Don't create empty cherry-picks

If the tree after a cherry-pick is identical to the new parent's tree
the newly picked commit is empty.  This can happen if the commit is
being cherry-picked on top of another version of itself, or was
already included in the target branch.

Change-Id: I7e3fe9c02ca0b5182635a6e202b1d38e4dc39405
(cherry picked from commit fd889d9db648e6f027ee5d2e295041cb1bc3591a)
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

1 files changed, 7 insertions, 9 deletions

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java
index 3468e9775d..b4992981ca 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/MergeUtil.java
@@ -221,20 +221,18 @@ public class MergeUtil {
 
     m.setBase(originalCommit.getParent(0));
     if (m.merge(mergeTip, originalCommit)) {
+      ObjectId tree = m.getResultTreeId();
+      if (tree.equals(mergeTip.getTree())) {
+        return null;
+      }
 
-      final CommitBuilder mergeCommit = new CommitBuilder();
-
-      mergeCommit.setTreeId(m.getResultTreeId());
+      CommitBuilder mergeCommit = new CommitBuilder();
+      mergeCommit.setTreeId(tree);
       mergeCommit.setParentId(mergeTip);
       mergeCommit.setAuthor(originalCommit.getAuthorIdent());
       mergeCommit.setCommitter(cherryPickCommitterIdent);
       mergeCommit.setMessage(commitMsg);
-
-      final ObjectId id = commit(inserter, mergeCommit);
-      final CodeReviewCommit newCommit =
-          (CodeReviewCommit) rw.parseCommit(id);
-
-      return newCommit;
+      return (CodeReviewCommit) rw.parseCommit(commit(inserter, mergeCommit));
     } else {
       return null;
     }