Run every command on the build VM over SSH (M1.3)

execution.SSHHost.execute is now the only way the agent runs anything
(L13). Commands travel as a script on SSH's stdin, so the secrets handed
to a call are never on a command line, and every value handed out is
redacted from the output. Timeouts kill the remote process group, and
an unreachable host is reported as such, not as a failed command.

Workspace and GitRepo run on the build VM. The clone uses the
repository's SSH URL, so no credential is stored in it (#89); fetches
and pushes get the push key over stdin, in a short-lived ssh-agent,
and PRs are still opened from the agent's host with its API token.
The bench runs on the build host too.

Tests reach "the build VM" through a throwaway local sshd (nisse_dev
.testing); CI installs openssh-server. The live push test now pushes
with the sandbox's deploy key (NISSE_PUSH_KEY).

Fixes #89.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/src/nisse_dev/execution.py

@@ -0,0 +1,207 @@
+"""Running commands on a tool VM over SSH (architecture L13).
+
+``SSHHost.execute`` is the only way the agent runs anything: there is no
+local execution path, so the host the agent lives on never runs a command a
+model chose. The harness binds the host; a model never names one.
+
+How a command travels. The remote side runs ``bash -s`` and reads a small
+script from SSH's stdin: the secrets handed to this one call as ``export``
+lines, a ``cd``, then the command under ``timeout``. Nothing secret is ever on
+a command line (``ps`` and ``/proc/*/cmdline`` would show it); the values exist
+only in that shell and the command's environment. The command's own stdin is
+``/dev/null``, or a here-document carrying ``stdin``, so it can never read the
+rest of the script.
+
+Every value handed out is redacted from what comes back (L12): a test that
+prints its environment must not put a key into the transcript.
+
+The connection is reused across calls (OpenSSH's ControlMaster), so a tool
+call costs one round trip, not a handshake.
+"""
+
+import os
+import secrets as _random
+import shlex
+import subprocess
+import tempfile
+from collections.abc import Iterable, Mapping
+from dataclasses import dataclass
+from pathlib import Path
+
+# The remote command's exit code is passed back as is, except that 255 is
+# what ssh itself uses for "could not connect"; a command exiting 255 is
+# reported as 254 so the two can't be confused.
+SSH_FAILED = 255
+_TIMED_OUT = 124  # coreutils `timeout`
+_KILL_GRACE_S = 5
+# The local backstop, beyond the remote timeout, for a connection that hangs.
+_LOCAL_SLACK_S = 30
+
+REDACTED = "[redacted]"
+
+
+class HostUnavailable(RuntimeError):
+    """The host could not be reached: not something the command did."""
+
+
+@dataclass(frozen=True)
+class Result:
+    exit_code: int
+    stdout: str
+    stderr: str
+    timed_out: bool = False
+
+    @property
+    def ok(self) -> bool:
+        return self.exit_code == 0 and not self.timed_out
+
+
+def redact(text: str, values: Iterable[str]) -> str:
+    """Mask every non-trivial secret value in ``text``. Longest first, so a
+    value containing another is masked whole."""
+    for value in sorted((v for v in values if len(v) >= 4), key=len, reverse=True):
+        text = text.replace(value, REDACTED)
+    return text
+
+
+def _heredoc(body: str) -> tuple[str, str]:
+    """A here-document redirection whose delimiter can't occur in ``body``."""
+    while True:
+        tag = f"NISSE_EOF_{_random.token_hex(8)}"
+        if tag not in body:
+            return f"<<'{tag}'", f"\n{body}\n{tag}"
+
+
+def _script(
+    command: str,
+    cwd: str | None,
+    secrets: Mapping[str, str],
+    stdin: str | None,
+    timeout_s: int,
+) -> str:
+    lines = [f"export {name}={shlex.quote(value)}" for name, value in secrets.items()]
+    if cwd is not None:
+        lines.append(f"cd -- {shlex.quote(cwd)} || exit 125")
+    redirect, body = ("< /dev/null", "") if stdin is None else _heredoc(stdin)
+    # `timeout` runs the command in its own process group and signals the
+    # whole group, so a backgrounded grandchild doesn't outlive it.
+    lines.append(
