create new error module, which displays errors before migration

scm-webapp/src/main/java/sonia/scm/boot/BootstrapContextListener.java

@@ -69,7 +69,6 @@ import javax.xml.bind.annotation.XmlAccessType;
 import javax.xml.bind.annotation.XmlAccessorType;
 import javax.xml.bind.annotation.XmlElement;
 import javax.xml.bind.annotation.XmlRootElement;
-import java.io.Closeable;
 import java.io.File;
 import java.io.IOException;
 import java.net.MalformedURLException;
@@ -126,9 +125,7 @@ public class BootstrapContextListener implements ServletContextListener {
   public void contextInitialized(ServletContextEvent sce) {
     context = sce.getServletContext();
 
-    File pluginDirectory = getPluginDirectory();
-
-    createContextListener(pluginDirectory);
+    createContextListener();
 
     contextListener.contextInitialized(sce);
 
@@ -140,12 +137,23 @@ public class BootstrapContextListener implements ServletContextListener {
     }
   }
 
-  private void createContextListener(File pluginDirectory) {
+  private void createContextListener() {
+    Throwable startupError = SCMContext.getContext().getStartupError();
+    if (startupError != null) {
+      contextListener = SingleView.error(startupError);
+    } else {
+      createMigrationOrNormalContextListener();
+    }
+  }
+
+  private void createMigrationOrNormalContextListener() {
     ClassLoader cl;
     Set<PluginWrapper> plugins;
     PluginLoader pluginLoader;
 
     try {
+      File pluginDirectory = getPluginDirectory();
+
       renameOldPluginsFolder(pluginDirectory);
 
       if (!isCorePluginExtractionDisabled()) {

scm-webapp/src/main/java/sonia/scm/boot/SingleView.java

@@ -0,0 +1,109 @@
+package sonia.scm.boot;
+
+import com.google.common.base.Throwables;
+import com.google.common.collect.ImmutableMap;
+import com.google.inject.Guice;
+import com.google.inject.Injector;
+import com.google.inject.multibindings.Multibinder;
+import com.google.inject.servlet.GuiceServletContextListener;
+import com.google.inject.servlet.ServletModule;
+import sonia.scm.Default;
+import sonia.scm.SCMContext;
+import sonia.scm.SCMContextProvider;
+import sonia.scm.template.MustacheTemplateEngine;
+import sonia.scm.template.TemplateEngine;
+import sonia.scm.template.TemplateEngineFactory;
+
+import javax.servlet.ServletContext;
+import javax.servlet.ServletContextListener;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+final class SingleView {
+
+  private SingleView() {
+  }
+
+  static ServletContextListener error(Throwable throwable) {
+    String error = Throwables.getStackTraceAsString(throwable);
+
+    ViewController controller = new SimpleViewController("/templates/error.mustache", request -> {
+      Object model = ImmutableMap.of(
+        "contextPath", request.getContextPath(),
+        "error", error
+      );
+      return new View(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, model);
+    });
+    return new SingleViewContextListener(controller);
+  }
+
+  private static class SingleViewContextListener extends GuiceServletContextListener {
+
+    private final ViewController controller;
+
+    private SingleViewContextListener(ViewController controller) {
+      this.controller = controller;
+    }
+
+    @Override
+    protected Injector getInjector() {
+      return Guice.createInjector(new SingleViewModule(controller));
+    }
+  }
+
+  private static class SingleViewModule extends ServletModule {
+
+    private final ViewController viewController;
+
+    private SingleViewModule(ViewController viewController) {
+      this.viewController = viewController;
+    }
+
+    @Override
+    protected void configureServlets() {
+      SCMContextProvider context = SCMContext.getContext();
+
+      bind(SCMContextProvider.class).toInstance(context);
+      bind(ViewController.class).toInstance(viewController);
+
+      Multibinder<TemplateEngine> engineBinder =
+        Multibinder.newSetBinder(binder(), TemplateEngine.class);
+
+      engineBinder.addBinding().to(MustacheTemplateEngine.class);
+      bind(TemplateEngine.class).annotatedWith(Default.class).to(
+        MustacheTemplateEngine.class);
+      bind(TemplateEngineFactory.class);
+
+      bind(ServletContext.class).annotatedWith(Default.class).toInstance(getServletContext());
+
+      serve("/images/*", "/styles/*", "/favicon.ico").with(StaticResourceServlet.class);
+      serve("/*").with(SingleViewServlet.class);
+    }
+  }
+
+  private static class SimpleViewController implements ViewController {
+
+    private final String template;
+    private final SimpleViewFactory viewFactory;
+
+    private SimpleViewController(String template, SimpleViewFactory viewFactory) {
+      this.template = template;
+      this.viewFactory = viewFactory;
+    }
+
+    @Override
+    public String getTemplate() {
+      return template;
+    }
+
+    @Override
+    public View createView(HttpServletRequest request) {
+      return viewFactory.create(request);
+    }
+  }
+
+  @FunctionalInterface
+  interface SimpleViewFactory {
+    View create(HttpServletRequest request);
+  }
+}

