Read ~/.ssh/config in the SSH pre-check

The paramiko pre-check ignored HostName, Port and IdentityFile, so hosts that rsync's ssh reached through ~/.ssh/config failed before the backup started.

tests/test_remote.py

@@ -365,7 +365,7 @@ class FakeClient:
         self.policies.append(type(policy).__name__)
 
     def connect(self, hostname: str, **kwargs: Any) -> None:
-        self.connects.append(kwargs)
+        self.connects.append({'hostname': hostname, **kwargs})
         outcome = self.outcomes.pop(0)
 
         if isinstance(outcome, UserWarning):
@@ -404,8 +404,35 @@ class TestLogin:
         assert remote_backup()._ssh_connect(str(tmp_path)) is client
         assert client.known_hosts == f'{tmp_path}/.ssh/known_hosts'
         assert client.connects[0]['timeout'] == sb.SSH_TIMEOUT
+        assert client.connects[0]['hostname'] == 'server'
+        assert 'port' not in client.connects[0]
         assert not client.closed
 
+    def test_ssh_config(self, client: FakeClient, tmp_path: Path) -> None:
+        (tmp_path / '.ssh').mkdir()
+        key = tmp_path / '.ssh' / 'id_backup'
+        key.touch()
+        (tmp_path / '.ssh' / 'config').write_text(
+            f'Host server\n  HostName backup.example.org\n  Port 2222\n'
+            f'  IdentityFile {key}\n  IdentityFile {tmp_path}/.ssh/missing\n')
+        client.outcomes = [ssh_error(), None]
+
+        assert remote_backup()._ssh_connect(str(tmp_path)) is client
+
+        for params in client.connects:
+            assert (params['hostname'], params['port']) == ('backup.example.org', 2222)
+            assert params['key_filename'] == [str(key)]
+
+    def test_bad_ssh_config(self, client: FakeClient, tmp_path: Path, caplog: pytest.LogCaptureFixture) -> None:
+        (tmp_path / '.ssh').mkdir()
+        (tmp_path / '.ssh' / 'config').write_text('Host server\n  Port abc\n')
+        client.outcomes = [None]
+
+        assert remote_backup()._ssh_connect(str(tmp_path)) is client
+        assert client.connects[0]['hostname'] == 'server'
+        assert 'port' not in client.connects[0]
+        assert 'Cannot read' in caplog.text
+
     @pytest.mark.parametrize('answer, connected', [('y', True), (' Yes', True), ('n', False), ('', False)])
     def test_unknown_host_key(self, client: FakeClient, monkeypatch: pytest.MonkeyPatch,
                               answer: str, connected: bool) -> None: