Add ZLibStream polyfill

[Copilot]

System.IO.Compression.ZLibStream (added in .NET 6) had no polyfill for older target frameworks.

Implementation

• Added ZLibStream as a type polyfill (PolyShim/Net60/ZLibStream.cs), enabled on .NET Framework 4.5+, .NET Standard 1.1+, and .NET Core/.NET below 6.0.
• The real ZLibStream relies on an internal zlib window-bits constructor of DeflateStream that isn't publicly exposed, so it can't be wrapped directly. Instead, the polyfill wraps the public raw DeflateStream (headerless deflate) and manually frames the data as a standard zlib stream:
  • Writes the 2-byte zlib header (CMF/FLG) lazily on first write, computed from the requested CompressionLevel to match native zlib output exactly.
  • Tracks an Adler-32 checksum of the uncompressed data and appends it as the 4-byte trailer on dispose.
  • On decompression, reads and validates the 2-byte header before delegating the remaining bytes to a raw inflate DeflateStream.
• Unsupported members (Seek, SetLength, Length, Position) throw NotSupportedException, matching the real type.

using System.IO;
using System.IO.Compression;

using var compressed = new MemoryStream();
using (var zlib = new ZLibStream(compressed, CompressionLevel.Optimal, leaveOpen: true))
    zlib.Write(data, 0, data.Length);
// compressed now holds a standard zlib stream (0x78 0x9C header + deflate payload + Adler-32 trailer)

Tests

• Added PolyShim.Tests/Net60/ZLibStreamTests.cs covering compression/decompression round-trips, header bytes per compression level, interop with DeflateStream-produced payloads, invalid header detection, and leaveOpen behavior.

[codecov]

Codecov Report

❌ Patch coverage is 79.83871% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.84%. Comparing base (37a7ea6) to head (cb3bb0c).
⚠️ Report is 1 commits behind head on prime.

Files with missing lines	Patch %	Lines
PolyShim/Net60/ZLibStream.cs	69.32%	34 Missing and 16 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #142      +/-   ##
==========================================
- Coverage   88.08%   87.84%   -0.24%     
==========================================
  Files         336      338       +2     
  Lines        7199     7447     +248     
  Branches      513      541      +28     
==========================================
+ Hits         6341     6542     +201     
- Misses        706      738      +32     
- Partials      152      167      +15     

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[Copilot]

Copilot review overview

🟡 Changes recommended

Framing, validation, checksum integrity, and disposal paths contain correctness issues.

Review effort: Balanced
Findings: 1 · 4

Open (5)

• Validate the zlib Adler-32 integrity trailer · New
• Validate reads before consuming the zlib header · New
• Validate writes before emitting the zlib header · New
• Dispose the base stream even when finalization fails · New
• Correctly handle empty, truncated, and unsupported zlib headers · New

What changed in this PR

Adds a ZLibStream polyfill for pre-.NET 6 targets using manually framed DeflateStream data.

Changes:

• Implements zlib framing, Adler-32 checksums, and stream behavior.
• Adds compression, decompression, and lifecycle tests.
• Registers the new polyfilled type.

File	Description
Signatures.md	Registers ZLibStream.
PolyShim/​Net60/​ZLibStream.cs	Implements the polyfill.
PolyShim.Tests/​Net60/​ZLibStreamTests.cs	Adds behavioral tests.

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[Tyrrrz]

@copilot please evaluate review comments

[Copilot]

@copilot please evaluate review comments

All five review comments were valid, and I fixed them in the latest commit.

• Adler-32 trailer: decompression now checks it. A small wrapper holds back the last 4 bytes of the source, and they are compared with the checksum of the data returned. Corrupted or truncated input now throws InvalidDataException.
• Read/write arguments: the buffer range is checked before any framing. Zero-length reads and writes are no-ops, so they no longer consume or emit a header.
• Disposal: the owned base stream is now closed in a finally block, after the deflate stream is finalized and the trailer is written.
• Header validation: an empty source reads as an empty stream. A truncated header throws. A header with CINFO > 7 or FDICT set throws too.

I checked the changes with a standalone harness against .NET 8's native ZLibStream: round-trip, native interop, empty source, a corrupted trailer and a truncated stream. The existing 13 ZLibStream tests pass only on net10, where the polyfill isn't compiled, so they don't exercise this code. I didn't add tests for the new cases to the repo.

[Copilot]

Copilot review overview

🟡 Changes recommended

Decompression can block or consume trailing data, and the polyfill omits required API behavior.

Review effort: Balanced
Findings: 1

Open (1)

• Add negative tests for Adler-32 trailer validation · New

Resolved since last review (5)

• Validate the zlib Adler-32 integrity trailer
• Correctly handle empty, truncated, and unsupported zlib headers
• Dispose the base stream even when finalization fails
• Validate writes before emitting the zlib header
• Validate reads before consuming the zlib header

Previously missed (1)

In code that hasn't changed since last review

Expose the wrapped stream through BaseStream

PolyShim/​Net60/​ZLibStream.cs:75

The .NET 6 ZLibStream API includes a public read-only BaseStream property. Omitting it makes code that accesses the underlying stream compile against the real type but fail on polyfill targets. Mirror the framework property through the wrapped DeflateStream.

This issue also appears in the following locations of the same file:

• line 308
• line 364

[Tyrrrz]

@copilot a few review comments

[Tyrrrz]

@copilot review comments

[Tyrrrz]

@copilot please regenerate Signatures.md to resolve conflicts

[Copilot]

@copilot please regenerate Signatures.md to resolve conflicts

Ran ./GenerateSignatures.csx; it regenerated Signatures.md with 591 signatures, and the output matches the checked-in file, so there was no diff to commit.

[Tyrrrz]

@copilot please also merge in latest prime. Also, tests are failing for some reason.

[Copilot]

@copilot please also merge in latest prime. Also, tests are failing for some reason.

Merged the latest prime in cb3bb0c, regenerated Signatures.md, and fixed the Windows compile failure by renaming the compression stream locals. Focused ZLibStream tests pass 7/7. The full suite had three unrelated network-dependent HttpClient failures in this environment.