以下的分析基於libvirt 3.0版本。
libvirt是一套免費，開源的支持linux下的主流虛擬化管理工具，目前有大量的應用程序構建在libvirt之上，很多虛擬化產品的開發都是靈活調用libvirt的API接口去實現的。對於應用程序，libvirt提供一套非阻塞調用的框架。
涉及相關的API：
virInitialize：初始化libvirt庫，主要針對多線程編程
virEventRegisterDefaultImpl：基於poll系統調用註冊默認事件實現，這是一個通用實現。
virEventRunDefaultImpl：循環運行事件，需要在線程中單獨運行該函數
virConnectOpen：連接libvirt服務端，即libvirtd
virConnectSetKeepAlive：設置保活週期，此函數控制客戶端發送keepalive消息。
相關的實現代碼如下：
if (virInitialize()< 0) {
printf("callvirInitialize initialize libvirt fail\n");
return 1;
}
/* register defaultlibvirt event implement */
if (virEventRegisterDefaultImpl() < 0) { //必須
printf("failed to registerdefault event implementation\n");
return 2;
}
//創建線程去分發事件，必須
void*libvirt_thread_cb(void *data)
{
(void)data;
while (!isexit) {
virEventRunDefaultImpl();
}
pthread_exit((void*)"libvirt pthreadwill exit!!!");
return (void*)0;
}
//連接libvirt服務端，並且設計保活機制
contor =virConnectOpen(NULL);//參數爲空，表示連接本地的libvirtd服務端
if (contor) {
//第二個參數，心跳發送週期； 第三個參數，心跳參數，當超過該次數時，連接斷開
if (virConnectSetKeepAlive(contor, 10, 6)< 0) {
printf("failed to set connkeep alive config\n");
virConnectClose(contor);
contor = NULL;
}
}
通過以上的實現，後續就可以非阻塞調用libvirt其它的API接口，當libvirtd阻塞時，能夠超時返回。
對於libvirtd端相關的心跳週期保存在libvirtd.conf文件中，可以修改參數，然後再重啓libvirtd即可生效：
keepalive_interval = 10 //default is 5s
keepalive_count = 6 //default is 5s
相關的源碼分析：
virEventRegisterDefaultImpl函數分析，源碼如下：
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;
}
通過源碼分析，調用virEventRegisterImpl函數對全局函數賦值，爲什麼會這樣做，下一步再分析
virConnectSetKeepAlive函數分析：
通過源碼分析，最終會調用virKeepAliveStart函數去啓用定時器，發送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) {//如果定時器存在，不做處理
VIR_DEBUG("Keepalive messages already enabled");
ret = 0;
goto cleanup;
}
if (interval > 0) {
if (ka->interval > 0) {//心跳週期已設置，不在設置
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) {//心跳週期小於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);//創建心跳定時器
if (ka->timer < 0)
goto cleanup;
/* the timer now has another reference to this object */
virObjectRef(ka);
ret = 0;
cleanup:
virObjectUnlock(ka);
return ret;
}
繼續分析virEventAddTimeout函數，源碼如下：
int
virEventAddTimeout(int timeout,
virEventTimeoutCallback cb,
void *opaque,
virFreeCallback ff)
{
if (!addTimeoutImpl)
return -1;
return addTimeoutImpl(timeout, cb, opaque, ff);
}
該函數實現很簡單，調用全局的函數去設置定時器，addTimeoutImpl是全局的函數接口，這個函數的賦值是應用程式調用virEventRegisterDefaultImpl函數去設置的，這是爲什麼需要調用virEventRegisterDefaultImpl函數的原因。
如果沒有提供以上的設置，爲什麼會阻塞，阻塞在哪個地方，通過gdb的調用，發現阻塞在virNetClientIOEventLoop函數中的poll系統調用上。堆棧如下：
(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函數分析：
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，導致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;
}
通過源碼可以分析，由於應用程序沒有調用virConnectSetKeepAlive函數設置心跳保活機制，導致client->keepalive爲NULL，分析virKeepAliveTimeout函數可知，當client->keepalive爲NULL時，直接返回爲-1；導致poll系統調用一直阻塞，直到有事件響應。
總結：通過以上設置，調用virConnectOpen函數連接libvirt的時，就可以實現非阻塞調用libvirt其它的API，當libvirt阻塞時，不會導致調用者阻塞。
當libvirt主線程阻塞時，上述的設置並不能解決virConnectOpen阻塞的問題，需要修改libvirt相關的代碼。至於爲什麼，自己去思考，怎麼解決這個問題？