bpo-43422: Revert _decimal C API addition (GH-24960)

Stefan Krah requested the reversal of these (unreleased) changes, quoting him:
> The capsule API does not meet my testing standards, since I've focused
on the upstream mpdecimal in the last couple of months.
> Additionally, I'd like to refine the API, perhaps together with the
Arrow community.

Automerge-Triggered-By: GH:pitrou

10 files changed, 20 insertions, 1119 deletions

diff --git a/Doc/c-api/concrete.rst b/Doc/c-api/concrete.rst
index bf263d6..c1d9fa1 100644
--- a/Doc/c-api/concrete.rst
+++ b/Doc/c-api/concrete.rst
@@ -115,4 +115,3 @@ Other Objects
    coro.rst
    contextvars.rst
    datetime.rst
-   decimal.rst
diff --git a/Doc/c-api/decimal.rst b/Doc/c-api/decimal.rst
deleted file mode 100644
index 94cc4a7..0000000
--- a/Doc/c-api/decimal.rst
+++ /dev/null
@@ -1,231 +0,0 @@
-.. sectionauthor:: Stefan Krah
-
-.. highlight:: c
-
-
-Decimal capsule API
-===================
-
-Capsule API functions can be used in the same manner as regular library
-functions, provided that the API has been initialized.
-
-
-Initialize
-----------
-
-Typically, a C extension module that uses the decimal API will do these
-steps in its init function:
-
-.. code-block:: c
-
-    #include "pydecimal.h"
-
-    static int decimal_initialized = 0;
-    if (!decimal_initialized) {
-        if (import_decimal() < 0) {
-            return NULL;
-        }
-
-        decimal_initialized = 1;
-    }
-
-
-Type checking, predicates, accessors
-------------------------------------
-
-.. c:function:: int PyDec_TypeCheck(const PyObject *dec)
-
-   Return 1 if ``dec`` is a Decimal, 0 otherwise.  This function does not set
-   any exceptions.
-
-
-.. c:function:: int PyDec_IsSpecial(const PyObject *dec)
-
-   Return 1 if ``dec`` is ``NaN``, ``sNaN`` or ``Infinity``, 0 otherwise.
-
-   Set TypeError and return -1 if ``dec`` is not a Decimal.  It is guaranteed that
-   this is the only failure mode, so if ``dec`` has already been type-checked, no
-   errors can occur and the function can be treated as a simple predicate.
-
-
-.. c:function:: int PyDec_IsNaN(const PyObject *dec)
-
-   Return 1 if ``dec`` is ``NaN`` or ``sNaN``, 0 otherwise.
-
-   Set TypeError and return -1 if ``dec`` is not a Decimal.  It is guaranteed that
-   this is the only failure mode, so if ``dec`` has already been type-checked, no
-   errors can occur and the function can be treated as a simple predicate.
-
-
-.. c:function:: int PyDec_IsInfinite(const PyObject *dec)
-
-   Return 1 if ``dec`` is ``Infinity``, 0 otherwise.
-
-   Set TypeError and return -1 if ``dec`` is not a Decimal.  It is guaranteed that
-   this is the only failure mode, so if ``dec`` has already been type-checked, no
-   errors can occur and the function can be treated as a simple predicate.
-
-
-.. c:function:: int64_t PyDec_GetDigits(const PyObject *dec)
-
-   Return the number of digits in the coefficient.  For ``Infinity``, the
-   number of digits is always zero.  Typically, the same applies to ``NaN``
-   and ``sNaN``, but both of these can have a payload that is equivalent to
-   a coefficient.  Therefore, ``NaNs`` can have a nonzero return value.
-
-   Set TypeError and return -1 if ``dec`` is not a Decimal.  It is guaranteed that
-   this is the only failure mode, so if ``dec`` has already been type-checked, no
-   errors can occur and the function can be treated as a simple accessor.
-
-
-Exact conversions between decimals and primitive C types
---------------------------------------------------------
-
-This API supports conversions for decimals with a coefficient up to 38 digits.
-
-Data structures
-~~~~~~~~~~~~~~~
-
-The conversion functions use the following status codes and data structures:
-
-.. code-block:: c
-
-   /* status cases for getting a triple */
-   enum mpd_triple_class {
-     MPD_TRIPLE_NORMAL,
-     MPD_TRIPLE_INF,
-     MPD_TRIPLE_QNAN,
-     MPD_TRIPLE_SNAN,
-     MPD_TRIPLE_ERROR,
-   };
-
-   typedef struct {
-     enum mpd_triple_class tag;
-     uint8_t sign;
-     uint64_t hi;
-     uint64_t lo;
-     int64_t exp;
-   } mpd_uint128_triple_t;
-
-The status cases are explained below.  ``sign`` is 0 for positive and 1 for negative.
-``((uint128_t)hi << 64) + lo`` is the coefficient, ``exp`` is the exponent.
-
-The data structure is called "triple" because the decimal triple (sign, coeff, exp)
-is an established term and (``hi``, ``lo``) represents a single ``uint128_t`` coefficient.
-
-
-Functions
-~~~~~~~~~
-
-.. c:function:: mpd_uint128_triple_t PyDec_AsUint128Triple(const PyObject *dec)
