How to handle simultaneous FOR-IN UPDATE-RETURNING loops?

Good afternoon,

in PostgreSQL 9.5 with pgbouncer (having "pool_mode = session" and
"server_reset_query = DISCARD ALL") 2-player games are stored in the
following table:

CREATE TABLE words_games (
gid SERIAL PRIMARY KEY,

created timestamptz NOT NULL,
finished timestamptz,

player1 integer REFERENCES words_users(uid) ON DELETE CASCADE NOT
NULL CHECK (player1 <> player2),
player2 integer REFERENCES words_users(uid) ON DELETE CASCADE,

played1 timestamptz,
played2 timestamptz
);

And every hour I run a custom PL/pgSQL function to forcibly finish games,
where one of the players hasn't played any move since more than 24h:
https://gist.github.com/afarber/416da460e5722ab1e3ed25385cea6cae (also
copy-pasted at the bottom of this mail).

However there is a problem: I can not use a "single-instance" cronjob to
run words_expire_games hourly.

I have to use the HttpServlet (so that I can send notifications to the
websocket-clients) and as result multiple servlet's might end up running at
the same time.

My question is if I should ensure that only 1 servlet runs the custom
PL/pgSQL function by using "synchronized" in Java as I do it right now:

private static final String SQL_EXPIRE_GAMES =
"SELECT " +
"out_uid AS uid, " +
"out_gid AS gid, " +
"out_fcm AS fcm, " +
"out_apns AS apns, " +
"out_sns AS sns, " +
"out_note AS note " +
"FROM words_expire_games()";

// the timestamp in milliseconds of the last successful hourly job run
private static long sLastRun = 0L;

// this method is run every time the servlet is called (i.e. very often)
private void hourlyJob() throws SQLException, IOException {
if (System.currentTimeMillis() - sLastRun < ONE_HOUR) {
return;
}

synchronized (MyListener.class) {
if (System.currentTimeMillis() - sLastRun < ONE_HOUR) {
return;
}

try (PreparedStatement st =
mDatabase.prepareStatement(SQL_EXPIRE_GAMES)) {
try (ResultSet rs = st.executeQuery()) {
while (rs.next()) {
Notification n = new Notification(
rs.getInt(KEY_UID),
rs.getInt(KEY_GID),
true,
rs.getString(KEY_FCM),
rs.getString(KEY_APNS),
rs.getString(KEY_SNS),
rs.getString(KEY_NOTE)
);
sendNotification(n); // send notifications about
forcibly finished games via websockets
}
}
}

sLastRun = System.currentTimeMillis();
}
}

Or if maybe there is some condition (maybe "UPDATE SKIP LOCKED"?) I could
add to my custom function copy-pasted below? -

Thank you for any insights
Alex

CREATE OR REPLACE FUNCTION words_expire_games(
) RETURNS TABLE (
out_uid integer, -- the player to be notified
out_gid integer, -- which game has expired
out_fcm text,
out_apns text,
out_sns text,
out_note text
) AS
$func$
DECLARE
_gid integer;
_loser integer;
_winner integer;
BEGIN
FOR _gid, _loser, _winner IN
UPDATE words_games
SET finished = CURRENT_TIMESTAMP
WHERE finished IS NULL
AND played1 IS NOT NULL
AND played2 IS NOT NULL
AND (played1 < CURRENT_TIMESTAMP - INTERVAL '24 hours'
OR played2 < CURRENT_TIMESTAMP - INTERVAL '24 hours')
RETURNING
gid,
CASE WHEN played1 < played2 THEN player1 ELSE
player2 END,
CASE WHEN played1 < played2 THEN player2 ELSE
player1 END
LOOP
-- log the last "move"
INSERT INTO words_moves (
action,
gid,
uid,
played,
tiles
) VALUES (
'expire',
_gid,
_loser,
CURRENT_TIMESTAMP,
null
);

-- notify the loser
SELECT
uid,
_gid,
fcm,
apns,
sns,
'You have lost (game expired)!'
FROM words_users
WHERE uid = _loser
INTO STRICT
out_uid,
out_gid,
out_fcm,
out_apns,
out_sns,
out_note;
RETURN NEXT;

-- notify the winner
SELECT
uid,
_gid,
fcm,
apns,
sns,
'You have won (game expired)!'
FROM words_users
WHERE uid = _winner
INTO STRICT
out_uid,
out_gid,
out_fcm,
out_apns,
out_sns,
out_note;
RETURN NEXT;

END LOOP;
END
$func$ LANGUAGE plpgsql;