Component used: Snapshot, SnapshotStateObserver, mutableStateOf

Version used: a02b52bd (androidx-main, 01 Jul 2021)

There is a race condition when we apply snapshot in one thread and try to read the snapshot value in another.

This code:

import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.compose.runtime.snapshots.Snapshot
import androidx.compose.runtime.snapshots.SnapshotStateObserver
import kotlin.concurrent.thread

fun main() {
    println("START")

    thread {
        while (true) {
            Snapshot.sendApplyNotifications()
        }
    }

    for (i in 1..10000) {
        val state1 by mutableStateOf(0)
        var state2 by mutableStateOf(true)
        val observer = SnapshotStateObserver({}).apply {
            start()
        }
        repeat(1000) {
            observer.observeReads(Unit, {}) {
                state1
                if (state2) {
                    state2 = false
                }
            }
        }
    }
}

crashes with exception:

Exception in thread “Thread-1” java.lang.NullPointerException
	at androidx.compose.runtime.collection.IdentityScopeMap.find(IdentityScopeMap.kt:319)
	at androidx.compose.runtime.collection.IdentityScopeMap.access$find(IdentityScopeMap.kt:26)
	at androidx.compose.runtime.snapshots.SnapshotStateObserver$applyObserver$1.invoke(SnapshotStateObserver.kt:284)
	at androidx.compose.runtime.snapshots.SnapshotStateObserver$applyObserver$1.invoke(SnapshotStateObserver.kt:26)
	at androidx.compose.runtime.snapshots.SnapshotKt.advanceGlobalSnapshot(Snapshot.kt:1440)
	at androidx.compose.runtime.snapshots.SnapshotKt.advanceGlobalSnapshot(Snapshot.kt:1459)
	at androidx.compose.runtime.snapshots.SnapshotKt.access$advanceGlobalSnapshot(Snapshot.kt:1)
	at androidx.compose.runtime.snapshots.Snapshot$Companion.sendApplyNotifications(Snapshot.kt:462)
	at androidx.compose.desktop.examples.example1.Main_jvmKt$main$1.invoke(Main.jvm.kt:28)
	at androidx.compose.desktop.examples.example1.Main_jvmKt$main$1.invoke(Main.jvm.kt:26)
	at kotlin.concurrent.ThreadsKt$thread$thread$1.run(Thread.kt:30)

The race condition is probably between map.removeValueIf { scope -> scope in invalidated } and currentMap!!.addValue(state) in SnapshotStateObserver