fix api hangs if no shards allocated

src/main/java/org/elasticsearch/action/admin/indices/seal/TransportSealIndicesAction.java

@@ -61,8 +61,13 @@ public class TransportSealIndicesAction extends HandledTransportAction<SealIndic
         ClusterState state = clusterService.state();
         String[] concreteIndices = state.metaData().concreteIndices(request.indicesOptions(), request.indices());
         GroupShardsIterator primaries = state.routingTable().activePrimaryShardsGrouped(concreteIndices, false);
-        final CountDown countDown = new CountDown(primaries.size());
         final Set<SyncedFlushService.SyncedFlushResult> results = ConcurrentCollections.newConcurrentSet();
+        if (primaries.size() == 0) {
+            // no active primary available
+            listener.onResponse(new SealIndicesResponse(results));
+            return;
+        }
+        final CountDown countDown = new CountDown(primaries.size());
         for (final ShardIterator shard : primaries) {
             final ShardId shardId = shard.shardId();
             syncedFlushService.attemptSyncedFlush(shardId, new ActionListener<SyncedFlushService.SyncedFlushResult>() {

src/test/java/org/elasticsearch/indices/SealTests.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.indices;
+
+import org.elasticsearch.action.admin.indices.seal.SealIndicesResponse;
+import org.elasticsearch.test.ElasticsearchIntegrationTest;
+import org.junit.Test;
+
+import static org.hamcrest.Matchers.equalTo;
+
+@ElasticsearchIntegrationTest.ClusterScope(scope = ElasticsearchIntegrationTest.Scope.SUITE, numDataNodes = 1)
+public class SealTests extends ElasticsearchIntegrationTest {
+
+    @Test
+    public void testUnallocatedShardsDoesNotHang() throws InterruptedException {
+        // create an index but because no data nodes are available no shards will be allocated
+        createIndex("test");
+        // this should not hang but instead immediately return with empty result set
+        SealIndicesResponse sealIndicesResponse = client().admin().indices().prepareSealIndices("test").get();
+        assertThat(sealIndicesResponse.results().size(), equalTo(0));
+    }
+}