add support for is_write_index in put-alias body parsing (#31674)

* add support for is_write_index in put-alias body parsing

The Rest Put-Alias Action does separate parsing of the alias body
to construct the IndicesAliasesRequest. This extra parsing
was missed in #30703.

* test flag was not just ignored by the parser

* disable backcompat tests

build.gradle

@@ -173,8 +173,8 @@ task verifyVersions {
  * the enabled state of every bwc task. It should be set back to true
  * after the backport of the backcompat code is complete.
  */
-final boolean bwc_tests_enabled = true
-final String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
+final boolean bwc_tests_enabled = false
+final String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/31674" /* place a PR link here when committing bwc changes */
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")

rest-api-spec/src/main/resources/rest-api-spec/test/indices.put_alias/10_basic.yml

@@ -56,3 +56,27 @@
       indices.put_alias:
         index: test_index
         name: foo
+
+---
+"Can set is_write_index":
+
+  - skip:
+      version: " - 6.3.99"
+      reason: "is_write_index is only available from 6.4.0 on"
+
+  - do:
+      indices.create:
+        index: test_index
+
+  - do:
+      indices.put_alias:
+        index: test_index
+        name: test_alias
+        body:
+          is_write_index: true
+
+  - do:
+      indices.get_alias:
+        index: test_index
+        name: test_alias
+  - match: {test_index.aliases.test_alias: { 'is_write_index': true }}

server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestIndexPutAliasAction.java

@@ -66,6 +66,7 @@ public class RestIndexPutAliasAction extends BaseRestHandler {
         String routing = null;
         String indexRouting = null;
         String searchRouting = null;
+        Boolean writeIndex = null;
 
         if (request.hasContent()) {
             try (XContentParser parser = request.contentParser()) {
@@ -90,6 +91,8 @@ public class RestIndexPutAliasAction extends BaseRestHandler {
                         } else if ("searchRouting".equals(currentFieldName)
                                 || "search-routing".equals(currentFieldName) || "search_routing".equals(currentFieldName)) {
                             searchRouting = parser.textOrNull();
+                        } else if ("is_write_index".equals(currentFieldName)) {
+                            writeIndex = parser.booleanValue();
                         }
                     } else if (token == XContentParser.Token.START_OBJECT) {
                         if ("filter".equals(currentFieldName)) {
@@ -117,6 +120,9 @@ public class RestIndexPutAliasAction extends BaseRestHandler {
         if (filter != null) {
             aliasAction.filter(filter);
         }
+        if (writeIndex != null) {
+            aliasAction.writeIndex(writeIndex);
+        }
         indicesAliasesRequest.addAliasAction(aliasAction);
         return channel -> client.admin().indices().aliases(indicesAliasesRequest, new RestToXContentListener<>(channel));
     }