Problem Description

zstd-ruby currently exports all ZSTD symbols with default visibility, which causes symbol collisions when used alongside other gems that dynamically link to system libzstd (e.g., rdkafka-ruby).

Reproduction Steps

1. Install both zstd-ruby and rdkafka-ruby in the same application
2. Run code that triggers ZSTD compression in rdkafka
3. Observe intermittent segmentation faults

Root Cause

zstd-ruby statically links libzstd but exports all ZSTD functions globally due to:

// In ext/zstdruby/libzstd/zstd.h
#define ZSTDLIB_VISIBLE __attribute__ ((visibility ("default")))
#define ZSTDLIB_API ZSTDLIB_VISIBLE

This creates a symbol collision where:

1. rdkafka-ruby expects to use system libzstd (/lib/x86_64-linux-gnu/libzstd.so.1)
2. Dynamic linker resolves ZSTD function calls to zstd-ruby's symbols instead
3. Memory allocated by system libzstd gets freed by zstd-ruby's libzstd implementation
4. Results in segfaults like:

   #0  edata_szind_set (szind=36, edata=0x0) at include/jemalloc/internal/edata.h:472
   ...
   #8  0x00007a1243a49528 in ZSTD_freeCCtx () from zstd-ruby/zstdruby.so
   #9  0x00007a125eace71d in rd_kafka_zstd_compress (...) at rdkafka_zstd.c:220
   

Proposed solution

• Modify the build configuration to hide ZSTD symbols
• Add RUBY_FUNC_EXPORTED macro to Init_zstdruby to explicitly mark it as publicly visible