bpo-42403: Simplify importlib external bootstrap (GH-23397)

Simplify the importlib external bootstrap code:
importlib._bootstrap_external now uses regular imports to import
builtin modules. When it is imported, the builtin __import__()
function is already fully working and so can be used to import
builtin modules like sys.

3 files changed, 55 insertions, 65 deletions

diff --git a/Lib/importlib/__init__.py b/Lib/importlib/__init__.py
index bea37d7..03ff714 100644
--- a/Lib/importlib/__init__.py
+++ b/Lib/importlib/__init__.py
@@ -34,7 +34,7 @@ try:
     import _frozen_importlib_external as _bootstrap_external
 except ImportError:
     from . import _bootstrap_external
-    _bootstrap_external._setup(_bootstrap)
+    _bootstrap_external._set_bootstrap_module(_bootstrap)
     _bootstrap._bootstrap_external = _bootstrap_external
 else:
     _bootstrap_external.__name__ = 'importlib._bootstrap_external'
diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index e00b27e..854b603 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -22,8 +22,15 @@ work. One should use importlib as the public-facing version of this module.
 
 # Bootstrap-related code ######################################################
 
+# Modules injected manually by _setup()
+_thread = None
+_warnings = None
+_weakref = None
+
+# Import done by _install_external_importers()
 _bootstrap_external = None
 
+
 def _wrap(new, old):
     """Simple substitute for functools.update_wrapper."""
     for replace in ['__module__', '__name__', '__qualname__', '__doc__']:
diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 5c30a67..a0236c4 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -19,6 +19,36 @@ work. One should use importlib as the public-facing version of this module.
 # reference any injected objects! This includes not only global code but also
 # anything specified at the class level.
 
+# Module injected manually by _set_bootstrap_module()
+_bootstrap = None
+
+# Import builtin modules
+import _imp
+import _io
+import sys
+import _warnings
+import marshal
+
+
+_MS_WINDOWS = (sys.platform == 'win32')
+if _MS_WINDOWS:
+    import nt as _os
+    import winreg
+else:
+    import posix as _os
+
+
+if _MS_WINDOWS:
+    path_separators = ['\\', '/']
+else:
+    path_separators = ['/']
+# Assumption made in _path_join()
+assert all(len(sep) == 1 for sep in path_separators)
+path_sep = path_separators[0]
+path_separators = ''.join(path_separators)
+_pathseps_with_colon = {f':{s}' for s in path_separators}
+
+
 # Bootstrap-related code ######################################################
 _CASE_INSENSITIVE_PLATFORMS_STR_KEY = 'win',
 _CASE_INSENSITIVE_PLATFORMS_BYTES_KEY = 'cygwin', 'darwin'
@@ -42,6 +72,8 @@ def _make_relax_case():
             return False
     return _relax_case
 
+_relax_case = _make_relax_case()
+
 
 def _pack_uint32(x):
     """Convert a 32-bit integer to little-endian."""
@@ -294,7 +326,11 @@ _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 _PYCACHE = '__pycache__'
 _OPT = 'opt-'
 
-SOURCE_SUFFIXES = ['.py']  # _setup() adds .pyw as needed.
+SOURCE_SUFFIXES = ['.py']
+if _MS_WINDOWS:
+    SOURCE_SUFFIXES.append('.pyw')
+
+EXTENSION_SUFFIXES = _imp.extension_suffixes()
 
 BYTECODE_SUFFIXES = ['.pyc']
 # Deprecated.
@@ -469,15 +505,18 @@ def _check_name(method):
             raise ImportError('loader for %s cannot handle %s' %
                                 (self.name, name), name=name)
         return method(self, name, *args, **kwargs)
-    try:
+
+    # FIXME: @_check_name is used to define class methods before the
+    # _bootstrap module is set by _set_bootstrap_module().
+    if _bootstrap is not None:
         _wrap = _bootstrap._wrap
-    except NameError:
-        # XXX yuck
+    else:
         def _wrap(new, old):
             for replace in ['__module__', '__name__', '__qualname__', '__doc__']:
                 if hasattr(old, replace):
                     setattr(new, replace, getattr(old, replace))
             new.__dict__.update(old.__dict__)
+
     _wrap(_check_name_wrapper, method)
     return _check_name_wrapper
 
