refactor: extract s_write_field helper in post_process s_save_data

[SVS87]

Description

Collapse the 38 repeated fill->name->write->clear stanzas in s_save_data into calls to a new private helper s_write_field. The helper optionally slices a scalar_field into out%q_sf, writes varname to the database, and clears varname on return.

Also fixes the ib_markers block which was missing the varname clear, and moves the elasticity/hyperelasticity varname clear inside the prim_vars_wrt guard where it belongs.

Part of #1523

Type of change (delete unused ones)

• Bug fix
• Refactor

Testing

Ran ./mfc.sh test -j 4 locally on Ubuntu/WSL with GNU compiler and MPI. 578 tests passed, 0 failed. Output-neutral by construction — the helper performs the exact same slice, write, and clear in the same order, so database files are bit-identical.

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

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[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.42%. Comparing base (05eda2d) to head (78508fe).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/post_process/m_start_up.fpp	88.88%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1600      +/-   ##
==========================================
- Coverage   60.48%   60.42%   -0.07%     
==========================================
  Files          83       83              
  Lines       19888    19835      -53     
  Branches     2951     2952       +1     
==========================================
- Hits        12030    11985      -45     
+ Misses       5864     5856       -8     
  Partials     1994     1994              

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[sbryngelson]

Reviewed as a behavior-equivalence audit of the refactor. Traced all 38 converted stanzas against their originals:

• Every src-passing call uses the same field, index expression, and x_beg:x_end,y_beg:y_end,z_beg:z_end slice as the original inline fill (checked the index-bearing ones: mom, vel, Y, alpha_rho_e, tau, xi, alpha, color_function, all qbmm rs/vs/ps/ms(i)).
• Every no-src call still has out%q_sf populated beforehand by a retained inline slice, an s_derive_* call, or a manual loop — none orphaned.
• s_write_field is a sibling module procedure; out resolves via use-association from m_data_output (public module var), and name_len/scalar_field/wp from their respective modules. ✓
• The varname-clear relocation is inert: every DB write is preceded by a full internal write(varname,...), which blank-pads the whole fixed-length scalar, so the clears were always redundant.

Output-neutral, as claimed. One latent (non-blocking) note inline.

[Copilot]

Pull request overview

Note

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Refactors s_save_data in post-process startup to reduce duplicated “slice → write → clear” sequences by introducing a helper routine, while also fixing a missing varname clear in the ib_markers output path and tightening variable clearing for elasticity/hyperelasticity.

Changes:

• Extract repeated output logic into a new helper subroutine s_write_field.
• Replace many manual out%q_sf slice + write + varname clear stanzas with calls to s_write_field.
• Fix ib_markers to clear varname consistently and adjust clearing placement for elasticity/hyperelasticity to be inside prim_vars_wrt guards.

[github-actions]

Claude Code Review

Head SHA: 78508fe

Files changed:

• 1
• src/post_process/m_start_up.fpp

Findings

Precision discipline: real(..., wp) used for stp storage field

src/post_process/m_start_up.fpp — ib_markers conversion hunk:

-                     & -offset_z%beg:p + offset_z%end))
+                     & -offset_z%beg:p + offset_z%end), wp)

out%q_sf is a scalar_field%sf, which uses stp per project convention. The conversion real(ib_markers%sf(...), wp) produces a wp-precision value that is then stored into the stp field. In --mixed precision mode (wp=double, stp=half) this silently materialises a double-precision intermediate that is truncated to half on assignment — real(..., stp) is the correct form. The old code (real(...) without kind) was already wrong for the same reason; this change picks the wrong explicit kind.

Fix: real(ib_markers%sf(...), stp).