resolved joins from main thread as illegal

src/main/java/carpet/script/CarpetExpression.java

@@ -10,6 +10,7 @@ import carpet.script.exception.CarpetExpressionException;
 import carpet.script.exception.ExitStatement;
 import carpet.script.exception.ExpressionException;
 import carpet.script.exception.InternalExpressionException;
+import carpet.CarpetSettings;
 import carpet.script.value.BlockValue;
 import carpet.script.value.EntityValue;
 import carpet.script.value.FunctionValue;
@@ -19,8 +20,8 @@ import carpet.script.value.NBTSerializableValue;
 import carpet.script.value.NullValue;
 import carpet.script.value.NumericValue;
 import carpet.script.value.StringValue;
+import carpet.script.value.ThreadValue;
 import carpet.script.value.Value;
-import carpet.CarpetSettings;
 import carpet.utils.BlockInfo;
 import carpet.utils.Messenger;
 import com.google.common.collect.Sets;
@@ -101,7 +102,6 @@ import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.UUID;
-import java.util.concurrent.CompletableFuture;
 import java.util.function.BiFunction;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
@@ -3006,6 +3006,17 @@ public class CarpetExpression
             return (_c, _t) -> res; // pass through for variables
         });
 
+        //"overidden" native call to cancel if on main thread
+        this.expr.addLazyFunction("join_task", 1, (c, t, lv) -> {
+            if (((CarpetContext)c).s.getMinecraftServer().isOnThread())
+                throw new InternalExpressionException("'join_task' cannot be called from main thread to avoid deadlocks");
+            Value v = lv.get(0).evalValue(c);
+            if (!(v instanceof ThreadValue))
+                throw new InternalExpressionException("'join_task' could only be used with a task value");
+            Value ret =  ((ThreadValue) v).join();
+            return (_c, _t) -> ret;
+        });
+
         this.expr.addLazyFunction("logger", 1, (c, t, lv) ->
         {
             Value res = lv.get(0).evalValue(c);