sync: Make nudge + config jobs reliable in SyncerThread2

chrome/browser/sync/engine/syncer_thread2.cc

Index: chrome/browser/sync/engine/syncer_thread2.cc
diff --git a/chrome/browser/sync/engine/syncer_thread2.cc b/chrome/browser/sync/engine/syncer_thread2.cc
index e94215e1b5f809ac682d54865f0e2085d812261a..9d198b8afc6c0ac0bed68190bbae1c14224923b1 100644
--- a/chrome/browser/sync/engine/syncer_thread2.cc
+++ b/chrome/browser/sync/engine/syncer_thread2.cc
@@ -23,37 +23,6 @@ using sync_pb::GetUpdatesCallerInfo;
 
 namespace s3 {
 
-struct SyncerThread::WaitInterval {
-  enum Mode {
-    // A wait interval whose duration has been affected by exponential
-    // backoff.
-    // EXPONENTIAL_BACKOFF intervals are nudge-rate limited to 1 per interval.
-    EXPONENTIAL_BACKOFF,
-    // A server-initiated throttled interval.  We do not allow any syncing
-    // during such an interval.
-    THROTTLED,
-  };
-  Mode mode;
-
-  // This bool is set to true if we have observed a nudge during this
-  // interval and mode == EXPONENTIAL_BACKOFF.
-  bool had_nudge;
-  base::TimeDelta length;
-  base::OneShotTimer<SyncerThread> timer;
-  WaitInterval(Mode mode, base::TimeDelta length);
-};
-
-struct SyncerThread::SyncSessionJob {
-  SyncSessionJobPurpose purpose;
-  base::TimeTicks scheduled_start;
-  linked_ptr<sessions::SyncSession> session;
-
-  // This is the location the nudge came from. used for debugging purpose.
-  // In case of multiple nudges getting coalesced this stores the first nudge
-  // that came in.
-  tracked_objects::Location nudge_location;
-};
-
 SyncerThread::DelayProvider::DelayProvider() {}
 SyncerThread::DelayProvider::~DelayProvider() {}
 
@@ -62,6 +31,23 @@ TimeDelta SyncerThread::DelayProvider::GetDelay(
   return SyncerThread::GetRecommendedDelay(last_delay);
 }
 
+GetUpdatesCallerInfo::GetUpdatesSource GetUpdatesFromNudgeSource(
+    NudgeSource source) {
+  switch (source) {
+    case NUDGE_SOURCE_NOTIFICATION:
+      return GetUpdatesCallerInfo::NOTIFICATION;
+    case NUDGE_SOURCE_LOCAL:
+      return GetUpdatesCallerInfo::LOCAL;
+    case NUDGE_SOURCE_CONTINUATION:
+      return GetUpdatesCallerInfo::SYNC_CYCLE_CONTINUATION;
+    case NUDGE_SOURCE_UNKNOWN:
+      return GetUpdatesCallerInfo::UNKNOWN;
+    default:
+      NOTREACHED();
+      return GetUpdatesCallerInfo::UNKNOWN;
+  }
+}
+
 SyncerThread::WaitInterval::WaitInterval(Mode mode, TimeDelta length)
     : mode(mode), had_nudge(false), length(length) { }
 
@@ -96,6 +82,7 @@ void SyncerThread::CheckServerConnectionManagerStatus(
     server_connection_ok_ = false;
   } else if (HttpResponse::SERVER_CONNECTION_OK == code) {
     server_connection_ok_ = true;
+    ExecutePendingJob();
   }
 }
 
@@ -140,74 +127,115 @@ void SyncerThread::StartImpl(Mode mode,
   AdjustPolling(NULL);  // Will kick start poll timer if needed.
   if (callback.get())
     callback->Run();
+
+  ExecutePendingJob();

[tim (not reviewing), 2011/04/09 23:52:58]

this kind of looks out of place.  at the very least it needs a comment.

[lipalani1, 2011/04/12 02:33:00]

Done.

