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>

harness/engine_ffi.py

@@ -0,0 +1,202 @@
+"""
+ctypes bindings for the multi-peer engine ABI (include/engine.h).
+
+The engine owns a pool of event-loop threads; torrents are pinned to a loop and
+every connection of a torrent lives there. Each loop has its own arena, so a
+delivered block names both the loop and the slot. We wrap each loop's arena once
+as a zero-copy ``memoryview`` and slice per block; returning the slot via
+``release()`` is what lets the engine issue new requests (credit-based flow
+control).
+"""
+from __future__ import annotations
+
+import ctypes as C
+import os
+
+# peer_state / peer_error mirrors of include/engine.h
+STATE_IDLE, STATE_CONNECTING, STATE_HANDSHAKE, STATE_CHOKED, \
+    STATE_RUNNING, STATE_STOPPED, STATE_ERROR = range(7)
+STATE_NAMES = ["IDLE", "CONNECTING", "HANDSHAKE", "CHOKED",
+               "RUNNING", "STOPPED", "ERROR"]
+ERROR_NAMES = ["OK", "CONNECT", "HANDSHAKE", "CLOSED", "PROTOCOL", "IO", "NOMEM"]
+
+BLOCK_SIZE = 16384
+
+
+class EngineConfig(C.Structure):
+    _fields_ = [
+        ("loop_count", C.c_uint32),
+        ("slots_per_loop", C.c_uint32),
+        ("max_pipeline", C.c_uint32),
+        ("request_timeout_ms", C.c_uint32),
+        ("recv_buffer_bytes", C.c_uint32),
+        ("encryption", C.c_uint32),
+        ("utp", C.c_uint32),
+        ("connect_timeout_ms", C.c_uint32),
+        ("fallback", C.c_uint32),
+    ]
+
+
+class EngineBlock(C.Structure):
+    _fields_ = [
+        ("torrent", C.c_uint32),
+        ("piece", C.c_uint32),
+        ("begin", C.c_uint32),
+        ("len", C.c_uint32),
+        ("loop", C.c_uint32),
+        ("slot", C.c_uint32),
+    ]
+
+
+class TorrentStatus(C.Structure):
+    _fields_ = [
+        ("state", C.c_int32),
+        ("error", C.c_int32),
+        ("bytes_received", C.c_uint64),
+        ("blocks_received", C.c_uint64),
+        ("peers", C.c_uint32),
+        ("peers_connected", C.c_uint32),
+        ("peers_failed", C.c_uint32),
+        ("outstanding", C.c_uint32),
+        ("free_slots", C.c_uint32),
+        ("pipeline_target", C.c_uint32),
+        ("rate_bps", C.c_double),
+        ("rtt_min_ms", C.c_double),
+    ]
+
+
+def _default_lib_path() -> str:
+    here = os.path.dirname(os.path.abspath(__file__))
+    cand = [
+        os.path.join(here, "..", "build", "libtorrentpeer.so"),
+        os.path.join(here, "..", "build", "lib", "libtorrentpeer.so"),
+    ]
+    for p in cand:
+        if os.path.exists(p):
+            return os.path.abspath(p)
+    return os.path.abspath(cand[0])
+
+
+def _load(lib_path: str | None) -> C.CDLL:
+    lib = C.CDLL(lib_path or _default_lib_path())
+    lib.engine_create.restype = C.c_void_p
+    lib.engine_create.argtypes = [C.POINTER(EngineConfig)]
+    lib.engine_destroy.restype = None
+    lib.engine_destroy.argtypes = [C.c_void_p]
+    lib.engine_add_torrent.restype = C.c_int32
+    lib.engine_add_torrent.argtypes = [
+        C.c_void_p, C.POINTER(C.c_uint8), C.POINTER(C.c_uint8),
+        C.c_uint64, C.c_uint64, C.c_uint32,
+    ]
+    lib.engine_add_peer.restype = C.c_int
+    lib.engine_add_peer.argtypes = [C.c_void_p, C.c_uint32, C.c_char_p, C.c_uint16]
+    lib.engine_set_priorities.restype = C.c_int
+    lib.engine_set_priorities.argtypes = [
+        C.c_void_p, C.c_uint32, C.POINTER(C.c_uint8), C.c_uint32]
+    lib.engine_set_priority.restype = C.c_int
+    lib.engine_set_priority.argtypes = [C.c_void_p, C.c_uint32, C.c_uint32, C.c_uint8]
+    lib.engine_request_piece.restype = C.c_int
+    lib.engine_request_piece.argtypes = [C.c_void_p, C.c_uint32, C.c_uint32]
+    lib.engine_poll_ready.restype = C.c_uint32
+    lib.engine_poll_ready.argtypes = [C.c_void_p, C.POINTER(EngineBlock), C.c_uint32]
+    lib.engine_release_slot.restype = None
+    lib.engine_release_slot.argtypes = [C.c_void_p, C.c_uint32, C.c_uint32]
+    lib.engine_wait.restype = C.c_int
+    lib.engine_wait.argtypes = [C.c_void_p, C.c_int]
+    lib.engine_arena_base.restype = C.c_void_p
+    lib.engine_arena_base.argtypes = [C.c_void_p, C.c_uint32]
+    lib.engine_arena_bytes.restype = C.c_uint64
+    lib.engine_arena_bytes.argtypes = [C.c_void_p, C.c_uint32]
+    lib.engine_loop_count.restype = C.c_uint32
+    lib.engine_loop_count.argtypes = [C.c_void_p]
+    lib.engine_torrent_status.restype = None
+    lib.engine_torrent_status.argtypes = [C.c_void_p, C.c_uint32, C.POINTER(TorrentStatus)]
+    return lib
+
+
+class Engine:
+    """Pythonic wrapper around one engine instance (a pool of loops)."""
