Prevent ConcurrentModificationException during cascading delete

hapi-fhir-jpaserver-base/src/main/java/ca/uhn/fhir/jpa/delete/DeleteConflictList.java

@@ -24,12 +24,17 @@ import ca.uhn.fhir.jpa.util.DeleteConflict;
 import org.apache.commons.lang3.Validate;
 import org.hl7.fhir.instance.model.api.IIdType;
 
-import java.util.*;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Set;
 import java.util.function.Predicate;
 
 public class DeleteConflictList implements Iterable<DeleteConflict> {
 	private final List<DeleteConflict> myList = new ArrayList<>();
 	private final Set<String> myResourceIdsMarkedForDeletion;
+	private int myRemoveModCount;
 
 	/**
 	 * Constructor
@@ -69,11 +74,38 @@ public class DeleteConflictList implements Iterable<DeleteConflict> {
 
 	@Override
 	public Iterator<DeleteConflict> iterator() {
-		return myList.iterator();
+		// Note that handlers may add items to this list, so we're using a special iterator
+		// that is ok with this. Only removals from the list should trigger a concurrent modification
+		// issue
+		return new Iterator<DeleteConflict>() {
+
+			private final int myOriginalRemoveModCont = myRemoveModCount;
+			private int myNextIndex = 0;
+
+			@Override
+			public boolean hasNext() {
+				checkForCoModification();
+				return myNextIndex < myList.size();
+			}
+
+			@Override
+			public DeleteConflict next() {
+				checkForCoModification();
+				return myList.get(myNextIndex++);
+			}
+
+			private void checkForCoModification() {
+				Validate.isTrue(myOriginalRemoveModCont == myRemoveModCount);
+			}
+		};
 	}
 
 	public boolean removeIf(Predicate<DeleteConflict> theFilter) {
-		return myList.removeIf(theFilter);
+		boolean retVal = myList.removeIf(theFilter);
+		if (retVal) {
+			myRemoveModCount++;
+		}
+		return retVal;
 	}
 
 	public void addAll(DeleteConflictList theNewConflicts) {

hapi-fhir-jpaserver-base/src/test/java/ca/uhn/fhir/jpa/provider/r4/CascadingDeleteInterceptorR4Test.java

@@ -64,6 +64,10 @@ public class CascadingDeleteInterceptorR4Test extends BaseResourceProviderR4Test
 		e.setSubject(new Reference(myPatientId));
 		myEncounterId = ourClient.create().resource(e).execute().getId().toUnqualifiedVersionless();
 
+		CarePlan cp = new CarePlan();
+		cp.setEncounter(new Reference(myEncounterId));
+		ourClient.create().resource(cp).execute();
+
 		Observation o = new Observation();
 		o.setStatus(Observation.ObservationStatus.FINAL);
 		o.setSubject( new Reference(myPatientId));

src/changes/changes.xml

@@ -50,6 +50,10 @@
 				ValueSet Precalculation sometimes failed on Oracle DBs due to an invalid SQL function. This
 				has been corrected.
 			</action>
+			<action type="fix">
+				A ConcurrentModificationException was sometimes thrown when performing a cascading delete.
+				This has been corrected.
+			</action>
 		</release>
 		<release version="4.1.0" date="2019-11-13" description="Jitterbug">
 			<action type="add">