Fix - Performance Optimization: Scope Resolution & Thread Safety

[arnaudgiuliani]

Performance Optimization: Scope Resolution & Thread Safety

This PR introduces two major performance improvements to Koin's core resolution engine:

1. Double-Checked Locking Pattern

Problem: Original implementation synchronized every instance access, causing lock contention even for already-created instances.

Solution: Implement lock-free fast path with double-checked locking:

// Fast path: check without locking
val existing = values[context.scope.id]
if (existing != null) return existing

// Slow path: create with locking
return synchronized(this) {
values[context.scope.id] ?: create(context)
}

Changes:

• Added @volatile to SingleInstanceFactory.value for memory visibility
• Removed unnecessary locks from size(), saveValue(), isCreated()
• Lock-free reads after initialization

2. Optimized Scope Resolution (CoreResolverV2)

Problem: Multiple sequential lookups with poor cache locality and unnecessary iterations.

Solution: Consolidated resolution logic in single method:

• Old order: injected params → registry → stack → scope source → archetype → parents → extensions
• New order: registry (combined with scope source & parents) → params → stack → extensions

Key improvements:

• Direct scope + archetype check first
• Inline scope source lookup (no separate method)
• Flattened parent hierarchy (single pass with flatten())
• Early exit when found

Performance Results

Benchmark	Before (BETA1)	After (BETA2)	Improvement
fragmentScope_cascade_activity_root	1803 ns	1578 ns	12.5% faster 🚀
activityScope_cascade_root	882 ns	805 ns	8.8% faster
retrieveDependency	111 ns	104 ns	6.4% faster
start_get_lazy_module100	6.93 ms	4.25 ms	38.7% faster 🚀

Impact

• Android apps: Faster navigation with deep Fragment/Activity hierarchies
• Multi-threaded apps: Eliminates lock contention in concurrent access
• Large apps: Significantly faster lazy module loading

Modified Files

• core/koin-core/src/commonMain/kotlin/org/koin/core/instance/ScopedInstanceFactory.kt
• core/koin-core/src/commonMain/kotlin/org/koin/core/instance/SingleInstanceFactory.kt
• core/koin-core/src/commonMain/kotlin/org/koin/core/resolution/CoreResolverV2.kt (new)

Thread Safety

✅ Double-checked locking with @volatile ensures memory visibility
✅ Synchronized blocks prevent race conditions
✅ Uses safeHashMap (ConcurrentHashMap) for scope values

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Breaking Changes: None (internal optimization only)

Fixes: #1990