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"

docs/fix-scope-parent-cycle.md

@@ -0,0 +1,34 @@
+## 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.

lynglib/src/androidMain/kotlin/net/sergeych/lyng/ScopePoolAndroid.kt

@@ -39,12 +39,18 @@ actual object ScopePool {
     actual fun borrow(parent: Scope, args: Arguments, pos: Pos, thisObj: Obj): Scope {
         val pool = pool()
         val s = if (pool.isNotEmpty()) pool.removeLast() else Scope(parent, args, pos, thisObj)
-        if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
-            s.resetForReuse(parent, args, pos, thisObj)
-        } else {
-            s.frameId = nextFrameId()
+        return try {
+            if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
+                s.resetForReuse(parent, args, pos, thisObj)
+            } else {
+                s.frameId = nextFrameId()
+            }
+            s
+        } catch (e: IllegalStateException) {
+            if (e.message?.contains("cycle") == true && e.message?.contains("scope parent chain") == true) {
+                Scope(parent, args, pos, thisObj)
+            } else throw e
         }
-        return s
     }
 
     actual fun release(scope: Scope) {

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Scope.kt

@@ -302,11 +302,9 @@ open class Scope(
      * Clears locals and slots, assigns new frameId, and sets parent/args/pos/thisObj.
      */
     fun resetForReuse(parent: Scope?, args: Arguments, pos: Pos, thisObj: Obj) {
-        ensureNoCycle(parent)
-        this.parent = parent
-        this.args = args
-        this.pos = pos
-        this.thisObj = thisObj
+        // Fully detach from any previous chain/state first to avoid residual ancestry
+        // that could interact badly with the new parent and produce a cycle.
+        this.parent = null
         this.skipScopeCreation = false
         // fresh identity for PIC caches
         this.frameId = nextFrameId()
@@ -315,6 +313,12 @@ open class Scope(
         slots.clear()
         nameToSlot.clear()
         localBindings.clear()
+        // Now safe to validate and re-parent
+        ensureNoCycle(parent)
+        this.parent = parent
+        this.args = args
+        this.pos = pos
+        this.thisObj = thisObj
         // Pre-size local slots for upcoming parameter assignment where possible
         reserveLocalCapacity(args.list.size + 4)
     }

lynglib/src/commonTest/kotlin/ScopeCycleRegressionTest.kt

@@ -0,0 +1,34 @@
+/*
+ * Regression test for scope parent-chain cycle when reusing pooled frames
+ */
+
+import kotlinx.coroutines.test.runTest
+import net.sergeych.lyng.eval
+import kotlin.test.Test
+
+class ScopeCycleRegressionTest {
+    @Test
+    fun instanceMethodCallDoesNotCycle() = runTest {
+        eval(
+            (
+                """
+                class Whatever {
+                    fun something() {
+                        trace("something")
+                    }
+                }
+
+                fun ll() { Whatever() }
+
+                fun callTest1() {
+                    val l = ll()
+                    l.something()
+                    "ok"
+                }
+
+                assertEquals("ok", callTest1())
+                """
+                ).trimIndent()
+        )
+    }
+}

lynglib/src/jsMain/kotlin/net/sergeych/lyng/ScopePoolJs.kt

@@ -29,12 +29,18 @@ actual object ScopePool {
 
     actual fun borrow(parent: Scope, args: Arguments, pos: Pos, thisObj: Obj): Scope {
         val s = if (pool.isNotEmpty()) pool.removeLast() else Scope(parent, args, pos, thisObj)
-        if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
-            s.resetForReuse(parent, args, pos, thisObj)
-        } else {
-            s.frameId = nextFrameId()
+        return try {
+            if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
+                s.resetForReuse(parent, args, pos, thisObj)
+            } else {
+                s.frameId = nextFrameId()
+            }
+            s
+        } catch (e: IllegalStateException) {
+            if (e.message?.contains("cycle") == true && e.message?.contains("scope parent chain") == true) {
+                Scope(parent, args, pos, thisObj)
+            } else throw e
         }
-        return s
     }
 
     actual fun release(scope: Scope) {

lynglib/src/jvmMain/kotlin/net/sergeych/lyng/ScopePoolJvm.kt

@@ -33,9 +33,19 @@ actual object ScopePool {
     actual fun borrow(parent: Scope, args: Arguments, pos: Pos, thisObj: Obj): Scope {
         val pool = threadLocalPool.get()
         val s = if (pool.isNotEmpty()) pool.removeLast() else Scope(parent, args, pos, thisObj)
-        // Always reset state on borrow to guarantee fresh-frame semantics
-        s.resetForReuse(parent, args, pos, thisObj)
-        return s
+        return try {
+            // Always reset state on borrow to guarantee fresh-frame semantics
+            s.resetForReuse(parent, args, pos, thisObj)
+            s
+        } catch (e: IllegalStateException) {
+            // Defensive fallback: if a cycle in scope parent chain is detected during reuse,
+            // discard pooled instance for this call frame and allocate a fresh scope instead.
+            if (e.message?.contains("cycle") == true && e.message?.contains("scope parent chain") == true) {
+                Scope(parent, args, pos, thisObj)
+            } else {
+                throw e
+            }
+        }
     }
 
     actual fun release(scope: Scope) {

lynglib/src/nativeMain/kotlin/net/sergeych/lyng/ScopePoolNative.kt

@@ -29,12 +29,18 @@ actual object ScopePool {
 
     actual fun borrow(parent: Scope, args: Arguments, pos: Pos, thisObj: Obj): Scope {
         val s = if (pool.isNotEmpty()) pool.removeLast() else Scope(parent, args, pos, thisObj)
-        if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
-            s.resetForReuse(parent, args, pos, thisObj)
-        } else {
-            s.frameId = nextFrameId()
+        return try {
+            if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
+                s.resetForReuse(parent, args, pos, thisObj)
+            } else {
+                s.frameId = nextFrameId()
+            }
+            s
+        } catch (e: IllegalStateException) {
+            if (e.message?.contains("cycle") == true && e.message?.contains("scope parent chain") == true) {
+                Scope(parent, args, pos, thisObj)
+            } else throw e
         }
-        return s
     }
 
     actual fun release(scope: Scope) {

lynglib/src/wasmJsMain/kotlin/net/sergeych/lyng/ScopePoolWasm.kt

@@ -29,12 +29,18 @@ actual object ScopePool {
 
     actual fun borrow(parent: Scope, args: Arguments, pos: Pos, thisObj: Obj): Scope {
         val s = if (pool.isNotEmpty()) pool.removeLast() else Scope(parent, args, pos, thisObj)
-        if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
-            s.resetForReuse(parent, args, pos, thisObj)
-        } else {
-            s.frameId = nextFrameId()
+        return try {
+            if (s.parent !== parent || s.args !== args || s.pos !== pos || s.thisObj !== thisObj) {
+                s.resetForReuse(parent, args, pos, thisObj)
+            } else {
+                s.frameId = nextFrameId()
+            }
+            s
+        } catch (e: IllegalStateException) {
+            if (e.message?.contains("cycle") == true && e.message?.contains("scope parent chain") == true) {
+                Scope(parent, args, pos, thisObj)
+            } else throw e
         }
-        return s
     }
 
     actual fun release(scope: Scope) {