Follow-up: generate missing precomputed fmap masks via first-pass pathway

[oesteban]

Follow-up: generate missing precomputed fmap masks via first-pass pathway

Summary

When --derivatives provides precomputed fieldmaps without an explicit mask, dMRIPrep has to handle fmap_mask='MISSING' before coeff2epi_wf registration.

Current behavior in dmriprep/workflows/dwi/fit.py avoids registration-time failure via fallback handling. This should be revised.

Why this needs follow-up

coeff2epi_wf expects a valid mask input and uses it during registration. The long-term fix should generate the missing fieldmap mask with the same pathway/logic used when masks are created during first-pass fieldmap processing, instead of relying on fallback behavior.

fMRIPrep comparison

As of fMRIPrep 25.2.4, the same pattern exists upstream:

• precomputed masks may be absent and represented as 'MISSING'
• fmap_mask is still passed to coeff2epi_wf

So this is not yet resolved upstream either, but dMRIPrep should track and improve this locally.

Proposed follow-up

• Identify and reuse the exact mask-generation path used in first-pass fieldmap preprocessing.
• Replace fallback behavior for precomputed fieldmaps lacking mask derivatives with that generation path.
• Add integration tests for --derivatives reuse with missing mask and with complete precomputed fieldmaps.
• Re-check parity and possible upstream alignment with fMRIPrep/SDCFlows.

Scope

This issue tracks technical debt/improvement only. Current behavior is a defensive stopgap to avoid registration-time failure.