Use chunked encoding for RestGetSettingsAction (#90326)

This one needs chunked encoding as well, the response easily grows to O(10M)
for large deployments with many indices. Especially when settings are numerous or
have large values like e.g. settings from the Beats templates.

server/src/main/java/org/elasticsearch/action/admin/indices/settings/get/GetSettingsResponse.java

@@ -10,9 +10,11 @@
 
 import org.elasticsearch.action.ActionResponse;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.collect.Iterators;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.ChunkedToXContent;
 import org.elasticsearch.common.xcontent.XContentParserUtils;
 import org.elasticsearch.xcontent.ToXContent;
 import org.elasticsearch.xcontent.ToXContentObject;
@@ -23,10 +25,11 @@
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.util.HashMap;
+import java.util.Iterator;
 import java.util.Map;
 import java.util.Objects;
 
-public class GetSettingsResponse extends ActionResponse implements ToXContentObject {
+public class GetSettingsResponse extends ActionResponse implements ToXContentObject, ChunkedToXContent {
 
     private final Map<String, Settings> indexToSettings;
     private final Map<String, Settings> indexToDefaultSettings;
@@ -44,25 +47,12 @@ public GetSettingsResponse(StreamInput in) throws IOException {
 
     /**
      * Returns a map of index name to {@link Settings} object.  The returned {@link Settings}
-     * objects contain only those settings explicitly set on a given index.  Any settings
-     * taking effect as defaults must be accessed via {@link #getIndexToDefaultSettings()}.
+     * objects contain only those settings explicitly set on a given index.
      */
     public Map<String, Settings> getIndexToSettings() {
         return indexToSettings;
     }
 
-    /**
-     * If the originating {@link GetSettingsRequest} object was configured to include
-     * defaults, this will contain a mapping of index name to {@link Settings} objects.
-     * The returned {@link Settings} objects will contain only those settings taking
-     * effect as defaults.  Any settings explicitly set on the index will be available
-     * via {@link #getIndexToSettings()}.
-     * See also {@link GetSettingsRequest#includeDefaults(boolean)}
-     */
-    public Map<String, Settings> getIndexToDefaultSettings() {
-        return indexToDefaultSettings;
-    }
-
     /**
      * Returns the string value for the specified index and setting.  If the includeDefaults
      * flag was not set or set to false on the GetSettingsRequest, this method will only
@@ -154,38 +144,44 @@ public String toString() {
         try {
             ByteArrayOutputStream baos = new ByteArrayOutputStream();
             XContentBuilder builder = new XContentBuilder(JsonXContent.jsonXContent, baos);
-            toXContent(builder, ToXContent.EMPTY_PARAMS, false);
+            var iterator = toXContentChunked(false);
+            while (iterator.hasNext()) {
+                iterator.next().toXContent(builder, ToXContent.EMPTY_PARAMS);
+            }
             return Strings.toString(builder);
         } catch (IOException e) {
             throw new IllegalStateException(e); // should not be possible here
         }
     }
 
     @Override
-    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
-        return toXContent(builder, params, indexToDefaultSettings.isEmpty());
+    public Iterator<? extends ToXContent> toXContentChunked() {
+        final boolean omitEmptySettings = indexToDefaultSettings.isEmpty();
+        return toXContentChunked(omitEmptySettings);
     }
 
-    private XContentBuilder toXContent(XContentBuilder builder, Params params, boolean omitEmptySettings) throws IOException {
-        builder.startObject();
-        for (Map.Entry<String, Settings> cursor : getIndexToSettings().entrySet()) {
-            // no settings, jump over it to shorten the response data
-            if (omitEmptySettings && cursor.getValue().isEmpty()) {
-                continue;
-            }
-            builder.startObject(cursor.getKey());
-            builder.startObject("settings");
-            cursor.getValue().toXContent(builder, params);
-            builder.endObject();
-            if (indexToDefaultSettings.isEmpty() == false) {
-                builder.startObject("defaults");
-                indexToDefaultSettings.get(cursor.getKey()).toXContent(builder, params);
-                builder.endObject();
-            }
-            builder.endObject();
-        }
-        builder.endObject();
-        return builder;
+    private Iterator<ToXContent> toXContentChunked(boolean omitEmptySettings) {
+        final boolean indexToDefaultSettingsEmpty = indexToDefaultSettings.isEmpty();
+        return Iterators.concat(
+            Iterators.single((builder, params) -> builder.startObject()),
+            getIndexToSettings().entrySet()
+                .stream()
+                .filter(entry -> omitEmptySettings == false || entry.getValue().isEmpty() == false)
+                .map(entry -> (ToXContent) (builder, params) -> {
+                    builder.startObject(entry.getKey());
+                    builder.startObject("settings");
+                    entry.getValue().toXContent(builder, params);
+                    builder.endObject();
+                    if (indexToDefaultSettingsEmpty == false) {
+                        builder.startObject("defaults");
+                        indexToDefaultSettings.get(entry.getKey()).toXContent(builder, params);
+                        builder.endObject();
+                    }
+                    return builder.endObject();
+                })
+                .iterator(),
+            Iterators.single((builder, params) -> builder.endObject())
+        );
     }
 
     @Override

server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetSettingsAction.java

@@ -14,7 +14,7 @@
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
-import org.elasticsearch.rest.action.RestToXContentListener;
+import org.elasticsearch.rest.action.RestChunkedToXContentListener;
 
 import java.io.IOException;
 import java.util.List;
@@ -52,6 +52,6 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
             .names(names);
         getSettingsRequest.local(request.paramAsBoolean("local", getSettingsRequest.local()));
         getSettingsRequest.masterNodeTimeout(request.paramAsTime("master_timeout", getSettingsRequest.masterNodeTimeout()));
-        return channel -> client.admin().indices().getSettings(getSettingsRequest, new RestToXContentListener<>(channel));
+        return channel -> client.admin().indices().getSettings(getSettingsRequest, new RestChunkedToXContentListener<>(channel));
     }
 }

server/src/test/java/org/elasticsearch/action/admin/indices/settings/get/GetSettingsResponseTests.java

@@ -74,4 +74,15 @@ protected Predicate<String> getRandomFieldsExcludeFilter() {
         return f -> f.equals("") || f.contains(".settings") || f.contains(".defaults");
     }
 
+    public void testOneChunkPerIndex() {
+        final var instance = createTestInstance();
+        final var iterator = instance.toXContentChunked();
+        int chunks = 0;
+        while (iterator.hasNext()) {
+            chunks++;
+            iterator.next();
+        }
+        assertEquals(2 + instance.getIndexToSettings().size(), chunks);
+    }
+
 }