nixos-rebuild-ng: allow overriding default ssh opts via environment (#538973)

pkgs/by-name/ni/nixos-rebuild-ng/nixos-rebuild.8.scd

@@ -379,6 +379,11 @@ NIX_PATH
 NIX_SSHOPTS
 	Additional options to be passed to ssh on the command line.
 
+NIXOS_REBUILD_SSH_DEFAULT_OPTS
+	Replaces the built-in default ssh options (connection sharing via a
+	private _ControlMaster_ that is closed on exit). If empty, no
+	default options are added.
+
 NIX_SUDOOPTS
 	Additional options to be passed to sudo on the command line.
 

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py

@@ -26,7 +26,7 @@ from .models import (
     Profile,
     Remote,
 )
-from .process import SSH_DEFAULT_OPTS, run_wrapper
+from .process import run_wrapper, ssh_default_opts
 from .utils import Args, dict_to_flags
 
 FLAKE_FLAGS: Final = ["--extra-experimental-features", "nix-command flakes"]
@@ -193,7 +193,7 @@ def copy_closure(
     Also supports copying a closure from a remote to another remote."""
 
     sshopts = os.getenv("NIX_SSHOPTS", "")
-    env = {"NIX_SSHOPTS": " ".join(filter(lambda x: x, [sshopts, *SSH_DEFAULT_OPTS]))}
+    env = {"NIX_SSHOPTS": " ".join(filter(lambda x: x, [sshopts, *ssh_default_opts()]))}
 
     def nix_copy_closure(host: Remote, to: bool) -> None:
         run_wrapper(

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/process.py

@@ -31,6 +31,14 @@ SSH_DEFAULT_OPTS: Final = [
 ]
 
 
+def ssh_default_opts() -> list[str]:
+    "Default ssh options appended after NIX_SSHOPTS."
+    env = os.getenv("NIXOS_REBUILD_SSH_DEFAULT_OPTS")
+    if env is None:
+        return SSH_DEFAULT_OPTS
+    return shlex.split(env)
+
+
 @dataclass(frozen=True)
 class Remote:
     host: str
@@ -133,7 +141,7 @@ def run_wrapper(
         ssh_args: list[Arg] = [
             "ssh",
             *remote.opts,
-            *SSH_DEFAULT_OPTS,
+            *ssh_default_opts(),
             remote.ssh_host(),
             "--",
             *[_quote_remote_arg(a) for a in remote_run_args],
@@ -282,7 +290,7 @@ def _kill_long_running_ssh_process(args: Args, remote: Remote) -> None:
             [
                 "ssh",
                 *remote.opts,
-                *SSH_DEFAULT_OPTS,
+                *ssh_default_opts(),
                 remote.ssh_host(),
                 "--",
                 "pkill",

pkgs/by-name/ni/nixos-rebuild-ng/src/tests/test_nix.py

@@ -275,6 +275,17 @@ def test_copy_closure(monkeypatch: MonkeyPatch) -> None:
             },
         )
 
+    # NIXOS_REBUILD_SSH_DEFAULT_OPTS replaces the ControlMaster defaults
+    monkeypatch.setenv("NIX_SSHOPTS", "-oControlPath=/run/user/1000/%C")
+    monkeypatch.setenv("NIXOS_REBUILD_SSH_DEFAULT_OPTS", "")
+    with patch(get_qualified_name(n.run_wrapper, n), autospec=True) as mock_run:
+        n.copy_closure(closure, target_host)
+        mock_run.assert_called_with(
+            ["nix-copy-closure", "--to", "user@target.host", closure],
+            append_local_env={"NIX_SSHOPTS": "-oControlPath=/run/user/1000/%C"},
+        )
+    monkeypatch.delenv("NIXOS_REBUILD_SSH_DEFAULT_OPTS")
+
     monkeypatch.setenv("NIX_SSHOPTS", "--ssh build-target-opt")
     env = {"NIX_SSHOPTS": " ".join(["--ssh build-target-opt", *p.SSH_DEFAULT_OPTS])}
     with patch(get_qualified_name(n.run_wrapper, n), autospec=True) as mock_run:

pkgs/by-name/ni/nixos-rebuild-ng/src/tests/test_process.py

@@ -234,6 +234,52 @@ def test_remote_from_name(monkeypatch: MonkeyPatch) -> None:
     )
 
 
+def test_ssh_default_opts(monkeypatch: MonkeyPatch) -> None:
+    monkeypatch.delenv("NIXOS_REBUILD_SSH_DEFAULT_OPTS", raising=False)
+    assert p.ssh_default_opts() == p.SSH_DEFAULT_OPTS
+
+    monkeypatch.setenv(
+        "NIXOS_REBUILD_SSH_DEFAULT_OPTS", "-o ControlPath=/run/user/1000/%C"
+    )
+    assert p.ssh_default_opts() == ["-o", "ControlPath=/run/user/1000/%C"]
+
+    monkeypatch.setenv("NIXOS_REBUILD_SSH_DEFAULT_OPTS", "")
+    assert p.ssh_default_opts() == []
+
+
+@patch.dict(
+    p.os.environ,
+    {"PATH": "/path/to/bin", "NIXOS_REBUILD_SSH_DEFAULT_OPTS": ""},
+    clear=True,
+)
+@patch("subprocess.run", autospec=True)
+def test_run_wrapper_ssh_default_opts_override(mock_run: Any) -> None:
+    p.run_wrapper(
+        ["test"],
+        check=True,
+        remote=m.Remote("user@localhost", ["-p", "2222"], "ssh"),
+    )
+    mock_run.assert_called_with(
+        [
+            "ssh",
+            "-p",
+            "2222",
+            "user@localhost",
+            "--",
+            "/bin/sh",
+            "-c",
+            """'exec /usr/bin/env -i PATH="${PATH-}" "$@"'""",
+            "sh",
+            "test",
+        ],
+        check=True,
+        text=True,
+        errors="surrogateescape",
+        env=None,
+        input=None,
+    )
+
+
 def test_ssh_host() -> None:
     ssh_remotes = {
         "user@[fe80::1%25eth0]": "user@fe80::1%eth0",