METK-183 MarsLanguage immutable + lazy loading + caching

[danovaro]

Description

MarsLanguage is now immutable and values for inheritance are stored in a separate context (a MarsRequest)
Given its immutability, it can now be cached

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

📋 Metkit Documentation 📋
https://sites.ecmwf.int/docs/metkit/pull-requests/PR-270

[Copilot]

🟡 Changes recommended

Shared mutable caching is thread-unsafe, and inheritance context handling introduces cross-verb and sequential-expansion regressions.

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Pull request overview

Introduces cached MarsLanguage instances and moves inheritance state into request contexts.

Changes:

• Adds process-wide language caching and const interfaces.
• Refactors inheritance handling into MarsRequest contexts.
• Updates ODB integration and tests for the new API.

mars2grib documentation: ✅ in sync — no impacted files under src/metkit/mars2grib or tests/mars2grib.

File summaries

File	Description
src/metkit/mars/MarsLanguage.cc	Implements caching and context-based expansion.
src/metkit/mars/MarsLanguage.h	Exposes cached language API.
src/metkit/mars/MarsExpansion.cc	Manages expansion contexts.
src/metkit/mars/MarsExpansion.h	Removes owned languages and reset API.
src/metkit/mars/Type.cc	Removes inheritance state.
src/metkit/mars/Type.h	Updates type state interface.
src/metkit/mars/TypeEnum.cc	Removes redundant reset.
src/metkit/mars/TypeEnum.h	Removes reset declaration.
src/metkit/mars/TypeParam.cc	Removes redundant reset.
src/metkit/mars/TypeParam.h	Removes reset declaration.
src/metkit/odb/OdbDecoder.cc	Uses cached language.
src/metkit/odb/OdbMetadataDecoder.cc	Accepts const language.
src/metkit/odb/OdbMetadataDecoder.h	Stores const language reference.
src/metkit/odb/OdbToRequest.cc	Uses cached language.
tests/test_date.cc	Updates language access.
tests/test_expand.cc	Updates language access.
tests/test_language.cc	Updates language access.
tests/test_mars_language.cc	Updates language access.
tests/test_mars_language_strict.cc	Updates language access.
tests/test_obstype.cc	Updates language access.
tests/test_param_axis.cc	Supplies expansion context.
tests/test_step.cc	Updates language access.
tests/test_time.cc	Updates language access.
tests/test_type_levelist.cc	Updates language access.

Review details

Suppressed comments (2)

src/metkit/mars/MarsExpansion.cc:42

• This overload also shares one inheritance context across all verbs, changing the prior per-language behavior. Mixed-verb MarsRequest batches can inherit parameters from an unrelated verb; select a separate context keyed by lang.verb() for each item.

    MarsRequest ctx;

src/metkit/mars/MarsExpansion.cc:56

• This local context is discarded after every call. Previously a MarsExpansion retained inheritance in its per-verb MarsLanguage until reset(), so sequential expand(request) calls on the same object inherited from one another. Store per-verb contexts on MarsExpansion (and preserve an explicit way to clear them) so this public overload does not silently lose that behavior.

    MarsRequest ctx;
    return language(request.verb()).expand(request, ctx, inherit_, strict_);

• Files reviewed: 24/24 changed files
• Comments generated: 4
• Review effort level: Balanced

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.08264% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.63%. Comparing base (5f653d0) to head (ab14f35).

Files with missing lines	Patch %	Lines
src/metkit/mars/MarsExpansion.cc	50.00%	9 Missing ⚠️
src/metkit/mars/MarsLanguage.cc	91.17%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #270      +/-   ##
===========================================
- Coverage    46.70%   46.63%   -0.07%     
===========================================
  Files          471      471              
  Lines        21868    21859       -9     
  Branches      1531     1530       -1     
===========================================
- Hits         10214    10195      -19     
- Misses       11654    11664      +10     

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[Copilot]

🟡 Changes recommended

The Rust bridge attempts to copy a non-copyable MarsLanguage returned by const reference.

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Review details

• Files reviewed: 31/31 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[simondsmart]

This looks good. The only thing I would really consider is if the context should have its own type (i.e. there might be something other than the mars request that we want to carry along. And it means that there is no ambiguity as to the order of the arguments)

[simondsmart]

Should we wrap the context in its own type? Such that the signature becomes self-explanatory (currently it accepts two MarsRequest objects).