Replace AssemblyStore compression algorithm LZ4 with Zstd

[Copilot]

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.

The 12-byte compressed-blob header (magic / descriptor index / uncompressed length) is unchanged; only the algorithm and magic value change. No FORMAT_VERSION bump is needed since the store and runtime are always built together by the same SDK.

Managed (build-time)

• New Zstd.cs P/Invoke wrapper over libSystem.IO.Compression.Native (ZSTD_compress2 via a ZSTD_CCtx) using zstd's default compression level (3).
• AssemblyCompression.cs compresses through Zstd instead of LZ4Codec; intermediate output extension .lz4 → .zst.
• Follow-up hardening from review feedback:
  • CallingConvention.Cdecl on ZSTD_* imports.
  • Graceful handling of missing native entry points/libraries and ZSTD_* error returns.
  • Safer output-bound handling (MaximumOutputSize) and controlled encoding failure paths.

Magic value

• COMPRESSED_DATA_MAGIC / managic constant XALZ (0x5A4C4158) → XAZS (0x535A4158), applied consistently across managed code, both native headers, and the diagnostic tools.

Native (load-time)

• New runtime-base/zstd.hh declares the minimal entry points (ZSTD_decompress, ZSTD_isError, ZSTD_getErrorName) instead of pulling in zstd.h.
• CLR (assembly-store.cc) and Mono (embedded-assemblies.cc) decompress via ZSTD_decompress, with error handling on ZSTD_isError.

size_t result = ZSTD_decompress (out, cad.uncompressed_file_size, in, data_size - sizeof (header));
if (ZSTD_isError (result)) { /* ZSTD_getErrorName (result) */ }

Build wiring

• Runtimes link -lSystem.IO.Compression.Native instead of xa::lz4; removed HAVE_LZ4, the common/lz4 CMake library, the libxa-lz4 archive in NativeRuntimeComponents, and the LZ4 source references in Configuration.props / native.targets.

Tooling

• decompress-assemblies and tmt detect the new magic and decompress using System.IO.Compression.ZstandardDecoder; unused K4os.Compression.LZ4 references removed from the affected tool projects.
• tmt follow-up fixes from review feedback:
  • full-buffer reads for compressed payloads (no single-read assumption),
  • pooled-buffer return on all failure paths.

Out of scope

• fastdev (FastDeploy / xamarin.sync) still uses LZ4 for fast-deploy-over-adb. As a result external/lz4 and the external/xamarin-android-tools K4os reference are intentionally retained; fully removing the LZ4 dependency requires a follow-up to migrate fastdev.

Validation notes

• CI exercised the CoreCLR app-build wiring; the release arm64 CoreCLR size baselines (BuildReleaseArm64{Simple,XForms}DotNet.CoreCLR.apkdesc) were refreshed from the CI-produced APK descriptors. Versus the LZ4 baselines on main, Zstd shrinks libassembly-store.so (Simple: 3,461,344 → 2,903,016 bytes; XForms: 14,137,920 → 11,658,648 bytes).
• The standalone diagnostic tools now target net11.0 and use System.IO.Compression.ZstandardDecoder for Zstd payloads, avoiding native Zstd P/Invokes in the tools.

Benchmark: .NET MAUI app on device

Measured a Release .NET MAUI app (Microsoft.Maui.Controls 11.0.0-preview.2.26152.10, net11.0-android, android-arm64, CoreCLR, profiled AOT) built against two locally-built SDKs: this PR (Zstd) vs its merge-base on main (LZ4). Only the AssemblyStore compression differs. Device: Samsung Galaxy A16 (SM-A165F), Android 16 (API 36), arm64-v8a.

Metric	LZ4 (main)	Zstd (this PR)	Δ
libassembly-store.so (uncompressed)	16,771,520 B	13,744,600 B	−3,026,920 B (−18.0%)
APK size (extractNativeLibs=false)	35,195,691 B	32,099,115 B	−3,096,576 B (−8.8%)
Cold start TotalTime, median	831 ms	851 ms	+20 ms (+2.4%)
Cold start TotalTime, mean	823 ms	850 ms	+27 ms (+3.3%)

• Startup is 29 interleaved adb shell am start -S -W samples per variant (alternating launches to neutralise thermal drift; first launch discarded as warmup). Zstd's slightly slower cold start reflects Zstd's higher decompression cost vs LZ4, traded for the ~3 MB smaller store.
• Sizes use extractNativeLibs=false, which stores the store .so uncompressed and page-aligned — what Google Play serves for .aab on API 26+. With the default extractNativeLibs=true, the raw .apk is a poor proxy: the APK zip re-Deflates the store, and Deflate shrinks the more-redundant LZ4 payload further, so the naive .apk size can look larger for Zstd despite the smaller on-device footprint.
• Store magic verified in the built APKs: XALZ (LZ4, main) vs XAZS (Zstd, this PR).

