Add extensionName() to security extension

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/SecurityExtension.java

@@ -115,4 +115,8 @@ default AuthenticationFailureHandler getAuthenticationFailureHandler(SecurityCom
     default AuthorizationEngine getAuthorizationEngine(Settings settings) {
         return null;
     }
+
+    default String extensionName() {
+        return getClass().getName();
+    }
 }

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/Security.java

@@ -43,6 +43,7 @@
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.common.util.concurrent.ThreadContext;
 import org.elasticsearch.common.util.set.Sets;
+import org.elasticsearch.core.Nullable;
 import org.elasticsearch.xcontent.NamedXContentRegistry;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.env.Environment;
@@ -651,37 +652,15 @@ auditTrailService, failureHandler, threadPool, anonymousUser, getAuthorizationEn
     }
 
     private AuthorizationEngine getAuthorizationEngine() {
-        AuthorizationEngine authorizationEngine = null;
-        String extensionName = null;
-        for (SecurityExtension extension : securityExtensions) {
-            final AuthorizationEngine extensionEngine = extension.getAuthorizationEngine(settings);
-            if (extensionEngine != null && authorizationEngine != null) {
-                throw new IllegalStateException("Extensions [" + extensionName + "] and [" + extension.toString() + "] "
-                    + "both set an authorization engine");
-            }
-            authorizationEngine = extensionEngine;
-            extensionName = extension.toString();
-        }
-
-        if (authorizationEngine != null) {
-            logger.debug("Using authorization engine from extension [" + extensionName + "]");
-        }
-        return authorizationEngine;
+        return findValueFromExtensions("authorization engine", extension -> extension.getAuthorizationEngine(settings));
     }
 
     private AuthenticationFailureHandler createAuthenticationFailureHandler(final Realms realms,
                                                                             final SecurityExtension.SecurityComponents components) {
-        AuthenticationFailureHandler failureHandler = null;
-        String extensionName = null;
-        for (SecurityExtension extension : securityExtensions) {
-            AuthenticationFailureHandler extensionFailureHandler = extension.getAuthenticationFailureHandler(components);
-            if (extensionFailureHandler != null && failureHandler != null) {
-                throw new IllegalStateException("Extensions [" + extensionName + "] and [" + extension.toString() + "] "
-                        + "both set an authentication failure handler");
-            }
-            failureHandler = extensionFailureHandler;
-            extensionName = extension.toString();
-        }
+        AuthenticationFailureHandler failureHandler = findValueFromExtensions(
+            "authentication failure handler",
+            extension -> extension.getAuthenticationFailureHandler(components)
+        );
         if (failureHandler == null) {
             logger.debug("Using default authentication failure handler");
             Supplier<Map<String, List<String>>> headersSupplier = () -> {
@@ -718,12 +697,49 @@ private AuthenticationFailureHandler createAuthenticationFailureHandler(final Re
             getLicenseState().addListener(() -> {
                 finalDefaultFailureHandler.setHeaders(headersSupplier.get());
             });
-        } else {
-            logger.debug("Using authentication failure handler from extension [" + extensionName + "]");
         }
         return failureHandler;
     }
 
+    /**
+     * Calls the provided function for each configured extension and return the value that was generated by the extensions.
+     * If multiple extensions provide a value, throws {@link IllegalStateException}.
+     * If no extensions provide a value (or if there are no extensions) returns {@code null}.
+     */
+    @Nullable
+    private <T> T findValueFromExtensions(String valueType, Function<SecurityExtension, T> method) {
+        T foundValue = null;
+        String fromExtension = null;
+        for (SecurityExtension extension : securityExtensions) {
+            final T extensionValue = method.apply(extension);
+            if (extensionValue == null) {
+                continue;
+            }
+            if (foundValue == null) {
+                foundValue = extensionValue;
+                fromExtension = extension.extensionName();
+            }
+            else {
+                throw new IllegalStateException(
+                    "Extensions ["
+                        + fromExtension
+                        + "] and ["
+                        + extension.extensionName()
+                        + "] "
+                        + " both attempted to provide a value for ["
+                        + valueType
+                        + "]"
+                );
+            }
+        }
+        if (foundValue == null) {
+            return null;
+        } else {
+            logger.debug("Using [{}] [{}] from extension [{}]", valueType, foundValue, fromExtension);
+            return foundValue;
+        }
+    }
+
     @Override
     public Settings additionalSettings() {
         return additionalSettings(settings, enabled);

x-pack/qa/security-example-spi-extension/src/main/java/org/elasticsearch/example/ExampleSecurityExtension.java

@@ -42,6 +42,11 @@ public class ExampleSecurityExtension implements SecurityExtension {
         });
     }
 
+    @Override
+    public String extensionName() {
+        return "example";
+    }
+
     @Override
     public Map<String, Realm.Factory> getRealms(SecurityComponents components) {
         return Map.ofEntries(