Hotfix async vouchers.yml write task as large files cause lag [SD-8155]

I've some a TODO/FIXME explaining what to do. Wanna focus on bringing the tickets down first

src/main/java/com/songoda/epicvouchers/EpicVouchers.java

@@ -20,6 +20,8 @@ import com.songoda.epicvouchers.libraries.inventory.IconInv;
 import com.songoda.epicvouchers.listeners.PlayerCommandListener;
 import com.songoda.epicvouchers.listeners.PlayerInteractListener;
 import com.songoda.epicvouchers.settings.Settings;
+import com.songoda.epicvouchers.utils.Callback;
+import com.songoda.epicvouchers.utils.ThreadSync;
 import com.songoda.epicvouchers.voucher.CoolDownManager;
 import com.songoda.epicvouchers.voucher.Voucher;
 import com.songoda.epicvouchers.voucher.VoucherExecutor;
@@ -101,18 +103,33 @@ public class EpicVouchers extends SongodaPlugin {
 
     @Override
     public void onDataLoad() {
-        if (!new File(this.getDataFolder(), "vouchers.yml").exists())
+        if (!new File(this.getDataFolder(), "vouchers.yml").exists()) {
             saveResource("vouchers.yml", false);
+        }
+
+        synchronized (vouchersConfig) {
             vouchersConfig.load();
+        }
 
         loadVouchersFromFile();
 
         connections.openMySQL();
 
-        Bukkit.getScheduler().scheduleSyncRepeatingTask(this, this::saveVouchers, 6000, 6000);
+        // FIXME: Config system needs to be greatly redone and only write changes when changes were made - Maybe even split it into multiple smaler files
+        //        Issue https://support.songoda.com/browse/SD-8155 has been hotfixed by writing changes to the file async and blocking the main thread when needed. This requires the use of `synchronized`
+        //        and expects every modifying code to use it (thread-safety)
+        //        Large vouchers.yml files cause huge performance problems otherwise...
+        //        Example file for testing: https://support.songoda.com/secure/attachment/17258/17258_vouchers.yml
+        Bukkit.getScheduler().scheduleSyncRepeatingTask(this,
+                () -> saveVouchersAsync(ex -> {
+                    if (ex != null) {
+                        ex.printStackTrace();
+                    }
+                }), 5 * 60 * 20, 5 * 60 * 20);   // 5 minutes
     }
 
     private void loadVouchersFromFile() {
+        synchronized (vouchersConfig) {
             voucherManager.clearVouchers();
 
             if (vouchersConfig.contains("vouchers")) {
@@ -165,15 +182,36 @@ public class EpicVouchers extends SongodaPlugin {
                 }
             }
         }
+    }
 
     private void saveVouchers() {
+        ThreadSync tSync = new ThreadSync();
+
+        saveVouchersAsync(ex -> {
+            if (ex != null) {
+                ex.printStackTrace();
+            }
+
+            tSync.release();
+        });
+
+        tSync.waitForRelease();
+    }
+
+    private void saveVouchersAsync(Callback callback) {
+        new Thread(() -> {
+            try {
+                synchronized (vouchersConfig) {
                     Collection<Voucher> voucherList = voucherManager.getVouchers();
 
-        for (String voucherName : vouchersConfig.getConfigurationSection("vouchers").getKeys(false)) {
+                    ConfigurationSection cfgSec = vouchersConfig.getConfigurationSection("vouchers");
+                    if (cfgSec != null) {
+                        for (String voucherName : cfgSec.getKeys(false)) {
                             if (voucherList.stream().noneMatch(voucher -> voucher.getKey().equals(voucherName))) {
                                 vouchersConfig.set("vouchers." + voucherName, null);
                             }
                         }
+                    }
 
                     for (Voucher voucher : voucherList) {
                         String prefix = "vouchers." + voucher.getKey() + ".";
@@ -211,11 +249,20 @@ public class EpicVouchers extends SongodaPlugin {
                     }
 
                     vouchersConfig.saveChanges();
+
+                    callback.accept(null);
+                }
+            } catch (Exception ex) {
+                callback.accept(ex);
+            }
+        }, getName() + "-AsyncConfigSave").start();
     }
 
     @Override
     public void onConfigReload() {
+        synchronized (vouchersConfig) {
             vouchersConfig.load();
+        }
 
         loadVouchersFromFile();
 
@@ -240,10 +287,6 @@ public class EpicVouchers extends SongodaPlugin {
         return this.voucherExecutor;
     }
 
-    public Config getVouchersConfig() {
-        return this.vouchersConfig;
-    }
-
     public CommandManager getCommandManager() {
         return commandManager;
     }

src/main/java/com/songoda/epicvouchers/utils/Callback.java

@@ -0,0 +1,6 @@
+package com.songoda.epicvouchers.utils;
+
+// TODO: Copied from EpicAnchors - Move to SongodaCore (maybe rename too?)
+public interface Callback {
+    void accept(Exception ex);
+}

src/main/java/com/songoda/epicvouchers/utils/ThreadSync.java

@@ -0,0 +1,31 @@
+package com.songoda.epicvouchers.utils;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+// TODO: Copied from EpicAnchors - Move to SongodaCore
+public class ThreadSync {
+    private final Object syncObj = new Object();
+    private final AtomicReference<Boolean> waiting = new AtomicReference<>(true);
+
+    public void waitForRelease() {
+        synchronized (syncObj) {
+            while (waiting.get()) {
+                try {
+                    syncObj.wait();
+                } catch (Exception ignore) {
+                }
+            }
+        }
+    }
+
+    public void release() {
+        synchronized (syncObj) {
+            waiting.set(false);
+            syncObj.notifyAll();
+        }
+    }
+
+    public void reset() {
+        waiting.set(true);
+    }
+}