[simonrozsival]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[simonrozsival]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[simonrozsival]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[simonrozsival]

/review

[github-actions]

✅ Android PR Reviewer completed successfully!

[Copilot]

Pull request overview

This PR migrates the AssemblyStore per-assembly compression pipeline from LZ4 to Zstandard (Zstd), removing the managed LZ4 dependency and updating managed tooling + native runtimes to read/decompress the new payloads.

Changes:

• Switched build-time assembly compression to Zstd and updated compressed blob magic (XALZ → XAZS) and intermediate file extension (.lz4 → .zst).
• Updated MonoVM/CoreCLR native runtime code paths to decompress assemblies via ZSTD_decompress from libSystem.IO.Compression.Native, and removed the internal LZ4 native library wiring.
• Updated diagnostic tools (tmt, decompress-assemblies) to detect the new magic and decompress using System.IO.Compression.ZstandardDecoder; refreshed APK descriptor baselines.

Show a summary per file

File	Description
tools/tmt/tmt.csproj	Moves tool to $(DotNetTargetFramework) and removes K4os LZ4 dependency.
tools/tmt/ApkManagedTypeResolver.cs	Updates magic and decompression implementation to Zstd decoder.
tools/decompress-assemblies/main.cs	Updates magic and decompression implementation to Zstd decoder.
tools/decompress-assemblies/decompress-assemblies.csproj	Moves tool to $(DotNetTargetFramework) and removes K4os LZ4 dependency.
tools/assembly-store-reader/assembly-store-reader.csproj	Removes K4os LZ4 package reference.
tools/assembly-store-reader-mk2/assembly-store-reader.csproj	Removes K4os LZ4 package reference.
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets	Routes compression intermediate output to android\\zstd\\ and updates target comment.
src/Xamarin.Android.Build.Tasks/Utilities/Zstd.cs	Introduces managed P/Invoke wrapper for Zstd entry points in System.IO.Compression.Native.
src/Xamarin.Android.Build.Tasks/Utilities/NativeRuntimeComponents.cs	Drops libxa-lz4 archive from runtime component list.
src/Xamarin.Android.Build.Tasks/Utilities/AssemblyStoreAssemblyInfo.cs	Updates filename normalization for .zst compressed artifacts.
src/Xamarin.Android.Build.Tasks/Utilities/AssemblyCompression.cs	Uses Zstd wrapper for compression and emits .zst outputs + new magic.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64XFormsDotNet.CoreCLR.apkdesc	Updates expected APK contents/sizes after runtime + store changes.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64SimpleDotNet.CoreCLR.apkdesc	Updates expected APK contents/sizes after runtime + store changes.
src/Xamarin.Android.Build.Tasks/Tasks/CompressAssemblies.cs	Updates task summary comment to reflect Zstd.
src/native/native.targets	Removes LZ4 CMake and source inputs from native build wiring.
src/native/mono/xamarin-app-stub/xamarin-app.hh	Updates COMPRESSED_DATA_MAGIC constant to XAZS.
src/native/mono/monodroid/embedded-assemblies.cc	Switches runtime decompression from LZ4 to Zstd and removes HAVE_LZ4 gating.
src/native/mono/monodroid/CMakeLists.txt	Removes HAVE_LZ4 define and links -lSystem.IO.Compression.Native.
src/native/common/lz4/CMakeLists.txt	Deletes internal LZ4 static library definition.
src/native/common/include/runtime-base/zstd.hh	Adds minimal Zstd entry point declarations for native runtime usage.
src/native/common/include/runtime-base/timing-internal.hh	Updates timing description string to “Zstd decompression”.
src/native/CMakeLists.txt	Removes add_subdirectory(common/lz4).
src/native/clr/include/xamarin-app.hh	Updates COMPRESSED_DATA_MAGIC constant to XAZS.
src/native/clr/host/CMakeLists.txt	Removes HAVE_LZ4 define and links -lSystem.IO.Compression.Native.
src/native/clr/host/assembly-store.cc	Switches runtime decompression from LZ4 to Zstd and removes HAVE_LZ4 gating.
Configuration.props	Removes LZ4 source directory/full-path properties.

