Wrap PL/Python SPI calls into subtransactions

This allows the language-specific try/catch construct to catch and
handle exceptions arising from SPI calls, matching the behavior of
other PLs.

As an additional bonus you no longer get all the ugly "unrecognized
error in PLy_spi_execute_query" errors.

Jan Urbański, reviewed by Steve Singer

src/pl/plpython/sql/plpython_error.sql

@@ -130,3 +130,25 @@ return None
 	LANGUAGE plpythonu;
 
 SELECT valid_type('rick');
+
+/* manually starting subtransactions - a bad idea
+ */
+CREATE FUNCTION manual_subxact() RETURNS void AS $$
+plpy.execute("savepoint save")
+plpy.execute("create table foo(x integer)")
+plpy.execute("rollback to save")
+$$ LANGUAGE plpythonu;
+
+SELECT manual_subxact();
+
+/* same for prepared plans
+ */
+CREATE FUNCTION manual_subxact_prepared() RETURNS void AS $$
+save = plpy.prepare("savepoint save")
+rollback = plpy.prepare("rollback to save")
+plpy.execute(save)
+plpy.execute("create table foo(x integer)")
+plpy.execute(rollback)
+$$ LANGUAGE plpythonu;
+
+SELECT manual_subxact_prepared();