exp push: fails

Bug Report

Description

dvc exp push is failing.

Reproduce

1. Fork https://github.com/iterative/example-dvc-experiments and clone the fork.
2. Run experiments.
3. Try to push experiments.

Output:

$ dvc exp push origin exp-b270f
2021-11-02 20:08:40,190 DEBUG: git push experiment 'refs/exps/06/eed83b30c1e3d6cd7be76c0965d7e0ea56439e/exp-b270f' -> 'origin'
2021-11-02 20:08:40,233 ERROR: unexpected error - [Errno 49] Can't assign requested address
------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/dave/Code/dvc/dvc/main.py", line 55, in main
    ret = cmd.do_run()
  File "/Users/dave/Code/dvc/dvc/command/base.py", line 45, in do_run
    return self.run()
  File "/Users/dave/Code/dvc/dvc/command/experiments.py", line 728, in run
    self.repo.experiments.push(
  File "/Users/dave/Code/dvc/dvc/repo/experiments/__init__.py", line 1003, in push
    return push(self.repo, *args, **kwargs)
  File "/Users/dave/Code/dvc/dvc/repo/__init__.py", line 50, in wrapper
    return f(repo, *args, **kwargs)
  File "/Users/dave/Code/dvc/dvc/repo/scm_context.py", line 14, in run
    return method(repo, *args, **kw)
  File "/Users/dave/Code/dvc/dvc/repo/experiments/push.py", line 40, in push
    repo.scm.push_refspec(
  File "/Users/dave/Code/dvc/dvc/scm/git/__init__.py", line 296, in _backend_func
    return func(*args, **kwargs)
  File "/Users/dave/Code/dvc/dvc/scm/git/backend/dulwich/__init__.py", line 452, in push_refspec
    client.send_pack(
  File "/opt/homebrew/lib/python3.9/site-packages/dulwich/client.py", line 926, in send_pack
    proto, unused_can_read, stderr = self._connect(b"receive-pack", path)
  File "/opt/homebrew/lib/python3.9/site-packages/dulwich/client.py", line 1670, in _connect
    con = self.ssh_vendor.run_command(
  File "/opt/homebrew/lib/python3.9/site-packages/fsspec/asyn.py", line 91, in wrapper
    return sync(self.loop, func, *args, **kwargs)
  File "/opt/homebrew/lib/python3.9/site-packages/fsspec/asyn.py", line 71, in sync
    raise return_result
  File "/opt/homebrew/lib/python3.9/site-packages/fsspec/asyn.py", line 25, in _runner
    result[0] = await coro
  File "/Users/dave/Code/dvc/dvc/scm/git/backend/dulwich/asyncssh_vendor.py", line 80, in _run_command
    conn = await asyncssh.connect(
  File "/opt/homebrew/lib/python3.9/site-packages/asyncssh/connection.py", line 6855, in connect
    return await _connect(options, loop, flags, conn_factory,
  File "/opt/homebrew/lib/python3.9/site-packages/asyncssh/connection.py", line 297, in _connect
    _, conn = await loop.create_connection(conn_factory, host, port,
  File "/opt/homebrew/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/base_events.py", line 1056, in create_connection
    raise exceptions[0]
  File "/opt/homebrew/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/base_events.py", line 1041, in create_connection
    sock = await self._connect_sock(
  File "/opt/homebrew/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/base_events.py", line 955, in _connect_sock
    await self.sock_connect(sock, address)
  File "/opt/homebrew/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/selector_events.py", line 502, in sock_connect
    return await fut
  File "/opt/homebrew/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/selector_events.py", line 507, in _sock_connect
    sock.connect(address)
OSError: [Errno 49] Can't assign requested address
------------------------------------------------------------
2021-11-02 20:08:40,333 DEBUG: Version info for developers:
DVC version: 2.8.3.dev13+g5690015e
---------------------------------
Platform: Python 3.9.5 on macOS-12.0.1-arm64-arm-64bit
Supports:
        webhdfs (fsspec = 2021.9.0),
        http (aiohttp = 3.7.4.post0, aiohttp-retry = 2.4.5),
        https (aiohttp = 3.7.4.post0, aiohttp-retry = 2.4.5),
        s3 (s3fs = 2021.8.0, boto3 = 1.17.5)
Cache types: reflink, hardlink, symlink
Cache directory: apfs on /dev/disk3s1s1
Caches: local
Remotes: https
Workspace directory: apfs on /dev/disk3s1s1
Repo: dvc, git

Having any troubles? Hit us up at https://dvc.org/support, we are always happy to help!
2021-11-02 20:08:40,333 DEBUG: Analytics is disabled.

[pmrowla]

There is an additional issue with github’s SSH server when the user is using RSA SSH keys. GH’s server does not strictly comply with the appropriate RFC’s, which causes asyncssh’s client to error out (see linked PR).

When using non-RSA keys (i.e. ed25519 keys), the issue does not occur.

This appears to be specific to github (and not a general git-over-ssh thing). All of the keys I tested (including RSA) worked as expected with asyncssh+gitlab (with and without the monkey patch from the PR).

[pmrowla]

The bug is that the way we call asyncssh.connect() in our vendor explicitly disables using ssh-agent fallback when no SSH key is provided through the dulwich client args. The correct way to connect in git-ssh is to use fall back to using ssh-agent here (or the default openssh IdentityFile) when no explicit auth args are set in dulwich.