[ruby-core:115393] [Ruby master Feature#20005] Add C API to return symbols of native extensions resolved from features

Issue #20005 has been updated by tagomoris (Satoshi Tagomori).


@nobu No, I intended that the API retrieves `extern`-ed (or `EXPORTS`-ed in *.def) symbols. I missed to mention it.

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Feature #20005: Add C API to return symbols of native extensions resolved from features
https://bugs.ruby-lang.org/issues/20005#change-105330

* Author: tagomoris (Satoshi Tagomori)
* Status: Open
* Priority: Normal
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I want an API to resolve symbols of other native extensions by a feature name and a symbol name (just like `dlsym`).

(`rb_dln_resolve_symbol` is an example function name of this feature in the example below)
```c
// "a_client_open" is a function defined in "/..../a_client.so", that should be loaded in Ruby beforehand.

// in the extension to be "a.so"
VALUE (*a_client_open)(VALUE);

void Init_a(void)
{
  a_client_open = (VALUE(*)(VALUE))rb_dln_resolve_symbol("a_client", "a_client_open");
  // the return value may be NULL if the symbols is not found
}
```

This API is to replace direct reference of external symbols.

Currently, native extensions have to just call the function of other extensions directly on the assumption of:
* The dependency native extension is already loaded
* The symbol is defined correctly in the dependency (the version of dependency is correct&expected)
Otherwise, it crashes.

This API provides a way for developers to raise exceptions if the resolved symbol is `NULL`, and these exceptions can display meaningful messages and instructions to the users of extensions.




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