Extend IGlobalExtension lifecylce by 'initSpecs'

New lifecycle method initSpecs(Collection<SpecInfo> specs) is called
once, before visiting single specifications later on in `visitSpec`. It
enables global extensions to view all specifications as an ensemble,
e.g. for iterating over them and rearranging their execution order.

Relates to spockframework#1443.

docs/extensions.adoc

@@ -763,13 +763,17 @@ fully-qualified class name in a file `META-INF/services/org.spockframework.runti
 class path. As soon as these two conditions are satisfied, the extension is automatically loaded and used when Spock is
 running.
 
-`IGlobalExtension` has the following three methods:
+`IGlobalExtension` has the following four methods:
 
 `start()`::
   This is called once at the very start of the Spock execution.
 
+`initSpecs(Collection<SpecInfo> specs)`::
+  This is called once, before visiting single specifications later on in `visitSpec`. It enables global extensions to
+  view all specifications as an ensemble, e.g. for iterating over them and rearranging their execution order.
+
 `visitSpec(SpecInfo spec)`::
-  This is called once for each specification. In this method you can prepare a specification with your extension magic,
+  This is called once for each specification. In this method, you can prepare a specification with your extension magic,
   like attaching interceptors to various interception points as described in the chapter <<Interceptors>>.
 
 `stop()`::

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

@@ -14,6 +14,7 @@
 
 package org.spockframework.runtime;
 
+import org.junit.platform.engine.TestDescriptor;
 import org.spockframework.runtime.extension.*;
 import org.spockframework.runtime.model.*;
 import org.spockframework.util.Nullable;
@@ -30,6 +31,8 @@
  */
 @SuppressWarnings("rawtypes")
 public class ExtensionRunner {
+  private static final SpecInfo EMPTY_SPEC = new SpecInfo();
+
   private final SpecInfo spec;
   private final IExtensionRegistry extensionRegistry;
   private final IConfigurationRegistry configurationRegistry;
@@ -42,6 +45,20 @@ public ExtensionRunner(SpecInfo spec, IExtensionRegistry extensionRegistry, ICon
     this.configurationRegistry = configurationRegistry;
   }
 
+  public ExtensionRunner(IExtensionRegistry extensionRegistry, IConfigurationRegistry configurationRegistry) {
+    this(EMPTY_SPEC, extensionRegistry, configurationRegistry);
+  }
+
+  public void initGlobalExtensions(Set<? extends TestDescriptor> testDescriptors) {
+    List<SpecInfo> specs = testDescriptors.stream()
+      .filter(testDescriptor -> testDescriptor instanceof SpecNode)
+      .map(testDescriptor -> ((SpecNode) testDescriptor).getNodeInfo())
+      .collect(toList());
+    for (IGlobalExtension extension : extensionRegistry.getGlobalExtensions()) {
+      extension.initSpecs(specs);
+    }
+  }
+
   public void run() {
     runGlobalExtensions();
     runAnnotationDrivenExtensions();

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

@@ -81,6 +81,10 @@ public ExtensionRunner createExtensionRunner(SpecInfo spec) {
     return new ExtensionRunner(spec, globalExtensionRegistry, globalExtensionRegistry);
   }
 
+  public ExtensionRunner createExtensionRunner() {
+    return new ExtensionRunner(globalExtensionRegistry, globalExtensionRegistry);
+  }
+
   public PlatformParameterizedSpecRunner createSpecRunner(SpecInfo spec) {
     return new PlatformParameterizedSpecRunner(
       new MasterRunSupervisor(spec, createStackTraceFilter(spec), diffedObjectRenderer));

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

@@ -5,6 +5,8 @@
 
 import org.junit.platform.engine.*;
 
+import java.util.Set;
+
 import static java.util.Comparator.comparingInt;
 
 class SpockEngineDiscoveryPostProcessor {
@@ -14,7 +16,9 @@ class SpockEngineDiscoveryPostProcessor {
   SpockEngineDescriptor postProcessEngineDescriptor(UniqueId uniqueId, RunContext runContext,
     SpockEngineDescriptor engineDescriptor) {
     SpockEngineDescriptor processedEngineDescriptor = new SpockEngineDescriptor(uniqueId, runContext);
-    engineDescriptor.getChildren().stream()
+    Set<? extends TestDescriptor> testDescriptors = engineDescriptor.getChildren();
+    initSpecNodes(testDescriptors, runContext);
+    testDescriptors.stream()
       .map(child -> processSpecNode(child, runContext))
       .sorted(comparingInt(child -> child instanceof SpecNode ? ((SpecNode) child).getNodeInfo().getExecutionOrder() : 0))
       .forEach(processedEngineDescriptor::addChild);
@@ -46,6 +50,10 @@ private UniqueId toUniqueId(UniqueId parentId, FeatureInfo feature) {
     return parentId.append("feature", feature.getFeatureMethod().getReflection().getName());
   }
 
+  private void initSpecNodes(Set<? extends TestDescriptor> testDescriptors, RunContext runContext) {
+    runContext.createExtensionRunner().initGlobalExtensions(testDescriptors);
+  }
+
   private TestDescriptor processSpecNode(TestDescriptor child, RunContext runContext) {
     if (child instanceof SpecNode) {
       SpecNode specNode = (SpecNode) child;

spock-core/src/main/java/org/spockframework/runtime/extension/IGlobalExtension.java

@@ -18,10 +18,13 @@
 
 import org.spockframework.runtime.model.SpecInfo;
 
+import java.util.Collection;
+
 // TODO: start/stop lifecycle
 // TODO: design threading model
 public interface IGlobalExtension {
-  default void start() {};
-  default void visitSpec(SpecInfo spec){};
-  default void stop(){};
+  default void start() {}
+  default void initSpecs(Collection<SpecInfo> specs) {}
+  default void visitSpec(SpecInfo spec) {}
+  default void stop() {}
 }