 }
 
-bool SyncerThread::ShouldRunJob(SyncSessionJobPurpose purpose,
-    const TimeTicks& scheduled_start) {
-  DCHECK_EQ(MessageLoop::current(), thread_.message_loop());
+SyncerThread::JobProcessDecision SyncerThread::DecideWhileInWaitInterval(
+    const SyncSessionJob& job) {
 
-  // Check wait interval.
-  if (wait_interval_.get()) {
-    // TODO(tim): Consider different handling for CLEAR_USER_DATA (i.e. permit
-    // when throttled).
-    if (wait_interval_->mode == WaitInterval::THROTTLED)
-      return false;
+  DCHECK(wait_interval_.get());
+  DCHECK(job.purpose != CLEAR_USER_DATA);

[tim (not reviewing), 2011/04/09 23:52:58]

DCHECK_NE

[lipalani1, 2011/04/12 02:33:00]

Done.

 
-    DCHECK_EQ(wait_interval_->mode, WaitInterval::EXPONENTIAL_BACKOFF);
-    if ((purpose != NUDGE) || wait_interval_->had_nudge)
-      return false;
-  }
+  if (job.purpose == POLL)
+    return DROP;
 
-  // Mode / purpose contract (See 'Mode' enum in header). Don't run jobs that
-  // were intended for a normal sync if we are in configuration mode, and vice
-  // versa.
-  switch (mode_) {
-    case CONFIGURATION_MODE:
-      if (purpose != CONFIGURATION)
-        return false;
-      break;
-    case NORMAL_MODE:
-      if (purpose == CONFIGURATION)
-        return false;
-      break;
-    default:
-      NOTREACHED() << "Unknown SyncerThread Mode: " << mode_;
-      return false;
+  DCHECK(job.purpose == NUDGE || job.purpose == CONFIGURATION);
+  if (wait_interval_->mode == WaitInterval::THROTTLED)
+    return SAVE;
+
+  DCHECK_EQ(wait_interval_->mode, WaitInterval::EXPONENTIAL_BACKOFF);
+  if (job.purpose == NUDGE) {
+    if (mode_ == CONFIGURATION_MODE)
+      return SAVE;
+
+    // If we already had one nudge then just drop this nudge. We will retry
+    // later when the timer runs out.
+    return wait_interval_->had_nudge ? DROP : CONTINUE;
+  } else {
+    // This is a config job. If our timer ran out then continue else save.

[tim (not reviewing), 2011/04/09 23:52:58]

Saying it's a config job is fine but I'd avoid referencing the timer.  I think
the line of code is self commenting in that regard. 

Also, you should remove the 'else' clause.

[lipalani1, 2011/04/12 02:33:00]

Done.

+    return job.is_canary_job ? CONTINUE : SAVE;
   }
+}
+
+SyncerThread::JobProcessDecision SyncerThread::DecideOnJob(
+    const SyncSessionJob& job) {
+  if (job.purpose == CLEAR_USER_DATA)
+    return CONTINUE;
+
+  if (wait_interval_.get())
+    return DecideWhileInWaitInterval(job);
 
-  // Continuation NUDGE tasks have priority over POLLs because they are the
-  // only tasks that trigger exponential backoff, so this prevents them from
-  // being starved from running (e.g. due to a very, very low poll interval,
-  // such as 0ms). It's rare that this would ever matter in practice.
-  if (purpose == POLL && (pending_nudge_.get() &&
-      pending_nudge_->session->source().updates_source ==
-          GetUpdatesCallerInfo::SYNC_CYCLE_CONTINUATION)) {
-    return false;
+  if (mode_ == CONFIGURATION_MODE) {
+    if (job.purpose == NUDGE) {

[tim (not reviewing), 2011/04/09 23:52:58]

remove the { }s from this inner if as they're 1-liners.

[lipalani1, 2011/04/12 02:33:00]

Done.

