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I'm running the new Windows Subsystem for Linux 2 (WSL-2), with Ubuntu 18.04. It is really fast, and running great, except I can't seem to connect to an external PostgreSQL database using Python. It just hangs, never responding. Here's a minimal reproduction:

$ python3.6
Python 3.6.8 (default, Jan 14 2019, 11:02:34)
[GCC 8.0.1 20180414 (experimental) [trunk revision 259383]] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import psycopg2
>>> psycopg2.connect(host="my-pg-server.mydomain.com", port=5432, user="my_user", dbname="my_db", password="")
[...crickets... doesn't time out, just hangs forever (at least an hour)...]

It isn't a firewall issue, as I can connect with telnet to the same host:

$ telnet my-pg-server.mydomain.com 5432
Trying 123.456.789.100...
Connected to my-pg-server.mydomain.com.
Escape character is '^]'.

The other weird part is that I can connect to an external SQL Server database. I'm sure the credentials are correct for both servers, they're directly from a Django settings file that I'm using just fine on other systems. Any ideas? Is there something I have to do with psycopg2 specifically for WSL-2?

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So it turns out I was pointing the finger in the wrong direction.

I logged into the PostgreSQL server box with a colleague, and issued this command:

ps -ef --sort=start_time | fgrep [db host name] | more

It turns out, the existing connections to the server were fine, but something is jammed up. I have a bunch of idle processes, then a slew of ones saying "startup waiting" - over 100. Here's the output from the command:

[...about 100 idle processes, truncated...]
postgres 26815 48821  0 Aug16 ? 00:00:00 postgres: my-pg-server: web_user web 192.168.9.187(55972) idle
postgres 27525 48821  0 Aug16 ? 00:00:00 postgres: my-pg-server: web_user web 192.168.9.187(55976) idle
postgres 14781 48821  0 00:00 ? 00:00:00 postgres: my-pg-server: postgres jsmith_d [local] VACUUM waiting
postgres 22738 48821  0 00:01 ? 00:00:00 postgres: my-pg-server: other_user other_db 192.168.9.187(57692) startup waiting
postgres  7683 48821  0 00:15 ? 00:00:00 postgres: my-pg-server: yetanother_user yetanother_db 192.168.9.187(57694) startup waiting
postgres 15951 48821  0 00:30 ? 00:00:00 postgres: my-pg-server: yetanother_user yetanother_db 192.168.9.187(57696) startup waiting
[...and about another 100 startup waiting processes, truncated...]

Aha! The culprit it found:

postgres 14781 48821 0 00:00 ? 00:00:00 postgres: my-pg-server: postgres jsmith_d [local] VACUUM waiting

It appears something is jammed up in the VACUUM process, which is causing the new connections to just hang, without failing. Time to dig a little deeper and clean it up, but there's the answer to this behavior.

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