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Add functions for handling exponential backoff loops.
In a few places in libvirt we busy-wait for events, for example qemu creating a monitor socket. This is problematic because: - We need to choose a sufficiently small polling period so that libvirt doesn't add unnecessary delays. - We need to choose a sufficiently large polling period so that the effect of busy-waiting doesn't affect the system. The solution to this conflict is to use an exponential backoff. This patch adds two functions to hide the details, and modifies a few places where we currently busy-wait. Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
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@ -42,6 +42,7 @@
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#include "virfile.h"
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#include "configmake.h"
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#include "virstring.h"
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#include "virtime.h"
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#define VIR_FROM_THIS VIR_FROM_STREAMS
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@ -520,8 +521,7 @@ int virFDStreamConnectUNIX(virStreamPtr st,
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bool abstract)
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{
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struct sockaddr_un sa;
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size_t i = 0;
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int timeout = 3;
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virTimeBackOffVar timeout;
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int ret;
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int fd = socket(AF_UNIX, SOCK_STREAM, 0);
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@ -541,7 +541,9 @@ int virFDStreamConnectUNIX(virStreamPtr st,
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goto error;
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}
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do {
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if (virTimeBackOffStart(&timeout, 1, 3*1000 /* ms */) < 0)
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goto error;
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while (virTimeBackOffWait(&timeout)) {
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ret = connect(fd, (struct sockaddr *)&sa, sizeof(sa));
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if (ret == 0)
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break;
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@ -553,7 +555,7 @@ int virFDStreamConnectUNIX(virStreamPtr st,
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}
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goto error;
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} while ((++i <= timeout*5) && (usleep(.2 * 1000000) <= 0));
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}
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if (virFDStreamOpenInternal(st, fd, NULL, -1, 0) < 0)
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goto error;
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@ -2373,6 +2373,8 @@ virThreadPoolSendJob;
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# util/virtime.h
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virTimeBackOffStart;
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virTimeBackOffWait;
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virTimeFieldsNow;
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virTimeFieldsNowRaw;
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virTimeFieldsThen;
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@ -173,9 +173,8 @@ qemuAgentOpenUnix(const char *monitor, pid_t cpid, bool *inProgress)
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{
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struct sockaddr_un addr;
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int monfd;
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int timeout = 3; /* In seconds */
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int ret;
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size_t i = 0;
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virTimeBackOffVar timeout;
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int ret = -1;
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*inProgress = false;
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@ -207,7 +206,9 @@ qemuAgentOpenUnix(const char *monitor, pid_t cpid, bool *inProgress)
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goto error;
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}
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do {
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if (virTimeBackOffStart(&timeout, 1, 3*1000 /* ms */) < 0)
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goto error;
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while (virTimeBackOffWait(&timeout)) {
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ret = connect(monfd, (struct sockaddr *) &addr, sizeof(addr));
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if (ret == 0)
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@ -232,7 +233,7 @@ qemuAgentOpenUnix(const char *monitor, pid_t cpid, bool *inProgress)
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_("failed to connect to monitor socket"));
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goto error;
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} while ((++i <= timeout*5) && (usleep(.2 * 1000000) <= 0));
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}
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if (ret != 0) {
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virReportSystemError(errno, "%s",
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@ -42,6 +42,7 @@
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#include "virobject.h"
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#include "virprobe.h"
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#include "virstring.h"
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#include "virtime.h"
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#ifdef WITH_DTRACE_PROBES
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# include "libvirt_qemu_probes.h"
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@ -327,9 +328,8 @@ qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
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{
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struct sockaddr_un addr;
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int monfd;
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int timeout = 30; /* In seconds */
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int ret;
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size_t i = 0;
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virTimeBackOffVar timeout;
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int ret = -1;
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if ((monfd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
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virReportSystemError(errno,
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@ -345,7 +345,9 @@ qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
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goto error;
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}
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do {
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if (virTimeBackOffStart(&timeout, 1, 30*1000 /* ms */) < 0)
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goto error;
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while (virTimeBackOffWait(&timeout)) {
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ret = connect(monfd, (struct sockaddr *) &addr, sizeof(addr));
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if (ret == 0)
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@ -362,7 +364,7 @@ qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
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_("failed to connect to monitor socket"));
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goto error;
