CherryPick: Keep going in the face of a single merge conflict

In addition to verifying this behavior in a test, this also fixes an
issue caused by setting mergeTip to null. On the next iteration of the
loop we were incorrectly treating the null merge tip as a cherry-pick
onto an unborn root. Setting the commit as CLEAN_MERGE resulted in
rejecting the later ref update in MergeOp.

Change-Id: I82299ea96725a21430a0871199cde3e2f3af267b

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/change/SubmitByCherryPickIT.java

@@ -268,4 +268,36 @@ public class SubmitByCherryPickIT extends AbstractSubmit {
     assertThat(log.get(2).getShortMessage())
         .isEqualTo(initialHead.getShortMessage());
   }
+
+  @Test
+  public void submitChangeAfterParentFailsDueToConflict() throws Exception {
+    Git git = createProject();
+    RevCommit initialHead = getRemoteHead();
+
+    checkout(git, initialHead.getId().getName());
+    PushOneCommit.Result change2 = createChange(git, "Change 2", "b", "b1");
+    submit(change2.getChangeId());
+
+    checkout(git, initialHead.getId().getName());
+    PushOneCommit.Result change3 = createChange(git, "Change 3", "b", "b2");
+    assertThat(change3.getCommit().getParent(0)).isEqualTo(initialHead);
+    PushOneCommit.Result change4 = createChange(git, "Change 3", "c", "c3");
+
+    submitStatusOnly(change3.getChangeId());
+    submitStatusOnly(change4.getChangeId());
+
+    // Merge fails; change3 contains the delta "b1" -> "b2", which cannot be
+    // applied against tip.
+    submitWithConflict(change3.getChangeId());
+
+    // change4 is a clean merge, so should succeed in the same run where change3
+    // failed.
+    ChangeInfo info4 = get(change4.getChangeId());
+    assertThat(info4.status).isEqualTo(ChangeStatus.MERGED);
+    List<RevCommit> log = getRemoteLog();
+    assertThat(log.get(0).getShortMessage())
+        .isEqualTo(change4.getCommit().getShortMessage());
+    assertThat(log.get(1).getShortMessage())
+        .isEqualTo(change2.getCommit().getShortMessage());
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CherryPick.java

@@ -72,11 +72,11 @@ public class CherryPick extends SubmitStrategy {
       CodeReviewCommit n = sorted.remove(0);
       try {
         if (mergeTip.getCurrentTip() == null) {
-          mergeTip = cherryPickUnbornRoot(n);
+          cherryPickUnbornRoot(n, mergeTip);
         } else if (n.getParentCount() == 0) {
           cherryPickRootOntoBranch(n);
         } else if (n.getParentCount() == 1) {
-          mergeTip = cherryPickOne(n, mergeTip);
+          cherryPickOne(n, mergeTip);
         } else {
           cherryPickMultipleParents(n, mergeTip);
         }
@@ -87,11 +87,10 @@ public class CherryPick extends SubmitStrategy {
     return mergeTip;
   }
 
-  private MergeTip cherryPickUnbornRoot(CodeReviewCommit n) {
+  private void cherryPickUnbornRoot(CodeReviewCommit n, MergeTip mergeTip) {
     // The branch is unborn. Take fast-forward resolution to create the branch.
-    MergeTip mergeTip = new MergeTip(n, Lists.newArrayList(n));
+    mergeTip.moveTipTo(n, n);
     n.setStatusCode(CommitMergeStatus.CLEAN_MERGE);
-    return mergeTip;
   }
 
   private void cherryPickRootOntoBranch(CodeReviewCommit n) {
@@ -100,24 +99,24 @@ public class CherryPick extends SubmitStrategy {
     n.setStatusCode(CommitMergeStatus.CANNOT_CHERRY_PICK_ROOT);
   }
 
-  private MergeTip cherryPickOne(CodeReviewCommit n, MergeTip mergeTip)
+  private void cherryPickOne(CodeReviewCommit n, MergeTip mergeTip)
       throws NoSuchChangeException, OrmException, IOException {
-    // If there is only one parent, a cherry-pick can be done by
+    // If there is only one parent, a cherry-pick can be done by taking the
-    // taking the delta relative to that one parent and redoing
+    // delta relative to that one parent and redoing that on the current merge
-    // that on the current merge tip.
+    // tip.
+    //
+    // Keep going in the case of a single merge failure; the goal is to
+    // cherry-pick as many commits as possible.
     try {
       CodeReviewCommit merge =
           writeCherryPickCommit(mergeTip.getCurrentTip(), n);
       mergeTip.moveTipTo(merge, merge);
       newCommits.put(mergeTip.getCurrentTip().getPatchsetId()
           .getParentKey(), mergeTip.getCurrentTip());
-      return mergeTip;
     } catch (MergeConflictException mce) {
       n.setStatusCode(CommitMergeStatus.PATH_CONFLICT);
-      return null;
     } catch (MergeIdenticalTreeException mie) {
       n.setStatusCode(CommitMergeStatus.ALREADY_MERGED);
-      return null;
     }
   }