mirror of
https://github.com/lancedb/lancedb.git
synced 2026-08-18 12:08:35 +00:00
Weston Pace
a17c241e86
## Summary PyTorch's `DataLoader` uses fork-based multiprocessing by default on Linux, but threads do not survive `fork()`. LanceDB's Python bindings drive async work through two threaded layers, both of which become inert in a forked child: - `BackgroundEventLoop` runs an asyncio loop on a Python `threading.Thread`. - `pyo3-async-runtimes::tokio` holds a global multi-threaded tokio runtime whose worker threads also die on fork — and its runtime lives in a `OnceLock` that cannot be replaced after first use. As a result, any `Permutation` (or other async API) used inside a fork-based `DataLoader` worker hangs indefinitely. This PR makes both layers fork-safe so `Permutation` works as a `torch.utils.data.Dataset` with `num_workers > 0`. ## Approach ### Rust — new `python/src/runtime.rs` Mirrors the pattern used in [Lance's Python bindings](https://github.com/lance-format/lance/blob/456198cd6f42be07f99617a6d7e39d6209cdf3cc/python/src/lib.rs#L139), adapted for the async-bridge use case. - `LanceRuntime` implements `pyo3_async_runtimes::generic::Runtime + ContextExt`, backed by an `AtomicPtr<tokio::runtime::Runtime>` we own (sidestepping `pyo3-async-runtimes`'s frozen `OnceLock` global). - A `pthread_atfork(after_in_child)` handler nulls the pointer; the next `spawn` rebuilds the runtime in the child. The previous runtime is intentionally **leaked** — calling `Drop` would try to join now-dead worker threads and hang. - `runtime::future_into_py` is a drop-in for `pyo3_async_runtimes::tokio::future_into_py`. All ~80 call sites in `arrow.rs` / `connection.rs` / `permutation.rs` / `query.rs` / `table.rs` are updated to route through it. - `python/Cargo.toml` adds `libc = "0.2"` and the tokio `rt-multi-thread` feature. ### Python — `lancedb/background_loop.py` - Refactors `BackgroundEventLoop.__init__` to a reusable `_start()` method. - An `os.register_at_fork(after_in_child=…)` hook calls `LOOP._start()` to give the singleton a fresh asyncio loop and thread **in place**. This matters because the rest of the codebase imports `LOOP` via `from .background_loop import LOOP` — rebinding the module attribute would leave those references holding the dead loop. ### Python — `lancedb/__init__.py` Removes the `__warn_on_fork` pre-fork warning (and the now-unused `import warnings`). Fork is supported. ## Test plan - [x] New `test_permutation_dataloader_fork_workers` in `python/tests/test_torch.py`: runs a `Permutation` through `torch.utils.data.DataLoader(num_workers=2, multiprocessing_context="fork")` inside a spawn-isolated child with a 30s hang detector. **Pre-fix**: timed out at 36s. **Post-fix**: passes in ~3.6s. - [x] New `test_remote_connection_after_fork` in `python/tests/test_remote_db.py`: forks a child that creates a fresh `lancedb.connect(...)` against a mock HTTP server and calls `table_names()`; passes in <1s, validates the runtime reset is sufficient for fresh remote clients. - [x] All 62 tests in `test_torch.py` + `test_permutation.py` pass. - [x] All 35 tests in `test_remote_db.py` pass. - [x] `test_table.py` (87) + `test_db.py` + `test_query.py` (157, minus one unrelated `sentence_transformers` import skip) — 244 passing. - [x] `cargo clippy -p lancedb-python --tests` clean. - [x] `cargo fmt`, `ruff check`, `ruff format` all clean. ## Known limitation (follow-up) This PR makes a **freshly-built** `lancedb.connect(...)` work in a forked child. An **inherited** `Connection` from the parent still carries an inherited `reqwest::Client` whose hyper connection pool references socket FDs and TCP/TLS state shared with the parent — using it from the child after fork is unsafe (especially with HTTP/1.1 keep-alive). The recommended pattern for fork-based `DataLoader` workers that hit a remote DB is to construct a new connection inside the worker. Auto-clearing inherited HTTP client pools on fork would require tracking live `Connection` instances in `lancedb` core and is left for a follow-up PR. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>