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} while ((++i <= timeout*5) && (usleep(.2 * 1000000) <= 0));
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}
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if (ret != 0) {
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virReportSystemError(errno, "%s",
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@ -34,14 +34,18 @@
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#include <config.h>
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#include <stdio.h>
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#include <unistd.h>
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#include <sys/time.h>
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#include "virtime.h"
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#include "viralloc.h"
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#include "virerror.h"
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#include "virlog.h"
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#define VIR_FROM_THIS VIR_FROM_NONE
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VIR_LOG_INIT("util.time");
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/* We prefer clock_gettime if available because that is officially
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* async signal safe according to POSIX. Many platforms lack it
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* though, so fallback to gettimeofday everywhere else
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@ -363,3 +367,80 @@ virTimeLocalOffsetFromUTC(long *offset)
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*offset = current - utc;
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return 0;
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}
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/**
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* virTimeBackOffStart:
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* @var: Timeout variable (with type virTimeBackOffVar).
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* @first: Initial time to wait (milliseconds).
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* @timeout: Timeout (milliseconds).
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*
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* Initialize the timeout variable @var and start the timer running.
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*
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* Returns 0 on success, -1 on error and raises a libvirt error.
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*/
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int
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virTimeBackOffStart(virTimeBackOffVar *var,
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unsigned long long first, unsigned long long timeout)
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{
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if (virTimeMillisNow(&var->start_t) < 0)
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return -1;
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var->next = first;
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var->limit_t = var->start_t + timeout;
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return 0;
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}
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/**
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* virTimeBackOffWait
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* @var: Timeout variable (with type virTimeBackOffVar *).
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*
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* You must initialize @var first by calling the following function,
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* which also starts the timer:
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*
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* if (virTimeBackOffStart(&var, first, timeout) < 0) {
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* // handle errors
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* }
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*
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* Then you use a while loop:
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*
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* while (virTimeBackOffWait(&var)) {
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* //...
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* }
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*
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* The while loop that runs the body of the code repeatedly, with an
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* exponential backoff. It first waits for first milliseconds, then
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* runs the body, then waits for 2*first ms, then runs the body again.
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* Then 4*first ms, and so on.
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*
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* When timeout milliseconds is reached, the while loop ends.
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*
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* The body should use "break" or "goto" when whatever condition it is
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* testing for succeeds (or there is an unrecoverable error).
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*/
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bool
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virTimeBackOffWait(virTimeBackOffVar *var)
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{
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unsigned long long t, next;
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ignore_value(virTimeMillisNowRaw(&t));
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VIR_DEBUG("t=%llu, limit=%llu", t, var->limit_t);
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if (t > var->limit_t)
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return 0; /* ends the while loop */
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next = var->next;
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var->next *= 2;
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/* If sleeping would take us beyond the limit, then shorten the
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* sleep. This is so we always run the body just before the final
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* timeout.
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*/
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if (t + next > var->limit_t)
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next = var->limit_t - t;
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VIR_DEBUG("sleeping for %llu ms", next);
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usleep(next * 1000);
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return 1;
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}
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@ -64,4 +64,15 @@ char *virTimeStringThen(unsigned long long when);
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int virTimeLocalOffsetFromUTC(long *offset)
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ATTRIBUTE_NONNULL(1) ATTRIBUTE_RETURN_CHECK;
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typedef struct {
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unsigned long long start_t;
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unsigned long long next;
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unsigned long long limit_t;
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} virTimeBackOffVar;
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int virTimeBackOffStart(virTimeBackOffVar *var,
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unsigned long long first, unsigned long long timeout);
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bool virTimeBackOffWait(virTimeBackOffVar *var);
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#endif
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