Fix exception in rejectEmptyCommit check on root commit

Ied0c501a assumed that the commit always has one parent, which resulted
in ArrayIndexOutOfBoundsException. There happened to be no test coverage
for submitting a root commit with rejectEmptyCommit=true.

Fix this by skipping the empty-commit check on the root commit, even
when rejectEmptyCommit=true. This mirrors the fact that an empty commit
is created when createEmptyCommit=true even when rejectEmptyCommit=true,
and it provides a way to initialize the branch even if there should be
no real data there yet.

Change-Id: Ife12c2c472482d4602e85f5d350acc4a49d83c0d

Documentation/config-project-config.txt

diff --git a/Documentation/config-project-config.txt b/Documentation/config-project-config.txt
index ac303e9..c3244cc 100644
--- a/Documentation/config-project-config.txt
+++ b/Documentation/config-project-config.txt
@@ -256,7 +256,8 @@
 
 - 'rejectEmptyCommit': Defines whether empty commits should be rejected when a change is merged.
 Changes might not seem empty at first but when attempting to merge, rebasing can lead to an empty
-commit. If this option is set to 'true' the merge would fail.
+commit. If this option is set to 'true' the merge would fail. An empty commit is still allowed as
+the initial commit on a branch.
 
 Merge strategy
 

java/com/google/gerrit/server/submit/FastForwardOp.java

diff --git a/java/com/google/gerrit/server/submit/FastForwardOp.java b/java/com/google/gerrit/server/submit/FastForwardOp.java
index f1749f4..08f5abb 100644
--- a/java/com/google/gerrit/server/submit/FastForwardOp.java
+++ b/java/com/google/gerrit/server/submit/FastForwardOp.java
@@ -28,6 +28,7 @@
   @Override
   protected void updateRepoImpl(RepoContext ctx) throws IntegrationException {
     if (args.project.is(BooleanProjectConfig.REJECT_EMPTY_COMMIT)
+        && toMerge.getParentCount() > 0
         && toMerge.getTree().equals(toMerge.getParent(0).getTree())) {
       toMerge.setStatusCode(EMPTY_COMMIT);
       return;

javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java

diff --git a/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java b/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java
index 61a2d84..5580279 100644
--- a/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java
+++ b/javatests/com/google/gerrit/acceptance/rest/change/AbstractSubmit.java
@@ -27,6 +27,7 @@
 import static java.util.stream.Collectors.toList;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
@@ -154,11 +155,12 @@
   @Test
   @TestProjectInput(createEmptyCommit = false)
   public void submitToEmptyRepo() throws Exception {
-    RevCommit initialHead = getRemoteHead();
+    assertThat(getRemoteHead()).isNull();
     PushOneCommit.Result change = createChange();
+    assertThat(change.getCommit().getParents()).isEmpty();
     Map<Branch.NameKey, ObjectId> actual = fetchFromSubmitPreview(change.getChangeId());
     RevCommit headAfterSubmitPreview = getRemoteHead();
-    assertThat(headAfterSubmitPreview).isEqualTo(initialHead);
+    assertThat(headAfterSubmitPreview).isNull();
     assertThat(actual).hasSize(1);
 
     submit(change.getChangeId());
@@ -1070,6 +1072,45 @@
     change.current().submit();
   }
 
+  @Test
+  @TestProjectInput(createEmptyCommit = false, rejectEmptyCommit = InheritableBoolean.TRUE)
+  public void submitNonemptyCommitToEmptyRepoWithRejectEmptyCommit_allowed() throws Exception {
+    assertThat(getRemoteHead()).isNull();
+    PushOneCommit.Result change = createChange();
+    assertThat(change.getCommit().getParents()).isEmpty();
+    Map<Branch.NameKey, ObjectId> actual = fetchFromSubmitPreview(change.getChangeId());
+    RevCommit headAfterSubmitPreview = getRemoteHead();
+    assertThat(headAfterSubmitPreview).isNull();
+    assertThat(actual).hasSize(1);
+
+    submit(change.getChangeId());
+    assertThat(getRemoteHead().getId()).isEqualTo(change.getCommit());
+    assertTrees(project, actual);
+  }
+
+  @Test
+  @TestProjectInput(createEmptyCommit = false, rejectEmptyCommit = InheritableBoolean.TRUE)
+  public void submitEmptyCommitToEmptyRepoWithRejectEmptyCommit_allowed() throws Exception {
+    assertThat(getRemoteHead()).isNull();
+    PushOneCommit.Result change =
+        pushFactory
+            .create(db, admin.getIdent(), testRepo, "Change 1", ImmutableMap.of())
+            .to("refs/for/master");
+    change.assertOkStatus();
+    // TODO(dborowitz): Use EMPTY_TREE_ID after upgrading to https://git.eclipse.org/r/127473
+    assertThat(change.getCommit().getTree())
+        .isEqualTo(ObjectId.fromString("4b825dc642cb6eb9a060e54bf8d69288fbee4904"));
+
+    Map<Branch.NameKey, ObjectId> actual = fetchFromSubmitPreview(change.getChangeId());
+    RevCommit headAfterSubmitPreview = getRemoteHead();
+    assertThat(headAfterSubmitPreview).isNull();
+    assertThat(actual).hasSize(1);
+
+    submit(change.getChangeId());
+    assertThat(getRemoteHead().getId()).isEqualTo(change.getCommit());
+    assertTrees(project, actual);
+  }
+
   private void setChangeStatusToNew(PushOneCommit.Result... changes) throws Exception {
     for (PushOneCommit.Result change : changes) {
       try (BatchUpdate bu =