Shrunk indices should ignore templates

A shrunk index should ignore anything from templates and instead take
its mappings, aliases, and settings from the original index, plus any
new settings and aliases passed in with the shrink request. This commit
causes this to be the case.

Relates #25380

core/src/main/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexService.java

@@ -264,6 +264,9 @@ public class MetaDataCreateIndexService extends AbstractComponent {
                                 customs.put(entry.getKey(), entry.getValue());
                             }
 
+                            final Index shrinkFromIndex = request.shrinkFrom();
+
+                            if (shrinkFromIndex == null) {
                                 // apply templates, merging the mappings into the request mapping if exists
                                 for (IndexTemplateMetaData template : templates) {
                                     templateNames.add(template.getName());
@@ -312,11 +315,14 @@ public class MetaDataCreateIndexService extends AbstractComponent {
                                         templatesAliases.put(aliasMetaData.alias(), aliasMetaData);
                                     }
                                 }
+                            }
                             Settings.Builder indexSettingsBuilder = Settings.builder();
+                            if (shrinkFromIndex == null) {
                                 // apply templates, here, in reverse order, since first ones are better matching
                                 for (int i = templates.size() - 1; i >= 0; i--) {
                                     indexSettingsBuilder.put(templates.get(i).settings());
                                 }
+                            }
                             // now, put the request settings, so they override templates
                             indexSettingsBuilder.put(request.settings());
                             if (indexSettingsBuilder.get(SETTING_NUMBER_OF_SHARDS) == null) {
@@ -340,7 +346,6 @@ public class MetaDataCreateIndexService extends AbstractComponent {
                             }
                             indexSettingsBuilder.put(IndexMetaData.SETTING_INDEX_PROVIDED_NAME, request.getProvidedName());
                             indexSettingsBuilder.put(SETTING_INDEX_UUID, UUIDs.randomBase64UUID());
-                            final Index shrinkFromIndex = request.shrinkFrom();
                             final IndexMetaData.Builder tmpImdBuilder = IndexMetaData.builder(request.index());
 
                             final int routingNumShards;

core/src/test/java/org/elasticsearch/routing/PartitionedRoutingIT.java

@@ -67,6 +67,7 @@ public class PartitionedRoutingIT extends ESIntegTestCase {
         client().admin().indices().prepareCreate(index)
             .setSettings(Settings.builder()
                 .put("index.number_of_shards", currentShards)
+                .put("index.number_of_replicas", numberOfReplicas())
                 .put("index.routing_partition_size", partitionSize))
             .addMapping("type", "{\"type\":{\"_routing\":{\"required\":true}}}", XContentType.JSON)
             .execute().actionGet();
@@ -107,6 +108,7 @@ public class PartitionedRoutingIT extends ESIntegTestCase {
             client().admin().indices().prepareShrinkIndex(previousIndex, index)
                 .setSettings(Settings.builder()
                     .put("index.number_of_shards", currentShards)
+                    .put("index.number_of_replicas", numberOfReplicas())
                     .build()).get();
             ensureGreen();
         }

rest-api-spec/src/main/resources/rest-api-spec/test/indices.shrink/20_source_mapping.yml

@@ -0,0 +1,77 @@
+---
+"Shrink index ignores target template mapping":
+    - skip:
+        version: " - 5.99.99"
+        reason:  bug fixed in 6.0
+
+    - do:
+        cluster.state: {}
+        # Get master node id
+
+    - set: { master_node: master }
+
+  # create index
+    - do:
+        indices.create:
+          index: source
+          wait_for_active_shards: 1
+          body:
+            settings:
+              # ensure everything is allocated on a single node
+              index.routing.allocation.include._id: $master
+              number_of_replicas: 0
+            mappings:
+              test:
+                properties:
+                  count:
+                    type: text
+
+    # index document
+    - do:
+        index:
+          index: source
+          type:  test
+          id:    "1"
+          body:  { "count": "1" }
+
+    # create template matching shrink tagret
+    - do:
+        indices.put_template:
+          name: tpl1
+          body:
+            index_patterns: targ*
+            mappings:
+              test:
+                properties:
+                  count:
+                    type: integer
+
+    # make it read-only
+    - do:
+        indices.put_settings:
+          index: source
+          body:
+            index.blocks.write: true
+            index.number_of_replicas: 0
+
+    - do:
+        cluster.health:
+          wait_for_status: green
+          index: source
+
+    # now we do the actual shrink
+    - do:
+        indices.shrink:
+          index: "source"
+          target: "target"
+          wait_for_active_shards: 1
+          master_timeout: 10s
+          body:
+            settings:
+              index.number_of_replicas: 0
+
+    - do:
+        cluster.health:
+          wait_for_status: green
+
+