Polishing.

Update javadoc to reflect recent changes and split tests.

Original Pull Request: #4666

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationOptions.java

@@ -33,6 +33,10 @@ import com.mongodb.ReadPreference;
  * Holds a set of configurable aggregation options that can be used within an aggregation pipeline. A list of support
  * aggregation options can be found in the
  * <a href="https://docs.mongodb.org/manual/reference/command/aggregate/#aggregate">MongoDB reference documentation</a>.
+ * <p>
+ * As off 4.3 {@link #allowDiskUse} can be {@literal null}, indicating use of server default, and may only be applied if
+ * {@link #isAllowDiskUseSet() explicitly set}. For compatibility reasons {@link #isAllowDiskUse()} will remain
+ * returning {@literal false} if the no value has been set.
  *
  * @author Thomas Darimont
  * @author Oliver Gierke

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationOptionsTests.java

@@ -86,6 +86,12 @@ class AggregationOptionsTests {
 		assertThat(aggregationOptions.isAllowDiskUseSet()).isFalse();
 
 		assertThat(aggregationOptions.toDocument()).doesNotContainKey("allowDiskUse");
+	}
+
+	@Test // GH-4664
+	void applyOptionsDoesNotChangeAllowDiskUseDefault() {
+
+		aggregationOptions = AggregationOptions.fromDocument(new Document());
 
 		Document empty = new Document();
 		aggregationOptions.applyAndReturnPotentiallyChangedCommand(empty);
@@ -93,6 +99,17 @@ class AggregationOptionsTests {
 		assertThat(empty).doesNotContainKey("allowDiskUse");
 	}
 
+	@Test // GH-4664
+	void applyOptionsDoesNotChangeExistingAllowDiskUse() {
+
+		aggregationOptions = AggregationOptions.fromDocument(new Document());
+
+		Document existing = new Document("allowDiskUse", true);
+		aggregationOptions.applyAndReturnPotentiallyChangedCommand(existing);
+
+		assertThat(existing).containsEntry("allowDiskUse", true);
+	}
+
 	@Test // DATAMONGO-960, DATAMONGO-2153, DATAMONGO-1836
 	void aggregationOptionsToString() {