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_libtorrent.py

@@ -0,0 +1,84 @@
+from __future__ import annotations
+
+import argparse
+import os
+import signal
+import time
+
+import libtorrent as lt
+
+from common import touch_ready
+
+
+def settings_for(mode: str, port: int) -> dict:
+    tcp = mode in ("plain", "mse")
+    utp = mode in ("utp", "utp-mse")
+    encrypted = mode in ("mse", "utp-mse")
+    settings = {
+        "listen_interfaces": f"0.0.0.0:{port}",
+        "enable_dht": False,
+        "enable_lsd": False,
+        "enable_upnp": False,
+        "enable_natpmp": False,
+        "enable_outgoing_tcp": tcp,
+        "enable_incoming_tcp": tcp,
+        "enable_outgoing_utp": utp,
+        "enable_incoming_utp": utp,
+        "announce_to_all_trackers": False,
+        "announce_to_all_tiers": False,
+        "alert_mask": 0,
+    }
+    if encrypted:
+        settings.update({
+            "in_enc_policy": int(lt.enc_policy.forced),
+            "out_enc_policy": int(lt.enc_policy.forced),
+            "allowed_enc_level": int(lt.enc_level.rc4),
+            "prefer_rc4": True,
+        })
+    else:
+        settings.update({
+            "in_enc_policy": int(lt.enc_policy.disabled),
+            "out_enc_policy": int(lt.enc_policy.disabled),
+        })
+    return settings
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="Seed a fixture with libtorrent.")
+    ap.add_argument("--torrent", required=True)
+    ap.add_argument("--data", required=True)
+    ap.add_argument("--port", required=True, type=int)
+    ap.add_argument("--mode", required=True, choices=["plain", "mse", "utp", "utp-mse"])
+    args = ap.parse_args()
+
+    stop = False
+
+    def _stop(signum, frame):
+        nonlocal stop
+        stop = True
+
+    signal.signal(signal.SIGTERM, _stop)
+    signal.signal(signal.SIGINT, _stop)
+
+    ses = lt.session(settings_for(args.mode, args.port))
+    h = ses.add_torrent({
+        "ti": lt.torrent_info(args.torrent),
+        "save_path": args.data,
+        "flags": lt.torrent_flags.seed_mode,
+    })
+    deadline = time.time() + 60
+    while time.time() < deadline and not h.status().is_seeding:
+        time.sleep(0.25)
+    if not h.status().is_seeding:
+        raise TimeoutError(f"libtorrent {args.mode} did not enter seed mode")
+    touch_ready()
+    print(f"libtorrent {args.mode} seeding on {args.port}", flush=True)
+    while not stop:
+        time.sleep(1)
+    ses.remove_torrent(h)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
+