[ML] Tolerate a body without timestamp for get_buckets with a timestamp (elastic/x-pack-elasticsearch#2640)

When getting a single bucket, the get_buckets API can take a timestamp
either in the body or in the URL.  Prior to this change, if a timestamp
was specified in the URL but a body not containing a timestamp was specified
(either empty or containing other parameters like exclude_interim or sort)
then it would cause a bad_request exception.  This in turn causes problems
for clients that cannot send a body when GETting and always send a body when
POSTing.

This change fixes get_buckets to always read any timestamp in the URL, even
when a body is sent.

relates elastic/x-pack-elasticsearch#2515

Original commit: elastic/x-pack-elasticsearch@5c23dd972e

plugin/src/main/java/org/elasticsearch/xpack/ml/rest/results/RestGetBucketsAction.java

@@ -6,6 +6,7 @@
 package org.elasticsearch.xpack.ml.rest.results;
 
 import org.elasticsearch.client.node.NodeClient;
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.rest.BaseRestHandler;
@@ -45,21 +46,24 @@ public class RestGetBucketsAction extends BaseRestHandler {
     @Override
     protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
         String jobId = restRequest.param(Job.ID.getPreferredName());
+        String timestamp = restRequest.param(GetBucketsAction.Request.TIMESTAMP.getPreferredName());
         final GetBucketsAction.Request request;
         if (restRequest.hasContentOrSourceParam()) {
             XContentParser parser = restRequest.contentOrSourceParamParser();
             request = GetBucketsAction.Request.parseRequest(jobId, parser);
+
+            // A timestamp in the URL overrides any timestamp that may also have been set in the body
+            if (!Strings.isNullOrEmpty(timestamp)) {
+                request.setTimestamp(timestamp);
+            }
         } else {
             request = new GetBucketsAction.Request(jobId);
 
             // Check if the REST param is set first so mutually exclusive
-            // options will only cause an error if set
-            if (restRequest.hasParam(GetBucketsAction.Request.TIMESTAMP.getPreferredName())) {
-                String timestamp = restRequest.param(GetBucketsAction.Request.TIMESTAMP.getPreferredName());
-                if (timestamp != null && !timestamp.isEmpty()) {
+            // options will cause an error if set
+            if (!Strings.isNullOrEmpty(timestamp)) {
                 request.setTimestamp(timestamp);
             }
-            }
             // multiple bucket options
             if (restRequest.hasParam(PageParams.FROM.getPreferredName()) || restRequest.hasParam(PageParams.SIZE.getPreferredName())) {
                 request.setPageParams(

plugin/src/test/resources/rest-api-spec/test/ml/jobs_get_result_buckets.yml

@@ -170,6 +170,18 @@ setup:
   - match: { buckets.0.job_id: jobs-get-result-buckets }
   - match: { buckets.0.result_type: bucket}
 
+---
+"Test result single bucket api with empty body":
+  - do:
+      xpack.ml.get_buckets:
+        job_id: "jobs-get-result-buckets"
+        timestamp: "2016-06-01T00:00:00Z"
+        body: {}
+
+  - match: { buckets.0.timestamp: 1464739200000}
+  - match: { buckets.0.job_id: jobs-get-result-buckets }
+  - match: { buckets.0.result_type: bucket}
+
 ---
 "Test mutually-exclusive params":
   - do: