MergeUtil: Use InMemoryInserter for dry runs

As in Ifd9d768d, making flush a no-op is not sufficient to ensure
nothing gets written to the repo.

Change-Id: If08e8184b484a0e7ba41fd3e11130d6fb26ed6c6

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeKindCacheImpl.java

@@ -28,6 +28,7 @@ import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.server.cache.CacheModule;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.gerrit.server.git.InMemoryInserter;
 import com.google.gerrit.server.git.MergeUtil;
 import com.google.gerrit.server.project.ProjectCache;
 import com.google.gerrit.server.project.ProjectState;
@@ -40,6 +41,7 @@ import com.google.inject.name.Named;
 import org.eclipse.jgit.errors.LargeObjectException;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.merge.ThreeWayMerger;
 import org.eclipse.jgit.revwalk.RevCommit;
@@ -221,10 +223,10 @@ public class ChangeKindCacheImpl implements ChangeKindCache {
         // A trivial rebase can be detected by looking for the next commit
         // having the same tree as would exist when the prior commit is
         // cherry-picked onto the next commit's new first parent.
-        ThreeWayMerger merger = MergeUtil.newThreeWayMerger(
-            repo, MergeUtil.createDryRunInserter(repo), key.strategyName);
+        try (ObjectInserter ins = new InMemoryInserter(repo)) {
+          ThreeWayMerger merger =
+              MergeUtil.newThreeWayMerger(repo, ins, key.strategyName);
           merger.setBase(prior.getParent(0));
-        try {
           if (merger.merge(next.getParent(0), prior)
               && merger.getResultTreeId().equals(next.getTree())) {
             if (prior.getParentCount() == 1) {

gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java

@@ -68,12 +68,10 @@ import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevFlag;
 import org.eclipse.jgit.revwalk.RevSort;
 import org.eclipse.jgit.revwalk.RevWalk;
-import org.eclipse.jgit.transport.PackParser;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
-import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -361,9 +359,9 @@ public class MergeUtil {
       return false;
     }
 
-    ThreeWayMerger m = newThreeWayMerger(repo, createDryRunInserter(repo));
-    try {
-      return m.merge(new AnyObjectId[] {mergeTip, toMerge});
+    try (ObjectInserter ins = new InMemoryInserter(repo)) {
+      return newThreeWayMerger(repo, ins)
+          .merge(new AnyObjectId[] {mergeTip, toMerge});
     } catch (LargeObjectException e) {
       log.warn("Cannot merge due to LargeObjectException: " + toMerge.name());
       return false;
@@ -409,8 +407,8 @@ public class MergeUtil {
       // taking the delta relative to that one parent and redoing
       // that on the current merge tip.
       //
-      try {
-        ThreeWayMerger m = newThreeWayMerger(repo, createDryRunInserter(repo));
+      try (ObjectInserter ins = new InMemoryInserter(repo)) {
+        ThreeWayMerger m = newThreeWayMerger(repo, ins);
         m.setBase(toMerge.getParent(0));
         return m.merge(mergeTip, toMerge);
       } catch (IOException e) {
@@ -437,26 +435,6 @@ public class MergeUtil {
     }
   }
 
-  public static ObjectInserter createDryRunInserter(Repository db) {
-    final ObjectInserter delegate = db.newObjectInserter();
-    return new ObjectInserter.Filter() {
-      @Override
-      protected ObjectInserter delegate() {
-        return delegate;
-      }
-
-      @Override
-      public PackParser newPackParser(InputStream in) throws IOException {
-        throw new UnsupportedOperationException();
-      }
-
-      @Override
-      public void flush() throws IOException {
-        // Do nothing.
-      }
-    };
-  }
-
   public CodeReviewCommit mergeOneCommit(PersonIdent author,
       PersonIdent committer, Repository repo, CodeReviewRevWalk rw,
       ObjectInserter inserter, Branch.NameKey destBranch,