Split mergeability checks by priority

When a project config is updated or a change is merged, all open
changes on an entire project or branch need to be rechecked for
mergeability. For projects or branches with lots of open changes,
this can flood the queue with potentially slow checks. This will
block simpler cases, such as checking mergeability of a single change
during ReceiveCommits.

Define a new Priority enum on the MergeabilityChecksExecutor so we
can split the queue based on priority, allowing smaller, interactive
changes such as pushes to not wait for the entire queued backlog.

Change-Id: I57f9eb33dbbcd8a560adfe4458ce48d7b0817957

Documentation/config-gerrit.txt

@@ -823,6 +823,15 @@ changes is updated.
 +
 Default is 1.
 
+[[changeMerge.interactiveThreadPoolSize]]changeMerge.interactiveThreadPoolSize::
++
+Maximum size of the thread pool in which the mergeability flag of open
+changes is updated, when processing interactive user requests (e.g.
+pushes to refs/for/*). Set to 0 or negative to share the pool for
+background mergeability checks.
++
+Default is 1.
+
 [[commentlink]]
 === Section commentlink
 

gerrit-pgm/src/main/java/com/google/gerrit/pgm/Reindex.java

@@ -44,6 +44,7 @@ import com.google.gerrit.server.cache.h2.DefaultCacheFactory;
 import com.google.gerrit.server.change.ChangeKindCache;
 import com.google.gerrit.server.change.MergeabilityChecker;
 import com.google.gerrit.server.change.MergeabilityChecksExecutor;
+import com.google.gerrit.server.change.MergeabilityChecksExecutor.Priority;
 import com.google.gerrit.server.change.PatchSetInserter;
 import com.google.gerrit.server.config.CanonicalWebUrl;
 import com.google.gerrit.server.config.CanonicalWebUrlProvider;
@@ -290,11 +291,20 @@ public class Reindex extends SiteProgram {
 
     @Provides
     @Singleton
-    @MergeabilityChecksExecutor
+    @MergeabilityChecksExecutor(Priority.BACKGROUND)
     public WorkQueue.Executor createMergeabilityChecksExecutor(
         WorkQueue queues) {
       return queues.createQueue(1, "MergeabilityChecks");
     }
+
+    @Provides
+    @Singleton
+    @MergeabilityChecksExecutor(Priority.INTERACTIVE)
+    public WorkQueue.Executor createInteractiveMergeabilityChecksExecutor(
+        @MergeabilityChecksExecutor(Priority.BACKGROUND)
+          WorkQueue.Executor bg) {
+      return bg;
+    }
   }
 
   private int indexAll() throws Exception {

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

@@ -34,6 +34,7 @@ import com.google.gerrit.reviewdb.client.RefNames;
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.server.CurrentUser;
 import com.google.gerrit.server.IdentifiedUser;
+import com.google.gerrit.server.change.MergeabilityChecksExecutor.Priority;
 import com.google.gerrit.server.change.Mergeable.MergeableInfo;
 import com.google.gerrit.server.git.MetaDataUpdate;
 import com.google.gerrit.server.git.ProjectConfig;
@@ -85,11 +86,13 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
     private List<Project.NameKey> projects;
     private boolean force;
     private boolean reindex;
+    private boolean interactive;
 
     private Check() {
       changes = Lists.newArrayListWithExpectedSize(1);
       branches = Lists.newArrayListWithExpectedSize(1);
       projects = Lists.newArrayListWithExpectedSize(1);
+      interactive = true;
     }
 
     public Check addChange(Change change) {
@@ -99,11 +102,13 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
 
     public Check addBranch(Branch.NameKey branch) {
       branches.add(branch);
+      interactive = false;
       return this;
     }
 
     public Check addProject(Project.NameKey project) {
       projects.add(project);
+      interactive = false;
       return this;
     }
 
@@ -119,7 +124,12 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
       return this;
     }
 
