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

Documentation/config-gerrit.txt

@@ -1214,6 +1214,15 @@ performance improvements by reducing the number of refs in the repo.
 +
 Default is true.
 
+[[change.api.excludeMergeableInChangeInfo]]change.api.excludeMergeableInChangeInfo::
++
+If true, the mergeability bit in
+link:rest-api-changes.html#change-info[ChangeInfo] will never be set. It can
+be requested separately through the
+link:rest-api-changes.html#get-mergeable[get-mergeable] endpoint.
++
+Default is false.
+
 [[change.showAssigneeInChangesTable]]change.showAssigneeInChangesTable::
 +
 Show assignee field in changes table. If set to false, assignees will

Documentation/rest-api-changes.txt

@@ -309,6 +309,10 @@ default. Optional fields are:
 * `SKIP_MERGEABLE`: skip the `mergeable` field in
 link:#change-info[ChangeInfo]. For fast moving projects, this field must
 be recomputed often, which is slow for projects with big trees.
++
+This option is deprecated. In a future release, `mergeable` will not be
+populated in link:#change-info[ChangeInfo]. It can be requested separately
+by calling the link:#get-mergeable[get-mergeable] endpoint.
 --
 
 [[submittable]]
@@ -5765,7 +5769,9 @@ Not set for merged changes.
 |`mergeable`          |optional|
 Whether the change is mergeable. +
 Not set for merged changes, if the change has not yet been tested, or
-if the link:#skip_mergeable[skip_mergeable] option is set.
+if the link:#skip_mergeable[skip_mergeable] option is set or when
+link:config-gerrit.html#change.api.excludeMergeableInChangeInfo[change.api.excludeMergeableInChangeInfo]
+is set.
 |`submittable`        |optional|
 Whether the change has been approved by the project submit rules. +
 Only set if link:#submittable[requested].

java/com/google/gerrit/extensions/client/ListChangesOption.java

@@ -78,6 +78,7 @@ public enum ListChangesOption {
   TRACKING_IDS(21),
 
   /** Skip mergeability data */
+  @Deprecated
   SKIP_MERGEABLE(22);
 
   private final int value;

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)) {

javatests/com/google/gerrit/acceptance/api/change/ChangeIT.java

@@ -252,6 +252,14 @@ public class ChangeIT extends AbstractDaemonTest {
     assertThat(c.mergeable).isTrue();
   }
 
+  @Test
+  @GerritConfig(name = "change.api.excludeMergeableInChangeInfo", value = "true")
+  public void excludeMergeableInChangeInfo() throws Exception {
+    PushOneCommit.Result r = createChange();
+    ChangeInfo c = gApi.changes().id(r.getChangeId()).get();
+    assertThat(c.mergeable).isNull();
+  }
+
   @Test
   public void setPrivateByOwner() throws Exception {
     TestRepository<InMemoryRepository> userRepo = cloneProject(project, user);