Fix SimpleBlocksIT#testConcurrentAddBlock (#126918)

Only one of concurrent `prepareAddBlock` calls actually wins a race to
add the block, so  we need to check `assertIndexHasBlock` if
the add index block has been acknowledged

Resolve #122324

Co-authored-by: Oleksandr Kolomiiets <[email protected]>

Author: arteam

muted-tests.yml

@@ -143,9 +143,6 @@ tests:
 - class: org.elasticsearch.test.rest.ClientYamlTestSuiteIT
   method: test {yaml=snapshot.delete/10_basic/Delete a snapshot asynchronously}
   issue: https://github.com/elastic/elasticsearch/issues/122102
-- class: org.elasticsearch.blocks.SimpleBlocksIT
-  method: testConcurrentAddBlock
-  issue: https://github.com/elastic/elasticsearch/issues/122324
 - class: org.elasticsearch.action.admin.cluster.node.tasks.CancellableTasksIT
   method: testChildrenTasksCancelledOnTimeout
   issue: https://github.com/elastic/elasticsearch/issues/123568

server/src/internalClusterTest/java/org/elasticsearch/blocks/SimpleBlocksIT.java

@@ -413,8 +413,11 @@ public void testConcurrentAddBlock() throws InterruptedException, ExecutionExcep
         try {
             startInParallel(threadCount, i -> {
                 try {
-                    indicesAdmin().prepareAddBlock(block, indexName).get();
-                    assertIndexHasBlock(block, indexName);
+                    AddIndexBlockResponse response = indicesAdmin().prepareAddBlock(block, indexName).get();
+                    // Check that the block has been added only for the request that won the race
+                    if (response.isAcknowledged() && response.getIndices().isEmpty() == false) {
+                        assertIndexHasBlock(block, indexName);
+                    }
                 } catch (final ClusterBlockException e) {
                     assertThat(e.blocks(), hasSize(1));
                     assertTrue(e.blocks().stream().allMatch(b -> b.id() == block.getBlock().id()));