Implement the full semantics for aliasing global variables

* Invalidate an Assumption when a GlobalVariableStorage becomes stale
  as it the variable gets aliased to another.

Author: eregon

src/main/java/org/truffleruby/core/exception/CoreExceptions.java

@@ -698,15 +698,6 @@ public DynamicObject zeroDivisionError(Node currentNode, ArithmeticException exc
         return ExceptionOperations.createRubyException(context, exceptionClass, errorMessage, currentNode, exception);
     }
 
-    // NotImplementedError
-
-    @TruffleBoundary
-    public DynamicObject notImplementedError(String message, Node currentNode) {
-        DynamicObject exceptionClass = context.getCoreLibrary().getNotImplementedErrorClass();
-        DynamicObject errorMessage = StringOperations.createString(context, StringOperations.encodeRope(message, UTF8Encoding.INSTANCE));
-        return ExceptionOperations.createRubyException(context, exceptionClass, errorMessage, currentNode, null);
-    }
-
     // SyntaxError
 
     @TruffleBoundary

src/main/java/org/truffleruby/language/globals/AliasGlobalVarNode.java

@@ -26,17 +26,8 @@ public AliasGlobalVarNode(String oldName, String newName) {
 
     @Override
     public Object execute(VirtualFrame frame) {
-        checkExisting();
         getContext().getCoreLibrary().getGlobalVariables().alias(oldName, newName);
         return nil();
     }
 
-    @TruffleBoundary
-    private void checkExisting() {
-        if (getContext().getCoreLibrary().getGlobalVariables().contains(newName)) {
-            // TODO CS 4-Apr-18 This exception is non-standard
-            throw new RaiseException(getContext(), coreExceptions().notImplementedError(String.format("%s is already a global", newName), this), true);
-        }
-    }
-
 }

src/main/java/org/truffleruby/language/globals/GlobalVariables.java

@@ -30,7 +30,7 @@ public GlobalVariables(DynamicObject defaultValue) {
         this.defaultValue = defaultValue;
     }
 
-    public String getOriginalName(String name) {
+    private String getOriginalName(String name) {
         return aliases.getOrDefault(name, name);
     }
 
@@ -65,19 +65,22 @@ public GlobalVariableStorage put(String name, DynamicObject getter, DynamicObjec
         return storage;
     }
 
-    @TruffleBoundary
-    public boolean contains(String name) {
-        return variables.containsKey(name);
-    }
-
     @TruffleBoundary
     public void alias(String oldName, String newName) {
-        // TODO
         // Record an alias of an alias against the original.
-        oldName = aliases.getOrDefault(oldName, oldName);
+        oldName = getOriginalName(oldName);
         aliases.put(newName, oldName);
+
         final GlobalVariableStorage storage = getStorage(oldName);
-        variables.put(newName, storage);
+
+        GlobalVariableStorage previousStorage;
+        do {
+            previousStorage = variables.get(newName);
+        } while (!ConcurrentOperations.replace(variables, newName, previousStorage, storage));
+
+        if (previousStorage != null) {
+            previousStorage.getValidAssumption().invalidate();
+        }
     }
 
     @TruffleBoundary