Deadlock between concurrent index builds on different tables

I am attempting to build several indexes in parallel, guaranteeing that I
never build one on the same table twice. I understand I can't build two on
the same table at once or I will get a deadlock. However, I am also
getting a deadlock when doing several in parallel on different tables.

Here is an example of the error I am getting:

2017-12-22 15:48:07.669 CST,"CREATE INDEX",2017-12-22 15:48:02
CST,8/32,0,ERROR,40P01,"deadlock detected","Process 4470 waits for
ShareLock on virtual transaction 4/262; blocked by process 4466.
Process 4466 waits for ShareLock on virtual transaction 8/32; blocked by
process 4470.
Process 4470: CREATE INDEX CONCURRENTLY index_foo_on_created_at ON foo
USING btree (created_at);
Process 4466: CREATE INDEX CONCURRENTLY index_bar_on_id ON bar USING btree
(id);","See server log for query details.",,,,"CREATE INDEX CONCURRENTLY
index_foo_on_created_at ON foo USING btree (created_at);",,,""

Here is my process:

- Kick off one index build and background 1 second apart, using a queue
table to determine what indexes to build and what is finished
- When I determine if a index can be built, I first check if there is
another already in build on target table before starting another
- After the index is built in each workflow, I mark it as built in a
queue table

I assume that one of the indexes (the earlier transaction) is building and
taking so long that several finished indexes end up waiting, and there is
perhaps a limit to that?

Any insight appreciated.

Thanks,
Jeremy