bpo-39576: Prevent memory error for overly optimistic precisions (GH-18581)

Lib/test/test_decimal.py

@@ -5476,6 +5476,41 @@ class CWhitebox(unittest.TestCase):
             self.assertEqual(Decimal.from_float(cls(101.1)),
                              Decimal.from_float(101.1))
 
+    def test_maxcontext_exact_arith(self):
+
+        # Make sure that exact operations do not raise MemoryError due
+        # to huge intermediate values when the context precision is very
+        # large.
+
+        # The following functions fill the available precision and are
+        # therefore not suitable for large precisions (by design of the
+        # specification).
+        MaxContextSkip = ['logical_invert', 'next_minus', 'next_plus',
+                          'logical_and', 'logical_or', 'logical_xor',
+                          'next_toward', 'rotate', 'shift']
+
+        Decimal = C.Decimal
+        Context = C.Context
+        localcontext = C.localcontext
+
+        # Here only some functions that are likely candidates for triggering a
+        # MemoryError are tested.  deccheck.py has an exhaustive test.
+        maxcontext = Context(prec=C.MAX_PREC, Emin=C.MIN_EMIN, Emax=C.MAX_EMAX)
+        with localcontext(maxcontext):
+            self.assertEqual(Decimal(0).exp(), 1)
+            self.assertEqual(Decimal(1).ln(), 0)
+            self.assertEqual(Decimal(1).log10(), 0)
+            self.assertEqual(Decimal(10**2).log10(), 2)
+            self.assertEqual(Decimal(10**223).log10(), 223)
+            self.assertEqual(Decimal(10**19).logb(), 19)
+            self.assertEqual(Decimal(4).sqrt(), 2)
+            self.assertEqual(Decimal("40E9").sqrt(), Decimal('2.0E+5'))
+            self.assertEqual(divmod(Decimal(10), 3), (3, 1))
+            self.assertEqual(Decimal(10) // 3, 3)
+            self.assertEqual(Decimal(4) / 2, 2)
+            self.assertEqual(Decimal(400) ** -1, Decimal('0.0025'))
+
+
 @requires_docstrings
 @unittest.skipUnless(C, "test requires C version")
 class SignatureTest(unittest.TestCase):