Avoid allocations in DGMulti parabolic calc_boundary_flux!

[jlchan]

Human summary

Some manual testing showed that removing the recursive call to calc_boundary_flux!

Trixi.jl/src/solvers/dgmulti/dg_parabolic.jl

Lines 249 to 250 in ea79220

	calc_boundary_flux!(flux, u, t, operator_type, Base.tail(boundary_conditions),
	mesh, equations, dg, cache, cache_parabolic)

appeared to fix the excess allocations observed in #2729. This PR changes the implementation of calc_boundary_flux! to look more like the hyperbolic case. I'm not sure why I didn't consider this option earlier 🤷

This produces qualitatively correct solution plots for examples/dgmulti_2d/elixir_advection_diffusion.jl examples/dgmulti_2d/elixir_navierstokes_lid_driven_cavity.jl.

AI summary

Parabolic calc_boundary_flux! for DGMulti peeled NamedTuple boundary conditions with first / Base.tail recursion, which caused heap allocations in rhs_parabolic! (for example examples/dgmulti_2d/elixir_advection_diffusion.jl).

This change matches the hyperbolic DGMulti implementation in dg.jl: iterate with zip(keys(boundary_conditions), boundary_conditions) and call calc_single_boundary_flux! per segment. The empty-NamedTuple terminator is removed.

Verification

After trixi_include("examples/dgmulti_2d/elixir_advection_diffusion.jl"), @allocated Trixi.rhs_parabolic!(du, ode.u0, semi, 0.0) reports zero bytes (post warm-up).

Fixes #2729

Made with Cursor

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.13%. Comparing base (78c41a3) to head (96d2d91).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2947      +/-   ##
==========================================
- Coverage   97.13%   97.13%   -0.00%     
==========================================
  Files         623      623              
  Lines       48385    48384       -1     
==========================================
- Hits        46996    46995       -1     
  Misses       1389     1389              

Flag	Coverage Δ
unittests	97.13% <100.00%> (-<0.01%)	⬇️

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[DanielDoehring]

Given that this is the same implementation as for the hyperbolic case I am fine with this change.

[jlchan]

Thanks for the review!

[jlchan]

@ranocha OK if I merge assuming tests pass?