plpython: Add SPI cursor support

Add a function plpy.cursor that is similar to plpy.execute but uses an
SPI cursor to avoid fetching the entire result set into memory.

Jan Urbański, reviewed by Steve Singer

src/pl/plpython/expected/plpython_subtransaction_0.out

@@ -382,3 +382,73 @@ SELECT * FROM subtransaction_tbl;
 (0 rows)
 
 DROP TABLE subtransaction_tbl;
+-- cursor/subtransactions interactions
+CREATE FUNCTION cursor_in_subxact() RETURNS int AS $$
+with plpy.subtransaction():
+    cur = plpy.cursor("select * from generate_series(1, 20) as gen(i)")
+    cur.fetch(10)
+fetched = cur.fetch(10);
+return int(fetched[5]["i"])
+$$ LANGUAGE plpythonu;
+ERROR:  could not compile PL/Python function "cursor_in_subxact"
+DETAIL:  SyntaxError: invalid syntax (line 3)
+CREATE FUNCTION cursor_aborted_subxact() RETURNS int AS $$
+try:
+    with plpy.subtransaction():
+        cur = plpy.cursor("select * from generate_series(1, 20) as gen(i)")
+        cur.fetch(10);
+        plpy.execute("select no_such_function()")
+except plpy.SPIError:
+    fetched = cur.fetch(10)
+    return int(fetched[5]["i"])
+return 0 # not reached
+$$ LANGUAGE plpythonu;
+ERROR:  could not compile PL/Python function "cursor_aborted_subxact"
+DETAIL:  SyntaxError: invalid syntax (line 4)
+CREATE FUNCTION cursor_plan_aborted_subxact() RETURNS int AS $$
+try:
+    with plpy.subtransaction():
+        plpy.execute('create temporary table tmp(i) '
+                     'as select generate_series(1, 10)')
+        plan = plpy.prepare("select i from tmp")
+        cur = plpy.cursor(plan)
+        plpy.execute("select no_such_function()")
+except plpy.SPIError:
+    fetched = cur.fetch(5)
+    return fetched[2]["i"]
+return 0 # not reached
+$$ LANGUAGE plpythonu;
+ERROR:  could not compile PL/Python function "cursor_plan_aborted_subxact"
+DETAIL:  SyntaxError: invalid syntax (line 4)
+CREATE FUNCTION cursor_close_aborted_subxact() RETURNS boolean AS $$
+try:
+    with plpy.subtransaction():
+        cur = plpy.cursor('select 1')
+        plpy.execute("select no_such_function()")
+except plpy.SPIError:
+    cur.close()
+    return True
+return False # not reached
+$$ LANGUAGE plpythonu;
+ERROR:  could not compile PL/Python function "cursor_close_aborted_subxact"
+DETAIL:  SyntaxError: invalid syntax (line 4)
+SELECT cursor_in_subxact();
+ERROR:  function cursor_in_subxact() does not exist
+LINE 1: SELECT cursor_in_subxact();
+               ^
+HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
+SELECT cursor_aborted_subxact();
+ERROR:  function cursor_aborted_subxact() does not exist
+LINE 1: SELECT cursor_aborted_subxact();
+               ^
+HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
+SELECT cursor_plan_aborted_subxact();
+ERROR:  function cursor_plan_aborted_subxact() does not exist
+LINE 1: SELECT cursor_plan_aborted_subxact();
+               ^
+HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
+SELECT cursor_close_aborted_subxact();
+ERROR:  function cursor_close_aborted_subxact() does not exist
+LINE 1: SELECT cursor_close_aborted_subxact();
+               ^
+HINT:  No function matches the given name and argument types. You might need to add explicit type casts.