Get the correct previous sensor result

+++ b/tests/src/Kernel/MonitoringCoreKernelTest.php
@@ -770,4 +771,114 @@ class MonitoringCoreKernelTest extends MonitoringUnitTestBase {
+    //$this->assertSameResult($sensor_result_0, $previous_result_1);

Here there are 2 results asserted, but one is a SensorResultEntity and the other is SensorResult. So the function assertSameResultHelper would take an entity and a result object and try to compare them (right now it takes 2 entities instead). But I don't think this is necessary, because we are comparing 2 different objects and we only need to compare some properties between the entity and the result. I suggest we can do a regular assertion in this test only for the statuses?

Here is a patch with the solution suggested in #5, along with some minor changes:
1. Changed the test comments and some result comparison.
2. Removed monitoring_sensor_result_second_last() since it is not used anymore.

1. +++ b/monitoring.module
   @@ -266,30 +266,6 @@ function monitoring_sensor_result_last($sensor_name) {
   -function monitoring_sensor_result_second_last($sensor_name) {
   -  $result = \Drupal::entityQuery('monitoring_sensor_result')
   -    ->condition('sensor_name', $sensor_name)
   -    ->sort('timestamp', 'DESC')
   -    ->sort('record_id', 'DESC')
   -    ->range(1, 1)
   

   monitoring is now stable and this is a method in a non-experimental module.

   I suppose nobody uses it yet, but we can't just remove this now.

   So, keep it but add a @deprecated method like they exist on deprecated core methods. Schedule removel for 8.x-2.0 or something like that.

2. +++ b/src/Result/SensorResult.php
   @@ -65,6 +62,13 @@ class SensorResult implements SensorResultInterface {
   +   * The previous sensor result.
   +   *
   +   * @var \Drupal\monitoring\Entity\SensorResultEntity|null
   +   */
   +  protected $previousResult = NULL;
   +
   
   +++ b/src/Result/SensorResultInterface.php
   @@ -197,4 +195,22 @@ interface SensorResultInterface extends SensorResultDataInterface {
   +   * @param \Drupal\monitoring\Entity\SensorResultEntity|null $previous_result
   ...
   +   * @return \Drupal\monitoring\Entity\SensorResultEntity|null
   

   use the interface

3. +++ b/src/SensorRunner.php
   @@ -133,6 +129,7 @@ class SensorRunner {
        foreach ($sensors_config_all as $name => $sensor_config) {
          if ($result = $this->runSensor($sensor_config)) {
   +        $result->setPreviousResult(monitoring_sensor_result_last($result->getSensorId()));
            $results[$name] = $result;
          }
   

   what happens with cached sensors? are we now serializing the injected object when caching? We should avoid that and set it again or so.

We could drop setPreviousResult() and simply use call getPreviousResult() here that internally uses monitoring_sensor_result_last().
Then internally set the property and simply drop it on serialisation.

3. We checked that and the current logic is fine, so you can ignore that part.

1. Added the second_last function again.
2. Using the DataInterface for the previous result. (this makes it easier to assert results with Tramy's method - assertSameResult() ).

In general looks fine to me. :-)

However, lots of small unrelated changes... Reverted them.
And please don't touch things such as removing file headers. If you want to do this, submit a separate issue that does this for ALL files.
Otherwise, patches conflict with those clean-ups being present here and there.