[ML] Model should defend against times earlier than epoch 0

[droberts195]

I saw this in a Java test log:

[2019-02-13T11:42:03,268][WARN ][o.e.x.m.d.DatafeedManager] [node-2] [test-lookback-only-given-empty-index] Datafeed lookback retrieved no data
... SKIPPED 9 LINES ...
[2019-02-13T11:42:03,648][WARN ][o.e.t.OutboundHandler    ] [node-2] send message failed [channel: Netty4TcpChannel{localAddress=/127.0.0.1:34349, remoteAddress=/127.0.0.1:57094}]
java.lang.IllegalStateException: Negative longs unsupported, use writeLong or writeZLong for negative numbers [-3600000]
	at org.elasticsearch.common.io.stream.StreamOutput.writeVLong(StreamOutput.java:274) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]
	at org.elasticsearch.xpack.core.ml.job.process.autodetect.state.ModelSizeStats.writeTo(ModelSizeStats.java:156) ~[?:?]
	at org.elasticsearch.common.io.stream.StreamOutput.writeOptionalWriteable(StreamOutput.java:854) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]
	at org.elasticsearch.xpack.core.ml.action.GetJobsStatsAction$Response$JobStats.writeTo(GetJobsStatsAction.java:277) ~[?:?]
	at org.elasticsearch.common.io.stream.StreamOutput.lambda$writeCollection$28(StreamOutput.java:1056) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]
	at org.elasticsearch.common.io.stream.StreamOutput.writeCollection(StreamOutput.java:1075) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]
	at org.elasticsearch.common.io.stream.StreamOutput.writeCollection(StreamOutput.java:1056) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]
	at org.elasticsearch.common.io.stream.StreamOutput.writeList(StreamOutput.java:1063) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]
	at org.elasticsearch.xpack.core.ml.action.util.QueryPage.writeTo(QueryPage.java:57) ~[?:?]
	at org.elasticsearch.action.support.tasks.TransportTasksAction$NodeTasksResponse.writeTo(TransportTasksAction.java:441) ~[elasticsearch-6.7.0-SNAPSHOT.jar:6.7.0-SNAPSHOT]

However, this did not actually fail any test.

It shows that a model size stats document contained an epoch time of -3600 seconds = -3600000 milliseconds. The Java side cannot cope with this. We should change the C++ code so that it does not create such documents.

A request to generate results for times before the epoch could fail the job, which would then cause a test failure on the Java side that we could fix.

[davidkyle]

I saw the same error as above it's reproducible with the following:

./gradlew :x-pack:plugin:ml:qa:native-multi-node-tests:integTestRunner -Dtests.seed=DF62D7F71CC2EDB6 -Dtests.class=org.elasticsearch.xpack.ml.integration.DatafeedJobsRestIT -Dtests.method=testLookbackWithoutPermissionsAndRollup

[droberts195]

I've tracked down the problem. It's that we unconditionally call CAnomalyJob::refreshMemoryAndReport() when the autodetect process exits even if it didn't see any input data. It's very similar to the problem that was noted for quantiles in #393 and fixed in #437. I'll add a similar fix for model size stats.