Make DescriptorSchemaCache in Json thread-local on Native to avoid freezing problems in custom Json instances

Fixes #1450

Author: sandwwraith

formats/json/commonMain/src/kotlinx/serialization/json/Json.kt

@@ -56,7 +56,7 @@ public sealed class Json(
     override val serializersModule: SerializersModule
 ) : StringFormat {
 
-    internal val schemaCache: DescriptorSchemaCache = DescriptorSchemaCache()
+    internal val _schemaCache: DescriptorSchemaCache = DescriptorSchemaCache()
 
     /**
      * The default instance of [Json] with default configuration.
@@ -291,5 +291,10 @@ private class JsonImpl(configuration: JsonConfiguration, module: SerializersModu
     }
 }
 
+/**
+ * Emulate thread-locality of cache on Native
+ */
+internal expect val Json.schemaCache: DescriptorSchemaCache
+
 private const val defaultIndent = "    "
 private const val defaultDiscriminator = "type"

formats/json/jsMain/src/kotlinx/serialization/json/schemaCache.kt

@@ -0,0 +1,9 @@
+/*
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.json.internal.*
+
+internal actual val Json.schemaCache: DescriptorSchemaCache get() = this._schemaCache

formats/json/jvmMain/src/kotlinx/serialization/json/schemaCache.kt

@@ -0,0 +1,9 @@
+/*
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.json.internal.*
+
+internal actual val Json.schemaCache: DescriptorSchemaCache get() = this._schemaCache

formats/json/nativeMain/src/kotlinx/serialization/json/schemaCache.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.json.internal.*
+
+@ThreadLocal
+private val jsonToCache: MutableMap<Json, DescriptorSchemaCache> = mutableMapOf()
+
+/**
+ * Emulate thread locality of cache on Native
+ */
+internal actual val Json.schemaCache: DescriptorSchemaCache
+    get() = jsonToCache.getOrPut(this) { DescriptorSchemaCache() }

formats/json/nativeTest/src/kotlinx/serialization/json/MultiWorkerJsonTest.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.*
+import kotlin.native.concurrent.*
+import kotlin.test.*
+
+class MultiWorkerJsonTest {
+    @Serializable
+    data class PlainOne(val one: Int)
+
+    @Serializable
+    data class PlainTwo(val two: Int)
+
+
+    @Test
+    fun testJsonIsFreezeSafe() {
+        val json = Json {
+            isLenient = true
+            ignoreUnknownKeys = true
+            useAlternativeNames = true
+        }
+        val worker = Worker.start()
+        val operation = {
+            for (i in 0..999) {
+                assertEquals(PlainOne(42), json.decodeFromString("""{"one":42,"two":239}"""))
+            }
+        }
+        worker.executeAfter(1000, operation.freeze())
+        for (i in 0..999) {
+            assertEquals(PlainTwo(239), json.decodeFromString("""{"one":42,"two":239}"""))
+        }
+        worker.requestTermination()
+        // Should be completed successfully
+    }
+}