+    private ListeningExecutorService getExecutor() {
+      return interactive ? interactiveExecutor : backgroundExecutor;
+    }
+
     public CheckedFuture<?, IOException> runAsync() {
+      final ListeningExecutorService executor = getExecutor();
       ListenableFuture<List<Change>> getChanges;
       if (branches.isEmpty() && projects.isEmpty()) {
         getChanges = Futures.immediateFuture(changes);
@@ -201,7 +211,8 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
   private final ChangeControl.GenericFactory changeControlFactory;
   private final Provider<Mergeable> mergeable;
   private final ChangeIndexer indexer;
-  private final ListeningExecutorService executor;
+  private final ListeningExecutorService backgroundExecutor;
+  private final ListeningExecutorService interactiveExecutor;
   private final MergeabilityCheckQueue mergeabilityCheckQueue;
   private final MetaDataUpdate.Server metaDataUpdateFactory;
 
@@ -211,7 +222,10 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
       IdentifiedUser.GenericFactory identifiedUserFactory,
       ChangeControl.GenericFactory changeControlFactory,
       Provider<Mergeable> mergeable, ChangeIndexer indexer,
-      @MergeabilityChecksExecutor Executor executor,
+      @MergeabilityChecksExecutor(Priority.BACKGROUND)
+        Executor backgroundExecutor,
+      @MergeabilityChecksExecutor(Priority.INTERACTIVE)
+        Executor interactiveExecutor,
       MergeabilityCheckQueue mergeabilityCheckQueue,
       MetaDataUpdate.Server metaDataUpdateFactory) {
     this.tl = tl;
@@ -220,7 +234,10 @@ public class MergeabilityChecker implements GitReferenceUpdatedListener {
     this.changeControlFactory = changeControlFactory;
     this.mergeable = mergeable;
     this.indexer = indexer;
-    this.executor = MoreExecutors.listeningDecorator(executor);
+    this.backgroundExecutor =
+        MoreExecutors.listeningDecorator(backgroundExecutor);
+    this.interactiveExecutor =
+        MoreExecutors.listeningDecorator(interactiveExecutor);
     this.mergeabilityCheckQueue = mergeabilityCheckQueue;
     this.metaDataUpdateFactory = metaDataUpdateFactory;
   }

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

@@ -28,4 +28,9 @@ import java.lang.annotation.Retention;
 @Retention(RUNTIME)
 @BindingAnnotation
 public @interface MergeabilityChecksExecutor {
+  public enum Priority {
+    BACKGROUND, INTERACTIVE;
+  }
+
+  Priority value();
 }

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

@@ -14,6 +14,7 @@
 
 package com.google.gerrit.server.change;
 
+import com.google.gerrit.server.change.MergeabilityChecksExecutor.Priority;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.git.WorkQueue;
 import com.google.inject.AbstractModule;
@@ -22,7 +23,6 @@ import com.google.inject.Singleton;
 
 import org.eclipse.jgit.lib.Config;
 
-
 /** Module providing the {@link MergeabilityChecksExecutor}. */
 public class MergeabilityChecksExecutorModule extends AbstractModule {
   @Override
@@ -31,11 +31,27 @@ public class MergeabilityChecksExecutorModule extends AbstractModule {
 
   @Provides
   @Singleton
-  @MergeabilityChecksExecutor
+  @MergeabilityChecksExecutor(Priority.BACKGROUND)
   public WorkQueue.Executor createMergeabilityChecksExecutor(
       @GerritServerConfig Config config,
       WorkQueue queues) {
     int poolSize = config.getInt("changeMerge", null, "threadPoolSize", 1);
     return queues.createQueue(poolSize, "MergeabilityChecks");
   }
+
+  @Provides
+  @Singleton
+  @MergeabilityChecksExecutor(Priority.INTERACTIVE)
+  public WorkQueue.Executor createMergeabilityChecksExecutor(
+      @GerritServerConfig Config config,
+      WorkQueue queues,
+      @MergeabilityChecksExecutor(Priority.BACKGROUND)
+        WorkQueue.Executor backgroundExecutor) {
+    int poolSize =
+        config.getInt("changeMerge", null, "interactiveThreadPoolSize", 1);
+    if (poolSize <= 0) {
+      return backgroundExecutor;
+    }
+    return queues.createQueue(poolSize, "InteractiveMergeabilityChecks");
+  }
 }