Fix join keys ordering in a sequence (#76699) (#76912)

A sequence payload is constructed by providing one list of sequences and
one for the hits. When fetching the list of hits though, the list of
sequnces can be iterated in reverse order to build the hit references;
meaning that the original list of sequences provided to the sequence
payload needs reversing too.

Author: bpintea

x-pack/plugin/eql/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/eql/10_basic.yml

@@ -41,6 +41,42 @@ setup:
             user: SYSTEM
             id: 123
             valid: true
+          - index:
+              _index: eql_test
+              _id:    4
+          - event:
+              - category: network
+            "@timestamp": 2020-02-06T12:34:56Z
+            user: ADMIN
+            id: 123
+            valid: true
+          - index:
+              _index: eql_test
+              _id:    5
+          - event:
+              - category: network
+            "@timestamp": 2020-02-07T12:34:56Z
+            user: SYSTEM
+            id: 123
+            valid: true
+          - index:
+              _index: eql_test
+              _id:    6
+          - event:
+              - category: network
+            "@timestamp": 2020-02-08T12:34:56Z
+            user: ADMIN
+            id: 123
+            valid: true
+          - index:
+              _index: eql_test
+              _id:    7
+          - event:
+              - category: network
+            "@timestamp": 2020-02-09T12:34:56Z
+            user: SYSTEM
+            id: 123
+            valid: true
 
 ---
 # Testing round-trip and the basic shape of the response
@@ -382,3 +418,13 @@ setup:
       catch: missing
       eql.get_status:
         id: $id
+---
+"Sequence checking correct join key ordering.":
+
+  - do:
+      eql.search:
+        index: eql_test
+        body:
+          query: 'sequence by user [network where valid == true] [network where true]'
+  - match: {hits.sequences.0.join_keys.0: "ADMIN"}
+  - match: {hits.sequences.1.join_keys.0: "SYSTEM"}

x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/execution/sequence/TumblingWindow.java

@@ -26,6 +26,7 @@
 import org.elasticsearch.xpack.eql.util.ReversedIterator;
 import org.elasticsearch.xpack.ql.util.ActionListeners;
 
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -548,6 +549,9 @@ private void payload(ActionListener<Payload> listener) {
 
         // get results through search (to keep using PIT)
         client.fetchHits(hits(completed), ActionListeners.map(listener, listOfHits -> {
+            if (criteria.get(0).descending()) {
+                Collections.reverse(completed);
+            }
             SequencePayload payload = new SequencePayload(completed, listOfHits, false, timeTook());
             close(listener);
             return payload;