BUG #15074: psql client never returns when creating index (longrunning operation)

The following bug has been logged on the website:

Bug reference:      15074
Logged by:          Lars Vonk
Email address:      lars.vonk@gmail.com
PostgreSQL version: 9.6.7
Operating system:   ubuntu 14.04 LTS
Description:

When creating an index on a table containing approx. 100M records the psql
client never returns when the index is eventually created on the server.  
This happens both in a psql client (9.6.6) on mac osx and using the libpq
client via the ruby pg gem when using active records migrations. 

When running the command directly op the postgres database server using the
psql client it works as expected.

The logs from the server:
2018-02-16 16:11:05 CET:82.161.205.254(59584):REDACTED@REDACTED_DB:[23141]:
LOG:  duration: 999618.165 ms
2018-02-16 16:11:05 CET:82.161.205.254(59584):REDACTED@REDACTED_DB:[23141]:
LOG:  could not receive data from client: Connection reset by peer

The psql client just hangs, when trying to cancel the query client side
(ctrl-c) the server logs says:
2018-02-16 16:13:53 CET:82.161.205.254(59961):[unknown]@[unknown]:[25738]:
LOG:  PID 23141 in cancel request did not match any process