+      return SAVE;
+    } else if (job.purpose == CONFIGURATION) {
+      return CONTINUE;
+    } else {
+      return DROP;
+    }
   }
 
-  // Freshness condition.
-  if (purpose == NUDGE &&
-      (scheduled_start < last_sync_session_end_time_)) {
-    return false;
+  // We are in normal mode.
+  DCHECK(mode_ == NORMAL_MODE);

[tim (not reviewing), 2011/04/09 23:52:58]

DCHECK_EQ

[lipalani1, 2011/04/12 02:33:00]

Done.

+  DCHECK(job.purpose != CONFIGURATION);

[tim (not reviewing), 2011/04/09 23:52:58]

I don't think this should be a dcheck.  The mode changes asynchronously from job
requests, so nothing prevents a config task from trying to run in normal mode.
The previous ShouldRunJob function was designed to support this and just drop. 
There should have been tests that tried this :\ So I don't think we should
assert.

[lipalani1, 2011/04/12 02:33:00]

Not sure I follow this. Let me talk to you today when I get in.
On 2011/04/09 23:52:58, timsteele wrote:

+
+  // Freshness condition
+  if (job.scheduled_start < last_sync_session_end_time_)
+    return DROP;
+
+  if (server_connection_ok_)
+    return CONTINUE;
+
+  return job.purpose == NUDGE ? SAVE : DROP;
+}
+
+void SyncerThread::UpdatePendingJob(const SyncSessionJob& job) {

[tim (not reviewing), 2011/04/09 23:52:58]

I've looked at the callsites of this function and it leaves me kind of confused
as to its purpose; it seems each callsite is after precisely one of the two
cases in this method?  It might honestly be more clear to inline it instead.  I
realize this may have been an attempt to reduce the places we create a
SyncSessionJob.  Or what I might do is take the if (!pending_nudge_.get()) case
and put that in a method called CreatePendingJob, but remove the coalescing part
altogether and inline that.  It's helpful to be able to search for 'Coalesce'
and immediately find the places where that is performed.

[lipalani1, 2011/04/12 02:33:00]

There are 3 callsites. 2 of them(from savejob and handleconsecutiveerror could
get into both the if or else part. 
The one from ScheudleNudgeImpl can only get into the else part. I can move that
inline.
On 2011/04/09 23:52:58, timsteele wrote:

+  DCHECK(job.purpose != CONFIGURATION);
+  if (pending_nudge_.get() == NULL) {
+    SyncSession* s = job.session.get();
+    scoped_ptr<SyncSession> session(new SyncSession(s->context(),
+        s->delegate(), s->source(), s->routing_info(), s->workers()));
+
+    SyncSessionJob new_job = {NUDGE, job.scheduled_start,
+        make_linked_ptr(session.release()), false, job.nudge_location};
+    pending_nudge_.reset(new SyncSessionJob(new_job));
+
+    return;
   }
 
-  return server_connection_ok_;
+  pending_nudge_->session->Coalesce(*(job.session.get()));
+  pending_nudge_->scheduled_start = job.scheduled_start;
+
+  // Unfortunately the nudge location cannot be modified. So it stores the
+  // location of the first caller.
 }
 
-GetUpdatesCallerInfo::GetUpdatesSource GetUpdatesFromNudgeSource(
-    NudgeSource source) {
-  switch (source) {
-    case NUDGE_SOURCE_NOTIFICATION:
-      return GetUpdatesCallerInfo::NOTIFICATION;
-    case NUDGE_SOURCE_LOCAL:
-      return GetUpdatesCallerInfo::LOCAL;
-    case NUDGE_SOURCE_CONTINUATION:
-      return GetUpdatesCallerInfo::SYNC_CYCLE_CONTINUATION;
-    case NUDGE_SOURCE_UNKNOWN:
-      return GetUpdatesCallerInfo::UNKNOWN;
-    default:
-      NOTREACHED();
-      return GetUpdatesCallerInfo::UNKNOWN;
+bool SyncerThread::ShouldRunJob(const SyncSessionJob& job) {
+  JobProcessDecision decision = DecideOnJob(job);
+  if (decision != SAVE)
+    return decision == CONTINUE ? true : false;

[tim (not reviewing), 2011/04/09 23:52:58]

remove the ternary here, it is redundant to decision==CONTINUE.

