SubmittedTogether: Only show tab if there is more than one change

If the tab contains only the current change, the tab feels pointless
and a waste of screen estate. In that case we don't want to display the
tab. To do so we choose to modify the REST API for the /submitted_together
call, because this keeps the client side code from growing exceptions
around the REST API calls.

Change-Id: If1943f93f5f5f9bc2dd1e2135e7ab4406e91262e

Documentation/rest-api-changes.txt

@@ -1073,7 +1073,11 @@ message is contained in the response body.
 --
 
 Returns a list of all changes which are submitted when
-link:#submit-change[\{submit\}] is called for this change.
+link:#submit-change[\{submit\}] is called for this change,
+including the current change itself.
+
+An empty list is returned if this change will be submitted
+by itself (no other changes).
 
 .Request
 ----

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/server/change/SubmittedTogetherIT.java

@@ -51,14 +51,14 @@ public class SubmittedTogetherIT extends AbstractDaemonTest {
     String id2 = getChangeId(c2_1);
     pushHead(testRepo, "refs/for/master", false);
 
-    assertSubmittedTogether(id1, id1);
+    assertSubmittedTogether(id1);
     assertSubmittedTogether(id2, id2, id1);
   }
 
   @Test
   public void respectsWholeTopicAndAncestors() throws Exception {
     RevCommit initialHead = getRemoteHead();
-    // Create two independant commits and push.
+    // Create two independent commits and push.
     RevCommit c1_1 = commitBuilder()
         .add("a.txt", "1")
         .message("subject: 1")
@@ -78,8 +78,45 @@ public class SubmittedTogetherIT extends AbstractDaemonTest {
       assertSubmittedTogether(id1, id2, id1);
       assertSubmittedTogether(id2, id2, id1);
     } else {
-      assertSubmittedTogether(id1, id1);
+      assertSubmittedTogether(id1);
-      assertSubmittedTogether(id2, id2);
+      assertSubmittedTogether(id2);
+    }
+  }
+
+  @Test
+  public void testTopicChaining() throws Exception {
+    RevCommit initialHead = getRemoteHead();
+    // Create two independent commits and push.
+    RevCommit c1_1 = commitBuilder()
+        .add("a.txt", "1")
+        .message("subject: 1")
+        .create();
+    String id1 = getChangeId(c1_1);
+    pushHead(testRepo, "refs/for/master/" + name("connectingTopic"), false);
+
+    testRepo.reset(initialHead);
+    RevCommit c2_1 = commitBuilder()
+        .add("b.txt", "2")
+        .message("subject: 2")
+        .create();
+    String id2 = getChangeId(c2_1);
+    pushHead(testRepo, "refs/for/master/" + name("connectingTopic"), false);
+
+    RevCommit c3_1 = commitBuilder()
+        .add("b.txt", "2")
+        .message("subject: 2")
+        .create();
+    String id3 = getChangeId(c3_1);
+    pushHead(testRepo, "refs/for/master/" + name("unrelated-topic"), false);
+
+    if (isSubmitWholeTopicEnabled()) {
+      assertSubmittedTogether(id1, id2, id1);
+      assertSubmittedTogether(id2, id2, id1);
+      assertSubmittedTogether(id3, id3, id2, id1);
+    } else {
+      assertSubmittedTogether(id1);
+      assertSubmittedTogether(id2);
+      assertSubmittedTogether(id3, id3, id2);
     }
   }
 
@@ -102,8 +139,8 @@ public class SubmittedTogetherIT extends AbstractDaemonTest {
     String id2 = getChangeId(c2_1);
     pushHead(testRepo, "refs/for/master", false);
 
-    assertSubmittedTogether(id1, id1);
+    assertSubmittedTogether(id1);
-    assertSubmittedTogether(id2, id2);
+    assertSubmittedTogether(id2);
   }
 
   private void assertSubmittedTogether(String chId, String... expected)

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

@@ -32,6 +32,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.EnumSet;
 import java.util.List;
 
@@ -60,12 +61,16 @@ public class SubmittedTogether implements RestReadView<ChangeResource> {
     try {
       ChangeSet cs = mergeSuperSet.completeChangeSet(dbProvider.get(),
           ChangeSet.create(resource.getChange()));
+      if (cs.ids().size() > 1) {
         return json.create(EnumSet.of(
             ListChangesOption.CURRENT_REVISION,
             ListChangesOption.CURRENT_COMMIT,
             ListChangesOption.DETAILED_LABELS,
             ListChangesOption.LABELS))
           .format(cs.ids());
+      } else {
+        return Collections.emptyList();
+      }
     } catch (OrmException | IOException e) {
       log.error("Error on getting a ChangeSet", e);
       throw e;