Prevent deletion of default branch (#1827)

Adds a pre receive repository hook that prevents the deletion of the default branch.
Mirrored repositories will change their default branches to another branch, when it is deleted.

scm-webapp/src/main/java/sonia/scm/repository/DefaultBranchDeleteProtection.java

@@ -0,0 +1,92 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020-present Cloudogu GmbH and Contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package sonia.scm.repository;
+
+import com.github.legman.Subscribe;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import sonia.scm.EagerSingleton;
+import sonia.scm.plugin.Extension;
+import sonia.scm.repository.api.HookFeature;
+import sonia.scm.repository.api.RepositoryService;
+import sonia.scm.repository.api.RepositoryServiceFactory;
+import sonia.scm.repository.spi.CannotDeleteDefaultBranchException;
+
+import javax.inject.Inject;
+import java.io.IOException;
+import java.util.List;
+
+@Extension
+@EagerSingleton
+public class DefaultBranchDeleteProtection {
+
+  private static final Logger LOG = LoggerFactory.getLogger(DefaultBranchDeleteProtection.class);
+
+  private final RepositoryServiceFactory serviceFactory;
+
+  @Inject
+  public DefaultBranchDeleteProtection(RepositoryServiceFactory serviceFactory) {
+    this.serviceFactory = serviceFactory;
+  }
+
+  @Subscribe(async = false)
+  public void protectDefaultBranch(PreReceiveRepositoryHookEvent event) {
+    if (event.getContext().isFeatureSupported(HookFeature.BRANCH_PROVIDER)) {
+      List<String> deletedOrClosed = event.getContext().getBranchProvider().getDeletedOrClosed();
+      if (!deletedOrClosed.isEmpty()) {
+        checkDeletedBranches(event, deletedOrClosed);
+      }
+    }
+  }
+
+  private void checkDeletedBranches(PreReceiveRepositoryHookEvent event, List<String> deletedOrClosed) {
+    try (RepositoryService service = serviceFactory.create(event.getRepository())) {
+      getBranches(service)
+        .getBranches()
+        .stream()
+        .filter(Branch::isDefaultBranch)
+        .findFirst()
+        .ifPresent(
+          defaultBranch -> assertBranchNotDeleted(event, deletedOrClosed, defaultBranch)
+        );
+    }
+  }
+
+  private Branches getBranches(RepositoryService service) {
+    try {
+      return service.getBranchesCommand().setDisableCache(true).getBranches();
+    } catch (IOException e) {
+      LOG.warn("Could not read branches in repository {} to check for default branch", service.getRepository());
+      return new Branches();
+    }
+  }
+
+  private void assertBranchNotDeleted(PreReceiveRepositoryHookEvent event, List<String> deletedOrClosed, Branch defaultBranch) {
+    String defaultBranchName = defaultBranch.getName();
+    if (deletedOrClosed.stream().anyMatch(branch -> branch.equals(defaultBranchName))) {
+      throw new CannotDeleteDefaultBranchException(event.getRepository(), defaultBranchName);
+    }
+  }
+}