Fix #817 Allow specifying global extension order

spock-core/src/main/java/org/spockframework/runtime/GlobalExtensionRegistry.java

@@ -27,17 +27,32 @@
  * Maintains a registry of global Spock extensions and their configuration objects,
  * which can be used to configure other extensions.
  *
+ * <p>If an extension is specified in two or more {@code IGlobalExtension} files on the classpath, it will only
+ * be processed for the first time it is discovered.
+ * This allows processing other extensions (e.g. Spring) before custom ones by listing the order of extensions
+ * explicitly in the file.
+ *
+ * <p>
+ * For instance this will load and run {@code SpringExtension} before {@code  MyExtension}:
+ * <pre>{@code
+ * org.spockframework.spring.SpringExtension
+ * com.example.MyExtension}</pre>
+ * </p>
+ *
+ *
  * @author Peter Niederwieser
  */
 public class GlobalExtensionRegistry implements IExtensionRegistry, IConfigurationRegistry {
-  private final List<Class<?>> globalExtensionClasses;
+  private final Set<Class<?>> globalExtensionClasses;
   private final Map<Class<?>, Object> configurationsByType = new HashMap<>();
   private final Map<String, Object> configurationsByName = new HashMap<>();
 
-  private final List<IGlobalExtension> globalExtensions = new ArrayList<>();
+  private final Set<IGlobalExtension> globalExtensions = new LinkedHashSet<>();
 
   GlobalExtensionRegistry(List<Class<?>> globalExtensionClasses, List<?> initialConfigurations) {
-    this.globalExtensionClasses = globalExtensionClasses;
+    // Only add an extension the first time it is encountered, i.e
+    // turns [A, A, B, A,] into [A,B]
+    this.globalExtensionClasses = new LinkedHashSet<>(globalExtensionClasses);
     initializeConfigurations(initialConfigurations);
   }
 
@@ -72,7 +87,7 @@ public Object getConfigurationByName(String name) {
   }
 
   @Override
-  public List<IGlobalExtension> getGlobalExtensions() {
+  public Set<IGlobalExtension> getGlobalExtensions() {
     return globalExtensions;
   }
 

spock-core/src/main/java/org/spockframework/runtime/IExtensionRegistry.java

@@ -16,8 +16,8 @@
 
 import org.spockframework.runtime.extension.IGlobalExtension;
 
-import java.util.List;
+import java.util.Set;
 
 public interface IExtensionRegistry {
-  List<IGlobalExtension> getGlobalExtensions();
+  Set<IGlobalExtension> getGlobalExtensions();
 }

spock-specs/src/test/groovy/org/spockframework/runtime/GlobalExtensionRegistrySpec.groovy

@@ -161,6 +161,27 @@ class GlobalExtensionRegistrySpec extends Specification {
     e.message.contains("unknown configuration class")
   }
 
+  // See https://github.com/spockframework/spock/issues/817
+  def "Extensions discovered later in the classpath are ignored if already processed earlier"() {
+    when:
+    def registry = new GlobalExtensionRegistry(extensionClasses, [])
+    registry.initializeGlobalExtensions()
+
+    then:
+    registry.globalExtensions*.class == expectedExtensionClasses
+
+    where:
+    // Ignore subsequent additions, even if they appear later in the list
+    extensionClasses                                                 || expectedExtensionClasses
+    [MyExtension, MyExtension]                                       || [MyExtension]
+    [SpringExtension, MyExtension, SpringExtension]                  || [SpringExtension, MyExtension]
+    [SpringExtension, SpringExtension, MyExtension, SpringExtension] || [SpringExtension, MyExtension]
+  }
+
+  static class SpringExtension extends AbstractGlobalExtension {
+
+  }
+
   static class MyExtension extends AbstractGlobalExtension {
     static instantiated = false