Check for external merge tools during merge

Co-authored-by: Thomas Zerr<thomas.zerr@cloudogu.com>
Co-authored-by: René Pfeuffer<rene.pfeuffer@cloudogu.com>

gradle/changelog/external_merge_tools.yml

@@ -0,0 +1,2 @@
+- type: added
+  description: Check for external merge tools during merge

scm-core/src/main/java/sonia/scm/repository/api/MergeDryRunCommandResult.java

@@ -21,9 +21,12 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  * SOFTWARE.
  */
-    
+
 package sonia.scm.repository.api;
 
+import java.util.Collection;
+import java.util.List;
+
 /**
  * This class keeps the result of a merge dry run. Use {@link #isMergeable()} to check whether an automatic merge is
  * possible or not.
@@ -31,9 +34,25 @@ package sonia.scm.repository.api;
 public class MergeDryRunCommandResult {
 
   private final boolean mergeable;
+  private final Collection<MergePreventReason> reasons;
 
+  /**
+   * Creates a result and sets the reason to FILE_CONFLICTS.
+   *
+   * @deprecated Please use {@link MergeDryRunCommandResult#MergeDryRunCommandResult(boolean, Collection)}
+   * instead and specify a concrete reason.
+   */
+  @Deprecated
   public MergeDryRunCommandResult(boolean mergeable) {
+    this(mergeable, List.of(new MergePreventReason(MergePreventReasonType.FILE_CONFLICTS)));
+  }
+
+  /**
+   * @since 3.3.0
+   */
+  public MergeDryRunCommandResult(boolean mergeable, Collection<MergePreventReason> reasons) {
     this.mergeable = mergeable;
+    this.reasons = reasons;
   }
 
   /**
@@ -43,4 +62,11 @@ public class MergeDryRunCommandResult {
   public boolean isMergeable() {
     return mergeable;
   }
+
+  /**
+   * This will return the reasons why the merge via the internal merge command is not possible.
+   */
+  public Collection<MergePreventReason> getReasons() {
+    return reasons;
+  }
 }

scm-core/src/main/java/sonia/scm/repository/api/MergePreventReason.java

@@ -0,0 +1,35 @@
+/*
+ * 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.api;
+
+import lombok.Value;
+
+/**
+ * @since 3.3.0
+ */
+@Value
+public class MergePreventReason {
+  MergePreventReasonType type;
+}

scm-core/src/main/java/sonia/scm/repository/api/MergePreventReasonType.java

@@ -0,0 +1,44 @@
+/*
+ * 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.api;
+
+/**
+ * @since 3.3.0
+ */
+public enum MergePreventReasonType {
+  /**
+   File conflicts are the typical reason why merges are not possible. Common examples are:
+   - File added on both branches
+   - File modified on one branch but deleted on the other
+   - File modified on both branches
+   Reference: {@link sonia.scm.repository.spi.MergeConflictResult.ConflictTypes}
+   */
+  FILE_CONFLICTS,
+  /**
+   * It is also possible that we cannot perform a merge properly because files would be affected which have to be merged by an external merge tool.
+   * For git these merge tools are configured in the .gitattributes file inside the repository.
+   */
+  EXTERNAL_MERGE_TOOL
+}

scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/AttributeAnalyzer.java

@@ -0,0 +1,119 @@
+/*
+ * 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.spi;
+
+import com.google.inject.assistedinject.Assisted;
+import jakarta.inject.Inject;
+import lombok.extern.slf4j.Slf4j;
+import org.eclipse.jgit.attributes.Attributes;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.util.LfsFactory;
+import sonia.scm.NotFoundException;
+import sonia.scm.repository.GitUtil;
+import sonia.scm.repository.Modified;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Collectors;
+
+@Slf4j
+class AttributeAnalyzer {
+
+  private static final String MERGE_TOOL_ATTRIBUTE_KEY = "merge";
+  private final GitContext context;
+  private final GitModificationsCommand modificationsCommand;
+
+  @Inject
+  AttributeAnalyzer(@Assisted GitContext context, GitModificationsCommand modificationsCommand) {
+    this.context = context;
+    this.modificationsCommand = modificationsCommand;
+  }
+
+  Optional<Attributes> getAttributes(RevCommit commit, String path) throws NotFoundException {
+    try (Repository repository = context.open()) {
+      Attributes attributesForPath = LfsFactory.getAttributesForPath(repository, path, commit);
+      if (attributesForPath.isEmpty()) {
+        return Optional.empty();
+      }
+      return Optional.of(attributesForPath);
+    } catch (IOException e) {
+      log.debug("Failed to get attributes", e);
+      return Optional.empty();
+    }
+  }
+
+  boolean hasExternalMergeToolConflicts(String source, String target) {
+    try (Repository repo = context.open()) {
+      String commonAncestorRevision = GitUtil.computeCommonAncestor(repo, GitUtil.getRevisionId(repo, source), GitUtil.getRevisionId(repo, target)).name();
+      return findExternalMergeToolConflicts(source, target, commonAncestorRevision);
+    } catch (IOException | NotFoundException e) {
+      log.debug("Failed to read/parse '.gitattributes' files", e);
+      return false;
+    }
+  }
+
+  boolean findExternalMergeToolConflicts(String source, String target, String commonAncestor) throws IOException {
+    List<String> changesInBoth = getPossiblyAffectedPaths(source, target, commonAncestor);
+    RevCommit targetCommit = getTargetCommit(target);
+
+    for (String path : changesInBoth) {
+      Optional<Attributes> attributes = getAttributes(targetCommit, path);
+      if (attributes.isPresent() && attributes.get().get(MERGE_TOOL_ATTRIBUTE_KEY) != null) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  private List<String> getPossiblyAffectedPaths(String source, String target, String commonAncestor) {
+    Collection<String> fromSourceToAncestor = findModifiedPaths(source, commonAncestor);
+    Collection<String> fromTargetToAncestor = findModifiedPaths(target, commonAncestor);
+    return fromSourceToAncestor.stream().filter(fromTargetToAncestor::contains).toList();
+  }
+
+  private Collection<String> findModifiedPaths(String baseRevision, String targetRevision) {
+    return modificationsCommand.getModifications(baseRevision, targetRevision).getModified()
+      .stream().map(Modified::getPath).collect(Collectors.toSet());
+  }
+
+  RevCommit getTargetCommit(String target) throws IOException {
+    try (Repository repository = context.open()) {
+      RevWalk rw = new org.eclipse.jgit.revwalk.RevWalk(repository);
+      return GitUtil.getCommit(repository, rw, repository.findRef(target));
+    } catch (IOException e) {
+      log.debug("Failed to get target commit", e);
+      throw e;
+    }
+  }
+
+  public interface Factory {
+    AttributeAnalyzer create(GitContext context);
+  }
+}

scm-plugins/scm-git-plugin/src/main/java/sonia/scm/repository/spi/GitMergeCommand.java

@@ -35,7 +35,6 @@ import org.eclipse.jgit.lib.IndexDiff;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Repository;
-import org.eclipse.jgit.merge.ResolveMerger;
 import org.eclipse.jgit.treewalk.CanonicalTreeParser;
 import org.eclipse.jgit.treewalk.filter.PathFilter;
 import sonia.scm.repository.GitRepositoryHandler;
@@ -43,11 +42,15 @@ import sonia.scm.repository.GitWorkingCopyFactory;
 import sonia.scm.repository.InternalRepositoryException;
 import sonia.scm.repository.api.MergeCommandResult;
 import sonia.scm.repository.api.MergeDryRunCommandResult;
+import sonia.scm.repository.api.MergePreventReason;
+import sonia.scm.repository.api.MergePreventReasonType;
 import sonia.scm.repository.api.MergeStrategy;
 import sonia.scm.repository.api.MergeStrategyNotSupportedException;
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Set;
 
 import static org.eclipse.jgit.merge.MergeStrategy.RECURSIVE;
@@ -57,7 +60,7 @@ import static sonia.scm.NotFoundException.notFound;
 public class GitMergeCommand extends AbstractGitCommand implements MergeCommand {
 
   private final GitWorkingCopyFactory workingCopyFactory;
-
+  private final AttributeAnalyzer attributeAnalyzer;
   private static final Set<MergeStrategy> STRATEGIES = ImmutableSet.of(
     MergeStrategy.MERGE_COMMIT,
     MergeStrategy.FAST_FORWARD_IF_POSSIBLE,
@@ -66,13 +69,14 @@ public class GitMergeCommand extends AbstractGitCommand implements MergeCommand
   );
 
   @Inject
-  GitMergeCommand(@Assisted GitContext context, GitRepositoryHandler handler) {
+  GitMergeCommand(@Assisted GitContext context, GitRepositoryHandler handler, AttributeAnalyzer attributeAnalyzer) {
-    this(context, handler.getWorkingCopyFactory());
+    this(context, handler.getWorkingCopyFactory(), attributeAnalyzer);
   }
 
-  GitMergeCommand(@Assisted GitContext context, GitWorkingCopyFactory workingCopyFactory) {
+  GitMergeCommand(@Assisted GitContext context, GitWorkingCopyFactory workingCopyFactory, AttributeAnalyzer attributeAnalyzer) {
     super(context);
     this.workingCopyFactory = workingCopyFactory;
+    this.attributeAnalyzer = attributeAnalyzer;
   }
 
   @Override
@@ -86,7 +90,7 @@ public class GitMergeCommand extends AbstractGitCommand implements MergeCommand
   }
 
   private MergeCommandResult mergeWithStrategy(MergeCommandRequest request) {
-    switch(request.getMergeStrategy()) {
+    switch (request.getMergeStrategy()) {
       case SQUASH:
         return inClone(clone -> new GitMergeWithSquash(clone, request, context, repository), workingCopyFactory, request.getTargetBranch());
 
@@ -103,21 +107,32 @@ public class GitMergeCommand extends AbstractGitCommand implements MergeCommand
         throw new MergeStrategyNotSupportedException(repository, request.getMergeStrategy());
     }
   }
-
+  
   @Override
   public MergeDryRunCommandResult dryRun(MergeCommandRequest request) {
-    try {
+    try (Repository repository = context.open()) {
-      Repository repository = context.open();
+      List<MergePreventReason> mergePreventReasons = new ArrayList<>(2);
-      ResolveMerger merger = (ResolveMerger) RECURSIVE.newMerger(repository, true);
+      if (attributeAnalyzer.hasExternalMergeToolConflicts(request.getBranchToMerge(), request.getTargetBranch())) {
-      return new MergeDryRunCommandResult(
+        mergePreventReasons.add(new MergePreventReason(MergePreventReasonType.EXTERNAL_MERGE_TOOL));
-        merger.merge(
+      }
-          resolveRevisionOrThrowNotFound(repository, request.getBranchToMerge()),
+
-          resolveRevisionOrThrowNotFound(repository, request.getTargetBranch())));
+      if (!isMergeableWithoutFileConflicts(repository, request.getBranchToMerge(), request.getTargetBranch())) {
+        mergePreventReasons.add(new MergePreventReason(MergePreventReasonType.FILE_CONFLICTS));
+      }
+
+      return new MergeDryRunCommandResult(mergePreventReasons.isEmpty(), mergePreventReasons);
     } catch (IOException e) {
       throw new InternalRepositoryException(context.getRepository(), "could not clone repository for merge", e);
     }
   }
 
+  private boolean isMergeableWithoutFileConflicts(Repository repository, String sourceRevision, String targetRevision) throws IOException {
+    return RECURSIVE.newMerger(repository, true).merge(
+      resolveRevisionOrThrowNotFound(repository,sourceRevision),
+      resolveRevisionOrThrowNotFound(repository, targetRevision)
+    );
+  }
+
   @Override
   public boolean isSupported(MergeStrategy strategy) {
     return STRATEGIES.contains(strategy);

scm-plugins/scm-git-plugin/src/test/java/sonia/scm/repository/spi/AttributeAnalyzerTest.java

