Defer World modification to when the worlds are not being ticked on Paper

src/main/java/com/onarandombox/MultiverseCore/api/MVWorldManager.java

@@ -355,4 +355,13 @@ boolean addWorld(String name, Environment env, String seedString, WorldType type
      * @return A collection of world names that are deemed importable.
      */
     Collection<String> getPotentialWorlds();
+
+    /**
+     * Performs the given operation that creates and/or unloads a world. If this operation cannot run because the worlds are being ticked, the operation is delayed until the next tick.
+     *
+     * @param worldName The world being modified.
+     * @param operationName The name of the operation being done.
+     * @param worldModification The operation to perform
+     */
+    void addOrRemoveWorldSafely(String worldName, String operationName, Runnable worldModification);
 }

src/main/java/com/onarandombox/MultiverseCore/commands/CloneCommand.java

@@ -40,16 +40,19 @@ public CloneCommand(MultiverseCore plugin) {
     @Override
     public void runCommand(CommandSender sender, List<String> args) {
         String oldName = args.get(0);
-	    if (!this.worldManager.hasUnloadedWorld(oldName, true)) {
+        String newName = args.get(1);
+        if (!this.worldManager.hasUnloadedWorld(oldName, true)) {
             // If no world was found, we can't clone.
             sender.sendMessage("Sorry, Multiverse doesn't know about world " + oldName + ", so we can't clone it!");
             sender.sendMessage("Check the " + ChatColor.GREEN + "/mv list" + ChatColor.WHITE + " command to verify it is listed.");
             return;
         }
-        if (this.plugin.getMVWorldManager().cloneWorld(oldName, args.get(1))) {
-            sender.sendMessage(ChatColor.GREEN + "World cloned!");
-        } else {
-            sender.sendMessage(ChatColor.RED + "World could NOT be cloned!");
-        }
+        this.worldManager.addOrRemoveWorldSafely(oldName, "clone", () -> {
+            if (this.worldManager.cloneWorld(oldName, newName)) {
+                sender.sendMessage(ChatColor.GREEN + "World cloned!");
+            } else {
+                sender.sendMessage(ChatColor.RED + "World could NOT be cloned!");
+            }
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/CreateCommand.java

@@ -120,12 +120,17 @@ public void runCommand(CommandSender sender, List<String> args) {
                 return;
             }
         }
-        Command.broadcastCommandMessage(sender, "Starting creation of world '" + worldName + "'...");
 
-        if (this.worldManager.addWorld(worldName, environment, seed, type, allowStructures, generator, useSpawnAdjust)) {
-            Command.broadcastCommandMessage(sender, "Complete!");
-        } else {
-            Command.broadcastCommandMessage(sender, "FAILED.");
-        }
+        boolean finalAllowStructures = allowStructures;
+        boolean finalUseSpawnAdjust = useSpawnAdjust;
+        this.worldManager.addOrRemoveWorldSafely(worldName, "create", () -> {
+            Command.broadcastCommandMessage(sender, "Starting creation of world '" + worldName + "'...");
+
+            if (this.worldManager.addWorld(worldName, environment, seed, type, finalAllowStructures, generator, finalUseSpawnAdjust)) {
+                Command.broadcastCommandMessage(sender, "Complete!");
+            } else {
+                Command.broadcastCommandMessage(sender, "FAILED.");
+            }
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/DeleteCommand.java

@@ -47,14 +47,14 @@ public void runCommand(CommandSender sender, List<String> args) {
     private Runnable deleteRunnable(@NotNull CommandSender sender,
                                     @NotNull String worldName) {
 
-        return () -> {
+        return () -> this.plugin.getMVWorldManager().addOrRemoveWorldSafely(worldName, "delete", () -> {
             sender.sendMessage(String.format("Deleting world '%s'...", worldName));
             if (this.plugin.getMVWorldManager().deleteWorld(worldName)) {
                 sender.sendMessage(String.format("%sWorld %s was deleted!", ChatColor.GREEN, worldName));
                 return;
             }
             sender.sendMessage(String.format("%sThere was an issue deleting '%s'! Please check console for errors.",
                     ChatColor.RED, worldName));
-        };
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/ImportCommand.java

@@ -127,11 +127,14 @@ public void runCommand(CommandSender sender, List<String> args) {
             sender.sendMessage("That world environment did not exist.");
             sender.sendMessage("For a list of available world types, type: " + ChatColor.AQUA + "/mvenv");
         } else {
-            Command.broadcastCommandMessage(sender, String.format("Starting import of world '%s'...", worldName));
-            if (this.worldManager.addWorld(worldName, environment, null, null, null, generator, useSpawnAdjust))
-                Command.broadcastCommandMessage(sender, ChatColor.GREEN + "Complete!");
-            else
-                Command.broadcastCommandMessage(sender, ChatColor.RED + "Failed!");
+            boolean finalUseSpawnAdjust = useSpawnAdjust;
+            this.worldManager.addOrRemoveWorldSafely(worldName, "unload", () -> {
+                Command.broadcastCommandMessage(sender, String.format("Starting import of world '%s'...", worldName));
+                if (this.worldManager.addWorld(worldName, environment, null, null, null, generator, finalUseSpawnAdjust))
+                    Command.broadcastCommandMessage(sender, ChatColor.GREEN + "Complete!");
+                else
+                    Command.broadcastCommandMessage(sender, ChatColor.RED + "Failed!");
+            });
         }
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/LoadCommand.java

