Check for early termination in Driver (elastic#118188)

This change introduces support for periodically checking for early 
termination. This enables early exits in the following scenarios:

1. The query has accumulated sufficient data (e.g., reaching the LIMIT).

2. The query is stopped (either by users or due to failures).

Other changes will be addressed in follow-up PRs.

docs/changelog/118188.yaml

@@ -0,0 +1,5 @@
+pr: 118188
+summary: Check for early termination in Driver
+area: ES|QL
+type: enhancement
+issues: []

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/Driver.java

@@ -186,7 +186,13 @@ SubscribableListener<Void> run(TimeValue maxTime, int maxIterations, LongSupplie
         long nextStatus = startTime + statusNanos;
         int iter = 0;
         while (true) {
-            IsBlockedResult isBlocked = runSingleLoopIteration();
+            IsBlockedResult isBlocked = Operator.NOT_BLOCKED;
+            try {
+                isBlocked = runSingleLoopIteration();
+            } catch (DriverEarlyTerminationException unused) {
+                closeEarlyFinishedOperators();
+                assert isFinished() : "not finished after early termination";
+            }
             iter++;
             if (isBlocked.listener().isDone() == false) {
                 updateStatus(nowSupplier.getAsLong() - startTime, iter, DriverStatus.Status.ASYNC, isBlocked.reason());
@@ -242,39 +248,59 @@ public void abort(Exception reason, ActionListener<Void> listener) {
     }
 
     private IsBlockedResult runSingleLoopIteration() {
-        ensureNotCancelled();
+        driverContext.checkForEarlyTermination();
         boolean movedPage = false;
 
-        if (activeOperators.isEmpty() == false && activeOperators.getLast().isFinished() == false) {
-            for (int i = 0; i < activeOperators.size() - 1; i++) {
-                Operator op = activeOperators.get(i);
-                Operator nextOp = activeOperators.get(i + 1);
+        for (int i = 0; i < activeOperators.size() - 1; i++) {
+            Operator op = activeOperators.get(i);
+            Operator nextOp = activeOperators.get(i + 1);
 
-                // skip blocked operator
-                if (op.isBlocked().listener().isDone() == false) {
-                    continue;
-                }
+            // skip blocked operator
+            if (op.isBlocked().listener().isDone() == false) {
+                continue;
+            }
 
-                if (op.isFinished() == false && nextOp.needsInput()) {
-                    Page page = op.getOutput();
-                    if (page == null) {
-                        // No result, just move to the next iteration
-                    } else if (page.getPositionCount() == 0) {
-                        // Empty result, release any memory it holds immediately and move to the next iteration
+            if (op.isFinished() == false && nextOp.needsInput()) {
+                driverContext.checkForEarlyTermination();
+                Page page = op.getOutput();
+                if (page == null) {
+                    // No result, just move to the next iteration
+                } else if (page.getPositionCount() == 0) {
+                    // Empty result, release any memory it holds immediately and move to the next iteration
+                    page.releaseBlocks();
+                } else {
+                    // Non-empty result from the previous operation, move it to the next operation
+                    try {
+                        driverContext.checkForEarlyTermination();
+                    } catch (DriverEarlyTerminationException | TaskCancelledException e) {
                         page.releaseBlocks();
-                    } else {
-                        // Non-empty result from the previous operation, move it to the next operation
-                        nextOp.addInput(page);
-                        movedPage = true;
+                        throw e;
                     }
+                    nextOp.addInput(page);
+                    movedPage = true;
                 }
+            }
 
-                if (op.isFinished()) {
-                    nextOp.finish();
-                }
+            if (op.isFinished()) {
+                driverContext.checkForEarlyTermination();
+                nextOp.finish();
             }
         }
 
