IB force history: record each run's last step; don't overwrite on restart

[sbryngelson]

Description

Chaining a run with ib_force_wrt across restarts loses IB force history in two ways.

1. One step is missing at every restart. s_write_ib_force_history(t_step) runs at RK stage 1 of step N and records the force left by step N-1. The time loop in p_main exits as soon as t_step == t_step_stop, so the record for t_step_stop (the force after the run's last step) is never written. The next run deliberately skips its t_step_start, because at that point the force is still the one from before the run (the existing comment explains this). So no run ever writes that step. Our exports showed exactly one missing record at each restart step, e.g. steps 12735, 25470 and 38205 in a three-chunk run.
2. A restart wipes the history. s_open_ib_force_history deletes and recreates D/ib_forces.dat on every run. A restarted run therefore keeps only its own rows, and every earlier chunk's history is gone unless the user renamed the file in between.

Fix.

• p_main calls s_write_ib_force_history(t_step) once more after the time loop, which writes the run's last step. I did not write the resumed run's first step instead, because that would record the stale (zero) force the existing skip avoids.
• A resumed run writes D/ib_forces_<t_step_start>.dat (D/ib_forces_n<n_start>.dat with cfl_dt) instead of overwriting D/ib_forces.dat. A fresh run still writes D/ib_forces.dat. Within each file the documented layout is unchanged: row 0 is that run's first recorded step, there is no header, and offsets are fixed.
• docs/documentation/case.md describes the per-run files and the new last row.

Why not append into one file. That needs either absolute step-based rows, or a row base inferred from the existing file. Absolute rows leave NUL-filled holes whenever a run starts from a restart without the earlier history (the existing docs and comments avoid holes on purpose). An inferred base is silently misaligned after a chunk killed past its last restart dump. Separate files avoid both, and they still concatenate into the full history.

Impact on results and goldens

The physics is unchanged. Every run with ib_force_wrt now has one more row per body in D/ib_forces.dat. Ten goldens contain that file (135F548B, 49893269, 4BED9896, 5A22B45F, B317404C, C8AD6271, D6794F4C, E085CC5A, E5B66084, F200F862), and they are updated only on the D/ib_forces.dat line:

• I regenerated them on a Frontier CPU compute node (GNU 12.3 + Cray MPICH, Release).
• Using the harness's own packtol.compare at each test's tolerance (1e-10, Examples 1e-3), the regenerated values for every field, including the existing ib_forces rows, match the committed NVHPC 25.11 goldens.
• I then appended only the new final row(s) to each old ib_forces line. The old line is a string prefix of the new one, every other line and every golden-metadata.txt is untouched, and the change is one line per golden.
• The appended values come from the GNU run. If you would rather have them from your NVHPC lane, ./mfc.sh test --generate --only <those 10> reproduces them.

Verification

All runs were on a Frontier CPU compute node (GNU 12.3 + Cray MPICH, Release, 2 ranks). The case is a 2D circle moving at a prescribed velocity, with ib_force_wrt.

run	rows written	steps covered
straight 0→40, this branch	40	1..40
0→20 then restart 20→40, this branch	20 + 20 (ib_forces.dat, ib_forces_20.dat)	1..20, 21..40
0→20 then restart 20→40, master	19, then overwritten by 19	1..19, then only 21..39

On this branch the two files concatenated match the straight run's ib_forces.dat: the same 40 times, and a maximum absolute difference of 2e-17 over all columns.

With the updated goldens, on the same node:

code	the 10 ib_forces tests
this branch	10/10 pass
master	10/10 fail: Variable count didn't match for D/ib_forces.dat (the missing last row)

simulation compiles with CCE 19 (CPU) and GNU 12.3. Precheck passes, apart from two test_thermochem cases that fail on the Frontier login node because they compile with the system /usr/bin/gfortran (addressed by #1943).

Contribution Policy

We do not accept pull requests generated primarily by AI without genuine understanding or real-world usage context.

All contributions are expected to demonstrate:

• A clear understanding of the codebase
• Alignment with product direction
• Thoughtful reasoning behind changes
• Evidence of real-world usage or hands-on experience with the problem

If these expectations are not met, we would prefer to implement the changes ourselves rather than spend time reviewing low-effort submissions.

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Acknowledgement

• I confirm this PR meets the above expectations and reflects my own understanding and real-world context.

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This PR was prepared with the assistance of an AI tool (Claude Code). We hit this in chunked production runs on Frontier; the fix was exercised on the small cases above.

PR template credit: junegunn

[Copilot]

🟢 Approval recommended

The changes preserve existing sampling semantics, with reported restart and golden-test verification supporting correctness.

2 open findings

• Clarify final-record stride and stride-1 concatenation guarantees · New
• Add regression coverage for ib_force_wrt restart histories · New

What changed in this PR

Updates MFC’s immersed-boundary force output to preserve history across restarts without changing the physics.

Changes:

• Writes the final force sample when allowed by the configured stride.
• Uses restart-specific filenames to preserve earlier history.
• Documents per-run output and extends force-history goldens.

File	Description
tests/​F200F862/​golden.txt	Adds the final force record.
tests/​E5B66084/​golden.txt	Adds the final force record.
tests/​B317404C/​golden.txt	Adds the final force record.
src/​simulation/​p_main.fpp	Calls the history writer after time stepping.
src/​simulation/​m_data_output.fpp	Selects restart-specific history filenames.
docs/​documentation/​case.md	Describes endpoints and per-run history files.

🧠 Review effort: Balanced

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[github-actions]

Lines of Code

File	Lines	Diff
src/simulation/m_data_output.fpp	1584	+6
src/simulation/p_main.fpp	75	+2

Directory	Lines	Diff
simulation	28433	+8
total	47414	+8

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.79%. Comparing base (27cae2c) to head (2e08a89).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_data_output.fpp	40.00%	3 Missing ⚠️
src/simulation/p_main.fpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1950      +/-   ##
==========================================
- Coverage   61.79%   61.79%   -0.01%     
==========================================
  Files          86       86              
  Lines       22773    22778       +5     
  Branches     3353     3355       +2     
==========================================
+ Hits        14073    14075       +2     
- Misses       6211     6213       +2     
- Partials     2489     2490       +1     

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