Copilot's findings

• Files reviewed: 26/26 changed files
• Comments generated: 6

[github-actions]

🤖 Android PR Review — Replace AssemblyStore LZ4 with Zstd

Overall this is a clean, well-executed change. I formed an independent assessment and verified the load-bearing assumptions:

• ✅ The ZSTD exports are real on .NET 11. libSystem.IO.Compression.Native in the .NET 11 runtime pack exports the raw, unprefixed ZSTD_* entry points — dotnet/runtime's own Interop.Zstd.cs P/Invokes the same symbols from System.IO.Compression.Native, and System.IO.Compression.ZstandardDecoder exists. The TFM bumps for decompress-assemblies/tmt (DotNetStableTargetFramework → DotNetTargetFramework) are required for that decoder and are correct. (Note: this firmly ties the feature to .NET 11+; .NET 10 does not export these.)
• ✅ Magic value change is correct and consistent. XALZ (0x5A4C4158) → XAZS (0x535A4158), applied across the managed writer, both native headers (CLR + mono stub), and the tools. The 12-byte header layout is unchanged.
• ✅ Native decompression is sound. Both assembly-store.cc (CLR) and embedded-assemblies.cc (Mono) feed data_size - sizeof(CompressedAssemblyHeader) as the compressed size, check ZSTD_isError, surface ZSTD_getErrorName, and validate the produced size against cad.uncompressed_file_size. The #if defined(HAVE_LZ4) && defined(RELEASE) → #if defined(RELEASE) guard change tracks the HAVE_LZ4 removal.
• ✅ LZ4 removal is clean. No dangling K4os/LZ4Codec references remain in the changed scope; the assembly-store-reader* tools only had unused package refs removed, and external/lz4 is correctly retained for FastDev.

A few non-blocking 💡 suggestions (posted inline on Zstd.cs):

1. Add a fast round-trip unit test for the new Zstd interop so an ABI break is caught quickly in CI rather than only in RELEASE apkdesc/device runs.
2. Check the ZSTD_CCtx_setParameter return value so a failed level-set doesn't silently fall back to the default level.
3. Use a file-scoped namespace in the new file to match its siblings.
4. Confirm the build-time cost of the ultra compression level (22) on the Release hot path.

CI: builds were green / in-progress at review time with no failures attributable to these changes; the documented Release size-threshold artifacts reflect the smaller libassembly-store.so, not build breaks. Worth a final green CI before merge.

Nice work — the approach is correct and the diff is tidy. None of the above is blocking.

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[simonrozsival]

@copilot address code review - let's reduce the quality level to reduce time needed for decompression. I also like the suggestions regarding testing and code organization. Make sure you address all of the comments.

[Copilot]

@copilot address code review - let's reduce the quality level to reduce time needed for decompression. I also like the suggestions regarding testing and code organization. Make sure you address all of...

Addressed in commit 78d6a04.

Implemented all review-thread fixes:

• Reduced Zstd compression level and hardened Zstd.cs (CallingConvention.Cdecl, parameter/error checks, missing-entrypoint handling).
• Added guards in AssemblyCompression for full input reads and invalid max output size.
• Fixed tmt decompression robustness (full reads + pooled buffer return on all paths).
• Fixed _AssembliesToCompress comment typo/reference in Xamarin.Android.Common.targets.

Validation run: affected projects build successfully; targeted Xamarin.Android.Build.Tests filter run was attempted but fails in this environment due pre-existing test environment setup (TestEnvironment.DotNetPreviewDirectory null).

[simonrozsival]

/review

[github-actions]

✅ Android PR Reviewer completed successfully!

[github-actions]

@copilot Solid, well-scoped swap from LZ4 to Zstandard — and nice hardening in response to the earlier review round (Cdecl on the P/Invokes, -1/EncodingFailed instead of crashing on missing exports, pooled-buffer return + ReadFully).

Verified

• Header layout unchanged; the magic/extension swap (XALZ→XAZS, 0x5A4C4158→0x535A4158, .lz4→.zst) is applied consistently across AssemblyCompression.cs, the CLR/Mono native readers (assembly-store.cc, embedded-assemblies.cc), and the tmt/decompress-assemblies tools. No stale XALZ/LZ4 references remain outside the intentionally-retained fastdev path.
• Both native readers map ZSTD_decompress (dst, dstCapacity = uncompressed_length, src, srcSize = compressed_size) correctly and check ZSTD_isError before use.
• The managed Zstd P/Invokes and the native -lSystem.IO.Compression.Native link target the raw, unprefixed ZSTD_* exports of the runtime pack — matching how dotnet/runtime's own Interop.Zstd binds them — so the mechanism is sound. ZstandardDecoder.TryDecompress (net11) is used correctly, and the netstandard2.0 build-task wrapper is justified (the BCL Zstandard types aren't reachable from netstandard2.0).

Comments

• ⚠️ Inline (Zstd.cs): the Compress XML doc still says "maximum compression level" while the code uses level 3 (zstd's default). Please reconcile the doc with the chosen level.
• 💡 I couldn't find a fast managed round-trip test (Zstd.Compress → ZstandardDecoder.TryDecompress) — re-raising the earlier suggestion, since binding to internal runtime-pack exports means an ABI break would otherwise only surface late in a Release apkdesc or device run.

CI: Azure DevOps build 1482888 is still in_progress across all 38 checks, so this isn't validated green yet — worth confirming the Release apkdesc diffs and device suites pass before merge.

Submitting as comments only (not approving, per review policy).

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