Issue #29300: Convert _struct module to Argument Clinic

* The struct module now requires contiguous buffers.
* Convert most functions and methods of the _struct module to Argument Clinic
* Use "Py_buffer" type for the "buffer" argument. Argument Clinic is
  responsible to create and release the Py_buffer object.
* Use "PyStructObject *" type for self to avoid explicit conversions.
* Add an unit test on the _struct.Struct.unpack_from() method to test passing
  arguments as keywords.
* Rephrase docstrings.
* Rename "fmt" argument to "format" in docstrings and the documentation.

As a side effect, functions and methods which used METH_VARARGS calling
convention like struct.pack() now use the METH_FASTCALL calling convention
which avoids the creation of temporary tuple to pass positional arguments and
so is faster. For example, struct.pack("i", 1) becomes 1.56x faster (-36%)::

    $ ./python -m perf timeit \
        -s 'import struct; pack=struct.pack' 'pack("i", 1)' \
        --compare-to=../default-ref/python
    Median +- std dev: 119 ns +- 1 ns -> 76.8 ns +- 0.4 ns: 1.56x faster (-36%)
    Significant (t=295.91)

Patch co-written with Serhiy Storchaka.

Modules/clinic/_struct.c.h

@@ -0,0 +1,276 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(Struct___init____doc__,
+"Struct(format)\n"
+"--\n"
+"\n"
+"Create a compiled struct object.\n"
+"\n"
+"Return a new Struct object which writes and reads binary data according to\n"
+"the format string.\n"
+"\n"
+"See help(struct) for more on format strings.");
+
+static int
+Struct___init___impl(PyStructObject *self, PyObject *format);
+
+static int
+Struct___init__(PyObject *self, PyObject *args, PyObject *kwargs)
+{
+    int return_value = -1;
+    static const char * const _keywords[] = {"format", NULL};
+    static _PyArg_Parser _parser = {"O:Struct", _keywords, 0};
+    PyObject *format;
+
+    if (!_PyArg_ParseTupleAndKeywordsFast(args, kwargs, &_parser,
+        &format)) {
+        goto exit;
+    }
+    return_value = Struct___init___impl((PyStructObject *)self, format);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(Struct_unpack__doc__,
+"unpack($self, buffer, /)\n"
+"--\n"
+"\n"
+"Return a tuple containing unpacked values.\n"
+"\n"
+"Unpack according to the format string Struct.format. The buffer\'s size\n"
+"in bytes must be Struct.size.\n"
+"\n"
+"See help(struct) for more on format strings.");
+
+#define STRUCT_UNPACK_METHODDEF    \
+    {"unpack", (PyCFunction)Struct_unpack, METH_O, Struct_unpack__doc__},
+
+static PyObject *
+Struct_unpack_impl(PyStructObject *self, Py_buffer *buffer);
+
+static PyObject *
+Struct_unpack(PyStructObject *self, PyObject *arg)
+{
+    PyObject *return_value = NULL;
+    Py_buffer buffer = {NULL, NULL};
+
+    if (!PyArg_Parse(arg, "y*:unpack", &buffer)) {
+        goto exit;
+    }
+    return_value = Struct_unpack_impl(self, &buffer);
+
+exit:
+    /* Cleanup for buffer */
+    if (buffer.obj) {
+       PyBuffer_Release(&buffer);
+    }
+
+    return return_value;
+}
+
+PyDoc_STRVAR(Struct_unpack_from__doc__,
+"unpack_from($self, /, buffer, offset=0)\n"
+"--\n"
+"\n"
+"Return a tuple containing unpacked values.\n"
+"\n"
+"Values are unpacked according to the format string Struct.format.\n"
+"\n"
+"The buffer\'s size in bytes, minus offset, must be at least Struct.size.\n"
+"\n"
+"See help(struct) for more on format strings.");
+
+#define STRUCT_UNPACK_FROM_METHODDEF    \
+    {"unpack_from", (PyCFunction)Struct_unpack_from, METH_FASTCALL, Struct_unpack_from__doc__},
+
+static PyObject *
+Struct_unpack_from_impl(PyStructObject *self, Py_buffer *buffer,
+                        Py_ssize_t offset);
+
+static PyObject *
+Struct_unpack_from(PyStructObject *self, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"buffer", "offset", NULL};
+    static _PyArg_Parser _parser = {"y*|n:unpack_from", _keywords, 0};
+    Py_buffer buffer = {NULL, NULL};
+    Py_ssize_t offset = 0;
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+        &buffer, &offset)) {
+        goto exit;
+    }
+    return_value = Struct_unpack_from_impl(self, &buffer, offset);
+
+exit:
+    /* Cleanup for buffer */
+    if (buffer.obj) {
+       PyBuffer_Release(&buffer);
+    }
+
+    return return_value;
+}
+
+PyDoc_STRVAR(Struct_iter_unpack__doc__,
+"iter_unpack($self, buffer, /)\n"
