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_qbittorrent.py

@@ -0,0 +1,88 @@
+from __future__ import annotations
+
+import argparse
+import os
+import signal
+import subprocess
+import time
+
+from common import touch_ready, wait_for_tcp
+
+
+def write_config(profile: str, data_dir: str, port: int) -> None:
+    cfg_dir = os.path.join(profile, "qBittorrent", "config")
+    os.makedirs(cfg_dir, exist_ok=True)
+    # qBittorrent stores settings in an INI-like file with escaped keys.
+    # These disable internet/discovery paths and fix the peer port.
+    with open(os.path.join(cfg_dir, "qBittorrent.conf"), "w", encoding="utf-8") as f:
+        f.write(f"""[BitTorrent]
+Session\\AddTorrentPaused=false
+Session\\BTProtocol=TCP
+Session\\DHTEnabled=false
+Session\\DefaultSavePath={data_dir}
+Session\\DisableAutoTMMByDefault=true
+Session\\LSDEnabled=false
+Session\\PeXEnabled=false
+Session\\Port={port}
+Session\\QueueingSystemEnabled=false
+Session\\UPnP=false
+
+[LegalNotice]
+Accepted=true
+
+[Preferences]
+WebUI\\Enabled=false
+""")
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="Seed a fixture with qBittorrent-nox.")
+    ap.add_argument("--torrent", required=True)
+    ap.add_argument("--data", required=True)
+    ap.add_argument("--port", type=int, required=True)
+    args = ap.parse_args()
+
+    profile = "/tmp/qbt-profile"
+    write_config(profile, args.data, args.port)
+    cmd = [
+        "qbittorrent-nox",
+        f"--profile={profile}",
+        "--configuration=interop",
+        f"--torrenting-port={args.port}",
+        f"--save-path={args.data}",
+        "--add-paused=false",
+        "--skip-dialog=true",
+        "--skip-hash-check",
+        args.torrent,
+    ]
+    proc = subprocess.Popen(cmd)
+    stop = False
+
+    def _stop(signum, frame):
+        nonlocal stop
+        stop = True
+        proc.terminate()
+
+    signal.signal(signal.SIGTERM, _stop)
+    signal.signal(signal.SIGINT, _stop)
+
+    try:
+        wait_for_tcp("127.0.0.1", args.port, timeout=45)
+        if proc.poll() is not None:
+            return proc.returncode or 1
+        touch_ready()
+        print(f"qBittorrent seeding on {args.port}", flush=True)
+        while not stop and proc.poll() is None:
+            time.sleep(1)
+        return proc.returncode or 0
+    finally:
+        if proc.poll() is None:
+            proc.terminate()
+            try:
+                proc.wait(timeout=10)
+            except subprocess.TimeoutExpired:
+                proc.kill()
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())