@@ -0,0 +1,125 @@
+/*
+ * 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.spi;
+
+import org.eclipse.jgit.attributes.Attribute;
+import org.eclipse.jgit.attributes.Attributes;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.junit.MockitoJUnitRunner;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@RunWith(MockitoJUnitRunner.class)
+public class AttributeAnalyzerTest extends AbstractGitCommandTestBase {
+
+  private AttributeAnalyzer attributeAnalyzer;
+
+  @Before
+  public void initAnalyzer() {
+    attributeAnalyzer = new AttributeAnalyzer(createContext(), new GitModificationsCommand(createContext()));
+  }
+
+  @Test
+  public void shouldNotGetAttributesIfFileDoesNotExist() throws IOException {
+    RevCommit commit = attributeAnalyzer.getTargetCommit("main");
+    Optional<Attributes> attributes = attributeAnalyzer.getAttributes(commit,"text1234.txt");
+
+    assertThat(attributes).isNotPresent();
+  }
+
+  @Test
+  public void shouldNotGetAttributesIfDoesNotExistForFilePattern() throws IOException {
+    RevCommit commit = attributeAnalyzer.getTargetCommit("main");
+    Optional<Attributes> attributes = attributeAnalyzer.getAttributes(commit,"text.txt");
+
+    assertThat(attributes).isNotPresent();
+  }
+
+  @Test
+  public void shouldGetAttributes() throws IOException {
+    RevCommit commit = attributeAnalyzer.getTargetCommit("main");
+    Optional<Attributes> attributes = attributeAnalyzer.getAttributes(commit,"text.ipr");
+
+    assertThat(attributes).isPresent();
+    Collection<Attribute> attributeCollection = attributes.get().getAll();
+    Attribute firstAttribute = attributeCollection.iterator().next();
+    assertThat(firstAttribute.getKey()).isEqualTo("merge");
+    assertThat(firstAttribute.getValue()).isEqualTo("mps");
+  }
+
+  @Test
+  public void shouldCheckIfMergeIsPreventedByExternalMergeTools_WithNoGitAttributes() {
+    String source = "change_possibly_needing_merge_tool";
+    String target = "removed_git_attributes";
+    assertThat(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).isFalse();
+  }
+
+  @Test
+  public void shouldCheckIfMergeIsPreventedByExternalMergeTools_WithEmptyGitAttributes() {
+    String source = "change_possibly_needing_merge_tool";
+    String target = "empty_git_attributes";
+    assertThat(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).isFalse();
+  }
+
+  @Test
+  public void shouldCheckIfMergeIsPreventedByExternalMergeTools_PatternFoundButNoMergeToolConfigured() {
+    String source = "change_possibly_needing_merge_tool";
+    String target = "removed_merge_tool_from_attributes";
+    assertThat(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).isFalse();
+  }
+
+  @Test
+  public void shouldCheckIfMergeIsPreventedByExternalMergeTools_WithoutMergeToolRelevantChange() {
+    String source = "change_not_needing_merge_tool";
+    String target = "conflicting_change_not_needing_merge_tool";
+    assertThat(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).isFalse();
+  }
+
+  @Test
+  public void shouldCheckIfMergeIsPreventedByExternalMergeTools_WithMergeToolRelevantChangeButFastForwardable() {
+    String source = "change_possibly_needing_merge_tool";
+    String target = "main";
+    assertThat(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).isFalse();
+  }
+
+  @Test
+  public void shouldCheckIfMergeIsPreventedByExternalMergeTools_WithMergeToolRelevantChangeAndPossibleConflict() {
+    String source = "change_possibly_needing_merge_tool";
+    String target = "conflicting_change_possibly_needing_merge_tool";
+    assertThat(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).isTrue();
+  }
+
+  @Override
+  protected String getZippedRepositoryResource() {
+    return "sonia/scm/repository/spi/scm-git-attributes-spi-test.zip";
+  }
+}

scm-plugins/scm-git-plugin/src/test/java/sonia/scm/repository/spi/GitMergeCommandConflictTest.java

@@ -34,11 +34,14 @@ import sonia.scm.repository.work.WorkdirProvider;
 import java.io.IOException;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 import static sonia.scm.repository.spi.MergeConflictResult.ConflictTypes.BOTH_MODIFIED;
 import static sonia.scm.repository.spi.MergeConflictResult.ConflictTypes.DELETED_BY_THEM;
 import static sonia.scm.repository.spi.MergeConflictResult.ConflictTypes.DELETED_BY_US;
 
