sync: remove buggy freshness condition in SyncSchedulerImpl add defensive DCHECKs.

sync/engine/sync_scheduler_impl.cc

Index: sync/engine/sync_scheduler_impl.cc
diff --git a/sync/engine/sync_scheduler_impl.cc b/sync/engine/sync_scheduler_impl.cc
index ff177b4f8347ec1bcd5582ee5c4873bb1cf948ec..fb97a62e1004b9dbd954c4b69f8eb5ea929f1c0a 100644
--- a/sync/engine/sync_scheduler_impl.cc
+++ b/sync/engine/sync_scheduler_impl.cc
@@ -275,6 +275,13 @@ void SyncSchedulerImpl::Start(Mode mode) {
   if (old_mode != mode_) {
     // We just changed our mode. See if there are any pending jobs that we could
     // execute in the new mode.
+    if (mode_ == NORMAL_MODE) {
+      // It is illegal to switch to NORMAL_MODE if a previous CONFIGURATION job
+      // has not yet completed.
+      DCHECK(!wait_interval_.get() ||
+             !wait_interval_->pending_configure_job.get());
+    }
+
     DoPendingJobIfPossible(false);
   }
 }
@@ -451,7 +458,7 @@ SyncSchedulerImpl::JobProcessDecision SyncSchedulerImpl::DecideOnJob(
   DCHECK_NE(job.purpose, SyncSessionJob::CONFIGURATION);
 
   // Freshness condition
-  if (job.scheduled_start < last_sync_session_end_time_) {
+  if (job.scheduled_start < last_nudge_or_poll_end_time_) {
     SDVLOG(2) << "Dropping job because of freshness";
     return DROP;
   }
@@ -773,19 +780,30 @@ void SyncSchedulerImpl::FinishSyncSessionJob(const SyncSessionJob& job) {
   DCHECK_EQ(MessageLoop::current(), sync_loop_);
   // Update timing information for how often datatypes are triggering nudges.
   base::TimeTicks now = TimeTicks::Now();
-  if (!last_sync_session_end_time_.is_null()) {
+  if (!last_nudge_or_poll_end_time_.is_null()) {
+    // TODO(tim): We should probably track NUDGE time separately to avoid
+    // skewing the histogram?  Does it make sense to only compare against
+    // successful sessions?
     ModelTypePayloadMap::const_iterator iter;
+    DCHECK_EQ(job.purpose == SyncSessionJob::POLL,
+              job.session->source().types.empty());
     for (iter = job.session->source().types.begin();
          iter != job.session->source().types.end();
          ++iter) {
 #define PER_DATA_TYPE_MACRO(type_str) \
     SYNC_FREQ_HISTOGRAM("Sync.Freq" type_str, \
-                        now - last_sync_session_end_time_);
+                        now - last_nudge_or_poll_end_time_);
       SYNC_DATA_TYPE_HISTOGRAM(iter->first);
 #undef PER_DATA_TYPE_MACRO
     }
   }
-  last_sync_session_end_time_ = now;
+
+  // TODO(tim): Consolidate Succeeded() checks by breaking this Finish job
+  // flow into successful / unsuccessful handler methods.
+  if (job.session->Succeeded() && (job.purpose == SyncSessionJob::NUDGE ||
+                                   job.purpose == SyncSessionJob::POLL)) {
+    last_nudge_or_poll_end_time_ = now;
+  }
 
   // Now update the status of the connection from SCM. We need this to decide
   // whether we need to save/run future jobs. The notifications from SCM are not
@@ -974,10 +992,6 @@ void SyncSchedulerImpl::DoPendingJobIfPossible(bool is_canary_job) {
     job_to_execute = wait_interval_->pending_configure_job.get();
   } else if (mode_ == NORMAL_MODE && pending_nudge_.get()) {
     SDVLOG(2) << "Found pending nudge job";
-    // 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();
 
     scoped_ptr<SyncSession> session(CreateSyncSession(
         pending_nudge_->session->source()));