY_CARDINALITY(fds), timeout);        if (ret < 0 && (errno == EAGAIN || errno == EINTR))            goto repoll;

}

        通过源码可以分析，由于应用程序没有调用virConnectSetKeepAlive函数设置心跳保活机制，导致client->keepalive为NULL，分析virKeepAliveTimeout函数可知，当client->keepalive为NULL时，直接返回为-1；导致poll系统调用一直阻塞，直到有事件响应。

       总结：通过以上设置，调用virConnectOpen函数连接libvirt的时，就可以实现非阻塞调用libvirt其它的API，当libvirt阻塞时，不会导致调用者阻塞。

       当libvirt主线程阻塞时，上述的设置并不能解决virConnectOpen阻塞的问题，需要修改libvirt相关的代码。至于为什么，自己去思考，怎么解决这个问题？