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More advanced ThreadBox with affinity guards
// This is a class that attempts to stop you accessing variables outside a lock.
//
// It does not do a perfect job, but can catch some common kinds of mistake, in
// particular when you accidentally try to work with objects inside closures that
// end up running later, outside the locked region (or in a different thread).
// EXAMPLE
val bank = ThreadBox(object {
val accounts by arrayListOf(10, 0, 0, 0).guard()
})
fun transfer(from: Int, to: Int, amount: Int) {
bank.accounts[0] = 0 // Does not compile: accounts not in scope
bank.locked {
accounts[from] -= amount
accounts[to] += amount
val ref = accounts // Bad idea
thread {
ref[0] = 0 // Doesn't throw: guard was stripped
accounts[0] = 0 // Throws IllegalStateException
}.join()
}
}
// IMPLEMENTATION
class ThreadBox<T>(v: T) {
companion object {
val inLockedRegion: ThreadLocal<ThreadBox<*>?> = ThreadLocal()
val owners = WeakHashMap<Any, ThreadBox<*>>()
}
private val value = v
init {
owners[value] = this
}
synchronized fun locked<R>(f: T.() -> R): R {
inLockedRegion.set(this)
return try {
value.f()
} finally {
inLockedRegion.set(null)
}
}
}
class Guard<T>(initialValue: T) : ReadOnlyProperty<kotlin.Any, T> {
val value = initialValue
override fun get(thisRef: Any, desc: PropertyMetadata): T {
check(ThreadBox.inLockedRegion.get() identityEquals ThreadBox.owners[thisRef]) { "Locking error: ${ThreadBox.inLockedRegion.get()} vs ${thisRef}" }
return value
}
}
fun <T> T.guard(): Guard<T> = Guard(this)
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