Flushing a closed job should not return a 200 (elastic/elasticsearch#625)

Original commit: elastic/x-pack-elasticsearch@419389e1af

elasticsearch/src/main/java/org/elasticsearch/xpack/prelert/job/manager/AutodetectProcessManager.java

@@ -137,8 +137,9 @@ public class AutodetectProcessManager extends AbstractComponent implements DataP
         logger.debug("Flushing job {}", jobId);
         AutodetectCommunicator communicator = autoDetectCommunicatorByJob.get(jobId);
         if (communicator == null) {
-            logger.debug("Cannot flush: no active autodetect process for job {}", jobId);
-            return;
+            String message = String.format(Locale.ROOT, "[%s] Cannot flush: no active autodetect process for job", jobId);
+            logger.debug(message);
+            throw new IllegalArgumentException(message);
         }
         try {
             communicator.flushJob(params);

elasticsearch/src/test/resources/rest-api-spec/test/post_data.yaml

@@ -16,6 +16,20 @@ setup:
             }
           }
 
+  - do:
+      xpack.prelert.put_job:
+        body:  >
+          {
+            "job_id":"closed_job",
+            "description":"A closed job",
+            "analysis_config" : {
+                "detectors" :[{"function":"metric","field_name":"responsetime","by_field_name":"airline"}]
+            },
+            "data_description" : {
+                "format":"JSON"
+            }
+          }
+
   - do:
       xpack.prelert.open_job:
         job_id: farequote
@@ -149,3 +163,21 @@ setup:
       catch: missing
       xpack.prelert.close_job:
         job_id: not_a_job
+
+---
+"Test flushing, posting and closing a closed job":
+  - do:
+      catch: /illegal_argument_exception/
+      xpack.prelert.flush_job:
+        job_id: closed_job
+
+  - do:
+      catch: /illegal_argument_exception/
+      xpack.prelert.close_job:
+        job_id: closed_job
+
+  - do:
+      catch: /illegal_argument_exception/
+      xpack.prelert.job_data:
+        job_id: closed_job
+        body: {}