BUG #16440: pg_basebackup intermittently hangs waiting for input unless run with --checkpoint=fast option

The following bug has been logged on the website:

Bug reference: 16440
Logged by: Curt Kolovson
Email address: ckolovson(at)gmail(dot)com
PostgreSQL version: 10.9
Operating system: Linux 4.9.219-1.ph2 x86_64 (VMware Photon OS 2.0)
Description:

We have noticed what appears to be an intermittent bug in pg_basebackup.
Here is what we are occasionally seeing:
$ /opt/vmware/vpostgres/current/bin/pg_basebackup -l "repmgr base backup"
-D /var/vmware/vpostgres/current/pgdata -h 172.18.50.48 -p 5432 -U repmgr -X
stream --verbose --progress
pg_basebackup: initiating base backup, waiting for checkpoint to complete

And then it hangs indefinitely at this point. It makes no progress (0 CPU),
so it is hanging on some type of input. Here is ps output:
postgres 3386 3370 0 15:01 ? 00:00:00
/opt/vmware/vpostgres/current/bin/pg_basebackup -l repmgr base backup -D
/var/vmware/vpostgres/current/pgdata -h 172.18.50.48 -p 5432 -U repmgr -X
stream

We notice that this behavior only occurs intermittently, but when it does,
it happens repeatedly on that system. The only workarounds we have found are
either to run it with the --checkpoint=fast option, or to restart postgres
on the primary.

We are using synchronous streaming replication.