-public class GitMergeCommand_Conflict_Test extends AbstractGitCommandTestBase {
+public class GitMergeCommandConflictTest extends AbstractGitCommandTestBase {
 
   static final String DIFF_HEADER = "diff --git a/Main.java b/Main.java";
   static final String DIFF_FILE_CONFLICT = "--- a/Main.java\n" +
@@ -93,7 +96,9 @@ public class GitMergeCommand_Conflict_Test extends AbstractGitCommandTestBase {
   }
 
   private MergeConflictResult computeMergeConflictResult(String branchToMerge, String targetBranch) {
-    GitMergeCommand gitMergeCommand = new GitMergeCommand(createContext(), new SimpleGitWorkingCopyFactory(new NoneCachingWorkingCopyPool(new WorkdirProvider(null, repositoryLocationResolver)), new SimpleMeterRegistry()));
+    AttributeAnalyzer attributeAnalyzer = mock(AttributeAnalyzer.class);
+    when(attributeAnalyzer.hasExternalMergeToolConflicts(any(), any())).thenReturn(false);
+    GitMergeCommand gitMergeCommand = new GitMergeCommand(createContext(), new SimpleGitWorkingCopyFactory(new NoneCachingWorkingCopyPool(new WorkdirProvider(null, repositoryLocationResolver)), new SimpleMeterRegistry()), attributeAnalyzer);
     MergeCommandRequest mergeCommandRequest = new MergeCommandRequest();
     mergeCommandRequest.setBranchToMerge(branchToMerge);
     mergeCommandRequest.setTargetBranch(targetBranch);

scm-plugins/scm-git-plugin/src/test/java/sonia/scm/repository/spi/GitMergeCommandTest.java

@@ -40,6 +40,9 @@ import org.junit.BeforeClass;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.jupiter.api.Assertions;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
 import sonia.scm.NoChangesMadeException;
 import sonia.scm.NotFoundException;
 import sonia.scm.repository.Added;
@@ -47,6 +50,9 @@ import sonia.scm.repository.GitTestHelper;
 import sonia.scm.repository.GitWorkingCopyFactory;
 import sonia.scm.repository.Person;
 import sonia.scm.repository.api.MergeCommandResult;
+import sonia.scm.repository.api.MergeDryRunCommandResult;
+import sonia.scm.repository.api.MergePreventReason;
+import sonia.scm.repository.api.MergePreventReasonType;
 import sonia.scm.repository.api.MergeStrategy;
 import sonia.scm.repository.work.NoneCachingWorkingCopyPool;
 import sonia.scm.repository.work.WorkdirProvider;
@@ -61,7 +67,9 @@ import java.util.function.Consumer;
 import java.util.function.Function;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.when;
 
+@RunWith(MockitoJUnitRunner.class)
 @SubjectAware(configuration = "classpath:sonia/scm/configuration/shiro.ini", username = "admin", password = "secret")
 public class GitMergeCommandTest extends AbstractGitCommandTestBase {
 
@@ -71,6 +79,8 @@ public class GitMergeCommandTest extends AbstractGitCommandTestBase {
   public ShiroRule shiro = new ShiroRule();
   @Rule
   public BindTransportProtocolRule transportProtocolRule = new BindTransportProtocolRule();
+  @Mock
+  private AttributeAnalyzer attributeAnalyzer;
 
   @BeforeClass
   public static void setSigner() {
@@ -84,21 +94,60 @@ public class GitMergeCommandTest extends AbstractGitCommandTestBase {
     request.setBranchToMerge("mergeable");
     request.setTargetBranch("master");
 
-    boolean mergeable = command.dryRun(request).isMergeable();
+    MergeDryRunCommandResult result = command.dryRun(request);
 
-    assertThat(mergeable).isTrue();
+    assertThat(result.isMergeable()).isTrue();
+    assertThat(result.getReasons()).isEmpty();
   }
 
   @Test
-  public void shouldDetectNotMergeableBranches() {
+  public void shouldDetectNotMergeableBranches_FileConflict() {
     GitMergeCommand command = createCommand();
     MergeCommandRequest request = new MergeCommandRequest();
     request.setBranchToMerge("test-branch");
     request.setTargetBranch("master");
 
-    boolean mergeable = command.dryRun(request).isMergeable();
+    MergeDryRunCommandResult result = command.dryRun(request);
 
