BUG #15290: Stuck Parallel Index Scan query

The following bug has been logged on the website:

Bug reference: 15290
Logged by: Victor Yegorov
Email address: vyegorov(at)gmail(dot)com
PostgreSQL version: 10.4
Operating system: Debian GNU/Linux 8.7 (jessie)
Description:

We've just encountered an issue on the streaming replica of the client.

In short — query is active for 8 hours and we're not able to terminate it:
all processes (leader and workers) are ignoring all signals.
Symptoms are similar to the ones described in #15036. As a result, we had to
perform immediate shutdown f the instance.
After restart we've got another such stuck query within minutes, so it's not
a single occurrence..

Query in subject
----------------
-[ RECORD 1
]-----------------------------------------------------------------------------------------------------
ts_age | 08:32:17.286343
state | active
query_age | 08:32:17.286345
change_age | 08:32:17.286344
datname | coub
pid | 2877
usename | app
waiting | f
client_addr |
client_port | -1
query | select count(*) as value from coubs where type='Coub::Simple'
and is_done=false and in_process=false

QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
Finalize Aggregate (cost=666001.67..666001.72 rows=1 width=8)
-> Gather (cost=666001.20..666001.65 rows=8 width=8)
Workers Planned: 8
-> Partial Aggregate (cost=665901.20..665901.25 rows=1 width=8)
-> Parallel Index Scan using
coubs_type_is_done_partial_simple on coubs (cost=0.56..663016.01
rows=1154077 width=0)
Index Cond: (is_done = false)
Filter: ((NOT is_done) AND (NOT in_process))
(7 rows)

In reality this query worked with just 4 workers:
2877 ? ts 0:08 \_ postgres: 10/main: app coub [local] SELECT
3416 ? Ss 0:00 \_ postgres: 10/main: bgworker: parallel worker
for PID 2877
3417 ? Ss 0:00 \_ postgres: 10/main: bgworker: parallel worker
for PID 2877
3418 ? Ss 0:00 \_ postgres: 10/main: bgworker: parallel worker
for PID 2877
3419 ? Ss 0:00 \_ postgres: 10/main: bgworker: parallel worker
for PID 2877

We have backtraces of the main process and all workers, and also
`pg_stat_activity` snapshot — I will attach them to the new e-mail.