bpo-42990: Functions inherit current builtins (GH-24564)

The types.FunctionType constructor now inherits the current builtins
if the globals dictionary has no "__builtins__" key, rather than
using {"None": None} as builtins: same behavior as eval() and exec()
functions.

Defining a function with "def function(...): ..." in Python is not
affected, globals cannot be overriden with this syntax: it also
inherits the current builtins.

PyFrame_New(), PyEval_EvalCode(), PyEval_EvalCodeEx(),
PyFunction_New() and PyFunction_NewWithQualName() now inherits the
current builtins namespace if the globals dictionary has no
"__builtins__" key.

* Add _PyEval_GetBuiltins() function.
* _PyEval_BuiltinsFromGlobals() now uses _PyEval_GetBuiltins() if
  builtins cannot be found in globals.
* Add tstate parameter to _PyEval_BuiltinsFromGlobals().

1 files changed, 27 insertions, 0 deletions

diff --git a/Lib/test/test_funcattrs.py b/Lib/test/test_funcattrs.py
index 15cf250..77977d0 100644
--- a/Lib/test/test_funcattrs.py
+++ b/Lib/test/test_funcattrs.py
@@ -1,3 +1,4 @@
+import textwrap
 import types
 import unittest
 
@@ -78,6 +79,32 @@ class FunctionPropertiesTest(FuncAttrsTest):
         self.cannot_set_attr(self.b, '__builtins__', 2,
                              (AttributeError, TypeError))
 
+        # bpo-42990: If globals is specified and has no "__builtins__" key,
+        # a function inherits the current builtins namespace.
+        def func(s): return len(s)
+        ns = {}
+        func2 = type(func)(func.__code__, ns)
+        self.assertIs(func2.__globals__, ns)
+        self.assertIs(func2.__builtins__, __builtins__)
+
+        # Make sure that the function actually works.
+        self.assertEqual(func2("abc"), 3)
+        self.assertEqual(ns, {})
+
+        # Define functions using exec() with different builtins,
+        # and test inheritance when globals has no "__builtins__" key
+        code = textwrap.dedent("""
+            def func3(s): pass
+            func4 = type(func3)(func3.__code__, {})
+        """)
+        safe_builtins = {'None': None}
+        ns = {'type': type, '__builtins__': safe_builtins}
+        exec(code, ns)
+        self.assertIs(ns['func3'].__builtins__, safe_builtins)
+        self.assertIs(ns['func4'].__builtins__, safe_builtins)
+        self.assertIs(ns['func3'].__globals__['__builtins__'], safe_builtins)
+        self.assertNotIn('__builtins__', ns['func4'].__globals__)
+
     def test___closure__(self):
         a = 12
         def f(): print(a)