@@ -33,10 +33,13 @@ public LoadCommand(MultiverseCore plugin) {
 
     @Override
     public void runCommand(CommandSender sender, List<String> args) {
-        if (this.plugin.getMVWorldManager().loadWorld(args.get(0))) {
-            Command.broadcastCommandMessage(sender, "Loaded world '" + args.get(0) + "'!");
-        } else {
-            sender.sendMessage("Error trying to load world '" + args.get(0) + "'!");
-        }
+        String worldName = args.get(0);
+        this.plugin.getMVWorldManager().addOrRemoveWorldSafely(worldName, "load", () -> {
+            if (this.plugin.getMVWorldManager().loadWorld(worldName)) {
+                Command.broadcastCommandMessage(sender, "Loaded world '" + worldName + "'!");
+            } else {
+                sender.sendMessage("Error trying to load world '" + worldName + "'!");
+            }
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/RegenCommand.java

@@ -58,12 +58,12 @@ private Runnable doWorldRegen(@NotNull CommandSender sender,
                                   @NotNull String seed,
                                   boolean keepGamerules) {
 
-        return () -> {
+        return () -> plugin.getMVWorldManager().addOrRemoveWorldSafely(worldName, "regenerate", () -> {
             if (this.plugin.getMVWorldManager().regenWorld(worldName, useSeed, randomSeed, seed, keepGamerules)) {
                 sender.sendMessage(ChatColor.GREEN + "World Regenerated!");
                 return;
             }
             sender.sendMessage(ChatColor.RED + "World could NOT be regenerated!");
-        };
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/RemoveCommand.java

@@ -33,10 +33,13 @@ public RemoveCommand(MultiverseCore plugin) {
 
     @Override
     public void runCommand(CommandSender sender, List<String> args) {
-        if (this.plugin.getMVWorldManager().removeWorldFromConfig(args.get(0))) {
-            sender.sendMessage("World removed from config!");
-        } else {
-            sender.sendMessage("Error trying to remove world from config!");
-        }
+        String worldName = args.get(0);
+        this.plugin.getMVWorldManager().addOrRemoveWorldSafely(worldName, "remove", () -> {
+            if (this.plugin.getMVWorldManager().removeWorldFromConfig(worldName)) {
+                sender.sendMessage("World removed from config!");
+            } else {
+                sender.sendMessage("Error trying to remove world from config!");
+            }
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/UnloadCommand.java

@@ -33,10 +33,13 @@ public UnloadCommand(MultiverseCore plugin) {
 
     @Override
     public void runCommand(CommandSender sender, List<String> args) {
-        if (this.plugin.getMVWorldManager().unloadWorld(args.get(0))) {
-            Command.broadcastCommandMessage(sender, "Unloaded world '" + args.get(0) + "'!");
-        } else {
-            sender.sendMessage("Error trying to unload world '" + args.get(0) + "'!");
-        }
+        String worldName = args.get(0);
+        this.plugin.getMVWorldManager().addOrRemoveWorldSafely(worldName, "unload", () -> {
+            if (this.plugin.getMVWorldManager().unloadWorld(worldName)) {
+                Command.broadcastCommandMessage(sender, "Unloaded world '" + worldName + "'!");
+            } else {
+                sender.sendMessage("Error trying to unload world '" + worldName + "'!");
+            }
+        });
     }
 }

src/main/java/com/onarandombox/MultiverseCore/utils/WorldManager.java

@@ -32,10 +32,13 @@
 import org.bukkit.permissions.Permission;
 import org.bukkit.permissions.PermissionDefault;
 import org.bukkit.plugin.Plugin;
+import org.bukkit.scheduler.BukkitRunnable;
 
 import java.io.File;
 import java.io.FilenameFilter;
 import java.io.IOException;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -48,7 +51,6 @@
 import java.util.Stack;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ConcurrentHashMap;
-import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
 /**
@@ -1029,4 +1031,45 @@ public Collection<String> getPotentialWorlds() {
                 .map(File::getName)
                 .collect(Collectors.toList());
     }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void addOrRemoveWorldSafely(String worldName, String operationName, Runnable worldModification) {
+        Logging.finest("Checking if it is safe to perform world modification");
+        if (safeToAddOrRemoveWorld()) {
+            // Operation is fine to do now
+            Logging.finest("Clear to modify worlds");
+            worldModification.run();
+        } else {
+            // Operation needs to be delayed until worlds are not being ticked
+            Logging.fine("Worlds were being ticked while attempting to %s %s. Trying again in the next tick", operationName, worldName);
+            new BukkitRunnable() {
+                public void run() {
+                    worldModification.run();
+                }
+            }.runTask(plugin);
+        }
+    }
+
+    private boolean safeToAddOrRemoveWorld(){
+        Logging.finest("Checking for Paper");
+        Method isTickingWorlds;
+        try {
+            isTickingWorlds = Bukkit.class.getMethod("isTickingWorlds");
+        } catch (NoSuchMethodException e) {
+            // Paper fixes aren't active, so it is always considered safe to proceed
+            Logging.finest("Paper fixes inactive");
+            return true;
+        }
+        // Paper fixes are active, and Paper wants us to check Bukkit.isTickingWorlds()
+        Logging.finest("Paper fixes active");
+        try {
+            return !(boolean) isTickingWorlds.invoke(null);
+        } catch (InvocationTargetException | IllegalAccessException e) {
+            // Shouldn't happen, I know I'm using the method correctly
+            throw new RuntimeException(e);
+        }
+    }
 }