Add JNI-style global references and complete GC roots

[dlunch]

Summary

• add a typed, RAII-managed GlobalRef<T> backed by independent JVM global-reference entries
• keep object arguments and field, static, array, method, and exception results alive as frame-local references
• use a global reference while Thread.start() waits for its host spawn callback to attach
• release bootstrap-only local references after JVM initialization

Root cause

Thread.start() captured its Java Thread instance in a Rust callback, but the JVM garbage collector could not see Rust-owned references. If collection ran after the caller frame returned and before the spawned task attached, the thread object could be collected. The same local-reference gap existed for object values entering a frame through several JVM APIs.

Validation

• cargo test --workspace
• cargo test -p jvm --test test_garbage_collection
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 99.09091% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.11%. Comparing base (258cd4e) to head (7617a5a).

Files with missing lines	Patch %	Lines
java_runtime/src/classes/java/lang/thread.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #182 +/- ##
==========================================
+ Coverage 87.03% 87.11% +0.08% 
==========================================
Files 198 199 +1 Lines 17846 17947 +101 ==========================================
+ Hits 15532 15635 +103 + Misses 2314 2312 -2 

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR extends the JVM’s GC root tracking to cover Rust-owned references by introducing JNI-style global references and by treating certain API results/arguments as frame-local roots, addressing a real lifetime gap around Thread.start() and other object flows across native/Java boundaries.

Changes:

• Add RAII-managed GlobalRef<T> backed by a global-reference table that the GC scans as a root set.
• Record object arguments and returned object/exception results as frame-local references so they remain GC-visible across API boundaries.
• Update Thread.start() to hold a global reference until the spawned callback attaches; add targeted GC regression tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test_utils/src/lib.rs	Adds a queued-spawn mode to deterministically delay spawn callbacks in tests.
jvm/tests/test_garbage_collection.rs	Adds tests covering global refs, frame-local roots for various APIs, and Thread.start() lifetime.
jvm/src/thread.rs	Updates Java-frame creation to seed GC-visible locals from call arguments.
jvm/src/lib.rs	Wires in the new global_ref module and exports GlobalRef.
jvm/src/jvm.rs	Implements global refs, clears bootstrap locals post-init, and roots returned object/exception values + certain API results.
jvm/src/global_ref.rs	Introduces GlobalReferences + RAII GlobalRef<T> with Drop cleanup.
jvm/src/garbage_collector.rs	Treats global references as additional GC roots.
java_runtime/src/classes/java/lang/thread.rs	Uses GlobalRef<Thread> to keep the thread instance alive until the spawn callback runs.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.