+"--\n"
+"\n"
+"Return an iterator yielding tuples.\n"
+"\n"
+"Tuples are unpacked from the given bytes source, like a repeated\n"
+"invocation of unpack_from().\n"
+"\n"
+"Requires that the bytes length be a multiple of the struct size.");
+
+#define STRUCT_ITER_UNPACK_METHODDEF    \
+    {"iter_unpack", (PyCFunction)Struct_iter_unpack, METH_O, Struct_iter_unpack__doc__},
+
+PyDoc_STRVAR(_clearcache__doc__,
+"_clearcache($module, /)\n"
+"--\n"
+"\n"
+"Clear the internal cache.");
+
+#define _CLEARCACHE_METHODDEF    \
+    {"_clearcache", (PyCFunction)_clearcache, METH_NOARGS, _clearcache__doc__},
+
+static PyObject *
+_clearcache_impl(PyObject *module);
+
+static PyObject *
+_clearcache(PyObject *module, PyObject *Py_UNUSED(ignored))
+{
+    return _clearcache_impl(module);
+}
+
+PyDoc_STRVAR(calcsize__doc__,
+"calcsize($module, format, /)\n"
+"--\n"
+"\n"
+"Return size in bytes of the struct described by the format string.");
+
+#define CALCSIZE_METHODDEF    \
+    {"calcsize", (PyCFunction)calcsize, METH_O, calcsize__doc__},
+
+PyDoc_STRVAR(unpack__doc__,
+"unpack($module, format, inputstr, /)\n"
+"--\n"
+"\n"
+"Return a tuple containing values unpacked according to the format string.\n"
+"\n"
+"The buffer\'s size in bytes must be calcsize(format).\n"
+"\n"
+"See help(struct) for more on format strings.");
+
+#define UNPACK_METHODDEF    \
+    {"unpack", (PyCFunction)unpack, METH_FASTCALL, unpack__doc__},
+
+static PyObject *
+unpack_impl(PyObject *module, PyObject *format, PyObject *inputstr);
+
+static PyObject *
+unpack(PyObject *module, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    PyObject *format;
+    PyObject *inputstr;
+
+    if (!_PyArg_UnpackStack(args, nargs, "unpack",
+        2, 2,
+        &format, &inputstr)) {
+        goto exit;
+    }
+
+    if (!_PyArg_NoStackKeywords("unpack", kwnames)) {
+        goto exit;
+    }
+    return_value = unpack_impl(module, format, inputstr);
+
+exit:
+    return return_value;
+}
+
+PyDoc_STRVAR(unpack_from__doc__,
+"unpack_from($module, format, /, buffer, offset=0)\n"
+"--\n"
+"\n"
+"Return a tuple containing values unpacked according to the format string.\n"
+"\n"
+"The buffer\'s size, minus offset, must be at least calcsize(format).\n"
+"\n"
+"See help(struct) for more on format strings.");
+
+#define UNPACK_FROM_METHODDEF    \
+    {"unpack_from", (PyCFunction)unpack_from, METH_FASTCALL, unpack_from__doc__},
+
+static PyObject *
+unpack_from_impl(PyObject *module, PyObject *format, Py_buffer *buffer,
+                 Py_ssize_t offset);
+
+static PyObject *
+unpack_from(PyObject *module, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"", "buffer", "offset", NULL};
+    static _PyArg_Parser _parser = {"Oy*|n:unpack_from", _keywords, 0};
+    PyObject *format;
+    Py_buffer buffer = {NULL, NULL};
+    Py_ssize_t offset = 0;
+
+    if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
+        &format, &buffer, &offset)) {
+        goto exit;
+    }
+    return_value = unpack_from_impl(module, format, &buffer, offset);
+
+exit:
+    /* Cleanup for buffer */
+    if (buffer.obj) {
+       PyBuffer_Release(&buffer);
+    }
+
+    return return_value;
+}
+
+PyDoc_STRVAR(iter_unpack__doc__,
+"iter_unpack($module, format, buffer, /)\n"
+"--\n"
+"\n"
+"Return an iterator yielding tuples unpacked from the given bytes.\n"
+"\n"
+"The bytes are unpacked according to the format string, like\n"
+"a repeated invocation of unpack_from().\n"
+"\n"
+"Requires that the bytes length be a multiple of the format struct size.");
+
+#define ITER_UNPACK_METHODDEF    \
+    {"iter_unpack", (PyCFunction)iter_unpack, METH_FASTCALL, iter_unpack__doc__},
+
+static PyObject *
+iter_unpack_impl(PyObject *module, PyObject *format, PyObject *buffer);
+
+static PyObject *
+iter_unpack(PyObject *module, PyObject **args, Py_ssize_t nargs, PyObject *kwnames)
+{
+    PyObject *return_value = NULL;
+    PyObject *format;
+    PyObject *buffer;
+
+    if (!_PyArg_UnpackStack(args, nargs, "iter_unpack",
+        2, 2,
+        &format, &buffer)) {
+        goto exit;
+    }
+
+    if (!_PyArg_NoStackKeywords("iter_unpack", kwnames)) {
+        goto exit;
+    }
+    return_value = iter_unpack_impl(module, format, buffer);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=db8152ad222fa3d0 input=a9049054013a1b77]*/