bpo-35214: Skip test_io tests that'd cause a huge malloc under msan (#11385)

* skip test_constructor under msan.

* fix the others as well.

* reuse existing related news entry.

* typo fix

Lib/test/test_io.py

@@ -28,6 +28,7 @@ import pickle
 import random
 import signal
 import sys
+import sysconfig
 import threading
 import time
 import unittest
@@ -59,6 +60,13 @@ else:
     class EmptyStruct(ctypes.Structure):
         pass
 
+_cflags = sysconfig.get_config_var('CFLAGS') or ''
+_config_args = sysconfig.get_config_var('CONFIG_ARGS') or ''
+MEMORY_SANITIZER = (
+    '-fsanitize=memory' in _cflags or
+    '--with-memory-sanitizer' in _config_args
+)
+
 def _default_chunk_size():
     """Get the default TextIOWrapper chunk size"""
     with open(__file__, "r", encoding="latin-1") as f:
@@ -1496,6 +1504,8 @@ class BufferedReaderTest(unittest.TestCase, CommonBufferedTests):
 class CBufferedReaderTest(BufferedReaderTest, SizeofTest):
     tp = io.BufferedReader
 
+    @unittest.skipIf(MEMORY_SANITIZER, "MSan defaults to crashing "
+                     "instead of returning NULL for malloc failure.")
     def test_constructor(self):
         BufferedReaderTest.test_constructor(self)
         # The allocation can succeed on 32-bit builds, e.g. with more
@@ -1840,6 +1850,8 @@ class BufferedWriterTest(unittest.TestCase, CommonBufferedTests):
 class CBufferedWriterTest(BufferedWriterTest, SizeofTest):
     tp = io.BufferedWriter
 
+    @unittest.skipIf(MEMORY_SANITIZER, "MSan defaults to crashing "
+                     "instead of returning NULL for malloc failure.")
     def test_constructor(self):
         BufferedWriterTest.test_constructor(self)
         # The allocation can succeed on 32-bit builds, e.g. with more
@@ -2314,6 +2326,8 @@ class BufferedRandomTest(BufferedReaderTest, BufferedWriterTest):
 class CBufferedRandomTest(BufferedRandomTest, SizeofTest):
     tp = io.BufferedRandom
 
+    @unittest.skipIf(MEMORY_SANITIZER, "MSan defaults to crashing "
+                     "instead of returning NULL for malloc failure.")
     def test_constructor(self):
         BufferedRandomTest.test_constructor(self)
         # The allocation can succeed on 32-bit builds, e.g. with more

Misc/NEWS.d/next/Core and Builtins/2018-12-30-15-36-23.bpo-35214.GWDQcv.rst

 clang Memory Sanitizer build instrumentation was added to work around false
-positives from socket, time, and test_faulthandler.
+positives from socket, time, test_io, and test_faulthandler.