Test if we should add an autoload constant in the replace loop

* So there is no race between the test and set.

(cherry picked from commit 324239a)

Author: eregon

src/main/java/org/truffleruby/core/module/ModuleFields.java

@@ -343,6 +343,10 @@ public RubyConstant setConstant(RubyContext context, Node currentNode, String na
     @TruffleBoundary
     public void setAutoloadConstant(RubyContext context, Node currentNode, String name, RubyString filename) {
         RubyConstant autoloadConstant = setConstantInternal(context, currentNode, name, filename, true);
+        if (autoloadConstant == null) {
+            return;
+        }
+
         if (context.getOptions().LOG_AUTOLOAD) {
             RubyLanguage.LOGGER.info(() -> String.format(
                     "%s: setting up autoload %s with %s",
@@ -371,6 +375,10 @@ private RubyConstant setConstantInternal(RubyContext context, Node currentNode,
         RubyConstant newConstant;
         do {
             previous = constants.get(name);
+            if (autoload && previous != null && previous.hasValue()) {
+                // abort, do not set an autoload constant, the constant already has a value
+                return null;
+            }
             newConstant = newConstant(currentNode, name, value, autoload, previous);
         } while (!ConcurrentOperations.replace(constants, name, previous, newConstant));
 

src/main/java/org/truffleruby/core/module/ModuleNodes.java

@@ -580,11 +580,7 @@ protected Object autoload(RubyModule module, String name, RubyString filename) {
                 throw new RaiseException(getContext(), coreExceptions().argumentError("empty file name", this));
             }
 
-            final RubyConstant constant = module.fields.getConstant(name);
-            if (constant == null || !constant.hasValue()) {
-                module.fields.setAutoloadConstant(getContext(), this, name, filename);
-            }
-
+            module.fields.setAutoloadConstant(getContext(), this, name, filename);
             return nil;
         }
     }