-
-   Convert a decimal to a triple.  As above, it is guaranteed that the only
-   Python failure mode is a TypeError, checks can be omitted if the type is
-   known.
-
-   For simplicity, the usage of the function and all special cases are
-   explained in code form and comments:
-
-.. code-block:: c
-
-    triple = PyDec_AsUint128Triple(dec);
-    switch (triple.tag) {
-    case MPD_TRIPLE_QNAN:
-        /*
-         * Success: handle a quiet NaN.
-         *   1) triple.sign is 0 or 1.
-         *   2) triple.exp is always 0.
-         *   3) If triple.hi or triple.lo are nonzero, the NaN has a payload.
-         */
-        break;
-
-    case MPD_TRIPLE_SNAN:
-        /*
-         * Success: handle a signaling NaN.
-         *   1) triple.sign is 0 or 1.
-         *   2) triple.exp is always 0.
-         *   3) If triple.hi or triple.lo are nonzero, the sNaN has a payload.
-         */
-        break;
-
-    case MPD_TRIPLE_INF:
-        /*
-         * Success: handle Infinity.
-         *   1) triple.sign is 0 or 1.
-         *   2) triple.exp is always 0.
-         *   3) triple.hi and triple.lo are always zero.
-         */
-        break;
-
-    case MPD_TRIPLE_NORMAL:
-        /* Success: handle a finite value. */
-        break;
-
-    case MPD_TRIPLE_ERROR:
-        /* TypeError check: can be omitted if the type of dec is known. */
-        if (PyErr_Occurred()) {
-            return NULL;
-        }
-
-        /* Too large for conversion.  PyDec_AsUint128Triple() does not set an
-           exception so applications can choose themselves.  Typically this
-           would be a ValueError. */
-        PyErr_SetString(PyExc_ValueError,
-            "value out of bounds for a uint128 triple");
-        return NULL;
-    }
-
-.. c:function:: PyObject *PyDec_FromUint128Triple(const mpd_uint128_triple_t *triple)
-
-   Create a decimal from a triple.  The following rules must be observed for
-   initializing the triple:
-
-   1) ``triple.sign`` must always be 0 (for positive) or 1 (for negative).
-
-   2) ``MPD_TRIPLE_QNAN``: ``triple.exp`` must be 0.  If ``triple.hi`` or ``triple.lo``
-      are nonzero,  create a ``NaN`` with a payload.
-
-   3) ``MPD_TRIPLE_SNAN``: ``triple.exp`` must be 0. If ``triple.hi`` or ``triple.lo``
-      are nonzero,  create an ``sNaN`` with a payload.
-
-   4) ``MPD_TRIPLE_INF``: ``triple.exp``, ``triple.hi`` and ``triple.lo`` must be zero.
-
-   5) ``MPD_TRIPLE_NORMAL``: ``MPD_MIN_ETINY + 38 < triple.exp < MPD_MAX_EMAX - 38``.
-      ``triple.hi`` and ``triple.lo`` can be chosen freely.
-
-   6) ``MPD_TRIPLE_ERROR``: It is always an error to set this tag.
-
-
-   If one of the above conditions is not met, the function returns ``NaN`` if
-   the ``InvalidOperation`` trap is not set in the thread local context.  Otherwise,
-   it sets the ``InvalidOperation`` exception and returns NULL.
-
-   Additionally, though extremely unlikely give the small allocation sizes,
-   the function can set ``MemoryError`` and return ``NULL``.
-
-
-Advanced API
-------------
-
-This API enables the use of ``libmpdec`` functions.  Since Python is compiled with
-hidden symbols, the API requires an external libmpdec and the ``mpdecimal.h``
-header.
-
-
-Functions
-~~~~~~~~~
-
-.. c:function:: PyObject *PyDec_Alloc(void)
-
-   Return a new decimal that can be used in the ``result`` position of ``libmpdec``
-   functions.
-
-.. c:function:: mpd_t *PyDec_Get(PyObject *v)
-
-   Get a pointer to the internal ``mpd_t`` of the decimal.  Decimals are immutable,
-   so this function must only be used on a new Decimal that has been created by
-   PyDec_Alloc().
-
-.. c:function:: const mpd_t *PyDec_GetConst(const PyObject *v)
-
-   Get a pointer to the constant internal ``mpd_t`` of the decimal.
diff --git a/Include/pydecimal.h b/Include/pydecimal.h
deleted file mode 100644
index f82e41f..0000000
--- a/Include/pydecimal.h
+++ /dev/null
@@ -1,182 +0,0 @@
-/*
- * Copyright (c) 2020 Stefan Krah. All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- *
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- *
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- *
- * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
-
-#ifndef CPYTHON_DECIMAL_H_
-#define CPYTHON_DECIMAL_H_
-
-
-#include <Python.h>
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-/****************************************************************************/
-/*                               Libmpdec API                               */
-/****************************************************************************/
