Dangling indices strip aliases (#47581)

Importing dangling indices with aliases risks breaking functionalities
using those aliases. For instance, writing to an alias may break if
there is no is_write_index indication on the existing alias and the
dangling index import adds a second index to the alias. Or an
application could have an assumption about the alias only ever pointing
to one index and suddenly seeing the alias also linked to an old index
could break it. With this change we strip aliases of the index meta data
found before importing a dangling index.

Author: henningandersen

server/src/main/java/org/elasticsearch/gateway/DanglingIndicesState.java

@@ -144,7 +144,7 @@ Map<Index, IndexMetaData> findNewDanglingIndices(final MetaData metaData) {
                 } else {
                     logger.info("[{}] dangling index exists on local file system, but not in cluster metadata, " +
                                 "auto import to cluster state", indexMetaData.getIndex());
-                    newIndices.put(indexMetaData.getIndex(), indexMetaData);
+                    newIndices.put(indexMetaData.getIndex(), stripAliases(indexMetaData));
                 }
             }
             return newIndices;
@@ -154,6 +154,20 @@ Map<Index, IndexMetaData> findNewDanglingIndices(final MetaData metaData) {
         }
     }
 
+    /**
+     * Dangling importing indices with aliases is dangerous, it could for instance result in inability to write to an existing alias if it
+     * previously had only one index with any is_write_index indication.
+     */
+    private IndexMetaData stripAliases(IndexMetaData indexMetaData) {
+        if (indexMetaData.getAliases().isEmpty()) {
+            return indexMetaData;
+        } else {
+            logger.info("[{}] stripping aliases: {} from index before importing",
+                indexMetaData.getIndex(), indexMetaData.getAliases().keys());
+            return IndexMetaData.builder(indexMetaData).removeAllAliases().build();
+        }
+    }
+
     /**
      * Allocates the provided list of the dangled indices by sending them to the master node
      * for allocation.

server/src/test/java/org/elasticsearch/gateway/DanglingIndicesStateTests.java

@@ -19,6 +19,7 @@
 package org.elasticsearch.gateway;
 
 import org.elasticsearch.Version;
+import org.elasticsearch.cluster.metadata.AliasMetaData;
 import org.elasticsearch.cluster.metadata.IndexGraveyard;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.MetaData;
@@ -158,6 +159,28 @@ public void testDanglingIndicesNotImportedWhenTombstonePresent() throws Exceptio
         }
     }
 
+    public void testDanglingIndicesStripAliases() throws Exception {
+        try (NodeEnvironment env = newNodeEnvironment()) {
+            MetaStateService metaStateService = new MetaStateService(env, xContentRegistry());
+            DanglingIndicesState danglingState = createDanglingIndicesState(env, metaStateService);
+
+            final Settings.Builder settings = Settings.builder().put(indexSettings).put(IndexMetaData.SETTING_INDEX_UUID, "test1UUID");
+            IndexMetaData dangledIndex = IndexMetaData.builder("test1")
+                .settings(settings)
+                .putAlias(AliasMetaData.newAliasMetaDataBuilder("test_aliasd").build())
+                .build();
+            metaStateService.writeIndex("test_write", dangledIndex);
+            assertThat(dangledIndex.getAliases().size(), equalTo(1));
+
+            final MetaData metaData = MetaData.builder().build();
+            Map<Index, IndexMetaData> newDanglingIndices = danglingState.findNewDanglingIndices(metaData);
+            assertThat(newDanglingIndices.size(), equalTo(1));
+            Map.Entry<Index, IndexMetaData> entry = newDanglingIndices.entrySet().iterator().next();
+            assertThat(entry.getKey().getName(), equalTo("test1"));
+            assertThat(entry.getValue().getAliases().size(), equalTo(0));
+        }
+    }
+
     private DanglingIndicesState createDanglingIndicesState(NodeEnvironment env, MetaStateService metaStateService) {
         return new DanglingIndicesState(env, metaStateService, null, mock(ClusterService.class));
     }