Add Gerrit config to disable mergeability bit in ChangeInfo

Computing mergeability of open changes becomes very expensive for hosts
that have fast-moving branches and a lot of open changes. This commit
scales down this problem by adding a config to remove the bit from
ChangeInfo.

We will consult the mailing list to see if we can remove this bit from
ChangeInfo and the change index completely or add a new MERGEABLE list
option.

Change-Id: I28e90e17abaf369d03ff9d3c0d491903ecf30bf9

java/com/google/gerrit/server/change/ChangeJson.java

@@ -83,6 +83,7 @@ import com.google.gerrit.server.ReviewerStatusUpdate;
 import com.google.gerrit.server.StarredChangesUtil;
 import com.google.gerrit.server.account.AccountInfoComparator;
 import com.google.gerrit.server.account.AccountLoader;
+import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.TrackingFooters;
 import com.google.gerrit.server.index.change.ChangeField;
 import com.google.gerrit.server.notedb.ChangeNotes;
@@ -111,6 +112,7 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
 import java.util.function.Supplier;
+import org.eclipse.jgit.lib.Config;
 
 /**
  * Produces {@link ChangeInfo} (which is serialized to JSON afterwards) from {@link ChangeData}.
@@ -216,6 +218,7 @@ public class ChangeJson {
   private final Metrics metrics;
   private final RevisionJson revisionJson;
   private final Optional<PluginDefinedAttributesFactory> pluginDefinedAttributesFactory;
+  private final boolean excludeMergeableInChangeInfo;
   private final boolean lazyLoad;
 
   private AccountLoader accountLoader;
@@ -236,6 +239,7 @@ public class ChangeJson {
       TrackingFooters trackingFooters,
       Metrics metrics,
       RevisionJson.Factory revisionJsonFactory,
+      @GerritServerConfig Config cfg,
       @Assisted Iterable<ListChangesOption> options,
       @Assisted Optional<PluginDefinedAttributesFactory> pluginDefinedAttributesFactory) {
     this.userProvider = user;
@@ -252,6 +256,8 @@ public class ChangeJson {
     this.metrics = metrics;
     this.revisionJson = revisionJsonFactory.create(options);
     this.options = Sets.immutableEnumSet(options);
+    this.excludeMergeableInChangeInfo =
+        cfg.getBoolean("change", "api", "excludeMergeableInChangeInfo", false);
     this.lazyLoad = containsAnyOf(this.options, REQUIRE_LAZY_LOAD);
     this.pluginDefinedAttributesFactory = pluginDefinedAttributesFactory;
 
@@ -509,7 +515,7 @@ public class ChangeJson {
       if (str.isOk()) {
         out.submitType = str.type;
       }
-      if (!has(SKIP_MERGEABLE)) {
+      if (!excludeMergeableInChangeInfo && !has(SKIP_MERGEABLE)) {
         out.mergeable = cd.isMergeable();
       }
       if (has(SUBMITTABLE)) {