bpo-41718: regrtest saved_test_environment avoids imports (GH-24934)

Reduce the number of modules imported by libregrtest.

saved_test_environment no longer imports modules at startup, but try
to get them from sys.modules. If an module is missing, skip the test.
It also sets directly support.environment_altered.

runtest() now now two saved_test_environment instances: one before
importing the test module, one after importing it.

Remove imports from test.libregrtest.save_env:

* asyncio
* logging
* multiprocessing
* shutil
* sysconfig
* urllib.request
* warnings

When a test method imports a module (ex: warnings) and the test
has a side effect (ex: add a warnings filter), the side effect is not
detected, because the module was not imported when Python
enters the saved_test_environment context manager.

1 files changed, 56 insertions, 34 deletions

diff --git a/Lib/test/libregrtest/save_env.py b/Lib/test/libregrtest/save_env.py
index 4c9c692..f0bfcf3 100644
--- a/Lib/test/libregrtest/save_env.py
+++ b/Lib/test/libregrtest/save_env.py
@@ -1,21 +1,15 @@
-import asyncio
 import builtins
 import locale
-import logging
 import os
-import shutil
 import sys
-import sysconfig
 import threading
-import urllib.request
-import warnings
 from test import support
 from test.support import os_helper
 from test.libregrtest.utils import print_warning
-try:
-    import _multiprocessing, multiprocessing.process
-except ImportError:
-    multiprocessing = None
+
+
+class SkipTestEnvironment(Exception):
+    pass
 
 
 # Unit tests are supposed to leave the execution environment unchanged
@@ -33,15 +27,13 @@ class saved_test_environment:
             #stuff
 
     Unless quiet is True, a warning is printed to stderr if any of
-    the saved items was changed by the test.  The attribute 'changed'
-    is initially False, but is set to True if a change is detected.
+    the saved items was changed by the test. The support.environment_altered
+    attribute is set to True if a change is detected.
 
     If verbose is more than 1, the before and after state of changed
     items is also printed.
     """
 
-    changed = False
-
     def __init__(self, testname, verbose=0, quiet=False, *, pgo=False):
         self.testname = testname
         self.verbose = verbose
@@ -73,20 +65,36 @@ class saved_test_environment:
                  'urllib.requests._url_tempfiles', 'urllib.requests._opener',
                 )
 
+    def get_module(self, name):
+        # function for restore() methods
+        return sys.modules[name]
+
+    def try_get_module(self, name):
+        # function for get() methods
+        try:
+            return self.get_module(name)
+        except KeyError:
+            raise SkipTestEnvironment
+
     def get_urllib_requests__url_tempfiles(self):
-        return list(urllib.request._url_tempfiles)
+        urllib_request = self.try_get_module('urllib.request')
+        return list(urllib_request._url_tempfiles)
     def restore_urllib_requests__url_tempfiles(self, tempfiles):
         for filename in tempfiles:
             os_helper.unlink(filename)
 
     def get_urllib_requests__opener(self):
-        return urllib.request._opener
+        urllib_request = self.try_get_module('urllib.request')
+        return urllib_request._opener
     def restore_urllib_requests__opener(self, opener):
-        urllib.request._opener = opener
+        urllib_request = self.get_module('urllib.request')
+        urllib_request._opener = opener
 
     def get_asyncio_events__event_loop_policy(self):
+        self.try_get_module('asyncio')
         return support.maybe_get_event_loop_policy()
     def restore_asyncio_events__event_loop_policy(self, policy):
+        asyncio = self.get_module('asyncio')
         asyncio.set_event_loop_policy(policy)
 
     def get_sys_argv(self):
@@ -145,8 +153,10 @@ class saved_test_environment:
         builtins.__import__ = import_
 
     def get_warnings_filters(self):
+        warnings = self.try_get_module('warnings')
         return id(warnings.filters), warnings.filters, warnings.filters[:]
     def restore_warnings_filters(self, saved_filters):
+        warnings = self.get_module('warnings')
         warnings.filters = saved_filters[1]
         warnings.filters[:] = saved_filters[2]
 
