Fix potential UB in ByteString.hashCode

bytestring/common/src/-Platform.kt

@@ -0,0 +1,14 @@
+/*
+* Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+* Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE.txt file.
+*/
+package kotlinx.io.bytestring
+
+/**
+ * Annotation indicating that the marked property is the subject of benign data race.
+ * LLVM does not support this notion, so on K/N platforms we alias it into `@Volatile` to prevent potential OoTA.
+ */
+@OptionalExpectation
+@Target(AnnotationTarget.FIELD)
+@OptIn(ExperimentalMultiplatform::class)
+internal expect annotation class BenignDataRace()

bytestring/common/src/ByteString.kt

@@ -69,6 +69,7 @@ public class ByteString private constructor(
     public constructor(data: ByteArray, startIndex: Int = 0, endIndex: Int = data.size) :
             this(data.copyOfRange(startIndex, endIndex), null)
 
+    @BenignDataRace
     private var hashCode: Int = 0
 
     public companion object {

bytestring/native/src/-PlatformNative.kt

@@ -0,0 +1,10 @@
+/*
+* Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+* Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE.txt file.
+*/
+package kotlinx.io.bytestring
+
+import kotlin.concurrent.*
+
+@Suppress("ACTUAL_WITHOUT_EXPECT") // This suppress can be removed in 2.0: KT-59355
+internal actual typealias BenignDataRace = Volatile