Keep the simulator hooks of a VUnit object when another one is created

[ru551n]

Problem

The simulator hooks that VUnit packages register (#1221) were kept in a module-global registry, and every new VUnit object cleared it. A run script that creates a second VUnit object after add_package(...) therefore loses the hooks of the first one. Its tests then run without the flags and environment the package registered, even though the package's setup has run and its files exist.

This was found with vunit-python-bridge. Its GHDL run_env hook puts the folder of its native library on LD_LIBRARY_PATH. A run script that created another VUnit object made every test fail with:

python_bridge_pkg.vhd:28:19:error: cannot load VHPIDIRECT shared library 'libvunit_python_bridge.so'

NVC similarly loses --load=..., and ModelSim/Questa loses its process_flags and run_env. The docs already say the hooks "apply only to the project in which they are registered", so this makes the code do what the docs describe.

Changes

• vunit/sim_if/hooks.py: a SimulatorHooks class holds the hooks of one VUnit object (register, get_flags, get_run_env), replacing the global _HOOKS, register_hooks and clear_hooks. The module functions get_flags(simulator_interface, kind) and get_run_env(simulator_interface, env) keep their signatures. They use the registry the interface was given, or none for an interface not created by a VUnit object. The simulator interfaces' call sites are unchanged.
• VUnit object: it creates its own SimulatorHooks instead of clearing the global registry, and attaches it to the simulator interface it creates (simulator_if.simulator_hooks).
• PackageContext.register_simulator_hooks: now a method with the same keyword arguments, registering into the registry of the context's VUnit object.
• vsim based interfaces: VsimSimulatorMixin used to read process_flags and run_env in __init__, before the VUnit object can attach its registry. It now reads them when the persistent vsim process is started, through a weak reference so the closure still doesn't keep the interface alive. Without this, persistent ModelSim/Questa and Riviera-PRO processes would have lost their hooks.
• Docs: vunit_packages.rst now says that creating another VUnit object neither removes a project's hooks nor gives them to the new object.

Tests

• tests/unit/test_sim_if_hooks.py: the tests use a SimulatorHooks instance per test instead of the global registry. New tests check that:

  • a simulator interface gets the hooks of the VUnit object that created it,
  • a VUnit object's hooks are kept when another VUnit object is created, and the new one has none,
  • an interface not created by a VUnit object has no hooks.

  The persistent-vsim test attaches the hooks after constructing the interface, so it covers the deferred lookup.

• tests/unit/test_builtins.py: the mocked VUnit object gets a registry.

• Locally on Windows: pytest tests/unit (593 passed) and pytest tests/lint (pycodestyle, pylint, mypy, license) pass, and black is clean.

• End to end: vunit-python-bridge's feature tests on AlmaLinux 9.6 with Python 3.11 and GHDL 6.0.0 (mcode), with an extra unused VUnit.from_args(args) after add_package in the run script:

  VUnit	Feature tests
  5.0.0.dev13	0 of 108, all with cannot load VHPIDIRECT shared library
  this branch	108 of 108

  Without the extra object, both pass 108 of 108.

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