From c85be1a88fb3d312762052d1e6e39612c285296d Mon Sep 17 00:00:00 2001
From: Seppe Volkaerts <seppevolkaerts@hotmail.com>
Date: Tue, 15 Dec 2020 23:08:53 +0100
Subject: [PATCH] Improve continuation error handling.

---
 .../proxy/event/VelocityEventManager.java        | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/proxy/src/main/java/com/velocitypowered/proxy/event/VelocityEventManager.java b/proxy/src/main/java/com/velocitypowered/proxy/event/VelocityEventManager.java
index ced5e868e..f5f7ca518 100644
--- a/proxy/src/main/java/com/velocitypowered/proxy/event/VelocityEventManager.java
+++ b/proxy/src/main/java/com/velocitypowered/proxy/event/VelocityEventManager.java
@@ -476,7 +476,9 @@ public class VelocityEventManager implements EventManager {
       try {
         task.run(this);
       } catch (final Throwable t) {
-        resumeWithException(t);
+        // validateOnlyOnce false here so don't get an exception if the
+        // continuation was resumed before
+        resume(t, false);
       }
       return !CONTINUATION_TASK_STATE.compareAndSet(
           this, TASK_STATE_EXECUTING, TASK_STATE_DEFAULT);
@@ -484,18 +486,22 @@ public class VelocityEventManager implements EventManager {
 
     @Override
     public void resume() {
-      resume(null);
+      resume(null, true);
     }
 
-    void resume(final @Nullable Throwable exception) {
+    void resume(final @Nullable Throwable exception, final boolean validateOnlyOnce) {
+      final boolean changed = CONTINUATION_TASK_RESUMED.compareAndSet(this, false, true);
       // Only allow the continuation to be resumed once
-      if (!CONTINUATION_TASK_RESUMED.compareAndSet(this, false, true)) {
+      if (!changed && validateOnlyOnce) {
         throw new IllegalStateException("The continuation can only be resumed once.");
       }
       final HandlerRegistration registration = registrations[index];
       if (exception != null) {
         logHandlerException(registration, exception);
       }
+      if (!changed) {
+        return;
+      }
       if (index + 1 == registrations.length) {
         // Optimization: don't schedule a task just to complete the future
         if (future != null) {
@@ -511,7 +517,7 @@ public class VelocityEventManager implements EventManager {
 
     @Override
     public void resumeWithException(final Throwable exception) {
-      resume(requireNonNull(exception, "exception"));
+      resume(requireNonNull(exception, "exception"), true);
     }
   }