Add limit option to ScanCursor.

ScanCursor can be now limited to a maximum number of elements to be retrieved.

Closes #1575

src/main/java/org/springframework/data/redis/core/ConvertingCursor.java

@@ -122,4 +122,12 @@ public long getPosition() {
 		return delegate.getPosition();
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.redis.core.Cursor#limit(long)
+	 */
+	@Override
+	public Cursor<T> limit(long count) {
+		return new ConvertingCursor<>(delegate.limit(count), converter);
+	}
 }

src/main/java/org/springframework/data/redis/core/Cursor.java

@@ -16,14 +16,19 @@
 package org.springframework.data.redis.core;
 
 import java.io.Closeable;
-import java.util.Iterator;
+
+import org.springframework.data.redis.util.BoundedIterator;
 
 /**
+ * Cursor abstraction to scan over the keyspace or elements within a data structure using a variant of a {@code SCAN}
+ * command.
+ *
  * @author Christoph Strobl
+ * @author Mark Paluch
  * @param <T>
  * @since 1.4
  */
-public interface Cursor<T> extends Iterator<T>, Closeable {
+public interface Cursor<T> extends BoundedIterator<T>, Closeable {
 
 	/**
 	 * Get the reference cursor. <br>
@@ -50,4 +55,14 @@ public interface Cursor<T> extends Iterator<T>, Closeable {
 	 */
 	long getPosition();
 
+	/**
+	 * Limit the maximum number of elements to be returned from this cursor. The returned cursor object can be used to
+	 * iterate over the remaining items and to {@link #close() release} associated resources. The returned cursor is not
+	 * attached to the state of {@code this} cursor and this object should be no longer used.
+	 *
+	 * @return a new {@link Cursor} with detached iteration state.
+	 * @since 2.5
+	 */
+	@Override
+	Cursor<T> limit(long count);
 }

src/main/java/org/springframework/data/redis/core/ScanCursor.java

@@ -22,6 +22,7 @@
 
 import org.springframework.dao.InvalidDataAccessApiUsageException;
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 
 /**
@@ -43,6 +44,7 @@ public abstract class ScanCursor<T> implements Cursor<T> {
 	private @Nullable Iterator<T> delegate;
 	private @Nullable final ScanOptions scanOptions;
 	private long position;
+	private final long limit;
 
 	/**
 	 * Crates new {@link ScanCursor} with {@code id=0} and {@link ScanOptions#NONE}
@@ -81,6 +83,23 @@ public ScanCursor(long cursorId, ScanOptions options) {
 		this.cursorId = cursorId;
 		this.state = CursorState.READY;
 		this.delegate = Collections.<T> emptyList().iterator();
+		this.limit = -1;
+	}
+
+	/**
+	 * Crates a new {@link ScanCursor}.
+	 *
+	 * @param source
+	 * @param limit
+	 * @since 2.5
+	 */
+	private ScanCursor(ScanCursor<T> source, long limit) {
+
+		this.scanOptions = source.scanOptions;
+		this.cursorId = source.cursorId;
+		this.state = source.state;
+		this.delegate = source.delegate;
+		this.limit = limit;
 	}
 
 	private void scan(long cursorId) {
@@ -178,6 +197,10 @@ public boolean hasNext() {
 
 		assertCursorIsOpen();
 
+		if (limit != -1 && getPosition() > limit - 1) {
+			return false;
+		}
+
 		while (!delegate.hasNext() && !CursorState.FINISHED.equals(state)) {
 			scan(cursorId);
 		}
@@ -283,10 +306,43 @@ public long getPosition() {
 		return position;
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.redis.core.Cursor#limit(long)
+	 */
+	@Override
+	public ScanCursor<T> limit(long count) {
+
+		Assert.isTrue(count >= 0, "Count must be greater or equal to zero");
+
+		return new ScanCursorWrapper<>(this, count);
+	}
+
 	/**
 	 * @author Thomas Darimont
 	 */
 	enum CursorState {
 		READY, OPEN, FINISHED, CLOSED;
 	}
+
+	/**
+	 * Wrapper for a concrete {@link ScanCursor} forwarding {@link #doScan(long, ScanOptions)}.
+	 *
+	 * @param <T>
+	 * @since 2.5
+	 */
+	private static class ScanCursorWrapper<T> extends ScanCursor<T> {
+
+		private final ScanCursor<T> delegate;
+
+		public ScanCursorWrapper(ScanCursor<T> delegate, long limit) {
+			super(delegate, limit);
+			this.delegate = delegate;
+		}
+
+		@Override
+		protected ScanIteration<T> doScan(long cursorId, ScanOptions options) {
+			return delegate.doScan(cursorId, options);
+		}
+	}
 }

src/main/java/org/springframework/data/redis/util/BoundedIterator.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.redis.util;
+
+import java.util.Iterator;
+
+/**
+ * Extension to {@link Iterator} that can be {@link #limit(long) limited} to a maximum number of items.
+ *
+ * @author Mark Paluch
+ * @since 2.5
+ */
+public interface BoundedIterator<T> extends Iterator<T> {
+
+	/**
+	 * Limit the maximum number of elements to return. The limit is only applied to the returned instance and not applied
+	 * to {@code this} iterator.
+	 *
+	 * @param count the maximum number of elements of iterator to return. Must be greater or equal to zero.
+	 * @return a new instance of {@link BoundedIterator} with {@code count} applied.
+	 */
+	BoundedIterator<T> limit(long count);
+}

src/test/java/org/springframework/data/redis/core/ScanCursorUnitTests.java

@@ -17,7 +17,6 @@
 
 import static org.assertj.core.api.Assertions.*;
 
-import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -32,7 +31,10 @@
 import org.springframework.dao.InvalidDataAccessApiUsageException;
 
 /**
+ * Unit tests for {@link ScanCursor}.
+ *
  * @author Christoph Strobl
+ * @author Mark Paluch
  */
 class ScanCursorUnitTests {
 
@@ -198,6 +200,43 @@ void hasNextShouldStopCorrectlyWhenWholeScanIterationDoesNotReturnResultsAndStat
 		assertThat(cursor.getCursorId()).isEqualTo(0L);
 	}
 
+	@Test // GH-1575
+	void limitShouldApplyLimitation() {
+
+		LinkedList<ScanIteration<String>> values = new LinkedList<>();
+		values.add(createIteration(1, "spring"));
+		values.add(createIteration(2, "data"));
+		values.add(createIteration(3, "redis"));
+		values.add(createIteration(0));
+		Cursor<String> cursor = initCursor(values).limit(2);
+
+		List<String> result = new ArrayList<>();
+		while (cursor.hasNext()) {
+			result.add(cursor.next());
+		}
+
+		assertThat(result).hasSize(2).contains("spring", "data");
+	}
+
+	@Test // GH-1575
+	void limitShouldNotLimitOriginalCursor() {
+
+		LinkedList<ScanIteration<String>> values = new LinkedList<>();
+		values.add(createIteration(1, "spring"));
+		values.add(createIteration(2, "data"));
+		values.add(createIteration(3, "redis"));
+		values.add(createIteration(0));
+		Cursor<String> cursor = initCursor(values);
+		cursor.limit(1);
+
+		List<String> result = new ArrayList<>();
+		while (cursor.hasNext()) {
+			result.add(cursor.next());
+		}
+
+		assertThat(result).hasSize(3);
+	}
+
 	private CapturingCursorDummy initCursor(Queue<ScanIteration<String>> values) {
 		CapturingCursorDummy cursor = new CapturingCursorDummy(values);
 		cursor.open();