queueLookup API method throwing ConcurrentModificationException

[Krakenied]

List<String[]> queueLookup = api.queueLookup(block);

really often throws ConcurrentModificationException (probably especially on bigger servers)

[23:29:59 WARN]: java.util.ConcurrentModificationException
[23:29:59 WARN]:        at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1013)
[23:29:59 WARN]:        at java.base/java.util.ArrayList$Itr.next(ArrayList.java:967)
[23:29:59 WARN]:        at CoreProtect-22.2.jar//net.coreprotect.api.QueueLookup.performLookup(QueueLookup.java:52)
[23:29:59 WARN]:        at CoreProtect-22.2.jar//net.coreprotect.CoreProtectAPI.queueLookup(CoreProtectAPI.java:182)

The cause of the error is probably consumerData being modified while iterating. https://github.com/PlayPro/CoreProtect/blob/66b77ee75a24dac13f9cbf95242b504e1494dde7/src/main/java/net/coreprotect/api/QueueLookup.java#L52

[Intelli]

Switched the for loop for a ListIterator -- let me know if that resolves the issue.

[Krakenied]

@Intelli

It doesn't fix the issue. I wrote a simple class:

    public static void a() {
        final ArrayList<Object[]> list = new ArrayList<>();
        for (Object[] data : list) {
            System.out.println(data.length);
        }
    }

    public static void b() {
        final ArrayList<Object[]> list = new ArrayList<>();
        final ListIterator<Object[]> it = list.listIterator();
        while (it.hasNext()) {
            Object[] data = it.next();
            System.out.println(data.length);
        }
    }

And bytecode for both methods is nearly the same:

$ javap -c Test.class
Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void a();
    Code:
       0: new           #7                  // class java/util/ArrayList
       3: dup
       4: invokespecial #9                  // Method java/util/ArrayList."<init>":()V
       7: astore_0
       8: aload_0
       9: invokevirtual #10                 // Method java/util/ArrayList.iterator:()Ljava/util/Iterator;
      12: astore_1
      13: aload_1
      14: invokeinterface #14,  1           // InterfaceMethod java/util/Iterator.hasNext:()Z
      19: ifeq          43
      22: aload_1
      23: invokeinterface #20,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
      28: checkcast     #24                 // class "[Ljava/lang/Object;"
      31: astore_2
      32: getstatic     #26                 // Field java/lang/System.out:Ljava/io/PrintStream;
      35: aload_2
      36: arraylength
      37: invokevirtual #32                 // Method java/io/PrintStream.println:(I)V
      40: goto          13
      43: return

  public static void b();
    Code:
       0: new           #7                  // class java/util/ArrayList
       3: dup
       4: invokespecial #9                  // Method java/util/ArrayList."<init>":()V
       7: astore_0
       8: aload_0
       9: invokevirtual #38                 // Method java/util/ArrayList.listIterator:()Ljava/util/ListIterator;
      12: astore_1
      13: aload_1
      14: invokeinterface #42,  1           // InterfaceMethod java/util/ListIterator.hasNext:()Z
      19: ifeq          43
      22: aload_1
      23: invokeinterface #45,  1           // InterfaceMethod java/util/ListIterator.next:()Ljava/lang/Object;
      28: checkcast     #24                 // class "[Ljava/lang/Object;"
      31: astore_2
      32: getstatic     #26                 // Field java/lang/System.out:Ljava/io/PrintStream;
      35: aload_2
      36: arraylength
      37: invokevirtual #32                 // Method java/io/PrintStream.println:(I)V
      40: goto          13
      43: return
}

Also, as you can see, ListIterator is fail-fast too: It means, that will throw CME too: I'm currently using a custom CoreProtect fork on my server to prevent this error from occurring. My solution to this is:

            for (Object[] data : consumerData.toArray(Object[][]::new)) {

and it hasn't thrown any errors since the change.

[Krakenied]

@Intelli still not fixed

[Intelli]

Can you provide the code you're using which triggers this error?

[Krakenied]

Still present https://github.com/LMBishop/Quests/issues/707

Can you provide the code you're using which triggers this error?

Crucial step: have dozens of players online

You just need to invoke CoreProtectAPI#queueLookup which calls QueueLookup#performLookup. And this thing throws: https://github.com/PlayPro/CoreProtect/blob/c493a0dece5780aa975ac5bf19899e3c369ebc9b/src/main/java/net/coreprotect/api/QueueLookup.java#L55

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.