@@ -713,7 +752,7 @@ class WindowsRegistryFinder:
     REGISTRY_KEY_DEBUG = (
         'Software\\Python\\PythonCore\\{sys_version}'
         '\\Modules\\{fullname}\\Debug')
-    DEBUG_BUILD = False  # Changed in _setup()
+    DEBUG_BUILD = (_MS_WINDOWS and '_d.pyd' in EXTENSION_SUFFIXES)
 
     @classmethod
     def _open_registry(cls, key):
@@ -1060,10 +1099,6 @@ class SourcelessFileLoader(FileLoader, _LoaderBasics):
         return None
 
 
-# Filled in by _setup().
-EXTENSION_SUFFIXES = []
-
-
 class ExtensionFileLoader(FileLoader, _LoaderBasics):
 
     """Loader for extension modules.
@@ -1552,66 +1587,14 @@ def _get_supported_file_loaders():
     return [extensions, source, bytecode]
 
 
-def _setup(_bootstrap_module):
-    """Setup the path-based importers for importlib by importing needed
-    built-in modules and injecting them into the global namespace.
-
-    Other components are extracted from the core bootstrap module.
-
-    """
-    global sys, _imp, _bootstrap
+def _set_bootstrap_module(_bootstrap_module):
+    global _bootstrap
     _bootstrap = _bootstrap_module
-    sys = _bootstrap.sys
-    _imp = _bootstrap._imp
-
-    self_module = sys.modules[__name__]
-
-    # Directly load the os module (needed during bootstrap).
-    os_details = ('posix', ['/']), ('nt', ['\\', '/'])
-    for builtin_os, path_separators in os_details:
-        # Assumption made in _path_join()
-        assert all(len(sep) == 1 for sep in path_separators)
-        path_sep = path_separators[0]
-        if builtin_os in sys.modules:
-            os_module = sys.modules[builtin_os]
-            break
-        else:
-            try:
-                os_module = _bootstrap._builtin_from_name(builtin_os)
-                break
-            except ImportError:
-                continue
-    else:
-        raise ImportError('importlib requires posix or nt')
-
-    setattr(self_module, '_os', os_module)
-    setattr(self_module, 'path_sep', path_sep)
-    setattr(self_module, 'path_separators', ''.join(path_separators))
-    setattr(self_module, '_pathseps_with_colon', {f':{s}' for s in path_separators})
-
-    # Directly load built-in modules needed during bootstrap.
-    builtin_names = ['_io', '_warnings', 'marshal']
-    if builtin_os == 'nt':
-        builtin_names.append('winreg')
-    for builtin_name in builtin_names:
-        if builtin_name not in sys.modules:
-            builtin_module = _bootstrap._builtin_from_name(builtin_name)
-        else:
-            builtin_module = sys.modules[builtin_name]
-        setattr(self_module, builtin_name, builtin_module)
-
-    # Constants
-    setattr(self_module, '_relax_case', _make_relax_case())
-    EXTENSION_SUFFIXES.extend(_imp.extension_suffixes())
-    if builtin_os == 'nt':
-        SOURCE_SUFFIXES.append('.pyw')
-        if '_d.pyd' in EXTENSION_SUFFIXES:
-            WindowsRegistryFinder.DEBUG_BUILD = True
 
 
 def _install(_bootstrap_module):
     """Install the path-based import components."""
-    _setup(_bootstrap_module)
+    _set_bootstrap_module(_bootstrap_module)
     supported_loaders = _get_supported_file_loaders()
     sys.path_hooks.extend([FileFinder.path_hook(*supported_loaders)])
     sys.meta_path.append(PathFinder)