+
+    def __init__(self, cfg: EngineConfig | None = None,
+                 lib_path: str | None = None, poll_batch: int = 1024):
+        self._lib = _load(lib_path)
+        self._e = self._lib.engine_create(C.byref(cfg) if cfg else None)
+        if not self._e:
+            raise RuntimeError("engine_create failed")
+
+        # One zero-copy memoryview per loop arena.
+        self.nloops = self._lib.engine_loop_count(self._e)
+        self._arenas = []
+        self.arenas = []
+        for i in range(self.nloops):
+            base = self._lib.engine_arena_base(self._e, i)
+            nbytes = self._lib.engine_arena_bytes(self._e, i)
+            buf = (C.c_char * nbytes).from_address(base)
+            self._arenas.append(buf)
+            self.arenas.append(memoryview(buf).cast("B"))
+
+        self._batch = poll_batch
+        self._blocks = (EngineBlock * poll_batch)()
+
+    def add_torrent(self, info_hash: bytes, peer_id: bytes, piece_length: int,
+                    total_size: int, num_pieces: int) -> int:
+        ih = (C.c_uint8 * 20).from_buffer_copy(info_hash)
+        pid = (C.c_uint8 * 20).from_buffer_copy(peer_id)
+        tid = self._lib.engine_add_torrent(self._e, ih, pid, piece_length,
+                                           total_size, num_pieces)
+        if tid < 0:
+            raise RuntimeError("engine_add_torrent failed")
+        return tid
+
+    def add_peer(self, torrent_id: int, ip: str, port: int) -> None:
+        if self._lib.engine_add_peer(self._e, torrent_id, ip.encode(), port) != 0:
+            raise RuntimeError("engine_add_peer failed")
+
+    def set_priorities(self, torrent_id: int, priorities) -> None:
+        buf = bytes(priorities)
+        arr = (C.c_uint8 * len(buf)).from_buffer_copy(buf)
+        if self._lib.engine_set_priorities(self._e, torrent_id, arr, len(buf)) != 0:
+            raise ValueError("set_priorities: length must equal num_pieces")
+
+    def set_priority(self, torrent_id: int, piece_index: int, priority: int) -> None:
+        if self._lib.engine_set_priority(self._e, torrent_id, piece_index, priority) != 0:
+            raise ValueError(f"set_priority({piece_index}) out of range")
+
+    def request_piece(self, torrent_id: int, piece_index: int) -> None:
+        if self._lib.engine_request_piece(self._e, torrent_id, piece_index) != 0:
+            raise ValueError(f"request_piece({piece_index}) out of range")
+
+    def poll_ready(self):
+        """Return a list of EngineBlock for completed blocks (may be empty)."""
+        n = self._lib.engine_poll_ready(self._e, self._blocks, self._batch)
+        return [self._blocks[i] for i in range(n)]
+
+    def block_data(self, loop: int, slot: int, length: int) -> memoryview:
+        off = slot * BLOCK_SIZE
+        return self.arenas[loop][off:off + length]
+
+    def release(self, loop: int, slot: int) -> None:
+        self._lib.engine_release_slot(self._e, loop, slot)
+
+    def wait(self, timeout_ms: int) -> int:
+        return self._lib.engine_wait(self._e, timeout_ms)
+
+    def status(self, torrent_id: int) -> TorrentStatus:
+        st = TorrentStatus()
+        self._lib.engine_torrent_status(self._e, torrent_id, C.byref(st))
+        return st
+
+    def close(self) -> None:
+        if self._e:
+            for mv in self.arenas:
+                mv.release()
+            self.arenas = []
+            self._arenas = []
+            self._lib.engine_destroy(self._e)
+            self._e = None
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        self.close()