Only compute submittable during submitted_together

The submittable field of ChangeInfo is undocumented and only used
by the Submitted Together tab of the web UI. It's also quite costly
to compute as the logic currently runs the Prolog rules in the
default "accurate" mode, ignoring the setFastEvalLabels(true) mode
used by DETAILED_LABELS.

Computing submittable for every search result is one of the reasons
dashboards are slow on gerrit-review. Each change is run through the
full set of rules, which requires group membership checks for every
single reviewer vote. These membership lookups take long enough that
the sequential evaluation adds up to a noticable delay.

Just disable the field, except during the submitted_together API.

Change-Id: Iffb893f31c5c01306ace4fb13a43f5f91dd9a2da

Documentation/rest-api-changes.txt

@@ -4474,6 +4474,9 @@ Not set for merged changes.
 |`mergeable`          |optional|
 Whether the change is mergeable. +
 Not set for merged changes, or if the change has not yet been tested.
+|`submittable`        |optional|
+Whether the change has been approved by the project submit rules. +
+Provided only by link:#submitted-together[submitted_together].
 |`insertions`         ||
 Number of inserted lines.
 |`deletions`          ||

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

@@ -481,9 +481,6 @@ public abstract class AbstractSubmit extends AbstractDaemonTest {
   }
 
   protected void assertSubmittable(String changeId) throws Exception {
-    assertThat(gApi.changes().id(changeId).info().submittable)
-        .named("submit bit on ChangeInfo")
-        .isEqualTo(true);
     RevisionResource rsrc = parseCurrentRevisionResource(changeId);
     UiAction.Description desc = submitHandler.getDescription(rsrc);
     assertThat(desc.isVisible()).named("visible bit on submit action").isTrue();

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

@@ -169,6 +169,7 @@ public class ChangeJson {
   private final ChangeKindCache changeKindCache;
 
   private AccountLoader accountLoader;
+  private boolean includeSubmittable;
   private Map<Change.Id, List<SubmitRecord>> submitRecords;
   private FixInput fix;
 
@@ -222,6 +223,11 @@ public class ChangeJson {
         : EnumSet.copyOf(options);
   }
 
+  public ChangeJson includeSubmittable(boolean include) {
+    includeSubmittable = include;
+    return this;
+  }
+
   public ChangeJson fix(FixInput fix) {
     this.fix = fix;
     return this;
@@ -421,14 +427,16 @@ public class ChangeJson {
     out.topic = in.getTopic();
     out.hashtags = cd.hashtags();
     out.changeId = in.getKey().get();
-    if (in.getStatus() != Change.Status.MERGED) {
+    if (in.getStatus().isOpen()) {
       SubmitTypeRecord str = cd.submitTypeRecord();
       if (str.isOk()) {
         out.submitType = str.type;
       }
       out.mergeable = cd.isMergeable();
+      if (includeSubmittable) {
+        out.submittable = Submit.submittable(cd);
+      }
     }
-    out.submittable = Submit.submittable(cd);
     Optional<ChangedLines> changedLines = cd.changedLines();
     if (changedLines.isPresent()) {
       out.insertions = changedLines.get().insertions;

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

@@ -126,6 +126,7 @@ public class SubmittedTogether implements RestReadView<ChangeResource> {
       info.changes = json.create(EnumSet.of(
           ListChangesOption.CURRENT_REVISION,
           ListChangesOption.CURRENT_COMMIT))
+        .includeSubmittable(true)
         .formatChangeDatas(cds);
       info.nonVisibleChanges = hidden;
       return info;