[lipalani1, 2011/04/12 02:33:00]

ha ha ! may be you got confused as I mentioned!!(or may be I made a mistake).
Both are needed. There are 3 choices: save, drop and continue. if not save and
continue then return true . if not save and not continue return false. 
On 2011/04/09 23:52:58, timsteele wrote:

+
+  DCHECK(job.purpose == NUDGE || job.purpose == CONFIGURATION);
+
+  SaveJob(job);
+  return false;
+}
+
+void SyncerThread::SaveJob(const SyncSessionJob& job) {
+  DCHECK(job.purpose != CLEAR_USER_DATA);
+  if (job.purpose == NUDGE || job.purpose == POLL) {

[tim (not reviewing), 2011/04/09 23:52:58]

We should never want to save a POLL.

[lipalani1, 2011/04/12 02:33:00]

The case here is when in config but a poll comes in.
On 2011/04/09 23:52:58, timsteele wrote:

[tim (not reviewing), 2011/04/12 06:09:29]

In that case I claim we want to drop the poll.  We shouldn't do any work at all
to make them reliable; they are a backup.

[lipalani1, 2011/04/13 00:07:29]

Actually the code was already dropping it!!
On 2011/04/12 06:09:29, timsteele wrote:
Done.

+    UpdatePendingJob(job);
+  } else {
+    DCHECK(wait_interval_.get());
+    DCHECK(mode_ == CONFIGURATION_MODE);
+
+    SyncSession* old = job.session.get();
+    SyncSession* s(new SyncSession(session_context_.get(), this,
+        old->source(), old->routing_info(), old->workers()));
+    SyncSessionJob new_job = {job.purpose, TimeTicks::Now(),
+                          make_linked_ptr(s), false, job.nudge_location};
+    wait_interval_->pending_configure_job.reset(new SyncSessionJob(new_job));
   }
 }
 
@@ -240,8 +268,9 @@ void SyncerThread::ScheduleNudge(const TimeDelta& delay,
   ModelTypePayloadMap types_with_payloads =
       syncable::ModelTypePayloadMapFromBitSet(types, std::string());
   thread_.message_loop()->PostTask(FROM_HERE, NewRunnableMethod(
-      this, &SyncerThread::ScheduleNudgeImpl, delay, source,
-      types_with_payloads, nudge_location));
+      this, &SyncerThread::ScheduleNudgeImpl, delay,
+      GetUpdatesFromNudgeSource(source), types_with_payloads, false,
+      nudge_location));
 }
 
 void SyncerThread::ScheduleNudgeWithPayloads(const TimeDelta& delay,
@@ -253,8 +282,9 @@ void SyncerThread::ScheduleNudgeWithPayloads(const TimeDelta& delay,
   }
 
   thread_.message_loop()->PostTask(FROM_HERE, NewRunnableMethod(
-      this, &SyncerThread::ScheduleNudgeImpl, delay, source,
-      types_with_payloads, nudge_location));
+      this, &SyncerThread::ScheduleNudgeImpl, delay,
+      GetUpdatesFromNudgeSource(source), types_with_payloads, false,
+      nudge_location));
 }
 
 void SyncerThread::ScheduleClearUserDataImpl() {
@@ -267,15 +297,10 @@ void SyncerThread::ScheduleClearUserDataImpl() {
 }
 
 void SyncerThread::ScheduleNudgeImpl(const TimeDelta& delay,
-    NudgeSource source, const ModelTypePayloadMap& types_with_payloads,
-    const tracked_objects::Location& nudge_location) {
+    GetUpdatesCallerInfo::GetUpdatesSource source,
+    const ModelTypePayloadMap& types_with_payloads,
+    bool is_canary_job, const tracked_objects::Location& nudge_location) {
   DCHECK_EQ(MessageLoop::current(), thread_.message_loop());
-  TimeTicks rough_start = TimeTicks::Now() + delay;
-  if (!ShouldRunJob(NUDGE, rough_start)) {
-    LOG(WARNING) << "Dropping nudge at scheduling time, source = "
-                 << source;
-    return;
-  }
 
