[adamreeve]

Adds a wrapper for the Arrow C++ ProxyMemoryPool class, and also exposes the native total_bytes_allocated and num_allocations methods on memory pools.

This implements part 1 of #729.

Unlike the existing memory pool types, a ProxyMemoryPool is non-static so must be kept alive while objects that use memory allocated through it remain alive. This required keeping references to the memory pool in things that depend on it.

In order to prevent use-after free when native memory allocations are freed, a native RefCountedProxyMemoryPool class was added that deletes itself only after all allocations made through it have been freed.

[Copilot]

🟡 Changes recommended

Critical pool-ownership gaps in properties getters and derived readers, streams, and batches allow live native consumers to outlast the owning proxy, causing use-after-free.

6 open issues

• Imported batches can outlive the file reader's pool proxy · New
• BufferReader does not retain the buffer's pool proxy · New
• ResizableBuffer stream drops pool ownership and omits it from Finish() · New
• ParquetFileWriter properties getter fails to retain the pool proxy · New
• ReaderProperties.MemoryPool drops the pool proxy reference · New
• WriterProperties.MemoryPool drops the pool proxy reference · New

What changed in this PR

Adds a managed ProxyMemoryPool wrapper for tracking native allocations independently per writer, implementing part 1 of #729. The PR also exposes allocation counters and adds pool-lifetime retention to dependent objects.

Changes:

• Adds disposable proxy pools and native bindings for cumulative allocation statistics.
• Retains pool references in properties, readers, writers, and buffers.
• Tests proxy statistics, nested pools, independent writer accounting, and lifetime behavior.

File	Description
csharp/​WriterPropertiesBuilder.cs	Passes the configured pool to built properties.
csharp/​WriterProperties.cs	Retains the pool alongside the native properties handle.
csharp/​ReaderProperties.cs	Retains the configured reader pool.
csharp/​PublicAPI.Unshipped.txt	Declares the proxy pool and allocation counters.
csharp/​ProxyMemoryPool.cs	Implements disposable proxy ownership using a safe handle.
csharp/​ParquetFileWriter.cs	Retains the configured pool through native writer release.
csharp/​ParquetFileReader.cs	Retains the configured pool through native reader release.
csharp/​MemoryPool.cs	Adds allocation counters and lifetime-retention helpers.
csharp/​IO/​ResizableBuffer.cs	Passes pool ownership to the buffer wrapper.
csharp/​IO/​BufferOutputStream.cs	Passes its stored pool to finished buffers.
csharp/​IO/​Buffer.cs	Retains the pool through native buffer release.
csharp/​Arrow/​FileWriter.cs	Retains the configured pool for Arrow writers.
csharp/​Arrow/​FileReader.cs	Retains the configured pool for Arrow readers.
csharp.test/​TestProxyMemoryPool.cs	Tests accounting, ownership, and disposal behavior.
cpp/​MemoryPool.cpp	Exports proxy creation, deletion, and allocation counters.

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

A critical use-after-free risk remains because both buffer constructors pass the pool’s raw pointer to native code without keeping memoryPool alive through construction; both need GC.KeepAlive(memoryPool).

1 open issue

• Keep the memory pool alive during native construction · New

6 resolved since last review

• WriterProperties.MemoryPool drops the pool proxy reference
• ReaderProperties.MemoryPool drops the pool proxy reference
• ParquetFileWriter properties getter fails to retain the pool proxy
• ResizableBuffer stream drops pool ownership and omits it from Finish()
• BufferReader does not retain the buffer's pool proxy
• Imported batches can outlive the file reader's pool proxy

_{Review effort: Balanced}

[marcin-krystianc]

LGTM