source/codedesk/runner.py

"""Per-episode workspaces and command execution for the coding tasks.

The server runs as root inside its container; every command the policy runs, and every test run, executes as the
unprivileged `nobody` user in the episode's workspace, with CPU, memory, file-size and process limits applied by a
shell prelude (no preexec_fn, which can deadlock in threaded servers). The server's code and the bug catalog are
root-only, so the policy cannot read hidden tests or the original sources. Outside a root container (local
development) commands run as the current user.
"""

import os
import shutil
import signal
import subprocess
import tempfile
import threading
from pathlib import Path

NOBODY = 65534
LIMITS = "ulimit -t 120 -v 4000000 -f 100000 -u 512 2>/dev/null; "
MAX_OUTPUT = 4000
_SLOTS = threading.BoundedSemaphore(4)
AS_ROOT = hasattr(os, "geteuid") and os.geteuid() == 0


def new_workspace(prefix: str = "ws-") -> Path:
    path = Path(tempfile.mkdtemp(prefix=prefix))
    if AS_ROOT:
        os.chown(path, NOBODY, NOBODY)
    path.chmod(0o755 if not AS_ROOT else 0o700)
    return path


def give_to_nobody(root: Path):
    if not AS_ROOT:
        return
    for dirpath, dirnames, filenames in os.walk(root):
        os.chown(dirpath, NOBODY, NOBODY)
        for name in filenames:
            p = os.path.join(dirpath, name)
            if not os.path.islink(p):
                os.chown(p, NOBODY, NOBODY)


def remove(path: Path):
    shutil.rmtree(path, ignore_errors=True)


def _drain(stream, chunks, keep=200_000):
    kept = 0
    while block := stream.read(8192):
        if kept < keep:
            chunks.append(block[:keep - kept])
            kept += len(chunks[-1])


def run(command: str, cwd: Path, timeout: int = 60, env_extra: dict | None = None) -> tuple[int, str]:
    """Run a shell command as nobody in cwd. Returns (exit code, combined output, tail-trimmed)."""
    env = {"PATH": "/usr/local/bin:/usr/bin:/bin", "HOME": str(cwd), "LANG": "C.UTF-8", "PYTHONDONTWRITEBYTECODE": "1",
           "PYTHONPATH": f"{cwd}:{cwd}/src", "TMPDIR": str(cwd), "MPLBACKEND": "Agg", **(env_extra or {})}
    kwargs = dict(user=NOBODY, group=NOBODY, extra_groups=[]) if AS_ROOT else {}
    with _SLOTS:
        proc = subprocess.Popen(["/bin/sh", "-c", LIMITS + command], cwd=cwd, env=env, stdin=subprocess.DEVNULL,
                                stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True, errors="replace",
                                start_new_session=True, **kwargs)
        chunks = []
        reader = threading.Thread(target=_drain, args=(proc.stdout, chunks), daemon=True)
        reader.start()
        try:
            proc.wait(timeout=timeout)
            timed_out = False
        except subprocess.TimeoutExpired:
            timed_out = True
        try:  # stop everything the command started
            os.killpg(proc.pid, signal.SIGKILL)
        except (ProcessLookupError, PermissionError):
            pass
        proc.wait()
        reader.join(2)
    out = "".join(chunks)
    if timed_out:
        out += f"\n[timed out after {timeout} s]"
    return (124 if timed_out else proc.returncode), out


def trim(out: str, limit: int = MAX_OUTPUT) -> str:
    if len(out) <= limit:
        return out
    return f"[... {len(out) - limit} characters cut ...]\n" + out[-limit:]