Improve native initialization logic, again

native/src/main/java/com/velocitypowered/natives/util/NativeCodeLoader.java

@@ -16,13 +16,14 @@ public class NativeCodeLoader<T> implements Supplier<T> {
 
     @Override
     public T get() {
-        if (selected == null) {
-            selected = tryLoad();
-        }
-        return selected.object;
+        return tryLoad().object;
     }
 
     private Variant<T> tryLoad() {
+        if (selected != null) {
+            return selected;
+        }
+
         synchronized (this) {
             if (selected != null) {
                 return selected;
@@ -41,18 +42,15 @@ public class NativeCodeLoader<T> implements Supplier<T> {
     }
 
     public String getLoadedVariant() {
-        if (selected == null) {
-            selected = tryLoad();
-        }
-        return selected.name;
+        return tryLoad().name;
     }
 
     static class Variant<T> {
-        private boolean available;
+        private volatile boolean available;
         private final Runnable setup;
         private final String name;
         private final T object;
-        private boolean hasBeenSetup = false;
+        private volatile boolean hasBeenSetup = false;
 
         Variant(BooleanSupplier available, Runnable setup, String name, T object) {
             this.available = available.getAsBoolean();
@@ -61,27 +59,34 @@ public class NativeCodeLoader<T> implements Supplier<T> {
             this.object = object;
         }
 
-        private void setup() {
-            if (available && !hasBeenSetup) {
-                try {
-                    setup.run();
-                    hasBeenSetup = true;
-                } catch (Exception e) {
-                    available = false;
+        public T get() {
+            if (!available) {
+                return null;
+            }
+
+            // Make sure setup happens only once
+            if (!hasBeenSetup) {
+                synchronized (this) {
+                    // We change availability if need be below, may as well check it again here for sanity.
+                    if (!available) {
+                        return null;
+                    }
+
+                    // Okay, now try the setup if we haven't done so yet.
+                    if (!hasBeenSetup) {
+                        try {
+                            setup.run();
+                            hasBeenSetup = true;
+                            return object;
+                        } catch (Exception e) {
+                            available = false;
+                            return null;
+                        }
+                    }
                 }
             }
-        }
 
-        public T get() {
-            if (!hasBeenSetup) {
-                setup();
-            }
-
-            if (available) {
-                return object;
-            }
-
-            return null;
+            return object;
         }
     }