+        f"timeout --kill-after={_KILL_GRACE_S} {timeout_s} "
+        f"bash -c {shlex.quote(command)} {redirect}{body}"
+    )
+    lines.append("rc=$?; [ $rc -eq 255 ] && rc=254; exit $rc")
+    return "\n".join(lines) + "\n"
+
+
+class SSHHost:
+    """One tool VM, reached as ``user@address`` with a private key."""
+
+    def __init__(
+        self,
+        address: str,
+        *,
+        key: Path,
+        state_dir: Path,
+        user: str = "root",
+        port: int = 22,
+    ) -> None:
+        self.address = address
+        self.user = user
+        self.port = port
+        self._key = Path(key)
+        state_dir = Path(state_dir)
+        state_dir.mkdir(parents=True, exist_ok=True)
+        self._known_hosts = state_dir / "known_hosts"
+        # A Unix socket's path is limited to about 100 bytes, so the shared
+        # connection's socket lives in a short private directory rather than
+        # beside the state, named by %C, ssh's hash of the connection.
+        sockets = Path(tempfile.gettempdir()) / f"nisse-ssh-{os.getuid()}"
+        sockets.mkdir(mode=0o700, exist_ok=True)
+        self._control = sockets / "%C"
+
+    def describe(self) -> str:
+        return f"{self.user}@{self.address}"
+
+    def _ssh(self) -> list[str]:
+        return [
+            "ssh",
+            "-i",
+            str(self._key),
+            "-p",
+            str(self.port),
+            "-o",
+            "BatchMode=yes",
+            "-o",
+            "IdentitiesOnly=yes",
+            "-o",
+            f"UserKnownHostsFile={self._known_hosts}",
+            "-o",
+            "StrictHostKeyChecking=accept-new",
+            "-o",
+            "ControlMaster=auto",
+            "-o",
+            f"ControlPath={self._control}",
+            "-o",
+            "ControlPersist=120",
+            "-o",
+            "ConnectTimeout=15",
+            "-o",
+            "ServerAliveInterval=15",
+            "-o",
+            "LogLevel=ERROR",
+            f"{self.user}@{self.address}",
+            "bash -s",
+        ]
+
+    def execute(
+        self,
+        command: str,
+        *,
+        cwd: str | None = None,
+        secrets: Mapping[str, str] | None = None,
+        stdin: str | None = None,
+        timeout_s: int = 120,
+    ) -> Result:
+        """Run ``command`` with bash in ``cwd``. ``secrets`` become environment
+        variables of this one command and are redacted from its output.
+        Raises HostUnavailable when the host can't be reached."""
+        handed_out = dict(secrets or {})
+        script = _script(command, cwd, handed_out, stdin, timeout_s)
+        try:
+            completed = subprocess.run(
+                self._ssh(),
+                input=script.encode(),
+                capture_output=True,
+                timeout=timeout_s + _KILL_GRACE_S + _LOCAL_SLACK_S,
+                check=False,
+            )
+        except subprocess.TimeoutExpired as exc:
+            raise HostUnavailable(
+                f"{self.describe()} stopped responding during a command"
+            ) from exc
+        except OSError as exc:
+            raise HostUnavailable(f"could not run ssh: {exc}") from exc
+        stdout = completed.stdout.decode(errors="replace")
+        stderr = completed.stderr.decode(errors="replace")
+        if completed.returncode == SSH_FAILED:
+            raise HostUnavailable(
+                f"could not reach {self.describe()}: "
+                + redact(stderr.strip() or "ssh failed", handed_out.values())
+            )
+        values = handed_out.values()
+        return Result(
+            exit_code=completed.returncode,
+            stdout=redact(stdout, values),
+            stderr=redact(stderr, values),
+            timed_out=completed.returncode in (_TIMED_OUT, 137),
+        )
+
+    def close(self) -> None:
+        """End the shared connection, if one is open."""
+        args = self._ssh()[:-2]  # without the destination and the command
+        subprocess.run(
+            [*args, "-O", "exit", f"{self.user}@{self.address}"],
+            capture_output=True,
+            timeout=10,
+            check=False,
+        )