-
-#ifndef LIBMPDEC_MPDECIMAL_H_
-struct mpd_t;  /* ABI-stable in the libmpdec-2.x series */
-
-/* status cases for getting a triple */
-enum mpd_triple_class {
-  MPD_TRIPLE_NORMAL,
-  MPD_TRIPLE_INF,
-  MPD_TRIPLE_QNAN,
-  MPD_TRIPLE_SNAN,
-  MPD_TRIPLE_ERROR,
-};
-
-typedef struct {
-  enum mpd_triple_class tag;
-  uint8_t sign;
-  uint64_t hi;
-  uint64_t lo;
-  int64_t exp;
-} mpd_uint128_triple_t;
-#endif
-
-
-/****************************************************************************/
-/*                                Capsule API                               */
-/****************************************************************************/
-
-#define PyDec_CAPSULE_NAME "_decimal._API"
-
-/* Simple API */
-#define PyDec_TypeCheck_INDEX 0
-#define PyDec_TypeCheck_RETURN int
-#define PyDec_TypeCheck_ARGS (const PyObject *)
-
-#define PyDec_IsSpecial_INDEX 1
-#define PyDec_IsSpecial_RETURN int
-#define PyDec_IsSpecial_ARGS (const PyObject *)
-
-#define PyDec_IsNaN_INDEX 2
-#define PyDec_IsNaN_RETURN int
-#define PyDec_IsNaN_ARGS (const PyObject *)
-
-#define PyDec_IsInfinite_INDEX 3
-#define PyDec_IsInfinite_RETURN int
-#define PyDec_IsInfinite_ARGS (const PyObject *)
-
-#define PyDec_GetDigits_INDEX 4
-#define PyDec_GetDigits_RETURN int64_t
-#define PyDec_GetDigits_ARGS (const PyObject *)
-
-#define PyDec_AsUint128Triple_INDEX 5
-#define PyDec_AsUint128Triple_RETURN mpd_uint128_triple_t
-#define PyDec_AsUint128Triple_ARGS (const PyObject *)
-
-#define PyDec_FromUint128Triple_INDEX 6
-#define PyDec_FromUint128Triple_RETURN PyObject *
-#define PyDec_FromUint128Triple_ARGS (const mpd_uint128_triple_t *triple)
-
-/* Advanced API */
-#define PyDec_Alloc_INDEX 7
-#define PyDec_Alloc_RETURN PyObject *
-#define PyDec_Alloc_ARGS (void)
-
-#define PyDec_Get_INDEX 8
-#define PyDec_Get_RETURN mpd_t *
-#define PyDec_Get_ARGS (PyObject *)
-
-#define PyDec_GetConst_INDEX 9
-#define PyDec_GetConst_RETURN const mpd_t *
-#define PyDec_GetConst_ARGS (const PyObject *)
-
-#define CPYTHON_DECIMAL_MAX_API 10
-
-
-#ifdef CPYTHON_DECIMAL_MODULE
-/* Simple API */
-static PyDec_TypeCheck_RETURN PyDec_TypeCheck PyDec_TypeCheck_ARGS;
-static PyDec_IsSpecial_RETURN PyDec_IsSpecial PyDec_IsSpecial_ARGS;
-static PyDec_IsNaN_RETURN PyDec_IsNaN PyDec_IsNaN_ARGS;
-static PyDec_IsInfinite_RETURN PyDec_IsInfinite PyDec_IsInfinite_ARGS;
-static PyDec_GetDigits_RETURN PyDec_GetDigits PyDec_GetDigits_ARGS;
-static PyDec_AsUint128Triple_RETURN PyDec_AsUint128Triple PyDec_AsUint128Triple_ARGS;
-static PyDec_FromUint128Triple_RETURN PyDec_FromUint128Triple PyDec_FromUint128Triple_ARGS;
-
-/* Advanced API */
-static PyDec_Alloc_RETURN PyDec_Alloc PyDec_Alloc_ARGS;
-static PyDec_Get_RETURN PyDec_Get PyDec_Get_ARGS;
-static PyDec_GetConst_RETURN PyDec_GetConst PyDec_GetConst_ARGS;
-#else
-static void **_decimal_api;
-
-/* Simple API */
-#define PyDec_TypeCheck \
-    (*(PyDec_TypeCheck_RETURN (*)PyDec_TypeCheck_ARGS) _decimal_api[PyDec_TypeCheck_INDEX])
-
-#define PyDec_IsSpecial \
-    (*(PyDec_IsSpecial_RETURN (*)PyDec_IsSpecial_ARGS) _decimal_api[PyDec_IsSpecial_INDEX])
-
-#define PyDec_IsNaN \
-    (*(PyDec_IsNaN_RETURN (*)PyDec_IsNaN_ARGS) _decimal_api[PyDec_IsNaN_INDEX])
-
-#define PyDec_IsInfinite \
-    (*(PyDec_IsInfinite_RETURN (*)PyDec_IsInfinite_ARGS) _decimal_api[PyDec_IsInfinite_INDEX])
-
-#define PyDec_GetDigits \
-    (*(PyDec_GetDigits_RETURN (*)PyDec_GetDigits_ARGS) _decimal_api[PyDec_GetDigits_INDEX])
-
-#define PyDec_AsUint128Triple \
-    (*(PyDec_AsUint128Triple_RETURN (*)PyDec_AsUint128Triple_ARGS) _decimal_api[PyDec_AsUint128Triple_INDEX])
-
-#define PyDec_FromUint128Triple \
-    (*(PyDec_FromUint128Triple_RETURN (*)PyDec_FromUint128Triple_ARGS) _decimal_api[PyDec_FromUint128Triple_INDEX])
-
-/* Advanced API */
-#define PyDec_Alloc \
-    (*(PyDec_Alloc_RETURN (*)PyDec_Alloc_ARGS) _decimal_api[PyDec_Alloc_INDEX])
-
-#define PyDec_Get \
-    (*(PyDec_Get_RETURN (*)PyDec_Get_ARGS) _decimal_api[PyDec_Get_INDEX])
-
-#define PyDec_GetConst \
-    (*(PyDec_GetConst_RETURN (*)PyDec_GetConst_ARGS) _decimal_api[PyDec_GetConst_INDEX])
-
-
-static int
-import_decimal(void)
-{
-    _decimal_api = (void **)PyCapsule_Import(PyDec_CAPSULE_NAME, 0);
-    if (_decimal_api == NULL) {
-        return -1;
-    }
-
-    return 0;
-}
-#endif
-
-#ifdef __cplusplus
-}
-#endif
-
-#endif  /* CPYTHON_DECIMAL_H_ */
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index dbd58e8..6aa4a9b 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -43,13 +43,6 @@ import random
 import inspect
 import threading
 