@@ -161,23 +171,28 @@ class saved_test_environment:
             asyncore.socket_map.update(saved_map)
 
     def get_shutil_archive_formats(self):
+        shutil = self.try_get_module('shutil')
         # we could call get_archives_formats() but that only returns the
         # registry keys; we want to check the values too (the functions that
         # are registered)
         return shutil._ARCHIVE_FORMATS, shutil._ARCHIVE_FORMATS.copy()
     def restore_shutil_archive_formats(self, saved):
+        shutil = self.get_module('shutil')
         shutil._ARCHIVE_FORMATS = saved[0]
         shutil._ARCHIVE_FORMATS.clear()
         shutil._ARCHIVE_FORMATS.update(saved[1])
 
     def get_shutil_unpack_formats(self):
+        shutil = self.try_get_module('shutil')
         return shutil._UNPACK_FORMATS, shutil._UNPACK_FORMATS.copy()
     def restore_shutil_unpack_formats(self, saved):
+        shutil = self.get_module('shutil')
         shutil._UNPACK_FORMATS = saved[0]
         shutil._UNPACK_FORMATS.clear()
         shutil._UNPACK_FORMATS.update(saved[1])
 
     def get_logging__handlers(self):
+        logging = self.try_get_module('logging')
         # _handlers is a WeakValueDictionary
         return id(logging._handlers), logging._handlers, logging._handlers.copy()
     def restore_logging__handlers(self, saved_handlers):
@@ -185,6 +200,7 @@ class saved_test_environment:
         pass
 
     def get_logging__handlerList(self):
+        logging = self.try_get_module('logging')
         # _handlerList is a list of weakrefs to handlers
         return id(logging._handlerList), logging._handlerList, logging._handlerList[:]
     def restore_logging__handlerList(self, saved_handlerList):
@@ -208,32 +224,34 @@ class saved_test_environment:
 
     # Same for Process objects
     def get_multiprocessing_process__dangling(self):
-        if not multiprocessing:
-            return None
+        multiprocessing_process = self.try_get_module('multiprocessing.process')
         # Unjoined process objects can survive after process exits
-        multiprocessing.process._cleanup()
+        multiprocessing_process._cleanup()
         # This copies the weakrefs without making any strong reference
-        return multiprocessing.process._dangling.copy()
+        return multiprocessing_process._dangling.copy()
     def restore_multiprocessing_process__dangling(self, saved):
-        if not multiprocessing:
-            return
-        multiprocessing.process._dangling.clear()
-        multiprocessing.process._dangling.update(saved)
+        multiprocessing_process = self.get_module('multiprocessing.process')
+        multiprocessing_process._dangling.clear()
+        multiprocessing_process._dangling.update(saved)
 
     def get_sysconfig__CONFIG_VARS(self):
         # make sure the dict is initialized
+        sysconfig = self.try_get_module('sysconfig')
         sysconfig.get_config_var('prefix')
         return (id(sysconfig._CONFIG_VARS), sysconfig._CONFIG_VARS,
                 dict(sysconfig._CONFIG_VARS))
     def restore_sysconfig__CONFIG_VARS(self, saved):
+        sysconfig = self.get_module('sysconfig')
         sysconfig._CONFIG_VARS = saved[1]
         sysconfig._CONFIG_VARS.clear()
         sysconfig._CONFIG_VARS.update(saved[2])
 
     def get_sysconfig__INSTALL_SCHEMES(self):
+        sysconfig = self.try_get_module('sysconfig')
         return (id(sysconfig._INSTALL_SCHEMES), sysconfig._INSTALL_SCHEMES,
                 sysconfig._INSTALL_SCHEMES.copy())
     def restore_sysconfig__INSTALL_SCHEMES(self, saved):
+        sysconfig = self.get_module('sysconfig')
         sysconfig._INSTALL_SCHEMES = saved[1]
         sysconfig._INSTALL_SCHEMES.clear()
         sysconfig._INSTALL_SCHEMES.update(saved[2])
@@ -264,8 +282,10 @@ class saved_test_environment:
             locale.setlocale(lc, setting)
 
     def get_warnings_showwarning(self):
+        warnings = self.try_get_module('warnings')
         return warnings.showwarning
     def restore_warnings_showwarning(self, fxn):
+        warnings = self.get_module('warnings')
         warnings.showwarning = fxn
 
     def resource_info(self):
@@ -276,26 +296,28 @@ class saved_test_environment:
             yield name, getattr(self, get_name), getattr(self, restore_name)
 
     def __enter__(self):
-        self.saved_values = dict((name, get()) for name, get, restore
-                                                   in self.resource_info())
+        self.saved_values = []
+        for name, get, restore in self.resource_info():
+            try:
+                original = get()
+            except SkipTestEnvironment:
+                continue
+
+            self.saved_values.append((name, get, restore, original))
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
         saved_values = self.saved_values
-        del self.saved_values
+        self.saved_values = None
 
         # Some resources use weak references
         support.gc_collect()
 
-        # Read support.environment_altered, set by support helper functions
-        self.changed |= support.environment_altered
-
-        for name, get, restore in self.resource_info():
+        for name, get, restore, original in saved_values:
             current = get()
-            original = saved_values.pop(name)
             # Check for changes to the resource's value
             if current != original:
-                self.changed = True
+                support.environment_altered = True
                 restore(original)
                 if not self.quiet and not self.pgo:
                     print_warning(f"{name} was modified by {self.testname}")