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lyng/docs/fix-scope-parent-cycle.md
sergeych d15dfb6087 core: prevent scope parent-chain cycles when reusing pooled frames 
- Scope.resetForReuse: fully detach before re-parenting (clear state, parent=null, new frameId), then validate with ensureNoCycle and assign parent/args/pos/thisObj
- ScopePool.borrow (JVM/Android/JS/Native/Wasm): defensive fallback to fresh Scope allocation if resetForReuse detects a cycle
- docs: add docs/fix-scope-parent-cycle.md describing the change and expectations
- test: add ScopeCycleRegressionTest to ensure instance method call pattern does not crash and returns "ok"
2025-12-11 00:50:46 +01:00

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Fix: prevent cycles in scope parent chain during pooled frame reuse

What changed

  • Scope.resetForReuse now fully detaches a reused scope from its previous chain/state before re-parenting:
    • sets parent = null and regenerates frameId
    • clears locals/slots/bindings caches
    • only after that, validates the new parent with ensureNoCycle and assigns it
  • ScopePool.borrow on all targets (JVM, Android, JS, Native, Wasm) now has a defensive fallback:
    • if resetForReuse throws IllegalStateException indicating a parent-chain cycle, the pool allocates a fresh Scope instead of failing.

Why

In some nested call patterns (notably instance method calls where an instance is produced by another function and immediately used), the same pooled Scope object can be rebound into a chain that already (transitively) contains it. Reassigning parent in that case forms a structural cycle, which ensureNoCycle correctly detects and throws. This could surface as:

IllegalStateException: cycle detected in scope parent chain assignment
  at net.sergeych.lyng.Scope.ensureNoCycle(...)
  at net.sergeych.lyng.Scope.resetForReuse(...)
  at net.sergeych.lyng.ScopePool.borrow(...)
  ... during instance method invocation

The fix removes the failure mode by:

  1. Detaching the reused frame from its prior chain/state before validating and assigning the new parent.
  2. Falling back to a new frame allocation if a cycle is still detected (extremely rare and cheap vs. a crash).

Expected effects

  • Eliminates sporadic cycle detected in scope parent chain crashes during instance method invocation.
  • No change to public API or normal semantics.
  • Pooling remains enabled by default; the fallback only triggers on the detected cycle edge case.
  • Negligible performance impact: fresh allocation is used only when a cycle would have crashed the VM previously.

Notes

  • The fix is platform-wide (all ScopePool actuals are covered).
  • We recommend adding/keeping a regression test that exercises: a class with a method, a function returning an instance, and an exported function calling the instance method. The test should pass without exceptions.