Revert "Removed safety shutdown hook again"

Found a fix.
This reverts commit f880122add.

base/src/main/java/de/staropensource/sosengine/base/Engine.java

@@ -32,6 +32,7 @@ import de.staropensource.sosengine.base.exceptions.dependency.UnmetDependenciesE
 import de.staropensource.sosengine.base.internal.events.InternalEngineShutdownEvent;
 import de.staropensource.sosengine.base.internal.types.DependencySubsystemVector;
 import de.staropensource.sosengine.base.logging.CrashHandler;
+import de.staropensource.sosengine.base.logging.InitLogger;
 import de.staropensource.sosengine.base.logging.Logger;
 import de.staropensource.sosengine.base.logging.LoggerInstance;
 import de.staropensource.sosengine.base.types.DependencyVector;
@@ -40,6 +41,7 @@ import de.staropensource.sosengine.base.types.immutable.ImmutableLinkedList;
 import de.staropensource.sosengine.base.utility.DependencyResolver;
 import de.staropensource.sosengine.base.utility.Miscellaneous;
 import de.staropensource.sosengine.base.utility.PlaceholderEngine;
+import lombok.AccessLevel;
 import lombok.Getter;
 import lombok.Setter;
 import org.jetbrains.annotations.NotNull;
@@ -150,6 +152,41 @@ public final class Engine extends SubsystemClass {
     @Setter
     private @NotNull ShutdownHandler shutdownHandler = new Engine.JvmShutdownHandler();
 
+    /**
+     * Contains the JVM shutdown hook thread,
+     * which ensures that the engine is fully shut
+     * down before the JVM exits.
+     *
+     * @see EngineInternals#installSafetyShutdownHook(boolean)
+     * @since v1-alpha4
+     */
+    @Getter(AccessLevel.MODULE)
+    private final @NotNull Thread safetyShutdownHook = Thread.ofPlatform()
+            .name("Engine shutdown thread")
+            .group(getThreadGroup())
+            .unstarted(() -> {
+                // To avoid race condition or something
+                try {
+                    Thread.sleep(1000);
+                } catch (InterruptedException ignored) {}
+
+                // Check if already shutting down
+                switch (state) {
+                    case UNKNOWN, SHUTDOWN, CRASHED -> {
+                        return;
+                    }
+                }
+
+                // Print warning about shutdown
+                logger.warn("Trying to shut down engine using shutdown hook.\nThis approach to shutting down the engine and JVM is NOT RECOMMENDED, please use Engine#shutdown() instead.");
+
+                // Shutdown
+                Engine.getInstance().shutdown();
+
+                // Print last message
+                InitLogger.warn(getClass(), "ENGINE", EngineInformation.getVersioningCodename(), "Engine successfully shut down using shutdown hook. PLEASE USE Engine#shutdown() INSTEAD OF System#exit() or Runtime#exit()!");
+            });
+
     /**
      * Initializes the StarOpenSource Engine.
      *
@@ -172,6 +209,7 @@ public final class Engine extends SubsystemClass {
             initializeClasses();         // Initialize classes
             if (checkEnvironment())      // Check environment
                 throw new IllegalStateException("Running in an incompatible environment");
+            ensureEnvironment();         // Prepare the environment and ensure safety
             populateCrashContent();      // Populate crash content
             cacheEvents();               // Cache event listeners
             startThreads();              // Start threads
@@ -224,6 +262,15 @@ public final class Engine extends SubsystemClass {
         return false;
     }
 
+    /**
+     * Ensures the execution safety of the environment.
+     *
+     * @since v1-alpha4
+     */
+    private void ensureEnvironment() {
+        EngineInternals.getInstance().installSafetyShutdownHook(true);
+    }
+
     /**
      * This method populates {@link CrashHandler#crashContent} with content.
      *
@@ -432,6 +479,11 @@ public final class Engine extends SubsystemClass {
         // Flush log messages
         Logger.flushLogMessages();
 
+        // Disable safety shutdown hook
+        try {
+            Runtime.getRuntime().removeShutdownHook(safetyShutdownHook);
+        } catch (Exception ignored) {}
+
         // Send events
         logger.verb("Notifiying classes about shutdown");
         new EngineShutdownEvent().callEvent();

base/src/main/java/de/staropensource/sosengine/base/EngineInternals.java

@@ -19,6 +19,7 @@
 
 package de.staropensource.sosengine.base;
 
+import de.staropensource.sosengine.base.exceptions.NoAccessException;
 import de.staropensource.sosengine.base.logging.LoggerInstance;
 import de.staropensource.sosengine.base.types.InternalAccessArea;
 import lombok.Getter;
@@ -91,4 +92,26 @@ public final class EngineInternals {
 
         restrictedAreas.add(area);
     }
+
+    /**
+     * Installs or uninstalls the JVM shutdown
+     * hook, which prevents the JVM from exiting
+     * before the engine has fully shut down.
+     * Highly recommended to keep enabled.
+     *
+     * @param status {@code true} to install, {@code false} otherwise
+     * @throws NoAccessException when restricted
+     * @since v1-alpha4
+     */
+    public void installSafetyShutdownHook(boolean status) throws NoAccessException {
+        if (restrictedAreas.contains(InternalAccessArea.SAFETY_SHUTDOWN_HOOK))
+            throw new NoAccessException("The internal access area SAFETY_SHUTDOWN_HOOK has been restricted");
+
+        try {
+            if (status)
+                Runtime.getRuntime().addShutdownHook(Engine.getInstance().getSafetyShutdownHook());
+            else
+                Runtime.getRuntime().removeShutdownHook(Engine.getInstance().getSafetyShutdownHook());
+        } catch (IllegalArgumentException | IllegalStateException ignored) {}
+    }
 }

base/src/main/java/de/staropensource/sosengine/base/types/InternalAccessArea.java

@@ -35,4 +35,12 @@ public enum InternalAccessArea {
      * @since v1-alpha4
      */
     ALL,
+
+    /**
+     * Refers to the toggling of the JVM shutdown hook, which
+     * prevents JVM shutdowns without the engine first shutting down.
+     *
+     * @since v1-alpha4
+     */
+    SAFETY_SHUTDOWN_HOOK,
 }