Issue #21591: Handle exec backwards compatibility in the AST builder.

Instead of deferring until runtime. This makes sure we hit the right
conditions in dealing with unqualified exec statements.

Reviewed by Victor Stinner. Test follows in a later commit.

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 2.7.9?
 Core and Builtins
 -----------------
 
+- Issue #21591: Correctly handle qualified exec statements in tuple form by
+  moving compatibility layer from run-time to AST transformation.
+
 Library
 -------
 

Python/ast.c

@@ -2679,6 +2679,18 @@ ast_for_exec_stmt(struct compiling *c, const node *n)
     expr1 = ast_for_expr(c, CHILD(n, 1));
     if (!expr1)
         return NULL;
+
+    if (expr1->kind == Tuple_kind && n_children < 4 &&
+        (asdl_seq_LEN(expr1->v.Tuple.elts) == 2 ||
+         asdl_seq_LEN(expr1->v.Tuple.elts) == 3)) {
+        /* Backwards compatibility: pass exec args as a tuple */
+        globals = (expr_ty) asdl_seq_GET(expr1->v.Tuple.elts, 1);
+        if (asdl_seq_LEN(expr1->v.Tuple.elts) == 3) {
+            locals = (expr_ty) asdl_seq_GET(expr1->v.Tuple.elts, 2);
+        }
+        expr1 = (expr_ty) asdl_seq_GET(expr1->v.Tuple.elts, 0);
+    }
+
     if (n_children >= 4) {
         globals = ast_for_expr(c, CHILD(n, 3));
         if (!globals)

Python/ceval.c

@@ -4673,18 +4673,9 @@ static int
 exec_statement(PyFrameObject *f, PyObject *prog, PyObject *globals,
                PyObject *locals)
 {
-    int n;
     PyObject *v;
     int plain = 0;
 
-    if (PyTuple_Check(prog) && globals == Py_None && locals == Py_None &&
-        ((n = PyTuple_Size(prog)) == 2 || n == 3)) {
-        /* Backward compatibility hack */
-        globals = PyTuple_GetItem(prog, 1);
-        if (n == 3)
-            locals = PyTuple_GetItem(prog, 2);
-        prog = PyTuple_GetItem(prog, 0);
-    }
     if (globals == Py_None) {
         globals = PyEval_GetGlobals();
         if (locals == Py_None) {