Ensure we emit a warning when using the deprecated 'template' field. (#50831)

Since 6.0, the 'template' field has been deprecated in put template requests in
favour of index_patterns. Previously, the PutIndexTemplateRequest would accept
the 'template' field in its 'source' methods and silently convert it to
'index_patterns'. This meant that users specifying 'template' in the source
would not receive a deprecation warning from the server.

This PR makes a small change to no longer silently convert 'template' to
'index_patterns', which ensures that users receive a deprecation warning.

Follow-up to #49460.

Author: jtibshirani

client/rest-high-level/src/main/java/org/elasticsearch/client/indices/PutIndexTemplateRequest.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.action.admin.indices.alias.Alias;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.action.support.master.MasterNodeRequest;
+import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
@@ -64,6 +65,15 @@ public class PutIndexTemplateRequest extends MasterNodeRequest<PutIndexTemplateR
 
     private List<String> indexPatterns;
 
+    /**
+     * This field corresponds to the deprecated 'template' parameter, which was replaced by
+     * 'index_patterns' in 6.0. It is stored and rendered to xContent separately from
+     * 'index_patterns' to ensure that the server emits a deprecation warning when it's been set.
+     */
+    @Deprecated
+    @Nullable
+    private String template;
+
     private int order;
 
     private boolean create;
@@ -86,7 +96,7 @@ public PutIndexTemplateRequest(String name) {
     @Override
     public ActionRequestValidationException validate() {
         ActionRequestValidationException validationException = null;
-        if (indexPatterns == null || indexPatterns.size() == 0) {
+        if (template == null && (indexPatterns == null || indexPatterns.size() == 0)) {
             validationException = addValidationError("index patterns are missing", validationException);
         }
         return validationException;
@@ -288,7 +298,7 @@ public PutIndexTemplateRequest source(Map<String, Object> templateSource) {
             String name = entry.getKey();
             if (name.equals("template")) {
                 if(entry.getValue() instanceof String) {
-                    patterns(Collections.singletonList((String) entry.getValue()));
+                    this.template = (String) entry.getValue();
                 }
             } else if (name.equals("index_patterns")) {
                 if(entry.getValue() instanceof String) {
@@ -297,7 +307,7 @@ public PutIndexTemplateRequest source(Map<String, Object> templateSource) {
                     List<String> elements = ((List<?>) entry.getValue()).stream().map(Object::toString).collect(Collectors.toList());
                     patterns(elements);
                 } else {
-                    throw new IllegalArgumentException("Malformed [template] value, should be a string or a list of strings");
+                    throw new IllegalArgumentException("Malformed [index_patterns] value, should be a string or a list of strings");
                 }
             } else if (name.equals("order")) {
                 order(XContentMapValues.nodeIntegerValue(entry.getValue(), order()));
@@ -424,7 +434,12 @@ public IndicesOptions indicesOptions() {
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
-        builder.field("index_patterns", indexPatterns);
+        if (template != null) {
+            builder.field("template", template);
+        } else {
+            builder.field("index_patterns", indexPatterns);
+        }
+
         builder.field("order", order);
         if (version != null) {
             builder.field("version", version);

client/rest-high-level/src/test/java/org/elasticsearch/client/IndicesClientIT.java

@@ -92,6 +92,7 @@
 import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.common.xcontent.json.JsonXContent;
 import org.elasticsearch.common.xcontent.support.XContentMapValues;
@@ -1578,6 +1579,32 @@ public void testPutTemplate() throws Exception {
         assertThat((Map<String, String>) extractValue("my-template.aliases.{index}-write", templates), hasEntry("search_routing", "xyz"));
     }
 
+    public void testPutTemplateWithDeprecatedTemplateField() throws Exception {
+        PutIndexTemplateRequest putTemplateRequest = new PutIndexTemplateRequest("my-template")
+            .source(XContentFactory.jsonBuilder()
+                .startObject()
+                    .field("template", "name-*")
+                    .field("order", 10)
+                    .startObject("settings")
+                        .field("number_of_shards", 3)
+                        .field("number_of_replicas", 0)
+                    .endObject()
+                .endObject());
+
+        AcknowledgedResponse putTemplateResponse = execute(putTemplateRequest,
+            highLevelClient().indices()::putTemplate,
+            highLevelClient().indices()::putTemplateAsync,
+            expectWarnings("Deprecated field [template] used, replaced by [index_patterns]"));
+        assertThat(putTemplateResponse.isAcknowledged(), equalTo(true));
+
+        Map<String, Object> templates = getAsMap("/_template/my-template");
+        assertThat(templates.keySet(), hasSize(1));
+        assertThat(extractValue("my-template.order", templates), equalTo(10));
+        assertThat(extractRawValues("my-template.index_patterns", templates), contains("name-*"));
+        assertThat(extractValue("my-template.settings.index.number_of_shards", templates), equalTo("3"));
+        assertThat(extractValue("my-template.settings.index.number_of_replicas", templates), equalTo("0"));
+    }
+
     public void testPutTemplateWithTypesUsingUntypedAPI() throws Exception {
         PutIndexTemplateRequest putTemplateRequest = new PutIndexTemplateRequest("my-template")
             .patterns(Arrays.asList("pattern-1", "name-*"))