[ML] Do not write model size stats after autodetect no-op

Similar to the fix for state and quantiles in elastic#437,
if no input is received and time is not advanced then
there is no need to write model size stats when the
autodetect process exits. Doing this can actually
cause a problem for a job that has never ever seen
any input, as the unnecessary model size stats were
written with a negative timestamp. This change also
adds an extra defensive check to prevent that ever
happening, although the only situation when it is
thought to be possible should be prevented by the
first change.

Backport of elastic#512

docs/CHANGELOG.asciidoc

@@ -61,6 +61,12 @@ to the model. (See {ml-pull}214[#214].)
 
 == {es} version 7.0.0-alpha1
 
+== {es} version 6.8.2
+
+=== Bug Fixes
+
+* Don't write model size stats when job is closed without any input {ml-pull}512[#512] (issue: {ml-issue}394[#394])
+
 == {es} version 6.7.2
 
 === Enhancements

lib/api/CAnomalyJob.cc

@@ -221,14 +221,14 @@ bool CAnomalyJob::handleRecord(const TStrStrUMap& dataRowFields) {
 
 void CAnomalyJob::finalise() {
     // Persist final state of normalizer iff an input record has been handled or time has been advanced.
-    if (this->isPersistenceNeeded("quantiles state")) {
+    if (this->isPersistenceNeeded("quantiles state and model size stats")) {
         m_JsonOutputWriter.persistNormalizer(m_Normalizer, m_LastNormalizerPersistTime);
-    }
 
-    // Prune the models so that the final persisted state is as neat as possible
-    this->pruneAllModels();
+        // Prune the models so that the final persisted state is as neat as possible
+        this->pruneAllModels();
 
-    this->refreshMemoryAndReport();
+        this->refreshMemoryAndReport();
+    }
 
     // Wait for any ongoing periodic persist to complete, so that the data adder
     // is not used by both a periodic periodic persist and final persist at the
@@ -1267,6 +1267,12 @@ void CAnomalyJob::writeOutModelPlot(const TModelPlotDataVec& modelPlotData) {
 }
 
 void CAnomalyJob::refreshMemoryAndReport() {
+    if (m_LastFinalisedBucketEndTime < m_ModelConfig.bucketLength()) {
+        LOG_ERROR(<< "Cannot report memory usage because last finalized bucket end time ("
+                  << m_LastFinalisedBucketEndTime << ") is smaller than bucket span ("
+                  << m_ModelConfig.bucketLength() << ')');
+        return;
+    }
     // Make sure model size stats are up to date and then send a final memory
     // usage report
     for (const auto& detector_ : m_Detectors) {