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, 49 insertions, 21 deletions

diff --git a/src/pl/plpython/plpy_planobject.c b/src/pl/plpython/plpy_planobject.c
index 9427674d2f4..3e385555e5e 100644
--- a/src/pl/plpython/plpy_planobject.c
+++ b/src/pl/plpython/plpy_planobject.c
@@ -12,7 +12,7 @@
 #include "plpython.h"
 #include "utils/memutils.h"
 
-static void PLy_plan_dealloc(PyObject *arg);
+static void PLy_plan_dealloc(PLyPlanObject *self);
 static PyObject *PLy_plan_cursor(PyObject *self, PyObject *args);
 static PyObject *PLy_plan_execute(PyObject *self, PyObject *args);
 static PyObject *PLy_plan_status(PyObject *self, PyObject *args);
@@ -26,20 +26,37 @@ static PyMethodDef PLy_plan_methods[] = {
 	{NULL, NULL, 0, NULL}
 };
 
-static PyTypeObject PLy_PlanType = {
-	PyVarObject_HEAD_INIT(NULL, 0)
-	.tp_name = "PLyPlan",
-	.tp_basicsize = sizeof(PLyPlanObject),
-	.tp_dealloc = PLy_plan_dealloc,
-	.tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
-	.tp_doc = PLy_plan_doc,
-	.tp_methods = PLy_plan_methods,
+static PyType_Slot PLyPlan_slots[] =
+{
+	{
+		Py_tp_dealloc, PLy_plan_dealloc
+	},
+	{
+		Py_tp_doc, (char *) PLy_plan_doc
+	},
+	{
+		Py_tp_methods, PLy_plan_methods
+	},
+	{
+		0, NULL
+	}
 };
 
+static PyType_Spec PLyPlan_spec =
+{
+	.name = "PLyPlan",
+		.basicsize = sizeof(PLyPlanObject),
+		.flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+		.slots = PLyPlan_slots,
+};
+
+static PyTypeObject *PLy_PlanType;
+
 void
 PLy_plan_init_type(void)
 {
-	if (PyType_Ready(&PLy_PlanType) < 0)
+	PLy_PlanType = (PyTypeObject *) PyType_FromSpec(&PLyPlan_spec);
+	if (!PLy_PlanType)
 		elog(ERROR, "could not initialize PLy_PlanType");
 }
 
@@ -48,8 +65,12 @@ PLy_plan_new(void)
 {
 	PLyPlanObject *ob;
 
-	if ((ob = PyObject_New(PLyPlanObject, &PLy_PlanType)) == NULL)
+	if ((ob = PyObject_New(PLyPlanObject, PLy_PlanType)) == NULL)
 		return NULL;
+#if PY_VERSION_HEX < 0x03080000
+	/* Workaround for Python issue 35810; no longer necessary in Python 3.8 */
+	Py_INCREF(PLy_PlanType);
+#endif
 
 	ob->plan = NULL;
 	ob->nargs = 0;
@@ -63,25 +84,32 @@ PLy_plan_new(void)
 bool
 is_PLyPlanObject(PyObject *ob)
 {
-	return ob->ob_type == &PLy_PlanType;
+	return ob->ob_type == PLy_PlanType;
 }
 
 static void
-PLy_plan_dealloc(PyObject *arg)
+PLy_plan_dealloc(PLyPlanObject *self)
 {
-	PLyPlanObject *ob = (PLyPlanObject *) arg;
+#if PY_VERSION_HEX >= 0x03080000
+	PyTypeObject *tp = Py_TYPE(self);
+#endif
 
-	if (ob->plan)
+	if (self->plan)
 	{
-		SPI_freeplan(ob->plan);
-		ob->plan = NULL;
+		SPI_freeplan(self->plan);
+		self->plan = NULL;
 	}
-	if (ob->mcxt)
+	if (self->mcxt)
 	{
-		MemoryContextDelete(ob->mcxt);
-		ob->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
 }