Submit: Disable submit button when change is unmergable.

A user reported internally:
> It used to be the case that the submit button was greyed out if your
> repo required fast-forward and your CL was out of date.  Now there's
> no obvious way to tell if a submit is likely to succeed without trying
> it, which is a minor but daily annoyance.

This may have been introduced with the `submitWholeTopic` feature recently.
Looking at the code I suspect that there was a client side check for
enabling the submit button as well as there is a client side class
com.google.gerrit.client.changes.ChangeInfo.MergeableInfo.

Part of the `submitWholeTopic` feature was moving responsibility more to
the server and dumb down the client a bit. So now we also have to do this
computation at the server side.

Change-Id: I5dcd8c954446462660d45ead13eb38b6d8fa822a
Signed-off-by: Stefan Beller <sbeller@google.com>

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

@@ -18,10 +18,13 @@ import static com.google.common.truth.Truth.assertThat;
 
 import com.google.gerrit.acceptance.PushOneCommit;
 import com.google.gerrit.extensions.client.SubmitType;
+import com.google.gerrit.extensions.common.ActionInfo;
 
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.junit.Test;
 
+import java.util.Map;
+
 public class SubmitByFastForwardIT extends AbstractSubmit {
 
   @Override
@@ -51,6 +54,14 @@ public class SubmitByFastForwardIT extends AbstractSubmit {
     testRepo.reset(initialHead);
     PushOneCommit.Result change2 =
         createChange("Change 2", "b.txt", "other content");
+
+    approve(change2.getChangeId());
+    Map<String, ActionInfo> actions = getActions(change2.getChangeId());
+
+    assertThat(actions).containsKey("submit");
+    ActionInfo info = actions.get("submit");
+    assertThat(info.enabled).isNull();
+
     submitWithConflict(change2.getChangeId());
     assertThat(getRemoteHead()).isEqualTo(oldHead);
     assertSubmitter(change.getChangeId(), 1);

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

@@ -35,6 +35,7 @@ import com.google.gerrit.extensions.api.changes.SubmitInput;
 import com.google.gerrit.extensions.common.ChangeInfo;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.restapi.RestModifyView;
 import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
 import com.google.gerrit.extensions.webui.UiAction;
@@ -133,6 +134,7 @@ public class Submit implements RestModifyView<RevisionResource, SubmitInput>,
   private final ParameterizedString submitTopicTooltip;
   private final boolean submitWholeTopic;
   private final Provider<InternalChangeQuery> queryProvider;
+  private final Provider<Mergeable> mergeableProvider;
 
   @Inject
   Submit(@GerritPersonIdent PersonIdent serverIdent,
@@ -149,7 +151,8 @@ public class Submit implements RestModifyView<RevisionResource, SubmitInput>,
       ChangeIndexer indexer,
       LabelNormalizer labelNormalizer,
       @GerritServerConfig Config cfg,
-      Provider<InternalChangeQuery> queryProvider) {
+      Provider<InternalChangeQuery> queryProvider,
+      Provider<Mergeable> mergeableProvider) {
     this.serverIdent = serverIdent;
     this.dbProvider = dbProvider;
     this.repoManager = repoManager;
@@ -177,6 +180,7 @@ public class Submit implements RestModifyView<RevisionResource, SubmitInput>,
         cfg.getString("change", null, "submitTopicTooltip"),
         DEFAULT_TOPIC_TOOLTIP));
     this.queryProvider = queryProvider;
+    this.mergeableProvider = mergeableProvider;
   }
 
   @Override
@@ -290,6 +294,14 @@ public class Submit implements RestModifyView<RevisionResource, SubmitInput>,
         .setTitle("")
         .setVisible(false);
     }
+
+    boolean enabled;
+    try {
+      enabled = mergeableProvider.get().apply(resource).mergeable;
+    } catch (RestApiException | OrmException | IOException e) {
+      throw new OrmRuntimeException("Could not determine mergeability", e);
+    }
+
     List<ChangeData> changesByTopic = null;
     if (submitWholeTopic && !Strings.isNullOrEmpty(topic)) {
       changesByTopic = getChangesByTopic(topic);
@@ -313,7 +325,7 @@ public class Submit implements RestModifyView<RevisionResource, SubmitInput>,
           .setTitle(Strings.emptyToNull(
               submitTopicTooltip.replace(params)))
           .setVisible(true)
-          .setEnabled(true);
+          .setEnabled(enabled);
       }
     } else {
       RevId revId = resource.getPatchSet().getRevision();
@@ -324,7 +336,8 @@ public class Submit implements RestModifyView<RevisionResource, SubmitInput>,
       return new UiAction.Description()
         .setLabel(label)
         .setTitle(Strings.emptyToNull(titlePattern.replace(params)))
-        .setVisible(true);
+        .setVisible(true)
+        .setEnabled(enabled);
     }
   }