[3.8] Revert bpo-39576: Prevent memory error for overly optimistic precisions (GH-20747)

This reverts commit b627102.

Author: Stefan Krah

Lib/test/test_decimal.py

@@ -5476,41 +5476,6 @@ def __abs__(self):
  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):

Modules/_decimal/libmpdec/mpdecimal.c

@@ -3781,43 +3781,6 @@ mpd_qdiv(mpd_t *q, const mpd_t *a, const mpd_t *b,
  const mpd_context_t *ctx, uint32_t *status)
 {
  _mpd_qdiv(SET_IDEAL_EXP, q, a, b, ctx, status);
-
- if (*status & MPD_Malloc_error) {
- /* Inexact quotients (the usual case) fill the entire context precision,
- * which can lead to malloc() failures for very high precisions. Retry
- * the operation with a lower precision in case the result is exact.
- *
- * We need an upper bound for the number of digits of a_coeff / b_coeff
- * when the result is exact. If a_coeff' * 1 / b_coeff' is in lowest
- * terms, then maxdigits(a_coeff') + maxdigits(1 / b_coeff') is a suitable
- * bound.
- *
- * 1 / b_coeff' is exact iff b_coeff' exclusively has prime factors 2 or 5.
- * The largest amount of digits is generated if b_coeff' is a power of 2 or
- * a power of 5 and is less than or equal to log5(b_coeff') <= log2(b_coeff').
- *
- * We arrive at a total upper bound:
- *
- * maxdigits(a_coeff') + maxdigits(1 / b_coeff') <=
- * a->digits + log2(b_coeff) =
- * a->digits + log10(b_coeff) / log10(2) <=
- * a->digits + b->digits * 4;
- */
- uint32_t workstatus = 0;
- mpd_context_t workctx = *ctx;
- workctx.prec = a->digits + b->digits * 4;
- if (workctx.prec >= ctx->prec) {
- return; /* No point in retrying, keep the original error. */
- }
-
- _mpd_qdiv(SET_IDEAL_EXP, q, a, b, &workctx, &workstatus);
- if (workstatus == 0) { /* The result is exact, unrounded, normal etc. */
- *status = 0;
- return;
- }
-
- mpd_seterror(q, *status, status);
- }
 }
 
 /* Internal function. */
@@ -7739,9 +7702,9 @@ mpd_qinvroot(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
 /* END LIBMPDEC_ONLY */
 
 /* Algorithm from decimal.py */
-static void
-_mpd_qsqrt(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
-  uint32_t *status)
+void
+mpd_qsqrt(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
+ uint32_t *status)
 {
  mpd_context_t maxcontext;
  MPD_NEW_STATIC(c,0,0,0,0);
@@ -7873,40 +7836,6 @@ _mpd_qsqrt(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
  goto out;
 }
 
-void
-mpd_qsqrt(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
- uint32_t *status)
-{
- _mpd_qsqrt(result, a, ctx, status);
-
- if (*status & (MPD_Malloc_error|MPD_Division_impossible)) {
- /* The above conditions can occur at very high context precisions
- * if intermediate values get too large. Retry the operation with
- * a lower context precision in case the result is exact.
- *
- * If the result is exact, an upper bound for the number of digits
- * is the number of digits in the input.
- *
- * NOTE: sqrt(40e9) = 2.0e+5 /\ digits(40e9) = digits(2.0e+5) = 2
- */
- uint32_t workstatus = 0;
- mpd_context_t workctx = *ctx;
- workctx.prec = a->digits;
-
- if (workctx.prec >= ctx->prec) {
- return; /* No point in repeating this, keep the original error. */
- }
-
- _mpd_qsqrt(result, a, &workctx, &workstatus);
- if (workstatus == 0) {
- *status = 0;
- return;
- }
-
- mpd_seterror(result, *status, status);
- }
-}
-
 
 /******************************************************************************/
 /* Base conversions */