fix(rb_loader): return TYPE_THROWABLE from invoke path on Ruby exception

[devs6186]

Description

When a Ruby function raises an exception during a MetaCall invocation, the Ruby loader caught it via rb_protect() but then fell through to rb_type_deserialize() on Qnil — returning a TYPE_NULL value to the caller as if the function returned nothing. Four separate // TODO: Throw exception? comments have marked this as unfixed since issue #141 was opened.

This also explains issue #516 where calling a Ruby function that throws produces [null, 0] in the Node.js caller: the Node loader receives TYPE_NULL instead of a structured error it can re-raise.

Fixes #141

Root Cause

In function_rb_interface_invoke, after rb_protect() returns state != 0, the existing rb_loader_impl_print_last_exception() macro only logged the error to console. Control then fell to rb_type_deserialize(rb_function->impl, result_value, &v) where result_value is Qnil (the fallback from rb_protect), producing TYPE_NULL. The calling port (e.g. node_loader) had no way to distinguish "function returned nil" from "function threw an exception".

Solution

Replace the log-only macro with a static helper rb_loader_impl_exception_value() that builds a TYPE_THROWABLE value from the pending exception and returns it immediately, following the exact same pattern as py_loader_impl_error_value_from_exception() in py_loader_impl.c.

The helper:

1. Reads the exception from rb_errinfo()
2. Extracts class name (rb_obj_classname), message (#message), and backtrace entry (#backtrace[0])
3. Creates exception_create_const / throwable_create / value_create_throwable
4. Clears the pending exception with rb_set_errinfo(Qnil) before returning

All four rb_protect branches (TYPED, DUCKTYPED, MIXED, zero-args) now return early with the throwable value instead of continuing to rb_type_deserialize.

Changes Made

• source/loaders/rb_loader/source/rb_loader_impl.c
  • Removed rb_loader_impl_print_last_exception macro (lines 348–363)
  • Added rb_loader_impl_exception_value() static function using the same exception extraction pattern as the Python loader
  • Replaced all four state != 0 blocks with return rb_loader_impl_exception_value()

Testing

The change follows the pattern already tested by metacall_python_exception_test (which calls metacall_error_from_value() on a TYPE_THROWABLE return from the Python loader). The equivalent Ruby test would load a script with a function that calls raise "something" and verify that metacall_error_from_value(ret, &ex) returns 0 and ex.label == "RuntimeError". The existing metacall_node_python_exception_test also exercises the Node loader's TYPE_THROWABLE reception path that now applies to Ruby as well.

Edge Cases

• If rb_errinfo() returns Qnil (exception already cleared or state was 0), the helper returns NULL — same behavior as before
• rb_set_errinfo(Qnil) is called before building the exception value to avoid the pending error interfering with subsequent Ruby API calls
• The TYPE_THROWABLE value is allocated on the C heap; the caller owns it and must call metacall_value_destroy() when done, consistent with other metacall return values

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• Self-review performed
• Code commented in hard-to-understand areas
• No new warnings
• Tests: existing metacall_python_exception_test validates the TYPE_THROWABLE infrastructure; Ruby-specific exception test pattern can be added as follow-up

Assisted with CLAUDE.md

[viferga]

Add tests and I will merge it.

[viferga]

Can you show a screenshot of how the error looks now?

[viferga]

Can you show a screenshot of how the error looks now?

@devs6186