scm-webapp/src/main/java/sonia/scm/boot/SingleViewServlet.java

@@ -0,0 +1,63 @@
+package sonia.scm.boot;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import sonia.scm.template.Template;
+import sonia.scm.template.TemplateEngine;
+import sonia.scm.template.TemplateEngineFactory;
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+import java.io.PrintWriter;
+
+@Singleton
+public class SingleViewServlet extends HttpServlet {
+
+  private static final Logger LOG = LoggerFactory.getLogger(SingleViewServlet.class);
+
+  private final Template template;
+  private final ViewController controller;
+
+  @Inject
+  public SingleViewServlet(TemplateEngineFactory templateEngineFactory, ViewController controller) {
+    template = createTemplate(templateEngineFactory, controller.getTemplate());
+    this.controller = controller;
+  }
+
+  private Template createTemplate(TemplateEngineFactory templateEngineFactory, String template) {
+    TemplateEngine engine = templateEngineFactory.getEngineByExtension(template);
+    try {
+      return engine.getTemplate(template);
+    } catch (IOException e) {
+      throw new IllegalStateException("failed to parse template: " + template, e);
+    }
+  }
+
+  @Override
+  protected void doGet(HttpServletRequest req, HttpServletResponse resp) {
+    process(req, resp);
+  }
+
+  @Override
+  protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
+    process(req, resp);
+  }
+
+  private void process(HttpServletRequest request, HttpServletResponse response) {
+    View view = controller.createView(request);
+
+    response.setStatus(view.getStatusCode());
+    response.setContentType("text/html");
+    response.setCharacterEncoding("UTF-8");
+
+    try (PrintWriter writer = response.getWriter()) {
+      template.execute(writer, view.getModel());
+    } catch (IOException ex) {
+      LOG.error("failed to write view", ex);
+    }
+  }
+}

scm-webapp/src/main/java/sonia/scm/boot/StaticResourceServlet.java

@@ -0,0 +1,39 @@
+package sonia.scm.boot;
+
+import com.github.sdorra.webresources.CacheControl;
+import com.github.sdorra.webresources.WebResourceSender;
+import com.google.common.annotations.VisibleForTesting;
+import sonia.scm.util.HttpUtil;
+
+import javax.inject.Singleton;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+import java.net.MalformedURLException;
+import java.net.URL;
+
+@Singleton
+public class StaticResourceServlet extends HttpServlet {
+
+  private final WebResourceSender sender = WebResourceSender.create()
+    .withGZIP()
+    .withGZIPMinLength(512)
+    .withBufferSize(16384)
+    .withCacheControl(CacheControl.create().noCache());
+
+  @Override
+  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
+    URL resource = createResourceUrlFromRequest(request);
+    if (resource != null) {
+      sender.resource(resource).get(request, response);
+    } else {
+      response.setStatus(HttpServletResponse.SC_NOT_FOUND);
+    }
+  }
+
+  private URL createResourceUrlFromRequest(HttpServletRequest request) throws MalformedURLException {
+    String uri = HttpUtil.getStrippedURI(request);
+    return request.getServletContext().getResource(uri);
+  }
+}

scm-webapp/src/main/java/sonia/scm/boot/View.java

@@ -0,0 +1,20 @@
+package sonia.scm.boot;
+
+class View {
+
+  private final int statusCode;
+  private final Object model;
+
+  View(int statusCode, Object model) {
+    this.statusCode = statusCode;
+    this.model = model;
+  }
+
+  int getStatusCode() {
+    return statusCode;
+  }
+
+  Object getModel() {
+    return model;
+  }
+}

scm-webapp/src/main/java/sonia/scm/boot/ViewController.java

@@ -0,0 +1,11 @@
+package sonia.scm.boot;
+
+import javax.servlet.http.HttpServletRequest;
+
+public interface ViewController {
+
+  String getTemplate();
+
+  View createView(HttpServletRequest request);
+
+}