Unix Sockets broken

[maxadamo]

Bug report criteria

• This bug report is not security related, security issues should be disclosed privately via etcd maintainers.
• This is not a support request or question, support requests or questions should be raised in the etcd discussion forums.
• You have read the etcd bug reporting guidelines.
• Existing open issues along with etcd frequently asked questions have been checked and this is not a duplicate.

What happened?

The issue #17443 has not been resolved: unix sockets don't pass the scheme validation (host:port) and etcd doesn't start.
I think a ticket liked this has been closed multiple times, but the issue has never been resolved.

What did you expect to happen?

being able to use the unix socket, as described in the documentation https://etcd.io/docs/v3.4/op-guide/configuration/

How can we reproduce it (as minimally and precisely as possible)?

just use the unix socket as described in #17443

Anything else we need to know?

n/a

Etcd version (please run commands below)

$ etcd --version
# paste output here
etcd Version: 3.4.7
Git SHA: e694b7bb0
Go Version: go1.12.17
Go OS/Arch: linux/amd64
$ etcdctl version
# paste output here
etcdctl version: 3.4.7
API version: 3.4etcdctl version: 3.4.7
API version: 3.4

Etcd configuration (command line flags or environment variables)

---

data-dir: "/var/lib/etcd"
name: test-pgplayground-witness
initial-advertise-peer-urls: http://192.168.1.237:2380
listen-peer-urls: http://0.0.0.0:238,unix:///var/run/etcd.sock
listen-client-urls: http://0.0.0.0:2379,unix:///var/run/etcd.sock
advertise-client-urls: http://192.168.1.237:2379
initial-cluster-token: test-pgplayground-etcd
initial-cluster: test-pgplayground01=http://192.168.1.235:2380,test-pgplayground02=http://192.168.1.236:2380,test-pgplayground-witness=http://192.168.1.237:2380
initial-cluster-state: new

Etcd debug information (please run commands below, feel free to obfuscate the IP address or FQDN in the output)

n/a

Relevant log output

Apr 03 12:03:42 test-pgplayground-witness.geant.org systemd[1]: Starting etcd key-value store...
Apr 03 12:03:42 test-pgplayground-witness.geant.org etcd[336970]: unexpected error setting up listen-peer-urls: URL address does not have the form "host:port": unix:///var/run/etcd.sock
Apr 03 12:03:42 test-pgplayground-witness.geant.org systemd[1]: etcd.service: Main process exited, code=exited, status=1/FAILURE