Prepare for Python "Limited API" in PL/Python

Using the Python Limited API would allow building PL/Python against
any Python 3.x version and using another Python 3.x version at run
time.  This commit does not activate that, but it prepares the code to
only use APIs supported by the Limited API.

Implementation details:

- Convert static types to heap types
  (https://docs.python.org/3/howto/isolating-extensions.html#heap-types).

- Replace PyRun_String() with component functions.

- Replace PyList_SET_ITEM() with PyList_SetItem().

This was previously committed as c47e8df815c and then reverted because
it wasn't working under Python older than 3.8.  That has been fixed in
this version.  There was a Python API change/bugfix between 3.7 and
3.8 that directly affects this patch.  The relevant commit is
<https://github.com/python/cpython/commit/364f0b0f19c>.  The
workarounds described there have been applied in this patch, and it
has been confirmed to work with Python 3.6 and 3.7.

Reviewed-by: Jakob Egger <jakob@eggerapps.at>
Discussion: https://www.postgresql.org/message-id/flat/ee410de1-1e0b-4770-b125-eeefd4726a24@eisentraut.org

1 files changed, 59 insertions, 25 deletions

diff --git a/src/pl/plpython/plpy_cursorobject.c b/src/pl/plpython/plpy_cursorobject.c
index bb3fa8a3909..1c6be756120 100644
--- a/src/pl/plpython/plpy_cursorobject.c
+++ b/src/pl/plpython/plpy_cursorobject.c
@@ -20,7 +20,7 @@
 #include "utils/memutils.h"
 
 static PyObject *PLy_cursor_query(const char *query);
-static void PLy_cursor_dealloc(PyObject *arg);
+static void PLy_cursor_dealloc(PLyCursorObject *self);
 static PyObject *PLy_cursor_iternext(PyObject *self);
 static PyObject *PLy_cursor_fetch(PyObject *self, PyObject *args);
 static PyObject *PLy_cursor_close(PyObject *self, PyObject *unused);
@@ -33,22 +33,43 @@ static PyMethodDef PLy_cursor_methods[] = {
 	{NULL, NULL, 0, NULL}
 };
 
-static PyTypeObject PLy_CursorType = {
-	PyVarObject_HEAD_INIT(NULL, 0)
-	.tp_name = "PLyCursor",
-	.tp_basicsize = sizeof(PLyCursorObject),
-	.tp_dealloc = PLy_cursor_dealloc,
-	.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
-	.tp_doc = PLy_cursor_doc,
-	.tp_iter = PyObject_SelfIter,
-	.tp_iternext = PLy_cursor_iternext,
-	.tp_methods = PLy_cursor_methods,
+static PyType_Slot PLyCursor_slots[] =
+{
+	{
+		Py_tp_dealloc, PLy_cursor_dealloc
+	},
+	{
+		Py_tp_doc, (char *) PLy_cursor_doc
+	},
+	{
+		Py_tp_iter, PyObject_SelfIter
+	},
+	{
+		Py_tp_iternext, PLy_cursor_iternext
+	},
+	{
+		Py_tp_methods, PLy_cursor_methods
+	},
+	{
+		0, NULL
+	}
 };
 
+static PyType_Spec PLyCursor_spec =
+{
+	.name = "PLyCursor",
+		.basicsize = sizeof(PLyCursorObject),
+		.flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+		.slots = PLyCursor_slots,
+};
+
+static PyTypeObject *PLy_CursorType;
+
 void
 PLy_cursor_init_type(void)
 {
-	if (PyType_Ready(&PLy_CursorType) < 0)
+	PLy_CursorType = (PyTypeObject *) PyType_FromSpec(&PLyCursor_spec);
+	if (!PLy_CursorType)
 		elog(ERROR, "could not initialize PLy_CursorType");
 }
 
@@ -80,8 +101,12 @@ PLy_cursor_query(const char *query)
 	volatile MemoryContext oldcontext;
 	volatile ResourceOwner oldowner;
 
-	if ((cursor = PyObject_New(PLyCursorObject, &PLy_CursorType)) == NULL)
+	if ((cursor = PyObject_New(PLyCursorObject, PLy_CursorType)) == NULL)
 		return NULL;
+#if PY_VERSION_HEX < 0x03080000
+	/* Workaround for Python issue 35810; no longer necessary in Python 3.8 */
+	Py_INCREF(PLy_CursorType);
+#endif
 	cursor->portalname = NULL;
 	cursor->closed = false;
 	cursor->mcxt = AllocSetContextCreate(TopMemoryContext,
@@ -177,8 +202,12 @@ PLy_cursor_plan(PyObject *ob, PyObject *args)
 		return NULL;
 	}
 
-	if ((cursor = PyObject_New(PLyCursorObject, &PLy_CursorType)) == NULL)
+	if ((cursor = PyObject_New(PLyCursorObject, PLy_CursorType)) == NULL)
 		return NULL;
+#if PY_VERSION_HEX < 0x03080000
+	/* Workaround for Python issue 35810; no longer necessary in Python 3.8 */
+	Py_INCREF(PLy_CursorType);
+#endif
 	cursor->portalname = NULL;
 	cursor->closed = false;
 	cursor->mcxt = AllocSetContextCreate(TopMemoryContext,
@@ -272,30 +301,35 @@ PLy_cursor_plan(PyObject *ob, PyObject *args)
 }
 
 static void
-PLy_cursor_dealloc(PyObject *arg)
+PLy_cursor_dealloc(PLyCursorObject *self)
 {
-	PLyCursorObject *cursor;
+#if PY_VERSION_HEX >= 0x03080000
+	PyTypeObject *tp = Py_TYPE(self);
+#endif
 	Portal		portal;
 
-	cursor = (PLyCursorObject *) arg;
-
-	if (!cursor->closed)
+	if (!self->closed)
 	{
-		portal = GetPortalByName(cursor->portalname);
+		portal = GetPortalByName(self->portalname);
 
 		if (PortalIsValid(portal))
 		{
 			UnpinPortal(portal);
 			SPI_cursor_close(portal);
 		}
-		cursor->closed = true;
+		self->closed = true;
 	}
-	if (cursor->mcxt)
+	if (self->mcxt)
 	{
-		MemoryContextDelete(cursor->mcxt);
-		cursor->mcxt = NULL;
+		MemoryContextDelete(self->mcxt);
+		self->mcxt = NULL;
 	}
-	arg->ob_type->tp_free(arg);
+
+	PyObject_Free(self);
+#if PY_VERSION_HEX >= 0x03080000
+	/* This was not needed before Python 3.8 (Python issue 35810) */
+	Py_DECREF(tp);
+#endif
 }
 
 static PyObject *