[diffusion] fix: fix PicklingError with --backend diffusers on non-T2I models (#21472)

Co-authored-by: Yufeng He <40085740+universeplayer@users.noreply.github.com>

python/sglang/multimodal_gen/configs/pipeline_configs/diffusers_generic.py

@@ -78,3 +78,52 @@ class DiffusersGenericPipelineConfig(PipelineConfig):
         Pass through - diffusers handles frame count.
         """
         return num_frames
+
+
+# Static subclasses for each non-default task type.
+# These exist so that _get_diffusers_model_info() can swap the task_type without
+# dynamically creating a class via make_dataclass -- dynamic classes break pickle
+# when multiprocessing uses the 'spawn' start method (see #21453).
+
+
+@dataclass
+class DiffusersT2VPipelineConfig(DiffusersGenericPipelineConfig):
+    task_type: ModelTaskType = ModelTaskType.T2V
+
+
+@dataclass
+class DiffusersI2VPipelineConfig(DiffusersGenericPipelineConfig):
+    task_type: ModelTaskType = ModelTaskType.I2V
+
+
+@dataclass
+class DiffusersTI2VPipelineConfig(DiffusersGenericPipelineConfig):
+    task_type: ModelTaskType = ModelTaskType.TI2V
+
+
+@dataclass
+class DiffusersI2IPipelineConfig(DiffusersGenericPipelineConfig):
+    task_type: ModelTaskType = ModelTaskType.I2I
+
+
+@dataclass
+class DiffusersTI2IPipelineConfig(DiffusersGenericPipelineConfig):
+    task_type: ModelTaskType = ModelTaskType.TI2I
+
+
+@dataclass
+class DiffusersI2MPipelineConfig(DiffusersGenericPipelineConfig):
+    task_type: ModelTaskType = ModelTaskType.I2M
+
+
+DIFFUSERS_TASK_TYPE_TO_CONFIG: dict[
+    ModelTaskType, type[DiffusersGenericPipelineConfig]
+] = {
+    ModelTaskType.T2I: DiffusersGenericPipelineConfig,
+    ModelTaskType.T2V: DiffusersT2VPipelineConfig,
+    ModelTaskType.I2V: DiffusersI2VPipelineConfig,
+    ModelTaskType.TI2V: DiffusersTI2VPipelineConfig,
+    ModelTaskType.I2I: DiffusersI2IPipelineConfig,
+    ModelTaskType.TI2I: DiffusersTI2IPipelineConfig,
+    ModelTaskType.I2M: DiffusersI2MPipelineConfig,
+}

python/sglang/multimodal_gen/registry.py

@@ -11,7 +11,6 @@ import dataclasses
 import importlib
 import os
 import pkgutil
-import sys
 from functools import lru_cache
 from typing import (
     TYPE_CHECKING,
@@ -453,6 +452,7 @@ def _get_diffusers_model_info(
     works correctly even under the diffusers backend.
     """
     from sglang.multimodal_gen.configs.pipeline_configs.diffusers_generic import (
+        DIFFUSERS_TASK_TYPE_TO_CONFIG,
         DiffusersGenericPipelineConfig,
     )
     from sglang.multimodal_gen.configs.sample.diffusers_generic import (
@@ -465,36 +465,17 @@ def _get_diffusers_model_info(
     sampling_param_cls = DiffusersGenericSamplingParams
     pipeline_config_cls = DiffusersGenericPipelineConfig
 
-    # If there is a registered native config for this model, inherit its task_type
+    # If there is a registered native config for this model, inherit its task_type.
+    # We use pre-defined static subclasses instead of make_dataclass so the config
+    # class is pickle-safe for multiprocessing spawn (fixes #21453).
     if model_path is not None:
         config_info = _get_config_info(model_path, model_id=model_id)
         if config_info is not None:
             sampling_param_cls = config_info.sampling_param_cls
             native_task_type = config_info.pipeline_config_cls.task_type
             if native_task_type != DiffusersGenericPipelineConfig.task_type:
-                pipeline_config_cls = dataclasses.make_dataclass(
+                pipeline_config_cls = DIFFUSERS_TASK_TYPE_TO_CONFIG.get(
-                    "DiffusersGenericPipelineConfig",
+                    native_task_type, DiffusersGenericPipelineConfig
-                    [
-                        (
-                            "task_type",
-                            type(native_task_type),
-                            dataclasses.field(default=native_task_type),
-                        )
-                    ],
-                    bases=(DiffusersGenericPipelineConfig,),
-                )
-                # make_dataclass sets __module__="types"; fix for pickle.
-                pipeline_config_cls.__module__ = (
-                    DiffusersGenericPipelineConfig.__module__
-                )
-                pipeline_config_cls.__qualname__ = (
-                    DiffusersGenericPipelineConfig.__qualname__
-                )
-                parent_module = sys.modules[DiffusersGenericPipelineConfig.__module__]
-                setattr(
-                    parent_module,
-                    DiffusersGenericPipelineConfig.__name__,
-                    pipeline_config_cls,
                 )
                 logger.debug(
                     "Inherited task_type=%s from native config for diffusers backend",