-from _testcapi import decimal_is_special
-from _testcapi import decimal_is_nan
-from _testcapi import decimal_is_infinite
-from _testcapi import decimal_get_digits
-from _testcapi import decimal_as_triple
-from _testcapi import decimal_from_triple
-
 
 C = import_fresh_module('decimal', fresh=['_decimal'])
 P = import_fresh_module('decimal', blocked=['_decimal'])
@@ -4758,175 +4751,6 @@ class CFunctionality(unittest.TestCase):
         self.assertEqual(C.DecTraps,
                          C.DecErrors|C.DecOverflow|C.DecUnderflow)
 
-    def test_decimal_api_predicates(self):
-        # Capsule API
-
-        d = C.Decimal("0")
-        self.assertFalse(decimal_is_special(d))
-        self.assertFalse(decimal_is_nan(d))
-        self.assertFalse(decimal_is_infinite(d))
-
-        d = C.Decimal("NaN")
-        self.assertTrue(decimal_is_special(d))
-        self.assertTrue(decimal_is_nan(d))
-        self.assertFalse(decimal_is_infinite(d))
-
-        d = C.Decimal("sNaN")
-        self.assertTrue(decimal_is_special(d))
-        self.assertTrue(decimal_is_nan(d))
-        self.assertFalse(decimal_is_infinite(d))
-
-        d = C.Decimal("inf")
-        self.assertTrue(decimal_is_special(d))
-        self.assertFalse(decimal_is_nan(d))
-        self.assertTrue(decimal_is_infinite(d))
-
-    def test_decimal_api_get_digits(self):
-        # Capsule API
-
-        d = C.Decimal("0")
-        self.assertEqual(decimal_get_digits(d), 1)
-
-        d = C.Decimal("1234567890")
-        self.assertEqual(decimal_get_digits(d), 10)
-
-        d = C.Decimal("inf")
-        self.assertEqual(decimal_get_digits(d), 0)
-
-        d = C.Decimal("NaN")
-        self.assertEqual(decimal_get_digits(d), 0)
-
-        d = C.Decimal("sNaN")
-        self.assertEqual(decimal_get_digits(d), 0)
-
-        d = C.Decimal("NaN1234567890")
-        self.assertEqual(decimal_get_digits(d), 10)
-
-        d = C.Decimal("sNaN1234567890")
-        self.assertEqual(decimal_get_digits(d), 10)
-
-    def test_decimal_api_triple(self):
-        # Capsule API
-
-        def as_triple(d):
-            """Convert a decimal to a decimal triple with a split uint128_t
-               coefficient:
-
-                   (sign, hi, lo, exp)
-
-               It is called 'triple' because (hi, lo) are regarded as a single
-               uint128_t that is split because not all compilers support uint128_t.
-            """
-            sign, digits, exp = d.as_tuple()
-
-            s = "".join(str(d) for d in digits)
-            coeff = int(s) if s else 0
-
-            if coeff < 0 or coeff >= 2**128:
-                raise ValueError("value out of bounds for a uint128 triple");
-
-            hi, lo = divmod(coeff, 2**64)
-            return (sign, hi, lo, exp)
-
-        def from_triple(triple):
-            """Convert a decimal triple with a split uint128_t coefficient to a string.
-            """
-            sign, hi, lo, exp = triple
-            coeff = hi * 2**64 + lo
-
-            if coeff < 0 or coeff >= 2**128:
-                raise ValueError("value out of bounds for a uint128 triple");
-
-            digits = tuple(int(c) for c in str(coeff))
-
-            return P.Decimal((sign, digits, exp))
-
-        signs = ["", "-"]
-
-        coefficients = [
-            "000000000000000000000000000000000000000",
-
-            "299999999999999999999999999999999999999",
-            "299999999999999999990000000000000000000",
-            "200000000000000000009999999999999999999",
-            "000000000000000000009999999999999999999",
-
-            "299999999999999999999999999999000000000",
-            "299999999999999999999000000000999999999",
-            "299999999999000000000999999999999999999",
-            "299000000000999999999999999999999999999",
-            "000999999999999999999999999999999999999",
-
-            "300000000000000000000000000000000000000",
-            "310000000000000000001000000000000000000",
-            "310000000000000000000000000000000000000",
-            "300000000000000000001000000000000000000",
-
-            "340100000000100000000100000000100000000",
-            "340100000000100000000100000000000000000",
-            "340100000000100000000000000000100000000",
-            "340100000000000000000100000000100000000",
-            "340000000000100000000100000000100000000",
-
-            "340282366920938463463374607431768211455",
-        ]
-
-        exponents = [
-            "E+0", "E+1", "E-1",
-            "E+%s" % str(C.MAX_EMAX-38),
-            "E-%s" % str(C.MIN_ETINY+38),
-        ]
-
-        for sign in signs:
-            for coeff in coefficients:
-                for exp in exponents:
-                    s = sign + coeff + exp
-
-                    ctriple = decimal_as_triple(C.Decimal(s))
-                    ptriple = as_triple(P.Decimal(s))
-                    self.assertEqual(ctriple, ptriple)
-
-                    c = decimal_from_triple(ctriple)
-                    p = decimal_from_triple(ptriple)
-                    self.assertEqual(str(c), str(p))
-
-        for s in ["NaN", "-NaN", "sNaN", "-sNaN", "NaN123", "sNaN123", "inf", "-inf"]:
-            ctriple = decimal_as_triple(C.Decimal(s))
-            ptriple = as_triple(P.Decimal(s))
-            self.assertEqual(ctriple, ptriple)
-
-            c = decimal_from_triple(ctriple)
-            p = decimal_from_triple(ptriple)
-            self.assertEqual(str(c), str(p))
-
-    def test_decimal_api_errors(self):
-        # Capsule API
-
-        self.assertRaises(TypeError, decimal_as_triple, "X")
-        self.assertRaises(ValueError, decimal_as_triple, C.Decimal(2**128))
-        self.assertRaises(ValueError, decimal_as_triple, C.Decimal(-2**128))
-
-        self.assertRaises(TypeError, decimal_from_triple, "X")
-        self.assertRaises(ValueError, decimal_from_triple, ())
-        self.assertRaises(ValueError, decimal_from_triple, (1, 2, 3, 4, 5))
-        self.assertRaises(ValueError, decimal_from_triple, (2**8, 0, 0, 0))
-        self.assertRaises(OverflowError, decimal_from_triple, (0, 2**64, 0, 0))
-        self.assertRaises(OverflowError, decimal_from_triple, (0, 0, 2**64, 0))
-        self.assertRaises(OverflowError, decimal_from_triple, (0, 0, 0, 2**63))
-        self.assertRaises(OverflowError, decimal_from_triple, (0, 0, 0, -2**63-1))
-        self.assertRaises(ValueError, decimal_from_triple, (0, 0, 0, "X"))
-        self.assertRaises(TypeError, decimal_from_triple, (0, 0, 0, ()))
-
-        with C.localcontext(C.Context()):
-            self.assertRaises(C.InvalidOperation, decimal_from_triple, (2, 0, 0, 0))
-            self.assertRaises(C.InvalidOperation, decimal_from_triple, (0, 0, 0, 2**63-1))
-            self.assertRaises(C.InvalidOperation, decimal_from_triple, (0, 0, 0, -2**63))
-
-        self.assertRaises(TypeError, decimal_is_special, "X")
-        self.assertRaises(TypeError, decimal_is_nan, "X")
-        self.assertRaises(TypeError, decimal_is_infinite, "X")
-        self.assertRaises(TypeError, decimal_get_digits, "X")
-
 class CWhitebox(unittest.TestCase):
     """Whitebox testing for _decimal"""
 