+        closeEarlyFinishedOperators();
+
+        if (movedPage == false) {
+            return oneOf(
+                activeOperators.stream()
+                    .map(Operator::isBlocked)
+                    .filter(laf -> laf.listener().isDone() == false)
+                    .collect(Collectors.toList())
+            );
+        }
+        return Operator.NOT_BLOCKED;
+    }
+
+    private void closeEarlyFinishedOperators() {
         for (int index = activeOperators.size() - 1; index >= 0; index--) {
             if (activeOperators.get(index).isFinished()) {
                 /*
@@ -300,16 +326,6 @@ private IsBlockedResult runSingleLoopIteration() {
                 break;
             }
         }
-
-        if (movedPage == false) {
-            return oneOf(
-                activeOperators.stream()
-                    .map(Operator::isBlocked)
-                    .filter(laf -> laf.listener().isDone() == false)
-                    .collect(Collectors.toList())
-            );
-        }
-        return Operator.NOT_BLOCKED;
     }
 
     public void cancel(String reason) {
@@ -318,13 +334,6 @@ public void cancel(String reason) {
         }
     }
 
-    private void ensureNotCancelled() {
-        String reason = cancelReason.get();
-        if (reason != null) {
-            throw new TaskCancelledException(reason);
-        }
-    }
-
     public static void start(
         ThreadContext threadContext,
         Executor executor,
@@ -335,19 +344,36 @@ public static void start(
         driver.completionListener.addListener(listener);
         if (driver.started.compareAndSet(false, true)) {
             driver.updateStatus(0, 0, DriverStatus.Status.STARTING, "driver starting");
-            // Register a listener to an exchange sink to handle early completion scenarios:
-            // 1. When the query accumulates sufficient data (e.g., reaching the LIMIT).
-            // 2. When users abort the query but want to retain the current result.
-            // This allows the Driver to finish early without waiting for the scheduled task.
-            if (driver.activeOperators.isEmpty() == false) {
-                if (driver.activeOperators.getLast() instanceof ExchangeSinkOperator sinkOperator) {
-                    sinkOperator.addCompletionListener(ActionListener.running(driver.scheduler::runPendingTasks));
-                }
-            }
+            initializeEarlyTerminationChecker(driver);
             schedule(DEFAULT_TIME_BEFORE_YIELDING, maxIterations, threadContext, executor, driver, driver.completionListener);
         }
     }
 
+    private static void initializeEarlyTerminationChecker(Driver driver) {
+        // Register a listener to an exchange sink to handle early completion scenarios:
+        // 1. When the query accumulates sufficient data (e.g., reaching the LIMIT).
+        // 2. When users abort the query but want to retain the current result.
+        // This allows the Driver to finish early without waiting for the scheduled task.
+        final AtomicBoolean earlyFinished = new AtomicBoolean();
+        driver.driverContext.initializeEarlyTerminationChecker(() -> {
+            final String reason = driver.cancelReason.get();
+            if (reason != null) {
+                throw new TaskCancelledException(reason);
+            }
+            if (earlyFinished.get()) {
+                throw new DriverEarlyTerminationException("Exchange sink is closed");
+            }
+        });
+        if (driver.activeOperators.isEmpty() == false) {
+            if (driver.activeOperators.getLast() instanceof ExchangeSinkOperator sinkOperator) {
+                sinkOperator.addCompletionListener(ActionListener.running(() -> {
+                    earlyFinished.set(true);
+                    driver.scheduler.runPendingTasks();
+                }));
+            }
+        }
+    }
+
     // Drains all active operators and closes them.
     private void drainAndCloseOperators(@Nullable Exception e) {
         Iterator<Operator> itr = activeOperators.iterator();

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/DriverContext.java

@@ -60,6 +60,8 @@ public class DriverContext {
 
     private final WarningsMode warningsMode;
 
+    private Runnable earlyTerminationChecker = () -> {};
+
     public DriverContext(BigArrays bigArrays, BlockFactory blockFactory) {
         this(bigArrays, blockFactory, WarningsMode.COLLECT);
     }
@@ -175,6 +177,21 @@ public void removeAsyncAction() {
         asyncActions.removeInstance();
     }
 
