Initial commit: multi-peer torrent download engine

Reactor/loop-pool engine with TCP/µTP/MSE transports, per-connection
pipelining, priority-driven piece selection with endgame, and the Python
FFI test harness.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

interop/seed_rtorrent.py

@@ -0,0 +1,80 @@
+from __future__ import annotations
+
+import argparse
+import os
+import signal
+import subprocess
+import time
+
+from common import touch_ready, wait_for_tcp
+
+
+def write_rc(path: str, torrent: str, data: str, port: int, session: str) -> None:
+    os.makedirs(session, exist_ok=True)
+    with open(path, "w", encoding="utf-8") as f:
+        f.write(f"""
+directory.default.set = {data}
+session.path.set = {session}
+network.port_range.set = {port}-{port}
+network.port_random.set = no
+dht.mode.set = disable
+protocol.pex.set = no
+trackers.use_udp.set = no
+network.http.max_open.set = 0
+pieces.hash.on_completion.set = no
+""")
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="Seed a fixture with rTorrent.")
+    ap.add_argument("--torrent", required=True)
+    ap.add_argument("--data", required=True)
+    ap.add_argument("--port", type=int, required=True)
+    args = ap.parse_args()
+
+    rc = "/tmp/rtorrent.rc"
+    session = "/tmp/rtorrent-session"
+    write_rc(rc, args.torrent, args.data, args.port, session)
+    env = os.environ.copy()
+    env.setdefault("TERM", "xterm")
+    command = f"rtorrent -n -o import={rc}"
+    proc = subprocess.Popen(
+        ["script", "-q", "-e", "-c", command, "/dev/null"],
+        env=env,
+        stdin=subprocess.PIPE,
+    )
+    stop = False
+
+    def _stop(signum, frame):
+        nonlocal stop
+        stop = True
+        proc.terminate()
+        subprocess.run(["pkill", "-TERM", "rtorrent"], check=False)
+
+    signal.signal(signal.SIGTERM, _stop)
+    signal.signal(signal.SIGINT, _stop)
+
+    try:
+        wait_for_tcp("127.0.0.1", args.port, timeout=45)
+        if proc.stdin:
+            proc.stdin.write(b"\x7f" + args.torrent.encode("utf-8") + b"\n")
+            proc.stdin.flush()
+            time.sleep(2)
+        if proc.poll() is not None:
+            return proc.returncode or 1
+        touch_ready()
+        print(f"rTorrent seeding on {args.port}", flush=True)
+        while not stop:
+            if subprocess.run(["pgrep", "rtorrent"], stdout=subprocess.DEVNULL,
+                              stderr=subprocess.DEVNULL).returncode != 0:
+                return 1
+            time.sleep(1)
+        return 0
+    finally:
+        subprocess.run(["pkill", "-TERM", "rtorrent"], check=False)
+        if proc.poll() is None:
+            proc.terminate()
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())