Add support for Zstandard: dotnet/android tools/assembly-store-reader-mk2 @ 2e30614

[github-actions]

TLDR;

Microsoft are adding support for Zstd compression to Android Assembly Stores:

Switches the per-assembly AssemblyStore compression from LZ4 (dotnet/lz4 + the K4os managed wrapper) to Zstandard, using the ZSTD_* entry points already exported by libSystem.IO.Compression.Native in the .NET runtime pack. This drops an external dependency and trades for a better compression ratio.

Eventually, this will be good news as we'll be able to drop this dependency:

sentry-dotnet/src/Sentry.Android.AssemblyReader/Sentry.Android.AssemblyReader.csproj

Lines 8 to 10 in 65363ca

	<ItemGroup>
	<PackageReference Include="K4os.Compression.LZ4" Version="1.3.6" />
	</ItemGroup>

In the near term, we'll be able to conditionally drop/replace this for recent enough versions of .NET (likely .NET 11 or later). We'll need to do this before shipping the next major version of the Sentry SDK for .NET.

First Change Detected

Headline: Switched to LZ4

The code at dotnet/android/tools/assembly-store-reader-mk2 has a new commit since our last review.

Latest commit	2e30614
Path history	View history
Tracking Code	dotnet/android tools/assembly-store-reader-mk2 @ 2e30614

Our vendored copy lives in src/Sentry.Android.AssemblyReader/. We modified the upstream code significantly,
so a direct merge is unlikely to be appropriate — but the commit above may reveal logic
changes worth porting.

Second Change Detected

Headline: re-use cached copies of decompressed files

The code at dotnet/android/tools/assembly-store-reader-mk2 has a new commit since our last review.

Latest commit	b367d98
Path history	View history

Our vendored copy lives in src/Sentry.Android.AssemblyReader/. We modified the upstream code significantly,
so a direct merge is unlikely to be appropriate — but the commit above may reveal logic
changes worth porting.

Automatically opened by the Watch Upstream Changes workflow.

[linear-code]

NET-723

[jamescrosswell]

Upstream added Zstandard alongside LZ4 — it did not replace it

Re-checked upstream at the current tip while triaging #5478. The premise in the description above ("Microsoft are finally replacing the LZ4 compression", "we'll be able to drop this dependency") doesn't match what actually landed, so this issue needs redefining before anyone picks it up.

What upstream actually does

Compression was extracted out of Utils into a new AssemblyCompression.cs, which supports both formats side by side:

const uint Lz4Magic = 0x5A4C4158;        // 'XALZ', little-endian
const uint ZstandardMagic = 0x535A4158;  // 'XAZS', little-endian

int decoded = format switch {
    AssemblyCompressionFormat.Lz4 => LZ4Codec.Decode (...),
    AssemblyCompressionFormat.Zstandard => ZstandardDecoder.TryDecompress (...) ? bytesWritten : -1,
    ...
};

using K4os.Compression.LZ4; is still there upstream. Compression format is a per-assembly magic header, not a per-store-version switch, so LZ4 support can't be dropped in any case — existing/older stores keep producing XALZ payloads.

Two constraints the description doesn't capture

1. The entire decompress path is behind #if NET11_0_OR_GREATER, because System.IO.Compression.ZstandardDecoder is .NET 11+. Below that, upstream deliberately fails:

   #else // !NET11_0_OR_GREATER
       if (!AssemblyCompression.IsCompressed (stream)) { return stream; }
       stream.Dispose ();
       throw new NotSupportedException ("Assembly decompression requires .NET 11 or later");
   #endif

2. Sentry.Android.AssemblyReader targets $(PreviousTfm);$(LatestTfm) = net9.0;net10.0, so the BCL decoder isn't reachable for us today.

Revised scope

• Not "replace LZ4 with Zstandard / drop K4os.Compression.LZ4" — rather add XAZS/Zstandard support in addition to the existing LZ4 path.
• Until we ship a net11.0 TFM, Zstd needs either a third-party decoder (e.g. ZstdSharp) or a net11.0 target with a conditional compile mirroring upstream's #if.
• K4os.Compression.LZ4 stays either way, so the dependency-removal motivation goes away.
• The Tracking Code in the description (dotnet/android tools/assembly-store-reader-mk2 @ 2e30614) is stale — the path moved to .github/skills/read-assembly-store/src/AssemblyStore in dotnet/android#12108.

Related gap in our fork, worth noting here

Our decompression isn't in the vendored V2/ tree — it's ArchiveUtils.TryDecompressLZ4, which returns null on an unrecognised magic and then hands the raw bytes straight to PEReader:

var decompressedStream = TryDecompressLZ4(assemblyName, inputStream, logger); // Returns null if not compressed
return new PEReader(decompressedStream ?? inputStream);

So a XAZS-compressed assembly won't produce a clear "unsupported compression" diagnostic — it'll fail obscurely inside PEReader. Detecting the Zstd magic and failing loudly is a cheap defensive fix that's independent of the full port.