   // Note we currently nudge for all types regardless of the ones incurring
   // the nudge.  Doing different would throw off some syncer commands like
@@ -284,29 +309,38 @@ void SyncerThread::ScheduleNudgeImpl(const TimeDelta& delay,
   std::vector<ModelSafeWorker*> workers;
   session_context_->registrar()->GetModelSafeRoutingInfo(&routes);
   session_context_->registrar()->GetWorkers(&workers);
-  SyncSourceInfo info(GetUpdatesFromNudgeSource(source),
-                      types_with_payloads);
+  SyncSourceInfo info(source, types_with_payloads);
 
-  scoped_ptr<SyncSession> session(new SyncSession(
+  SyncSession* session(new SyncSession(
       session_context_.get(), this, info, routes, workers));
 
+  SyncSessionJob job = {NUDGE, TimeTicks::Now() + delay,
+                        make_linked_ptr(session), is_canary_job,
+                        nudge_location};
+
+  session = NULL;
+  if (!ShouldRunJob(job))
+    return;
+
   if (pending_nudge_.get()) {
     if (IsBackingOff() && delay > TimeDelta::FromSeconds(1))
       return;
 
-    pending_nudge_->session->Coalesce(*session.get());
+    UpdatePendingJob(job);

[tim (not reviewing), 2011/04/09 23:52:58]

I think this was more clear when we just did the call to Coalesce inline. 
Update just performs the same check anyway.  I'd prefer if we moved it back. 

[lipalani1, 2011/04/12 02:33:00]

Done.

 
     if (!IsBackingOff()) {
       return;
     } else {
       // Re-schedule the current pending nudge.
       SyncSession* s = pending_nudge_->session.get();
-      session.reset(new SyncSession(s->context(), s->delegate(), s->source(),
-          s->routing_info(), s->workers()));
+      job.session.reset(new SyncSession(s->context(), s->delegate(),
+          s->source(), s->routing_info(), s->workers()));
       pending_nudge_.reset();
     }
   }
-  ScheduleSyncSessionJob(delay, NUDGE, session.release(), nudge_location);
+
+  //  TODO(lipalani) - pass the job itself to ScheduleSyncSessionJob.
+  ScheduleSyncSessionJob(delay, NUDGE, job.session.release(), nudge_location);
 }
 
 // Helper to extract the routing info and workers corresponding to types in
@@ -354,21 +388,23 @@ void SyncerThread::ScheduleConfig(const ModelTypeBitSet& types) {
                              &routes, &workers);
 
   thread_.message_loop()->PostTask(FROM_HERE, NewRunnableMethod(
-      this, &SyncerThread::ScheduleConfigImpl, routes, workers));
+      this, &SyncerThread::ScheduleConfigImpl, routes, workers,
+      GetUpdatesCallerInfo::FIRST_UPDATE));
 }
 
 void SyncerThread::ScheduleConfigImpl(const ModelSafeRoutingInfo& routing_info,
-    const std::vector<ModelSafeWorker*>& workers) {
+    const std::vector<ModelSafeWorker*>& workers,
+    const sync_pb::GetUpdatesCallerInfo::GetUpdatesSource source) {
   DCHECK_EQ(MessageLoop::current(), thread_.message_loop());
 