diff --git a/Misc/NEWS.d/next/Library/2021-03-21-17-02-52.bpo-43422.POk6cU.rst b/Misc/NEWS.d/next/Library/2021-03-21-17-02-52.bpo-43422.POk6cU.rst
new file mode 100644
index 0000000..4de3060
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-03-21-17-02-52.bpo-43422.POk6cU.rst
@@ -0,0 +1 @@
+Revert the _decimal C API which was added in bpo-41324.
diff --git a/Modules/_decimal/_decimal.c b/Modules/_decimal/_decimal.c
index 664d45a..83e237d 100644
--- a/Modules/_decimal/_decimal.c
+++ b/Modules/_decimal/_decimal.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008-2020 Stefan Krah. All rights reserved.
+ * Copyright (c) 2008-2012 Stefan Krah. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -33,8 +33,6 @@
 
 #include <stdlib.h>
 
-#define CPYTHON_DECIMAL_MODULE
-#include "pydecimal.h"
 #include "docstrings.h"
 
 
@@ -5570,175 +5568,6 @@ static PyTypeObject PyDecContext_Type =
 };
 
 
-/****************************************************************************/
-/*                                   C-API                                  */
-/****************************************************************************/
-
-/* Simple API */
-static int
-PyDec_TypeCheck(const PyObject *v)
-{
-    return PyDec_Check(v);
-}
-
-static int
-PyDec_IsSpecial(const PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_IsSpecial: argument must be a Decimal");
-        return -1;
-    }
-
-    return mpd_isspecial(MPD(v));
-}
-
-static int
-PyDec_IsNaN(const PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_IsNaN: argument must be a Decimal");
-        return -1;
-    }
-
-    return mpd_isnan(MPD(v));
-}
-
-static int
-PyDec_IsInfinite(const PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_IsInfinite: argument must be a Decimal");
-        return -1;
-    }
-
-    return mpd_isinfinite(MPD(v));
-}
-
-static int64_t
-PyDec_GetDigits(const PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_GetDigits: argument must be a Decimal");
-        return -1;
-    }
-
-    return MPD(v)->digits;
-}
-
-static mpd_uint128_triple_t
-PyDec_AsUint128Triple(const PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        mpd_uint128_triple_t triple = { MPD_TRIPLE_ERROR, 0, 0, 0, 0 };
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_AsUint128Triple: argument must be a Decimal");
-        return triple;
-    }
-
-    return mpd_as_uint128_triple(MPD(v));
-}
-
-static PyObject *
-PyDec_FromUint128Triple(const mpd_uint128_triple_t *triple)
-{
-    PyObject *context;
-    PyObject *result;
-    uint32_t status = 0;
-
-    CURRENT_CONTEXT(context);
-
-    result = dec_alloc();
-    if (result == NULL) {
-        return NULL;
-    }
-
-    if (mpd_from_uint128_triple(MPD(result), triple, &status) < 0) {
-        if (dec_addstatus(context, status)) {
-            Py_DECREF(result);
-            return NULL;
-        }
-    }
-
-    return result;
-}
-
-/* Advanced API */
-static PyObject *
-PyDec_Alloc(void)
-{
-    return dec_alloc();
-}
-
-static mpd_t *
-PyDec_Get(PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_Get: argument must be a Decimal");
-        return NULL;
-    }
-
-    return MPD(v);
-}
-
-static const mpd_t *
-PyDec_GetConst(const PyObject *v)
-{
-    if (!PyDec_Check(v)) {
-        PyErr_SetString(PyExc_TypeError,
-            "PyDec_GetConst: argument must be a Decimal");
-        return NULL;
-    }
-
-    return MPD(v);
-}
-
-static void
-destroy_api(PyObject *capsule)
-{
-    void *capi = PyCapsule_GetPointer(capsule, PyDec_CAPSULE_NAME);
-    PyMem_Free(capi);
-}
-
-static PyObject *
-init_api(void)
-{
-    void **_decimal_api = PyMem_Calloc(CPYTHON_DECIMAL_MAX_API, sizeof(void *));
-    if (_decimal_api == NULL) {
-        PyErr_NoMemory();
-        return NULL;
-    }
-
-    /* Simple API */
-    _decimal_api[PyDec_TypeCheck_INDEX] = (void *)PyDec_TypeCheck;
-    _decimal_api[PyDec_IsSpecial_INDEX] = (void *)PyDec_IsSpecial;
-    _decimal_api[PyDec_IsNaN_INDEX] = (void *)PyDec_IsNaN;
-    _decimal_api[PyDec_IsInfinite_INDEX] = (void *)PyDec_IsInfinite;
-    _decimal_api[PyDec_GetDigits_INDEX] = (void *)PyDec_GetDigits;
-    _decimal_api[PyDec_AsUint128Triple_INDEX] = (void *)PyDec_AsUint128Triple;
-    _decimal_api[PyDec_FromUint128Triple_INDEX] = (void *)PyDec_FromUint128Triple;
-
-    /* Advanced API */
-    _decimal_api[PyDec_Alloc_INDEX] = (void *)PyDec_Alloc;
-    _decimal_api[PyDec_Get_INDEX] = (void *)PyDec_Get;
-    _decimal_api[PyDec_GetConst_INDEX] = (void *)PyDec_GetConst;
-
-    PyObject *capsule = PyCapsule_New(_decimal_api, PyDec_CAPSULE_NAME, destroy_api);
-    if (!capsule) {
-        PyMem_Free(_decimal_api);
-    }
-    return capsule;
-}
-
-
-/****************************************************************************/
-/*                                  Module                                  */
-/****************************************************************************/
-
 static PyMethodDef _decimal_methods [] =
 {
   { "getcontext", (PyCFunction)PyDec_GetCurrentContext, METH_NOARGS, doc_getcontext},
@@ -5849,27 +5678,17 @@ PyInit__decimal(void)
     DecCondMap *cm;
     struct ssize_constmap *ssize_cm;
     struct int_constmap *int_cm;
-    static PyObject *capsule = NULL;
-    static int initialized = 0;
     int i;
 
