Revert "Remove custom allocator."

[ericstj]

This reverts commit c62bc5b.

We had reports from a partner that they were seeing poor performance from .NET 9.0 that pointed to heap allocation in zlib.

| Name                                                           	Inc %	     Inc	Exc %	   Exc |
|  + system.io.compression.native!CompressionNative_DeflateInit2_	49.6	  52,302	 0.0	    37 |
|   + system.io.compression.native!deflateInit2                  	49.5	  52,130	 0.0	    18 |
|   |+ system.io.compression.native!alloc_deflate                	48.6	  51,218	 0.0	    39 |
|   ||+ module ucrtbase <<ucrtbase!_aligned_malloc>>             	48.1	  50,715	 0.0	    29 |
|   |||+ ntdll!RtlpAllocateHeapInternal                          	48.1	  50,686	 0.0	    23 |
|   ||| + ntdll!RtlpAllocateHeap                                 	48.1	  50,663	45.9	48,399 |

This was caused by native heap fragmentation. We tested the partner's service with a fix that added back the zlib custom allocator and it resolved the issue. We shipped 9.0 preview 7 and RC1 with the allocator, it was removed in RC2. This is a clean revert of the commit that removed the allocator.

We intend to ask for this fix for servicing in .NET 9.0 once it's in.

[Copilot]

Pull Request Overview

This PR reverts the removal of the custom allocator for zlib to address performance regression in .NET 9.0. A partner reported poor performance caused by native heap fragmentation when using the standard process heap for zlib allocations.

• Reintroduces custom allocator implementation with defense-in-depth security features
• Adds platform-specific allocator implementations for Windows and Unix systems
• Integrates custom allocator into zlib initialization

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
zlib_allocator_win.c	Windows-specific custom allocator with separate heap and security cookies
zlib_allocator_unix.c	Unix-specific custom allocator using malloc/calloc with security validation
zlib_allocator.h	Header file declaring the custom allocator function signatures
pal_zlib.c	Integration point to use custom allocator in zlib stream initialization
CMakeLists.txt	Build configuration to include platform-appropriate allocator source file

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[ericstj]

Performance results from this change compared to other iterations. Here Allocator is the initial version of this change, minAllocator is the current version of the change (including removal of zeroing and canary checks).

Method	Job	Mean	Error	StdDev
Test	9.0	304.1 ms	6.07 ms	15.00 ms
Test	9.0+Allocator	291.9 ms	5.83 ms	14.08 ms
Test	9.0+minAllocator	267.8 ms	4.56 ms	4.04 ms
Test	10.0	304.0 ms	7.97 ms	23.26 ms
Test	10.0+minAllocator	275.3 ms	5.39 ms	9.30 ms

[ericstj]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/16680374285