   // TODO(tim): config-specific GetUpdatesCallerInfo value?
   SyncSession* session = new SyncSession(session_context_.get(), this,
-      SyncSourceInfo(GetUpdatesCallerInfo::FIRST_UPDATE,
+      SyncSourceInfo(source,
           syncable::ModelTypePayloadMapFromRoutingInfo(
               routing_info, std::string())),
       routing_info, workers);
   ScheduleSyncSessionJob(TimeDelta::FromSeconds(0), CONFIGURATION, session,
-                         FROM_HERE);
+    FROM_HERE);
 }
 
 void SyncerThread::ScheduleSyncSessionJob(const base::TimeDelta& delay,
@@ -377,7 +413,7 @@ void SyncerThread::ScheduleSyncSessionJob(const base::TimeDelta& delay,
   DCHECK_EQ(MessageLoop::current(), thread_.message_loop());
 
   SyncSessionJob job = {purpose, TimeTicks::Now() + delay,
-                        make_linked_ptr(session), nudge_location};
+                        make_linked_ptr(session), false, nudge_location};
   if (purpose == NUDGE) {
     DCHECK(!pending_nudge_.get() || pending_nudge_->session.get() == session);
     pending_nudge_.reset(new SyncSessionJob(job));
@@ -409,11 +445,8 @@ void SyncerThread::SetSyncerStepsForPurpose(SyncSessionJobPurpose purpose,
 
 void SyncerThread::DoSyncSessionJob(const SyncSessionJob& job) {
   DCHECK_EQ(MessageLoop::current(), thread_.message_loop());
-  if (!ShouldRunJob(job.purpose, job.scheduled_start)) {
-    LOG(WARNING) << "Dropping nudge at DoSyncSessionJob, source = "
-        << job.session->source().updates_source;
+  if (!ShouldRunJob(job))
     return;
-  }
 
   if (job.purpose == NUDGE) {
     DCHECK(pending_nudge_.get());
@@ -427,13 +460,17 @@ void SyncerThread::DoSyncSessionJob(const SyncSessionJob& job) {
   SetSyncerStepsForPurpose(job.purpose, &begin, &end);
 
   bool has_more_to_sync = true;
-  while (ShouldRunJob(job.purpose, job.scheduled_start) && has_more_to_sync) {
+  while (ShouldRunJob(job) && has_more_to_sync) {
     VLOG(1) << "SyncerThread: Calling SyncShare.";
     // Synchronously perform the sync session from this thread.
     syncer_->SyncShare(job.session.get(), begin, end);
     has_more_to_sync = job.session->HasMoreToSync();
     if (has_more_to_sync)
       job.session->ResetTransientState();
+    if (IsSyncingCurrentlySilenced()) {
+      DCHECK_NE(job.purpose, CLEAR_USER_DATA);
+      SaveJob(job);

[tim (not reviewing), 2011/04/09 23:52:58]

This if-block seems out of place...we should be doing as little as possible. I
think if anything this should be moved to the IsSyncingCurrentlySilenced
if-block inside FinishSyncSessionJob; we shouldn't let that logic / decision
making creep out if possible.  Also add a comment there why it is needed... we
have to be careful because a job could be partially done if we managed to call
SyncShare.

Lastly though, the DCHECK is not needed because SaveJob does this.

[lipalani1, 2011/04/12 02:33:00]

Good point. Done.
On 2011/04/09 23:52:58, timsteele wrote:
Done.

+    }
   }
   VLOG(1) << "SyncerThread: Done SyncShare looping.";
   FinishSyncSessionJob(job);
@@ -519,8 +556,16 @@ void SyncerThread::ScheduleNextSync(const SyncSessionJob& old_job) {
   } else {
     // We weren't continuing and we aren't in backoff.  Schedule a normal
     // continuation.
-    ScheduleNudgeImpl(TimeDelta::FromSeconds(0), NUDGE_SOURCE_CONTINUATION,
-                      old_job.session->source().types, FROM_HERE);
+    if (old_job.purpose == CONFIGURATION) {
+      ScheduleConfigImpl(old_job.session->routing_info(),
+          old_job.session->workers(),
+          GetUpdatesFromNudgeSource(NUDGE_SOURCE_CONTINUATION));
+    } else  {
+      // For all other purposes(nudge and poll) we schedule a retry nudge.
+      ScheduleNudgeImpl(TimeDelta::FromSeconds(0),
+                        GetUpdatesFromNudgeSource(NUDGE_SOURCE_CONTINUATION),
+                        old_job.session->source().types, false, FROM_HERE);
+    }
   }
 }
 
