Plug another leak, and finally add a test for #1174606 (read() from /dev/zero).

The leak was the reason my previous attempts at testing failed...

Lib/test/test_io.py

@@ -505,6 +505,22 @@ class IOTest(unittest.TestCase):
         with open(support.TESTFN, "rb") as f:
             self.assertEqual(f.read(), b"abcxxx")
 
+    def test_unbounded_file(self):
+        # Issue #1174606: reading from an unbounded stream such as /dev/zero.
+        zero = "/dev/zero"
+        if not os.path.exists(zero):
+            raise unittest.SkipTest("{0} does not exist".format(zero))
+        if sys.maxsize > 0x7FFFFFFF:
+            raise unittest.SkipTest("test can only run in a 32-bit address space")
+        if support.real_max_memuse < support._2G:
+            raise unittest.SkipTest("test requires at least 2GB of memory")
+        with open(zero, "rb", buffering=0) as f:
+            self.assertRaises(OverflowError, f.read)
+        with open(zero, "rb") as f:
+            self.assertRaises(OverflowError, f.read)
+        with open(zero, "r") as f:
+            self.assertRaises(OverflowError, f.read)
+
 class CIOTest(IOTest):
     pass
 

Modules/_fileio.c

@@ -543,6 +543,7 @@ fileio_readall(PyFileIOObject *self)
 			PyErr_SetString(PyExc_OverflowError,
 				"unbounded read returned more bytes "
 				"than a Python string can hold ");
+			Py_DECREF(result);
 			return NULL;
 		}