+    /**
+     * Checks if the Driver associated with this DriverContext has been cancelled or early terminated.
+     */
+    public void checkForEarlyTermination() {
+        earlyTerminationChecker.run();
+    }
+
+    /**
+     * Initializes the early termination or cancellation checker for this DriverContext.
+     * This method should be called when associating this DriverContext with a driver.
+     */
+    public void initializeEarlyTerminationChecker(Runnable checker) {
+        this.earlyTerminationChecker = checker;
+    }
+
     /**
      * Evaluators should use this function to decide their warning behavior.
      * @return an appropriate {@link WarningsMode}

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/DriverEarlyTerminationException.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.compute.operator;
+
+import org.elasticsearch.ElasticsearchException;
+
+/**
+ * An exception indicates that a compute should be terminated early as the downstream pipeline has enough or no long requires more data.
+ */
+public final class DriverEarlyTerminationException extends ElasticsearchException {
+    public DriverEarlyTerminationException(String message) {
+        super(message);
+    }
+}

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/operator/DriverTests.java

@@ -19,6 +19,7 @@
 import org.elasticsearch.common.util.concurrent.ThreadContext;
 import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.compute.data.BasicBlockTests;
+import org.elasticsearch.compute.data.Block;
 import org.elasticsearch.compute.data.BlockFactory;
 import org.elasticsearch.compute.data.ElementType;
 import org.elasticsearch.compute.data.Page;
@@ -40,6 +41,7 @@
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.CyclicBarrier;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Function;
 import java.util.function.LongSupplier;
 
@@ -280,6 +282,49 @@ public Page getOutput() {
         }
     }
 
+    public void testEarlyTermination() {
+        DriverContext driverContext = driverContext();
+        ThreadPool threadPool = threadPool();
+        try {
+            int positions = between(1000, 5000);
+            List<Page> inPages = randomList(1, 100, () -> {
+                var block = driverContext.blockFactory().newConstantIntBlockWith(randomInt(), positions);
+                return new Page(block);
+            });
+            final var sourceOperator = new CannedSourceOperator(inPages.iterator());
+            final int maxAllowedRows = between(1, 100);
+            final AtomicInteger processedRows = new AtomicInteger(0);
+            var sinkHandler = new ExchangeSinkHandler(driverContext.blockFactory(), positions, System::currentTimeMillis);
+            var sinkOperator = new ExchangeSinkOperator(sinkHandler.createExchangeSink(), Function.identity());
+            final var delayOperator = new EvalOperator(driverContext.blockFactory(), new EvalOperator.ExpressionEvaluator() {
+                @Override
+                public Block eval(Page page) {
+                    for (int i = 0; i < page.getPositionCount(); i++) {
+                        driverContext.checkForEarlyTermination();
+                        if (processedRows.incrementAndGet() >= maxAllowedRows) {
+                            sinkHandler.fetchPageAsync(true, ActionListener.noop());
+                        }
+                    }
+                    return driverContext.blockFactory().newConstantBooleanBlockWith(true, page.getPositionCount());
+                }
+
+                @Override
+                public void close() {
+
+                }
+            });
+            Driver driver = new Driver(driverContext, sourceOperator, List.of(delayOperator), sinkOperator, () -> {});
+            ThreadContext threadContext = threadPool.getThreadContext();
+            PlainActionFuture<Void> future = new PlainActionFuture<>();
+
+            Driver.start(threadContext, threadPool.executor("esql"), driver, between(1, 1000), future);
+            future.actionGet(30, TimeUnit.SECONDS);
+            assertThat(processedRows.get(), equalTo(maxAllowedRows));
+        } finally {
+            terminate(threadPool);
+        }
+    }
+
     public void testResumeOnEarlyFinish() throws Exception {
         DriverContext driverContext = driverContext();
         ThreadPool threadPool = threadPool();