Run scheduled tasks from other plugins using API in sequence

This prevents two transactions executing at the same time.

Affects issues:
- Fixed #3147

Plan/common/src/main/java/com/djrapitops/plan/processing/Processing.java

@@ -38,6 +38,7 @@ public class Processing implements SubSystem {
     private final ErrorLogger errorLogger;
 
     private ExecutorService nonCriticalExecutor;
+    private ExecutorService nonCriticalSingleThreadExecutor;
     private ExecutorService criticalExecutor;
 
     @Inject
@@ -50,6 +51,7 @@ public Processing(
         this.logger = logger;
         this.errorLogger = errorLogger;
         nonCriticalExecutor = createExecutor(6, "Plan Non critical-pool-%d");
+        nonCriticalSingleThreadExecutor = createExecutor(1, "Plan Non critical-pool-single-threaded-%d");
         criticalExecutor = createExecutor(2, "Plan Critical-pool-%d");
     }
 
@@ -71,13 +73,18 @@ public void submit(Runnable runnable) {
     }
 
     public CompletableFuture<Boolean> submitNonCritical(Runnable runnable) {
-        if (runnable == null || nonCriticalExecutor.isShutdown()) {
+        return submitNonCritical(runnable, false);
+    }
+
+    public CompletableFuture<Boolean> submitNonCritical(Runnable runnable, boolean singleThreaded) {
+        ExecutorService executorService = singleThreaded ? nonCriticalSingleThreadExecutor : nonCriticalExecutor;
+        if (runnable == null || executorService.isShutdown()) {
             return null;
         }
         return CompletableFuture.supplyAsync(() -> {
             runnable.run();
             return true;
-        }, nonCriticalExecutor).handle(this::exceptionHandlerNonCritical);
+        }, executorService).handle(this::exceptionHandlerNonCritical);
     }
 
     public CompletableFuture<Boolean> submitCritical(Runnable runnable) {
@@ -138,21 +145,25 @@ public void enable() {
         if (nonCriticalExecutor.isShutdown()) {
             nonCriticalExecutor = createExecutor(6, "Plan Non critical-pool-%d");
         }
+        if (nonCriticalSingleThreadExecutor.isShutdown()) {
+            nonCriticalSingleThreadExecutor = createExecutor(1, "Plan Non critical-pool-single-threaded-%d");
+        }
         if (criticalExecutor.isShutdown()) {
             criticalExecutor = createExecutor(2, "Plan Critical-pool-%d");
         }
     }
 
     @Override
     public void disable() {
-        shutdownNonCriticalExecutor();
+        shutdownNonCriticalExecutors();
         shutdownCriticalExecutor();
         ensureShutdown();
         logger.info(locale.get().getString(PluginLang.DISABLED_PROCESSING_COMPLETE));
     }
 
-    private void shutdownNonCriticalExecutor() {
+    private void shutdownNonCriticalExecutors() {
         nonCriticalExecutor.shutdownNow();
+        nonCriticalSingleThreadExecutor.shutdownNow();
     }
 
     private void shutdownCriticalExecutor() {
@@ -184,12 +195,16 @@ private void ensureShutdown() {
             if (!nonCriticalExecutor.isTerminated()) {
                 nonCriticalExecutor.shutdownNow();
             }
+            if (!nonCriticalSingleThreadExecutor.isTerminated()) {
+                nonCriticalSingleThreadExecutor.shutdownNow();
+            }
             if (!criticalExecutor.isTerminated() && !criticalExecutor.awaitTermination(1, TimeUnit.SECONDS)) {
                 criticalExecutor.shutdownNow();
             }
         } catch (InterruptedException e) {
             logger.error("Processing shutdown thread interrupted: " + e.getMessage());
             nonCriticalExecutor.shutdownNow();
+            nonCriticalSingleThreadExecutor.shutdownNow();
             criticalExecutor.shutdownNow();
             Thread.currentThread().interrupt();
         }

Plan/common/src/main/java/com/djrapitops/plan/settings/SchedulerSvc.java

@@ -24,6 +24,8 @@
 @Singleton
 public class SchedulerSvc implements SchedulerService {
 
+    private static final boolean SINGLE_THREADED = true;
+
     private final Processing processing;
 
     @Inject
@@ -33,7 +35,7 @@ public SchedulerSvc(Processing processing) {
 
     @Override
     public void runAsync(Runnable runnable) {
-        processing.submitNonCritical(runnable);
+        processing.submitNonCritical(runnable, SINGLE_THREADED);
     }
 
     public void register() {