BUG #13671: pg_terminate_backend(pid) does not work

The following bug has been logged on the website:

Bug reference:      13671
Logged by:          alex
Email address:      oktogen@mail.ru
PostgreSQL version: 9.4.5
Operating system:   ubuntu
Description:

The command pg_terminate_backend(pid) does not terminate sql session
in case incorrect formed COPY command(without terminate-symbol)
Example
--SESSION #1
CREATE TABLE public.comments (
  user_id INTEGER NOT NULL,
  description TEXT NOT NULL,
  path public.ltree NOT NULL,
  CONSTRAINT comments_idx PRIMARY KEY(user_id, description, path)
)WITHOUT OIDS;

COPY comments (user_id, description, path) FROM stdin;
1    efbb974bcf5af8f2ba8f352225ecff63    0001
2    8dcdb42684773f413b40d63f5ef0722e    0001.0001.0001
--

--SESSION #2
-- Now SESSION # 1 can not be forced to complete the outside.
-- learn pid process which need  terminate
SELECT a.pid,
    a.usename,
    a.application_name,
    a.client_addr,
    clock_timestamp() - a.query_start AS duration,
    a.state,
    a.query,
    a.waiting
FROM pg_stat_activity a;

--attempt to finish the session with incorrect COPY command
SELECT pg_terminate_backend(6476) AS output;
--   output
-------------
-- TRUE

SELECT
    a.pid,
  a.state,
  a.query
FROM pg_stat_activity a
WHERE a.pid = 6476;
-- output(failed attempt)
+-----+--------+---------------------------------------------------------+
| pid | state  |                  query                                  |
+-----+--------+---------------------------------------------------------+
|6476 | active |  COPY comments (user_id, description, path) FROM stdin; |
+-----+--------+---------------------------------------------------------+