bpo-41103: Resurrect the old buffer protocol. (GH-27437)

Revert "bpo-41103: Remove old buffer protocol support (#21117)"

This reverts commit 6f8a6ee59c.

Doc/c-api/objbuffer.rst

@@ -0,0 +1,55 @@
+.. highlight:: c
+
+Old Buffer Protocol
+-------------------
+
+.. deprecated:: 3.0
+
+These functions were part of the "old buffer protocol" API in Python 2.
+In Python 3, this protocol doesn't exist anymore but the functions are still
+exposed to ease porting 2.x code.  They act as a compatibility wrapper
+around the :ref:`new buffer protocol <bufferobjects>`, but they don't give
+you control over the lifetime of the resources acquired when a buffer is
+exported.
+
+Therefore, it is recommended that you call :c:func:`PyObject_GetBuffer`
+(or the ``y*`` or ``w*`` :ref:`format codes <arg-parsing>` with the
+:c:func:`PyArg_ParseTuple` family of functions) to get a buffer view over
+an object, and :c:func:`PyBuffer_Release` when the buffer view can be released.
+
+
+.. c:function:: int PyObject_AsCharBuffer(PyObject *obj, const char **buffer, Py_ssize_t *buffer_len)
+
+   Returns a pointer to a read-only memory location usable as character-based
+   input.  The *obj* argument must support the single-segment character buffer
+   interface.  On success, returns ``0``, sets *buffer* to the memory location
+   and *buffer_len* to the buffer length.  Returns ``-1`` and sets a
+   :exc:`TypeError` on error.
+
+
+.. c:function:: int PyObject_AsReadBuffer(PyObject *obj, const void **buffer, Py_ssize_t *buffer_len)
+
+   Returns a pointer to a read-only memory location containing arbitrary data.
+   The *obj* argument must support the single-segment readable buffer
+   interface.  On success, returns ``0``, sets *buffer* to the memory location
+   and *buffer_len* to the buffer length.  Returns ``-1`` and sets a
+   :exc:`TypeError` on error.
+
+
+.. c:function:: int PyObject_CheckReadBuffer(PyObject *o)
+
+   Returns ``1`` if *o* supports the single-segment readable buffer interface.
+   Otherwise returns ``0``.  This function always succeeds.
+
+   Note that this function tries to get and release a buffer, and exceptions
+   which occur while calling corresponding functions will get suppressed.
+   To get error reporting use :c:func:`PyObject_GetBuffer()` instead.
+
+
+.. c:function:: int PyObject_AsWriteBuffer(PyObject *obj, void **buffer, Py_ssize_t *buffer_len)
+
+   Returns a pointer to a writable memory location.  The *obj* argument must
+   support the single-segment, character buffer interface.  On success,
+   returns ``0``, sets *buffer* to the memory location and *buffer_len* to the
+   buffer length.  Returns ``-1`` and sets a :exc:`TypeError` on error.
+