Enable opt-in R8 JNI name obfuscation for CoreCLR and NativeAOT

[simonrozsival]

Context

Advances #12535
Related to dotnet/maui#23027
Related to dotnet/docs-maui#2395

.NET for Android currently disables R8 name obfuscation because managed bindings embed Java/JNI class, method, field, and descriptor names. If R8 renames the Java side while managed metadata and IL retain the original names, JNI lookup and native registration fail.

This change implements build-time rewriting without a runtime lookup table. R8 remains the naming authority: a naming-only pass establishes names, managed assemblies are rewritten before ILLink or NativeAOT ILC, and final R8 performs normal shrinking and optimization with -applymapping.

The feature is opt-in through AndroidEnableR8JniNameObfuscation. It requires AndroidLinkTool=r8, AndroidTypeMapImplementation=trimmable, and either CoreCLR or NativeAOT.

Pipeline

1. Generate the pre-trim trimmable typemap and Java callable wrappers.
2. Compile the seed Java graph and run naming-only R8.
3. Rewrite managed JNI metadata, IL strings, descriptors, typemap attributes, aliases, and FieldRVA data using the seed mapping.
4. Feed collision-safe, path-hashed rewritten assemblies to ILLink or NativeAOT ILC.
5. Generate backend-specific final keep rules and a post-link reachability manifest.
6. Run final R8 with normal shrinking/optimization and -applymapping.
7. Validate seed/final name stability and required Java-symbol reachability.
8. Package the resulting DEX and optimized managed output normally.

NativeAOT's prebuilt host performs fixed native lookups against mono.android.Runtime, so that small bridge remains deliberately preserved.

Production hardening

• Rewritten assemblies are staged under source-path hashes, preventing duplicate-filename and ABI collisions while preserving downstream item metadata.
• Multi-RID builds use isolated per-RID rewrite outputs and manifests; the shared final Java/R8 pass validates against the first RID's equivalent managed manifest.
• Strong-named linker/compiler inputs retain their public key and assembly identity plus re-signable signature-directory space, while the stale StrongNameSigned flag is cleared. These private rewritten inputs are intentionally delay-signed because the original private keys are unavailable.
• NativeAOT multi-RID inner builds retain their normal parallelism.
• Generated ACW keep rules deduplicate repeated map values; one measured seed configuration decreased from 21,875 rules to 7,271.
• Seed compilation now runs after intermediate-directory cleanup/setup so clean inner builds have valid stamp directories.

Validation coverage

Automated Release integration tests cover CoreCLR and NativeAOT and verify:

• clean, no-op, missing-output recovery, and JNI-name invalidation builds;
• arm64 plus x64 multi-ABI builds with isolated rewritten inputs and manifests;
• referenced Android libraries containing registered Java peers;
• custom ProGuard configuration changes invalidating the seed pass, managed rewrite, and final R8;
• collision-safe hashed staging paths;
• seed/final mappings plus rewrite and reachability manifests;
• preservation of public-key assembly identity and delay-signature space.

The focused hardening matrix passes all 7 CoreCLR, NativeAOT, multi-ABI, project-reference/custom-rule, incrementality, and strong-name tests. Existing JNI mapping, rewriting, scanning, mapping-consistency, and reachability coverage remains in place.

Measurements

Metric	Baseline	Two-pass	Delta
CoreCLR signed APK	17,542,007 bytes	17,251,191 bytes	-290,816 bytes (-1.66%)
NativeAOT signed APK	18,348,247 bytes	18,016,471 bytes	-331,776 bytes (-1.81%)

Across 86 generated typemap assemblies, rewriting 13,031 real typemap entries reduced raw PE size by 1,016,832 bytes (9.42%) and gzip-equivalent size by 107,446 bytes (6.61%). NativeAOT's packaged saving came primarily from classes.dex, because those managed assemblies are compiled into the application native library.

Naming-only R8 remains the dominant added clean-build cost. Replacing it with a managed allocator would reproduce R8 inheritance, interface dispatch, override, keep-rule, and minifier semantics, so this design intentionally keeps R8 as the naming authority.

Remaining follow-up

Additional ACW class-name obfuscation is intentionally deferred. Allowing those classes to be renamed caused CoreCLR final R8 to remove reachable generated native declarations such as nctor_0; the reachability validator correctly rejected the output. Supporting this safely requires a separate native registration/retention design rather than weaker validation or another superficial ProGuard modifier.

[simonrozsival]

/review

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