-    assertThat(mergeable).isFalse();
+    assertThat(result.isMergeable()).isFalse();
+    assertThat(result.getReasons().size()).isEqualTo(1);
+    assertThat(result.getReasons().stream().toList().get(0).getType()).isEqualTo(MergePreventReasonType.FILE_CONFLICTS);
+  }
+
+  @Test
+  public void shouldDetectNotMergeableBranches_ExternalMergeTool() {
+    String source = "mergeable";
+    String target = "master";
+    when(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).thenReturn(true);
+    GitMergeCommand command = createCommand();
+    MergeCommandRequest request = new MergeCommandRequest();
+    request.setBranchToMerge(source);
+    request.setTargetBranch(target);
+
+    MergeDryRunCommandResult result = command.dryRun(request);
+
+    assertThat(result.isMergeable()).isFalse();
+    assertThat(result.getReasons().size()).isEqualTo(1);
+    assertThat(result.getReasons().stream().toList().get(0).getType()).isEqualTo(MergePreventReasonType.EXTERNAL_MERGE_TOOL);
+  }
+
+  @Test
+  public void shouldDetectNotMergeableBranches_ExternalMergeToolAndFileConflict() {
+    String source = "test-branch";
+    String target = "master";
+    when(attributeAnalyzer.hasExternalMergeToolConflicts(source, target)).thenReturn(true);
+    GitMergeCommand command = createCommand();
+    MergeCommandRequest request = new MergeCommandRequest();
+    request.setBranchToMerge(source);
+    request.setTargetBranch(target);
+
+    MergeDryRunCommandResult result = command.dryRun(request);
+
+    assertThat(result.isMergeable()).isFalse();
+    assertThat(result.getReasons().size()).isEqualTo(2);
+    List<MergePreventReason> reasons = result.getReasons().stream().toList();
+    assertThat(reasons.get(0).getType()).isEqualTo(MergePreventReasonType.EXTERNAL_MERGE_TOOL);
+    assertThat(reasons.get(1).getType()).isEqualTo(MergePreventReasonType.FILE_CONFLICTS);
   }
 
   @Test
@@ -192,7 +241,7 @@ public class GitMergeCommandTest extends AbstractGitCommandTestBase {
   }
 
   @Test
-  public void shouldNotMergeConflictingBranches() {
+  public void shouldNotMergeConflictingBranches_FileConflict() {
     GitMergeCommand command = createCommand();
     MergeCommandRequest request = new MergeCommandRequest();
     request.setBranchToMerge("test-branch");
@@ -419,7 +468,7 @@ public class GitMergeCommandTest extends AbstractGitCommandTestBase {
   }
 
   @Test(expected = NotFoundException.class)
-  public void shouldHandleNotExistingTargetBranchInDryRun() {
+  public void shouldHandleNotExistingTargetBranchInDryRun() throws IOException {
     GitMergeCommand command = createCommand();
     MergeCommandRequest request = new MergeCommandRequest();
     request.setTargetBranch("not_existing");
@@ -517,7 +566,7 @@ public class GitMergeCommandTest extends AbstractGitCommandTestBase {
   }
 
   private GitMergeCommand createCommand(Consumer<Git> interceptor) {
-    return new GitMergeCommand(createContext(), new SimpleGitWorkingCopyFactory(new NoneCachingWorkingCopyPool(new WorkdirProvider(null, repositoryLocationResolver)), new SimpleMeterRegistry())) {
+    return new GitMergeCommand(createContext(), new SimpleGitWorkingCopyFactory(new NoneCachingWorkingCopyPool(new WorkdirProvider(null, repositoryLocationResolver)), new SimpleMeterRegistry()), attributeAnalyzer) {
       @Override
       <R, W extends GitCloneWorker<R>> R inClone(Function<Git, W> workerSupplier, GitWorkingCopyFactory workingCopyFactory, String initialBranch) {
         Function<Git, W> interceptedWorkerSupplier = git -> {

scm-plugins/scm-git-plugin/src/test/resources/sonia/scm/repository/mergetool.gitattributes

@@ -0,0 +1,20 @@
+*.ipr text merge=mps
+*.bat text
+*.txt text
+*.iml text
+*.xml text
+*.java text
+*.mpr text merge=mps
+*.css text
+*.html text
+*.dtd text
+*.sh text
+dependencies text merge=mps
+generated text merge=mps
+*.mps text merge=mps
+trace.info text merge=mps
+*.mpl text merge=mps
+*.msd text merge=mps
+*.devkit text merge=mps
+*.mpsr text merge=mps
+*.model text merge=mps