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Use a managed `fast_transcoder_table` to cache results of src/dest encoding lookup. Without the cache, it could take a while because `transcode_search_path` does a BFS to get from src to dest encodings due to base encodings, aliases, etc. Since this managed table is a single-level table instead of 2 levels like `transcoder_table`, we don't need VM lock held and can use RCU. Make sure VM lock is NOT held when calling `load_transcoder_entry`, as that could call `rb_require_internal_silent`.
…e first Loading needs to be outside of VM lock.
Most of the time this should work fine, but if a native extension calls `setlocale(SOME_LOCALE)`, and that encoding is not loaded, it will load next time `rb_locale_encoding()` is called. This is called for methods on ENV like ENV#[]. Since these ENV methods acquire the VM lock, we need to make sure the encoding is loaded prior to locking it. Calling `setlocale()` like this is supported. The documentation for `rb_locale_encoding()` states: ``` * This is dynamic. If you change the process' locale by e.g. calling * `setlocale(3)`, that should also change the return value of this function. * * There is no official way for Ruby scripts to manipulate locales, though. rb_encoding *rb_locale_encoding(void); ```
This avoids taking the VM lock when doing "str".encode(Encoding::UTF_32), for example.
We want to avoid taking the VM lock at all on transcoding operations like `String#encode`, so now we have another managed table so we can avoid going through a two-level table that requires a VM lock. I think this is justified because looking up transcoder entries is done fairly often. Ractors are now quite fast when using `String#encode`.
Add managed_st_table functions and use them in transcode.c.
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