feat(config): add multiprocessing option to DataLoader context and sets spawn as default (#4139)

* Add dataloader_multiprocessing_context, default to spawn

Make the DataLoader multiprocessing start method configurable on
TrainPipelineConfig and default it to 'spawn'.

The previous default (fork on Linux) is unsafe with libraries that hold
non-fork-safe state in the parent process — common ones in this codebase
are PyAV, torchcodec, and the ffmpeg shared libs they wrap. Symptoms
reported in #2488, #2209, and observed locally include:

- multiprocessing.context.AuthenticationError: digest received was wrong
- RuntimeError: Pin memory thread exited unexpectedly
- RuntimeError: DataLoader worker exited unexpectedly
- Random SIGSEGV inside worker processes during video decode

Switching to spawn re-imports modules cleanly in each worker and
eliminates these failure modes. Added the setting as a config field
rather than hard-coding so users on platforms where fork is preferred
can opt back in via --dataloader-multiprocessing-context=fork.

* Address review: shorten config comment, note spawn startup tradeoff

Per @jashshah999, mention that spawn workers re-import modules and so
add some startup time vs fork. Also trim the failure-mode dump from
the inline comment — the linked issue covers the symptoms in detail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore(scripts): add multiprocessing_context safeguards

* chore(config): add libs note

---------

Co-authored-by: 0o8o0-blip <0o8o0-blip@users.noreply.github.com>

src/lerobot/configs/train.py

@@ -14,6 +14,7 @@
 import builtins
 import datetime as dt
 import json
+import multiprocessing
 import os
 import tempfile
 from dataclasses import dataclass, field
@@ -101,6 +102,12 @@ class TrainPipelineConfig(HubMixin):
     batch_size: int = 8
     prefetch_factor: int = 4
     persistent_workers: bool = True
+    # DataLoader worker start method. "spawn" is safer than "fork" with
+    # non-fork-safe libs (PyAV / torchcodec / ffmpeg), but adds some
+    # worker-startup time per run since workers re-import modules instead
+    # of inheriting parent state. Override with `--dataloader_multiprocessing_context=fork`
+    # when appropriate, or set it to `null` to use Python's platform default.
+    dataloader_multiprocessing_context: str | None = "spawn"
     steps: int = 100_000
     # Run policy in the simulation environment every N steps to measure reward/success (0 = disabled).
     env_eval_freq: int = 20_000
@@ -212,6 +219,17 @@ class TrainPipelineConfig(HubMixin):
             self.reward_model.pretrained_path = str(policy_dir)
 
     def validate(self) -> None:
+        available_contexts = multiprocessing.get_all_start_methods()
+        if (
+            self.dataloader_multiprocessing_context is not None
+            and self.dataloader_multiprocessing_context not in available_contexts
+        ):
+            raise ValueError(
+                "`dataloader_multiprocessing_context` must be None or one of "
+                f"{available_contexts} on this platform, got "
+                f"{self.dataloader_multiprocessing_context!r}."
+            )
+
         self._resolve_pretrained_from_cli()
 
         if self.policy is None and self.reward_model is None:

src/lerobot/scripts/lerobot_train.py

@@ -71,6 +71,16 @@ from lerobot.utils.utils import (
 from .lerobot_eval import eval_policy_all
 
 
+def _dataloader_worker_kwargs(cfg: TrainPipelineConfig) -> dict[str, Any]:
+    """Return worker-only DataLoader options, disabling them for single-process loading."""
+    workers_enabled = cfg.num_workers > 0
+    return {
+        "prefetch_factor": cfg.prefetch_factor if workers_enabled else None,
+        "persistent_workers": cfg.persistent_workers and workers_enabled,
+        "multiprocessing_context": cfg.dataloader_multiprocessing_context if workers_enabled else None,
+    }
+
+
 def update_policy(
     train_metrics: MetricsTracker,
     policy: PreTrainedPolicy,
@@ -473,8 +483,7 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         pin_memory=device.type == "cuda",
         drop_last=False,
         collate_fn=collate_fn,
-        prefetch_factor=cfg.prefetch_factor if cfg.num_workers > 0 else None,
-        persistent_workers=cfg.persistent_workers and cfg.num_workers > 0,
+        **_dataloader_worker_kwargs(cfg),
     )
 
     # Build eval dataloader if a held-out split exists
@@ -500,8 +509,7 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
             pin_memory=device.type == "cuda",
             drop_last=False,
             collate_fn=eval_collate_fn,
-            prefetch_factor=cfg.prefetch_factor if cfg.num_workers > 0 else None,
-            persistent_workers=cfg.persistent_workers and cfg.num_workers > 0,
+            **_dataloader_worker_kwargs(cfg),
         )
 
     # Prepare everything with accelerator