Make AbstractBlazeQueryEnv closable, shutdown SkyQEnv executor on close

Users of SkyQueryEnvironment ought to be able to shut down its executor
when they're finished with it.

RELNOTES: None.
PiperOrigin-RevId: 162262117

src/main/java/com/google/devtools/build/lib/query2/AbstractBlazeQueryEnvironment.java

@@ -48,7 +48,8 @@
  * {@link QueryEnvironment} that can evaluate queries to produce a result, and implements as much of
  * QueryEnvironment as possible while remaining mostly agnostic as to the objects being stored.
  */
-public abstract class AbstractBlazeQueryEnvironment<T> extends AbstractQueryEnvironment<T> {
+public abstract class AbstractBlazeQueryEnvironment<T> extends AbstractQueryEnvironment<T>
+    implements AutoCloseable {
   protected ErrorSensingEventHandler eventHandler;
   protected final boolean keepGoing;
   protected final boolean strictScope;
@@ -78,6 +79,9 @@ protected AbstractBlazeQueryEnvironment(
     this.extraFunctions = ImmutableList.copyOf(extraFunctions);
   }
 
+  @Override
+  public abstract void close();
+
   private static DependencyFilter constructDependencyFilter(
       Set<Setting> settings) {
     DependencyFilter specifiedFilter =

src/main/java/com/google/devtools/build/lib/query2/BlazeQueryEnvironment.java

@@ -111,6 +111,11 @@ public class BlazeQueryEnvironment extends AbstractBlazeQueryEnvironment<Target>
     this.labelVisitor = new LabelVisitor(targetProvider, dependencyFilter);
   }
 
+  @Override
+  public void close() {
+    // BlazeQueryEnvironment has no resources that need to be cleaned up.
+  }
+
   @Override
   public DigraphQueryEvalResult<Target> evaluateQuery(
       QueryExpression expr,

src/main/java/com/google/devtools/build/lib/query2/SkyQueryEnvironment.java

@@ -232,6 +232,14 @@ protected SkyQueryEnvironment(
             PrepareDepsOfPatternsFunction.getSkyKeys(universeKey, eventHandler));
   }
 
+  @Override
+  public void close() {
+    if (executor != null) {
+      executor.shutdownNow();
+      executor = null;
+    }
+  }
+
   private void beforeEvaluateQuery() throws InterruptedException {
     if (graph == null || !graphFactory.isUpToDate(universeKey)) {
       // If this environment is uninitialized or the graph factory needs to evaluate, do so. We