@@ -556,9 +601,21 @@ void SyncerThread::HandleConsecutiveContinuationError(
       IsBackingOff() ? wait_interval_->length : TimeDelta::FromSeconds(1));
   wait_interval_.reset(new WaitInterval(WaitInterval::EXPONENTIAL_BACKOFF,
                                         length));
-  SyncSessionJob job = {NUDGE, TimeTicks::Now() + length,
-                        make_linked_ptr(s), FROM_HERE};
-  pending_nudge_.reset(new SyncSessionJob(job));
+  if (old_job.purpose == CONFIGURATION) {
+    SyncSessionJob job = {old_job.purpose, TimeTicks::Now() + length,
+                        make_linked_ptr(s), false, FROM_HERE};
+    wait_interval_->pending_configure_job.reset(new SyncSessionJob(job));
+  } else {
+    // We are not in configuration mode. So wait_interval's pending job

[tim (not reviewing), 2011/04/09 23:52:58]

I'm rather concerned about the apparent connection this CL makes between
job.purpose == CONFIGURATION and mode_ == CONFIGURATION :\  That is not the
design.  They are orthogonal constructs so that they are *not* coupled to each
other.

[lipalani1, 2011/04/12 02:33:00]

Hmm.. not sure I follow. let me sync up with you.
On 2011/04/09 23:52:58, timsteele wrote:

+    // should be null.
+    DCHECK(wait_interval_->pending_configure_job.get() == NULL);
+
+    // No matter what type of job it is.(nudge or poll, it cannot be config)
+    // We are going to treat it as nudge when doing exponential back off
+    // retries.
+    // TODO(lipalani) - handle clear user data.
+    UpdatePendingJob(old_job);
+  }
   wait_interval_->timer.Start(length, this, &SyncerThread::DoCanaryJob);
 }
 
@@ -593,9 +650,38 @@ void SyncerThread::Stop() {
 }
 
 void SyncerThread::DoCanaryJob() {
-  DCHECK(pending_nudge_.get());
+  // We should not be here unless wait interval was initialized due to
+  // throttling or backing off.
+  DCHECK(wait_interval_.get());
+
   wait_interval_->had_nudge = false;
-  SyncSessionJob copy = *pending_nudge_;
+
+  // We should have one of 2 things. Otherwise we shouldnt be running the timer.
+  DCHECK(wait_interval_->pending_configure_job.get() || pending_nudge_.get());
+  ExecutePendingJob();
+}
+
+void SyncerThread::ExecutePendingJob() {
+  if (mode_ == CONFIGURATION_MODE) {
+    if (wait_interval_.get() && wait_interval_->pending_configure_job.get()) {
+      ExecuteJobByMakingACopy(wait_interval_->pending_configure_job.get());
+    }
+  } else {
+    if (pending_nudge_.get()) {
+      // Pending jobs mostly have time from the past. Reset it so this job
+      // will get executed.
+      if (pending_nudge_->scheduled_start < TimeTicks::Now())
+        pending_nudge_->scheduled_start = TimeTicks::Now();
+      // The pending nudge would be cleared in the DoSyncSessionJob function.
+      ExecuteJobByMakingACopy(pending_nudge_.get());
+    }
+  }
+}
+
+void SyncerThread::ExecuteJobByMakingACopy(SyncSessionJob* job) {
+  DCHECK(job);
+  SyncSessionJob copy = *job;
+  copy.is_canary_job = true;
   DoSyncSessionJob(copy);
 }
 
@@ -614,6 +700,7 @@ void SyncerThread::PollTimerCallback() {
 
 void SyncerThread::Unthrottle() {
   DCHECK_EQ(WaitInterval::THROTTLED, wait_interval_->mode);
+  DoCanaryJob();
   wait_interval_.reset();
 }