Fix AbstractBulkByScrollRequest slices parameter via Rest (#53068)

Currently the AbstractBulkByScrollRequest accepts slice values of 0 via its
`setSlices` method, denoting the "auto" slicing behaviour that is usable by
settting the "slices=auto" parameter on rest requests. When using the High Level
Rest Client, however, we send the 0 value as an integer, which is then rejected
as invalid by `AbstractBulkByScrollRequest#parseSlices`. Instead of making
parsing of the rest request more lenient, this PR opts for changing the
RequestConverter logic in the client to translate 0 values to "auto" on the rest
requests.

Closes #53044

client/rest-high-level/src/main/java/org/elasticsearch/client/RequestConverters.java

@@ -916,6 +916,10 @@ Params withRouting(String routing) {
         }
 
         Params withSlices(int slices) {
+            if (slices == 0) {
+                // translate to "auto" value in rest request so the receiving end doesn't throw error
+                return putParam("slices", AbstractBulkByScrollRequest.AUTO_SLICES_VALUE);
+            }
             return putParam("slices", String.valueOf(slices));
         }
 

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestConvertersTests.java

@@ -82,6 +82,7 @@
 import org.elasticsearch.index.rankeval.RankEvalSpec;
 import org.elasticsearch.index.rankeval.RatedRequest;
 import org.elasticsearch.index.rankeval.RestRankEvalAction;
+import org.elasticsearch.index.reindex.AbstractBulkByScrollRequest;
 import org.elasticsearch.index.reindex.DeleteByQueryRequest;
 import org.elasticsearch.index.reindex.ReindexRequest;
 import org.elasticsearch.index.reindex.RemoteInfo;
@@ -454,9 +455,13 @@ public void testReindex() throws IOException {
             reindexRequest.setSourceQuery(new TermQueryBuilder("foo", "fooval"));
         }
         if (randomBoolean()) {
-            int slices = randomInt(100);
+            int slices = randomIntBetween(0,4);
             reindexRequest.setSlices(slices);
-            expectedParams.put("slices", String.valueOf(slices));
+            if (slices == 0) {
+                expectedParams.put("slices", AbstractBulkByScrollRequest.AUTO_SLICES_VALUE);
+            } else {
+                expectedParams.put("slices", Integer.toString(slices));
+            }
         } else {
             expectedParams.put("slices", "1");
         }
@@ -524,7 +529,11 @@ public void testUpdateByQuery() throws IOException {
         }
         if (randomBoolean()) {
             int slices = randomIntBetween(0, 4);
-            expectedParams.put("slices", Integer.toString(slices));
+            if (slices == 0) {
+                expectedParams.put("slices", AbstractBulkByScrollRequest.AUTO_SLICES_VALUE);
+            } else {
+                expectedParams.put("slices", Integer.toString(slices));
+            }
             updateByQueryRequest.setSlices(slices);
         } else {
             expectedParams.put("slices", "1");
@@ -589,7 +598,11 @@ public void testDeleteByQuery() throws IOException {
         }
         if (randomBoolean()) {
             int slices = randomIntBetween(0, 4);
-            expectedParams.put("slices", Integer.toString(slices));
+            if (slices == 0) {
+                expectedParams.put("slices", AbstractBulkByScrollRequest.AUTO_SLICES_VALUE);
+            } else {
+                expectedParams.put("slices", Integer.toString(slices));
+            }
             deleteByQueryRequest.setSlices(slices);
         } else {
             expectedParams.put("slices", "1");

server/src/main/java/org/elasticsearch/index/reindex/AbstractBulkByScrollRequest.java

@@ -402,6 +402,7 @@ public TimeValue getScrollTime() {
 
     /**
      * The number of slices this task should be divided into. Defaults to 1 meaning the task isn't sliced into subtasks.
+     * A value of 0 is equivalent to the "auto" slices parameter of the Rest API.
      */
     public Self setSlices(int slices) {
         if (slices < 0) {