Add ReindexDatastreamIndexAction (elastic#116996) (elastic#118703)

Add an action to reindex a single index from a source index to a destination index. Unlike the reindex action, this action copies settings and mappings from the source index to the dest index before performing the reindex. This action is part of work to reindex data streams and will be called on each of the backing indices within a data stream.

(cherry picked from commit 0a6ce27)

Author: parkertimmins

docs/changelog/116996.yaml

@@ -0,0 +1,5 @@
+pr: 116996
+summary: Initial work on `ReindexDatastreamIndexAction`
+area: Data streams
+type: enhancement
+issues: []

server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java

@@ -1656,23 +1656,11 @@ static void prepareResizeIndexSettings(
             throw new IllegalStateException("unknown resize type is " + type);
         }
 
-        final Settings.Builder builder = Settings.builder();
+        final Settings.Builder builder;
         if (copySettings) {
-            // copy all settings and non-copyable settings and settings that have already been set (e.g., from the request)
-            for (final String key : sourceMetadata.getSettings().keySet()) {
-                final Setting<?> setting = indexScopedSettings.get(key);
-                if (setting == null) {
-                    assert indexScopedSettings.isPrivateSetting(key) : key;
-                } else if (setting.getProperties().contains(Setting.Property.NotCopyableOnResize)) {
-                    continue;
-                }
-                // do not override settings that have already been set (for example, from the request)
-                if (indexSettingsBuilder.keys().contains(key)) {
-                    continue;
-                }
-                builder.copy(key, sourceMetadata.getSettings());
-            }
+            builder = copySettingsFromSource(true, sourceMetadata.getSettings(), indexScopedSettings, indexSettingsBuilder);
         } else {
+            builder = Settings.builder();
             final Predicate<String> sourceSettingsPredicate = (s) -> (s.startsWith("index.similarity.")
                 || s.startsWith("index.analysis.")
                 || s.startsWith("index.sort.")
@@ -1690,6 +1678,36 @@ static void prepareResizeIndexSettings(
         }
     }
 
+    public static Settings.Builder copySettingsFromSource(
+        boolean copyPrivateSettings,
+        Settings sourceSettings,
+        IndexScopedSettings indexScopedSettings,
+        Settings.Builder indexSettingsBuilder
+    ) {
+        final Settings.Builder builder = Settings.builder();
+        for (final String key : sourceSettings.keySet()) {
+            final Setting<?> setting = indexScopedSettings.get(key);
+            if (setting == null) {
+                assert indexScopedSettings.isPrivateSetting(key) : key;
+                if (copyPrivateSettings == false) {
+                    continue;
+                }
+            } else if (setting.getProperties().contains(Setting.Property.NotCopyableOnResize)) {
+                continue;
+            } else if (setting.isPrivateIndex()) {
+                if (copyPrivateSettings == false) {
+                    continue;
+                }
+            }
+            // do not override settings that have already been set (for example, from the request)
+            if (indexSettingsBuilder.keys().contains(key)) {
+                continue;
+            }
+            builder.copy(key, sourceSettings);
+        }
+        return builder;
+    }
+
     /**
      * Returns a default number of routing shards based on the number of shards of the index. The default number of routing shards will
      * allow any index to be split at least once and at most 10 times by a factor of two. The closer the number or shards gets to 1024