Fixed decoding of huge JSON data for okio streams

Fixes #2006

formats/json-okio/commonMain/src/kotlinx/serialization/json/okio/internal/OkioJsonStreams.kt

@@ -50,7 +50,7 @@ internal class OkioSerialReader(private val source: BufferedSource): SerialReade
     override fun read(buffer: CharArray, bufferOffset: Int, count: Int): Int {
         var i = 0
         while (i < count && !source.exhausted()) {
-            buffer[i] = source.readUtf8CodePoint().toChar()
+            buffer[bufferOffset + i] = source.readUtf8CodePoint().toChar()
             i++
         }
         return if (i > 0) i else -1

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonHugeDataSerializationTest.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2017-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.encodeToString
+import kotlin.test.Test
+
+
+class JsonHugeDataSerializationTest : JsonTestBase() {
+
+    @Serializable
+    private data class Node(
+        val children: List<Node>?
+    )
+
+    private fun createNodes(count: Int, depth: Int): List<Node> {
+        val ret = mutableListOf<Node>()
+        if (depth == 0) return ret
+        for (i in 0 until count) {
+            ret.add(Node(createNodes(1, depth - 1)))
+        }
+        return ret
+    }
+
+    @Test
+    fun test() {
+        // create some huge instance
+        val rootNode = Node(createNodes(1000, 10))
+
+        //  Encoding will always be true for a standard `encodeToString` - we leave this assumption so as not to insert a huge string into the sources
+        val expectedJson = Json.encodeToString(rootNode)
+
+        assertJsonFormAndRestored(Node.serializer(), rootNode, expectedJson)
+    }
+}

formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt

@@ -184,7 +184,6 @@ internal abstract class AbstractJsonLexer {
 
     open fun consumeNextToken(expected: Char) {
         ensureHaveChars()
-        val source = source
         var cpos = currentPosition
         while (true) {
             cpos = prefetchOrEof(cpos)

formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/JsonLexer.kt

@@ -67,7 +67,9 @@ internal class ReaderJsonLexer(
 
     private fun preload(spaceLeft: Int) {
         val buffer = _source
-        buffer.copyInto(buffer, 0, currentPosition, currentPosition + spaceLeft)
+        if (spaceLeft != 0) {
+            buffer.copyInto(buffer, 0, currentPosition, currentPosition + spaceLeft)
+        }
         var read = spaceLeft
         val sizeTotal = _source.size
         while (read != sizeTotal) {