 
     /* Init libmpdec */
-    if (!initialized) {
-        mpd_traphandler = dec_traphandler;
-        mpd_mallocfunc = PyMem_Malloc;
-        mpd_reallocfunc = PyMem_Realloc;
-        mpd_callocfunc = mpd_callocfunc_em;
-        mpd_free = PyMem_Free;
-        mpd_setminalloc(_Py_DEC_MINALLOC);
-
-        capsule = init_api();
-        if (capsule == NULL) {
-            return NULL;
-        }
+    mpd_traphandler = dec_traphandler;
+    mpd_mallocfunc = PyMem_Malloc;
+    mpd_reallocfunc = PyMem_Realloc;
+    mpd_callocfunc = mpd_callocfunc_em;
+    mpd_free = PyMem_Free;
+    mpd_setminalloc(_Py_DEC_MINALLOC);
 
-        initialized = 1;
-    }
 
     /* Init external C-API functions */
     _py_long_multiply = PyLong_Type.tp_as_number->nb_multiply;
@@ -6094,10 +5913,6 @@ PyInit__decimal(void)
     CHECK_INT(PyModule_AddStringConstant(m, "__version__", "1.70"));
     CHECK_INT(PyModule_AddStringConstant(m, "__libmpdec_version__", mpd_version()));
 
-    /* Add capsule API */
-    if (PyModule_AddObjectRef(m, "_API", capsule) < 0) {
-        goto error;
-    }
 
     return m;
 
@@ -6121,7 +5936,6 @@ error:
     Py_CLEAR(basic_context_template); /* GCOV_NOT_REACHED */
     Py_CLEAR(extended_context_template); /* GCOV_NOT_REACHED */
     Py_CLEAR(m); /* GCOV_NOT_REACHED */
-    Py_CLEAR(capsule); /* GCOV_NOT_REACHED */
 
     return NULL; /* GCOV_NOT_REACHED */
 }
diff --git a/Modules/_decimal/tests/bench.py b/Modules/_decimal/tests/bench.py
index 88fd7b5..3726db1 100644
--- a/Modules/_decimal/tests/bench.py
+++ b/Modules/_decimal/tests/bench.py
@@ -7,10 +7,13 @@
 
 
 import time
+try:
+    from test.support import import_fresh_module
+except ImportError:
+    from test.test_support import import_fresh_module
 
-import _decimal as C
-import _pydecimal as P
-
+C = import_fresh_module('decimal', fresh=['_decimal'])
+P = import_fresh_module('decimal', blocked=['_decimal'])
 
 #
 # NOTE: This is the pi function from the decimal documentation, modified
diff --git a/Modules/_decimal/tests/deccheck.py b/Modules/_decimal/tests/deccheck.py
index 5de57d1..98ecd50 100644
--- a/Modules/_decimal/tests/deccheck.py
+++ b/Modules/_decimal/tests/deccheck.py
@@ -47,18 +47,14 @@ from subprocess import PIPE, STDOUT
 from queue import Queue, Empty
 from threading import Thread, Event, Lock
 
+from test.support import import_fresh_module
 from randdec import randfloat, all_unary, all_binary, all_ternary
 from randdec import unary_optarg, binary_optarg, ternary_optarg
 from formathelper import rand_format, rand_locale
 from _pydecimal import _dec_from_triple
 
-from _testcapi import decimal_as_triple
-from _testcapi import decimal_from_triple
-
-import _decimal as C
-import _pydecimal as P
-
-
+C = import_fresh_module('decimal', fresh=['_decimal'])
+P = import_fresh_module('decimal', blocked=['_decimal'])
 EXIT_STATUS = 0
 
 
@@ -162,45 +158,6 @@ TernaryRestricted = ['__pow__', 'context.power']
 
 
 # ======================================================================
-#                            Triple tests
-# ======================================================================
-
-def c_as_triple(dec):
-    sign, hi, lo, exp = decimal_as_triple(dec)
-
-    coeff = hi * 2**64 + lo
-    return (sign, coeff, exp)
-
-def c_from_triple(triple):
-    sign, coeff, exp = triple
-
-    hi = coeff // 2**64
-    lo = coeff % 2**64
-    return decimal_from_triple((sign, hi, lo, exp))
-
-def p_as_triple(dec):
-    sign, digits, exp = dec.as_tuple()
-
-    s = "".join(str(d) for d in digits)
-    coeff = int(s) if s else 0
-
-    if coeff < 0 or coeff >= 2**128:
-        raise ValueError("value out of bounds for a uint128 triple")
-
-    return (sign, coeff, exp)
-
-def p_from_triple(triple):
-    sign, coeff, exp = triple
-
-    if coeff < 0 or coeff >= 2**128:
-        raise ValueError("value out of bounds for a uint128 triple")
-
-    digits = tuple(int(c) for c in str(coeff))
-
-    return P.Decimal((sign, digits, exp))
-
-
-# ======================================================================
 #                            Unified Context
 # ======================================================================
 
