Standardise BaseSolver.solve() multiprocessing context to fork

[BradyPlanden]

Description

This PR updated the multiprocess pool to be constructed with context fork for darwin and linux platforms. It also adds an example and updates integration tests to use the list of dictionaries input functionality which should ensure this get tested in script formation.

Fixes #3974

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (c204bdd) to head (4047b37).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3975   +/-   ##
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  Coverage    99.58%   99.58%           
========================================
  Files          257      257           
  Lines        21251    21254    +3     
========================================
+ Hits         21163    21166    +3     
  Misses          88       88           

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[agriyakhetarpal]

The Windows tests are still failing, I guess it is related to the fact that fork() syscalls are not available on Windows (can we fix this just conditionally in case the MWE you posted works on Windows but fails on Unix-like systems?)

[BradyPlanden]

This is now ready for merge for those that have permissions.

It's worth noting that this only fixes #3974 for darwin systems and protects linux systems from the default context change in Python 3.14. Windows systems will still require the __main__ guard discussed in #3974.

Longer term, @martinjrobins and I discussed moving to a more robust solution by implementing the multiprocessing in C++.

[martinjrobins]

looks good, just need approval from @kratman, then can merge

[kratman]

It's worth noting that this only fixes #3974 for darwin systems and protects linux systems from the default context change in Python 3.14. Windows systems will still require the main guard

@BradyPlanden If this does not fix the problem on windows, is the task completed? Merging this PR will close the issue.

[BradyPlanden]

I would recommend we merge this and reopen (or unlink) #3974 to continue discussion on a longer-term solution that doesn't depend on the multiprocessing. Open to other suggestions though.

[kratman]

I would recommend we merge this and reopen (or unlink) #3974 to continue discussion on a longer-term solution that doesn't depend on the multiprocessing. Open to other suggestions though.

Yeah I think it should be unlinked or a new ticket opened for the windows side