Fix false-positive margin warning in forkserver workers (#4506)

Author: h-mayorquin

src/spikeinterface/core/__init__.py

@@ -83,6 +83,7 @@
     get_global_job_kwargs,
     set_global_job_kwargs,
     reset_global_job_kwargs,
+    is_set_global_job_kwargs_set,
 )
 
 # tools

src/spikeinterface/preprocessing/filter.py

@@ -3,7 +3,12 @@
 import numpy as np
 
 from spikeinterface.core.core_tools import define_function_handling_dict_from_class
-from spikeinterface.core import get_chunk_with_margin, ensure_chunk_size, get_global_job_kwargs
+from spikeinterface.core import (
+    get_chunk_with_margin,
+    ensure_chunk_size,
+    get_global_job_kwargs,
+    is_set_global_job_kwargs_set,
+)
 
 from .basepreprocessor import BasePreprocessor, BasePreprocessorSegment
 
@@ -118,7 +123,7 @@ def __init__(
         margin = int(margin_ms * fs / 1000.0)
 
         global_job_kwargs_chunk_size = ensure_chunk_size(recording, **get_global_job_kwargs())
-        if margin > MARGIN_TO_CHUNK_PERCENT_WARNING * global_job_kwargs_chunk_size:
+        if is_set_global_job_kwargs_set() and margin > MARGIN_TO_CHUNK_PERCENT_WARNING * global_job_kwargs_chunk_size:
             warnings.warn(
                 f"The margin size ({margin} samples) is more than {int(MARGIN_TO_CHUNK_PERCENT_WARNING * 100)}% "
                 f"of the global chunk size {global_job_kwargs_chunk_size} samples. This may lead to performance bottlenecks when "