Add check for invalid index in WildcardExpressionResolver (#26409)

This commit adds validation to the resolving of indexes in the wildcard
expression resolver. It no longer throws a 404 Not Found when resolving
invalid indices. It throws a 400 instead, as it is an invalid
index. This was the behavior of 5.x.

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

@@ -34,6 +34,7 @@ import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.indices.IndexClosedException;
+import org.elasticsearch.indices.InvalidIndexNameException;
 import org.joda.time.DateTimeZone;
 import org.joda.time.format.DateTimeFormat;
 import org.joda.time.format.DateTimeFormatter;
@@ -601,6 +602,7 @@ public class IndexNameExpressionResolver extends AbstractComponent {
                 if (Strings.isEmpty(expression)) {
                     throw indexNotFoundException(expression);
                 }
+                validateAliasOrIndex(expression);
                 if (aliasOrIndexExists(options, metaData, expression)) {
                     if (result != null) {
                         result.add(expression);
@@ -654,6 +656,16 @@ public class IndexNameExpressionResolver extends AbstractComponent {
             return result;
         }
 
+        private static void validateAliasOrIndex(String expression) {
+            // Expressions can not start with an underscore. This is reserved for APIs. If the check gets here, the API
+            // does not exist and the path is interpreted as an expression. If the expression begins with an underscore,
+            // throw a specific error that is different from the [[IndexNotFoundException]], which is typically thrown
+            // if the expression can't be found.
+            if (expression.charAt(0) == '_') {
+                throw new InvalidIndexNameException(expression, "must not start with '_'.");
+            }
+        }
+
         private static boolean aliasOrIndexExists(IndicesOptions options, MetaData metaData, String expression) {
             AliasOrIndex aliasOrIndex = metaData.getAliasAndIndexLookup().get(expression);
             //treat aliases as unavailable indices when ignoreAliases is set to true (e.g. delete index and update aliases api)

core/src/test/java/org/elasticsearch/cluster/metadata/IndexNameExpressionResolverTests.java

@@ -30,6 +30,7 @@ import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.indices.IndexClosedException;
+import org.elasticsearch.indices.InvalidIndexNameException;
 import org.elasticsearch.test.ESTestCase;
 
 import java.util.Arrays;
@@ -1126,4 +1127,14 @@ public class IndexNameExpressionResolverTests extends ESTestCase {
             assertEquals("test-index", indices[0]);
         }
     }
+
+    public void testInvalidIndex() {
+        MetaData.Builder mdBuilder = MetaData.builder().put(indexBuilder("test"));
+        ClusterState state = ClusterState.builder(new ClusterName("_name")).metaData(mdBuilder).build();
+        IndexNameExpressionResolver.Context context = new IndexNameExpressionResolver.Context(state, IndicesOptions.lenientExpandOpen());
+
+        InvalidIndexNameException iine = expectThrows(InvalidIndexNameException.class,
+            () -> indexNameExpressionResolver.concreteIndexNames(context, "_foo"));
+        assertEquals("Invalid index name [_foo], must not start with '_'.", iine.getMessage());
+    }
 }

rest-api-spec/src/main/resources/rest-api-spec/test/indices.get/10_basic.yml

@@ -160,3 +160,11 @@ setup:
   - is_true: test_index_2.settings
   - is_true: test_index_3.settings
 
+---
+"Should return an exception when querying invalid indices":
+
+ - do:
+     catch: bad_request
+     indices.get:
+       index: _foo
+