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Add a new command-line option '-module-assertions' that users can
enable developing or debugging a module.  If this option is present,
Emacs performs additional checks to verify that modules fulfill their
requirements.  These checks are expensive and crash Emacs if modules
are invalid, so disable them by default.

This is a command-line option instead of an ordinary variable because
changing it while Emacs is running would cause data structure
imbalances.

* src/emacs.c (main): New command line option '-module-assertions'.

* src/emacs-module.c (module_assert_main_thread)
(module_assert_runtime, module_assert_env, module_assert_value):
New functions to assert module requirements.
(syms_of_module): New uninterned variable 'module-runtimes'.
(init_module_assertions, in_main_thread, module_abort): New helper
functions.
(initialize_environment): Initialize value list.  If assertions are
enabled, use a heap-allocated environment object.
(finalize_environment): Add assertion that environment list is never
empty.
(finalize_runtime_unwind): Pop module runtime object stack.
(value_to_lisp): Assert that the value is valid.
(lisp_to_value): Record new value if assertions are enabled.
(mark_modules): Mark allocated object list.
(MODULE_FUNCTION_BEGIN_NO_CATCH)
(module_non_local_exit_check, module_non_local_exit_clear)
(module_non_local_exit_get, module_non_local_exit_signal)
(module_non_local_exit_throw): Assert thread and environment.
(module_get_environment): Assert thread and runtime.
(module_make_function, module_funcall, module_intern)
(module_funcall, module_make_integer, module_make_float)
(module_make_string, module_make_user_ptr, module_vec_get)
(funcall_module, Fmodule_load): Adapt callers.
(module_make_global_ref): If assertions are enabled, use the global
environment to store global values.
(module_free_global_ref): Remove value from global value list.

* test/Makefile.in (EMACSOPT): Enable module assertions when testing
modules.

* test/data/emacs-module/mod-test.c (Fmod_test_invalid_store)
(Fmod_test_invalid_load): New functions to test module assertions.
(emacs_module_init): Bind the new functions.

* test/src/emacs-module-tests.el (mod-test-emacs): New constant for
the Emacs binary file.
(mod-test-file): New constant for the test module file name.
(module--test-assertions): New unit test.