Thread safety bug in CodecFactory

The code for returning Compressor objects to the caller goes to some lengths to achieve thread safety, including keeping Codec objects in an Apache Commons pool that has thread-safe borrow semantics.  This is all undone by the BytesCompressor and BytesDecompressor Maps in org.apache.parquet.hadoop.CodecFactory which end up caching single compressor and decompressor instances due to code in CodecFactory@getCompressor and CodecFactory@getDecompressor.  When the caller runs multiple threads, those threads end up sharing compressor and decompressor instances.

For compressors based on Xerial Snappy this bug has no effect because that library is itself thread safe.  But when BuiltInGzipCompressor from Hadoop is selected for the CompressionCodecName.GZIP case, serious problems ensue.  That class is not thread safe and sharing one instance of it between threads produces both silent data corruption and JVM crashes.

To fix this situation, parquet-mr should stop caching single compressor and decompressor instances.

Reporter: James Turton

Related issues:

• Parquet CodecFactory thread safety bug (fixes)

_{Note: This issue was originally created as PARQUET-2126. Please see the migration documentation for further details.}

从 org.apache.parquet.hadoop.CodecFactory 开始，重点查看 getCompressor 和 getDecompressor，并追踪 BytesCompressor 和 BytesDecompressor 映射如何与 Apache Commons pool 交互。完成的标准是 compressor 和 decompressor 实例不再作为单个共享实例进行缓存，这样并发的 GZIP 调用方就无法共享非线程安全的 BuiltInGzipCompressor。