@@ -893,44 +850,12 @@ def verify(t, stat):
         t.presults.append(str(t.rp.imag))
         t.presults.append(str(t.rp.real))
 
-        ctriple = None
-        if str(t.rc) == str(t.rp): # see skip handler
-            try:
-                ctriple = c_as_triple(t.rc)
-            except ValueError:
-                try:
-                    ptriple = p_as_triple(t.rp)
-                except ValueError:
-                    pass
-                else:
-                    raise RuntimeError("ValueError not raised")
-            else:
-                cres = c_from_triple(ctriple)
-                t.cresults.append(ctriple)
-                t.cresults.append(str(cres))
-
-                ptriple = p_as_triple(t.rp)
-                pres = p_from_triple(ptriple)
-                t.presults.append(ptriple)
-                t.presults.append(str(pres))
-
         if t.with_maxcontext and isinstance(t.rmax, C.Decimal):
             t.maxresults.append(t.rmax.to_eng_string())
             t.maxresults.append(t.rmax.as_tuple())
             t.maxresults.append(str(t.rmax.imag))
             t.maxresults.append(str(t.rmax.real))
 
-            if ctriple is not None:
-                # NaN payloads etc. depend on precision and clamp.
-                if all_nan(t.rc) and all_nan(t.rmax):
-                    t.maxresults.append(ctriple)
-                    t.maxresults.append(str(cres))
-                else:
-                    maxtriple = c_as_triple(t.rmax)
-                    maxres = c_from_triple(maxtriple)
-                    t.maxresults.append(maxtriple)
-                    t.maxresults.append(str(maxres))
-
         nc = t.rc.number_class().lstrip('+-s')
         stat[nc] += 1
     else:
diff --git a/Modules/_decimal/tests/formathelper.py b/Modules/_decimal/tests/formathelper.py
index 482e02a..19b2aad 100644
--- a/Modules/_decimal/tests/formathelper.py
+++ b/Modules/_decimal/tests/formathelper.py
@@ -31,10 +31,11 @@
 
 import os, sys, locale, random
 import platform, subprocess
+from test.support import import_fresh_module
 from distutils.spawn import find_executable
 
-import _decimal as C
-import _pydecimal as P
+C = import_fresh_module('decimal', fresh=['_decimal'])
+P = import_fresh_module('decimal', blocked=['_decimal'])
 
 
 windows_lang_strings = [
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index ed59c32..20107f2 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -19,7 +19,6 @@
 
 #include "Python.h"
 #include "datetime.h"
-#include "pydecimal.h"
 #include "marshal.h"
 #include "structmember.h"         // PyMemberDef
 #include <float.h>
@@ -2764,252 +2763,6 @@ test_PyDateTime_DELTA_GET(PyObject *self, PyObject *obj)
     return Py_BuildValue("(lll)", days, seconds, microseconds);
 }
 
