Fix race condition when contexts share an engine

This patch includes fixes for a category of race conditions when
concurrently running code in two or more contexts that share an engine.

The PythonNAryClinicBuiltinNodes can have multiple threads call
castWithNode concurrently on the same object. After one thread has
assigned a particular castNode, another can overwrite the array with an
empty one, causing an NPE when re-reading it from the array.  Instead,
the methods can assign directly a local variable and then save the
values in the array. With this change, a thread doesn't risk reading a
null after having computed the value.

Author: Zach Olstein

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/function/builtins/PythonBinaryClinicBuiltinNode.java

@@ -68,15 +68,18 @@ private Object cast(ArgumentClinicProvider clinic, VirtualFrame frame, int argIn
     }
 
     protected Object castWithNode(ArgumentClinicProvider clinic, VirtualFrame frame, int argIndex, Object value) {
+        ArgumentCastNode castNode;
         if (castNodes == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
             castNodes = new ArgumentCastNode[2];
         }
-        if (castNodes[argIndex] == null) {
+        castNode = castNodes[argIndex];
+        if (castNode == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
-            castNodes[argIndex] = insert(clinic.createCastNode(argIndex, this));
+            castNode = insert(clinic.createCastNode(argIndex, this));
+            castNodes[argIndex] = castNode;
         }
-        return castNodes[argIndex].execute(frame, value);
+        return castNode.execute(frame, value);
     }
 
     @Override

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/function/builtins/PythonQuaternaryClinicBuiltinNode.java

@@ -66,15 +66,18 @@ private Object cast(ArgumentClinicProvider clinic, VirtualFrame frame, int argIn
     }
 
     protected Object castWithNode(ArgumentClinicProvider clinic, VirtualFrame frame, int argIndex, Object value) {
+        ArgumentCastNode castNode;
         if (castNodes == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
             castNodes = new ArgumentCastNode[4];
         }
-        if (castNodes[argIndex] == null) {
+        castNode = castNodes[argIndex];
+        if (castNode == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
-            castNodes[argIndex] = insert(clinic.createCastNode(argIndex, this));
+            castNode = insert(clinic.createCastNode(argIndex, this));
+            castNodes[argIndex] = castNode;
         }
-        return castNodes[argIndex].execute(frame, value);
+        return castNode.execute(frame, value);
     }
 
     @Override

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/function/builtins/PythonTernaryClinicBuiltinNode.java

@@ -66,15 +66,18 @@ private Object cast(ArgumentClinicProvider clinic, VirtualFrame frame, int argIn
     }
 
     protected Object castWithNode(ArgumentClinicProvider clinic, VirtualFrame frame, int argIndex, Object value) {
+        ArgumentCastNode castNode;
         if (castNodes == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
             castNodes = new ArgumentCastNode[3];
         }
-        if (castNodes[argIndex] == null) {
+        castNode = castNodes[argIndex];
+        if (castNode == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
-            castNodes[argIndex] = insert(clinic.createCastNode(argIndex, this));
+            castNode = insert(clinic.createCastNode(argIndex, this));
+            castNodes[argIndex] = castNode;
         }
-        return castNodes[argIndex].execute(frame, value);
+        return castNode.execute(frame, value);
     }
 
     @Override