BUG - MAG L1D IndexError on spin offsets

[maxinelasp]

Description of the issue

Seeing an index error during calculate_spin_offsets. Probably just needs a check on chunk_epoch to ensure the chunk has any values in it.

Steps to reproduce the issue

["--instrument","mag","--data-level","l1d","--descriptor","norm-srf","--start-date","20260109","--version","v002","--dependency","imap_mag_l1d_norm-srf-e9bfa7c8_20260109_v002.json","--upload-to-sdc"]

imap_mag_l1d_norm-srf-e9bfa7c8_20260109_v002.json

Expected vs Actual behavior

/usr/local/lib/python3.12/site-packages/imap_processing/mag/l1d/mag_l1d_data.py:515: RuntimeWarning: Mean of empty slice
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avg_x = np.nanmean(chunk_vectors[:, 0])
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/usr/local/lib/python3.12/site-packages/imap_processing/mag/l1d/mag_l1d_data.py:516: RuntimeWarning: Mean of empty slice
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avg_y = np.nanmean(chunk_vectors[:, 1])
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Traceback (most recent call last):
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File "/usr/local/bin/imap_cli", line 8, in <module>
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Processing MAG l1d
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sys.exit(main())
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^^^^^^
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File "/usr/local/lib/python3.12/site-packages/imap_processing/cli.py", line 1604, in main
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instrument.process()
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File "/usr/local/lib/python3.12/site-packages/imap_processing/cli.py", line 447, in process
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products = self.do_processing(dependencies)
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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File "/usr/local/lib/python3.12/site-packages/imap_processing/cli.py", line 1161, in do_processing
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datasets = mag_l1d(
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^^^^^^^^
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File "/usr/local/lib/python3.12/site-packages/imap_processing/mag/l1d/mag_l1d.py", line 85, in mag_l1d
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l1d_norm = MagL1d(
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^^^^^^^
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File "<string>", line 16, in __init__
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File "/usr/local/lib/python3.12/site-packages/imap_processing/mag/l1d/mag_l1d_data.py", line 189, in __post_init__
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self.spin_offsets = self.calculate_spin_offsets()
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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File "/usr/local/lib/python3.12/site-packages/imap_processing/mag/l1d/mag_l1d_data.py", line 530, in calculate_spin_offsets
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offset_epochs.append(chunk_epoch[0])
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~~~~~~~~~~~^^^
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IndexError: index 0 is out of bounds for axis 0 with size 0

Additional notes, affected areas, and suggested fixes

Add a check to see if chunk_epoch has a length of zero before continuing processing. Possibly, fail with a smarter error, although I think one chunk being empty is probably ok.