-/* Test decimal API */
-static int decimal_initialized = 0;
-static PyObject *
-decimal_is_special(PyObject *module, PyObject *dec)
-{
-    int is_special;
-
-    (void)module;
-    if (!decimal_initialized) {
-       if (import_decimal() < 0) {
-            return NULL;
-       }
-
-       decimal_initialized = 1;
-    }
-
-    is_special = PyDec_IsSpecial(dec);
-    if (is_special < 0) {
-        return NULL;
-    }
-
-    return PyBool_FromLong(is_special);
-}
-
-static PyObject *
-decimal_is_nan(PyObject *module, PyObject *dec)
-{
-    int is_nan;
-
-    (void)module;
-    if (!decimal_initialized) {
-       if (import_decimal() < 0) {
-            return NULL;
-       }
-
-       decimal_initialized = 1;
-    }
-
-    is_nan = PyDec_IsNaN(dec);
-    if (is_nan < 0) {
-        return NULL;
-    }
-
-    return PyBool_FromLong(is_nan);
-}
-
-static PyObject *
-decimal_is_infinite(PyObject *module, PyObject *dec)
-{
-    int is_infinite;
-
-    (void)module;
-    if (!decimal_initialized) {
-       if (import_decimal() < 0) {
-            return NULL;
-       }
-
-       decimal_initialized = 1;
-    }
-
-    is_infinite = PyDec_IsInfinite(dec);
-    if (is_infinite < 0) {
-        return NULL;
-    }
-
-    return PyBool_FromLong(is_infinite);
-}
-
-static PyObject *
-decimal_get_digits(PyObject *module, PyObject *dec)
-{
-    int64_t digits;
-
-    (void)module;
-    if (!decimal_initialized) {
-       if (import_decimal() < 0) {
-            return NULL;
-       }
-
-       decimal_initialized = 1;
-    }
-
-    digits = PyDec_GetDigits(dec);
-    if (digits < 0) {
-        return NULL;
-    }
-
-    return PyLong_FromLongLong(digits);
-}
-
-static PyObject *
-decimal_as_triple(PyObject *module, PyObject *dec)
-{
-    PyObject *tuple = NULL;
-    PyObject *sign, *hi, *lo;
-    mpd_uint128_triple_t triple;
-
-    (void)module;
-    if (!decimal_initialized) {
-       if (import_decimal() < 0) {
-            return NULL;
-       }
-
-       decimal_initialized = 1;
-    }
-
-    triple = PyDec_AsUint128Triple(dec);
-    if (triple.tag == MPD_TRIPLE_ERROR && PyErr_Occurred()) {
-        return NULL;
-    }
-
-    sign = PyLong_FromUnsignedLong(triple.sign);
-    if (sign == NULL) {
-        return NULL;
-    }
-
-    hi = PyLong_FromUnsignedLongLong(triple.hi);
-    if (hi == NULL) {
-        Py_DECREF(sign);
-        return NULL;
-    }
-
-    lo = PyLong_FromUnsignedLongLong(triple.lo);
-    if (lo == NULL) {
-        Py_DECREF(hi);
-        Py_DECREF(sign);
-        return NULL;
-    }
-
-    switch (triple.tag) {
-    case MPD_TRIPLE_QNAN:
-        assert(triple.exp == 0);
-        tuple = Py_BuildValue("(OOOs)", sign, hi, lo, "n");
-        break;
-
-    case MPD_TRIPLE_SNAN:
-        assert(triple.exp == 0);
-        tuple = Py_BuildValue("(OOOs)", sign, hi, lo, "N");
-        break;
-
-    case MPD_TRIPLE_INF:
-        assert(triple.hi == 0);
-        assert(triple.lo == 0);
-        assert(triple.exp == 0);
-        tuple = Py_BuildValue("(OOOs)", sign, hi, lo, "F");
-        break;
-
-    case MPD_TRIPLE_NORMAL:
-        tuple = Py_BuildValue("(OOOL)", sign, hi, lo, triple.exp);
-        break;
-
-    case MPD_TRIPLE_ERROR:
-        PyErr_SetString(PyExc_ValueError,
-            "value out of bounds for a uint128 triple");
-        break;
-
-    default:
-        PyErr_SetString(PyExc_RuntimeError,
-            "decimal_as_triple: internal error: unexpected tag");
-        break;
-    }
-
-    Py_DECREF(lo);
-    Py_DECREF(hi);
-    Py_DECREF(sign);
-
-    return tuple;
-}
-
-static PyObject *
-decimal_from_triple(PyObject *module, PyObject *tuple)
-{
-    mpd_uint128_triple_t triple = { MPD_TRIPLE_ERROR, 0, 0, 0, 0 };
-    PyObject *exp;
-    unsigned long sign;
-
-    (void)module;
-    if (!decimal_initialized) {
-       if (import_decimal() < 0) {
-            return NULL;
-       }
-
-       decimal_initialized = 1;
-    }
-
-    if (!PyTuple_Check(tuple)) {
-        PyErr_SetString(PyExc_TypeError, "argument must be a tuple");
-        return NULL;
-    }
-
-    if (PyTuple_GET_SIZE(tuple) != 4) {
-        PyErr_SetString(PyExc_ValueError, "tuple size must be 4");
-        return NULL;
-    }
-
-    sign = PyLong_AsUnsignedLong(PyTuple_GET_ITEM(tuple, 0));
-    if (sign == (unsigned long)-1 && PyErr_Occurred()) {
-        return NULL;
-    }
-    if (sign > UINT8_MAX) {
-        PyErr_SetString(PyExc_ValueError, "sign must be 0 or 1");
-        return NULL;
-    }
-    triple.sign = (uint8_t)sign;
-
-    triple.hi = PyLong_AsUnsignedLongLong(PyTuple_GET_ITEM(tuple, 1));
-    if (triple.hi == (unsigned long long)-1 && PyErr_Occurred()) {
-        return NULL;
-    }
-
-    triple.lo = PyLong_AsUnsignedLongLong(PyTuple_GET_ITEM(tuple, 2));
-    if (triple.lo == (unsigned long long)-1 && PyErr_Occurred()) {
-        return NULL;
-    }
-
-    exp = PyTuple_GET_ITEM(tuple, 3);
-    if (PyLong_Check(exp)) {
-        triple.tag = MPD_TRIPLE_NORMAL;
-        triple.exp = PyLong_AsLongLong(exp);
-        if (triple.exp == -1 && PyErr_Occurred()) {
-            return NULL;
-        }
-    }
-    else if (PyUnicode_Check(exp)) {
-        if (PyUnicode_CompareWithASCIIString(exp, "F") == 0) {
-            triple.tag = MPD_TRIPLE_INF;
-        }
-        else if (PyUnicode_CompareWithASCIIString(exp, "n") == 0) {
-            triple.tag = MPD_TRIPLE_QNAN;
-        }
-        else if (PyUnicode_CompareWithASCIIString(exp, "N") == 0) {
-            triple.tag = MPD_TRIPLE_SNAN;
-        }
-        else {
-            PyErr_SetString(PyExc_ValueError, "not a valid exponent");
-            return NULL;
-        }
-    }
-    else {
-        PyErr_SetString(PyExc_TypeError, "exponent must be int or string");
-        return NULL;
-    }
-
-    return PyDec_FromUint128Triple(&triple);
-}
-
 /* test_thread_state spawns a thread of its own, and that thread releases
  * `thread_done` when it's finished.  The driver code has to know when the
  * thread finishes, because the thread uses a PyObject (the callable) that
@@ -5725,12 +5478,6 @@ static PyMethodDef TestMethods[] = {
     {"PyDateTime_DATE_GET",        test_PyDateTime_DATE_GET,      METH_O},
     {"PyDateTime_TIME_GET",        test_PyDateTime_TIME_GET,      METH_O},
     {"PyDateTime_DELTA_GET",       test_PyDateTime_DELTA_GET,     METH_O},
-    {"decimal_is_special",      decimal_is_special,              METH_O},
-    {"decimal_is_nan",          decimal_is_nan,                  METH_O},
-    {"decimal_is_infinite",     decimal_is_infinite,             METH_O},
-    {"decimal_get_digits",      decimal_get_digits,              METH_O},
-    {"decimal_as_triple",       decimal_as_triple,               METH_O},
-    {"decimal_from_triple",     decimal_from_triple,             METH_O},
     {"test_list_api",           test_list_api,                   METH_NOARGS},
     {"test_dict_iteration",     test_dict_iteration,             METH_NOARGS},
     {"dict_getitem_knownhash",  dict_getitem_knownhash,          METH_VARARGS},