BUG #15026: Deadlock using GIST index

The following bug has been logged on the website:

Bug reference:      15026
Logged by:          Mark Scheffer
Email address:      pg2401k@pinkwin.com
PostgreSQL version: 10.1
Operating system:   SLES 12 sp3
Description:

Following (fabricated) example shows there is a path in the GIST code that
leads to deadlocks:

-- Session 1:
DROP TABLE IF EXISTS locked; 
CREATE TABLE locked
(
    key text NOT NULL,
    EXCLUDE USING gist (key WITH =)
);
BEGIN;
INSERT INTO locked(key) values('a');

-- Session 2:
INSERT INTO locked(key) values('a');
--> lock wait

-- Session 3:
INSERT INTO locked(key) values('a');
--> lock wait

-- Session 1:
ROLLBACK;
--> Session 3 generates deadlock:

ERROR:  deadlock detected
DETAIL:  Process 16079 waits for ShareLock on transaction 1420618256;
blocked by process 8594.
Process 8594 waits for ShareLock on transaction 1420618257; blocked by
process 16079.
HINT:  See server log for query details.
CONTEXT:  while checking exclusion constraint on tuple (0,2) in relation
"locked"

Note: Replacing the GIST with a unique index does not produce this error.
Note: Setting  deadlock_timeout to some high value will help analysis.

Version:  10.1-9.1 from OpenSuse repository
"PostgreSQL 10.1 on x86_64-suse-linux-gnu, compiled by gcc (SUSE Linux)
4.8.5, 64-bit"

Regards,

Mark.