Correctly throw SerializationException instead of IOOBE for some cases with EOF in streams (#1677)

Fixes #1675

Author: sandwwraith

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

@@ -196,7 +196,7 @@ internal abstract class AbstractJsonLexer {
 
     protected fun unexpectedToken(expected: Char) {
         --currentPosition // To properly handle null
-        if (expected == STRING && consumeStringLenient() == NULL) {
+        if (currentPosition >= 0 && expected == STRING && consumeStringLenient() == NULL) {
             fail("Expected string literal but 'null' literal was found.\n$coerceInputValuesHint", currentPosition - 4)
         }
         fail(charToTokenClass(expected))

formats/json/jvmMain/src/kotlinx/serialization/json/internal/JsonLexerJvm.kt

@@ -1,3 +1,7 @@
+/*
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
 package kotlinx.serialization.json.internal
 
 import java.io.*
@@ -93,7 +97,7 @@ internal class ReaderJsonLexer(
         if (position < source.length) return position
         currentPosition = position
         ensureHaveChars()
-        if (currentPosition != 0) return -1 // if something was loaded, then it would be zero.
+        if (currentPosition != 0 || source.isEmpty()) return -1 // if something was loaded, then it would be zero.
         return 0
     }
 

formats/json/jvmTest/src/kotlinx/serialization/features/JsonJvmStreamsTest.kt

@@ -1,13 +1,18 @@
+/*
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
 package kotlinx.serialization.features
 
-import kotlinx.serialization.StringData
+import kotlinx.serialization.*
 import kotlinx.serialization.builtins.serializer
 import kotlinx.serialization.json.Json
 import kotlinx.serialization.json.internal.BATCH_SIZE
 import kotlinx.serialization.test.decodeViaStream
 import kotlinx.serialization.test.encodeViaStream
 import org.junit.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
 
 class JsonJvmStreamsTest {
     private val strLen = BATCH_SIZE * 2 + 42
@@ -47,4 +52,17 @@ class JsonJvmStreamsTest {
         assertEquals(str, json.decodeViaStream(StringData.serializer(), input).data)
         assertEquals(str, json.decodeViaStream(String.serializer(), str))
     }
+
+    @Test
+    fun testThrowsCorrectExceptionOnEof() {
+        assertFailsWith<SerializationException> {
+            Json.decodeViaStream(StringData.serializer(), """{"data":""")
+        }
+        assertFailsWith<SerializationException> {
+            Json.decodeViaStream(StringData.serializer(), "")
+        }
+        assertFailsWith<SerializationException> {
+            Json.decodeViaStream(String.serializer(), "\"")
+        }
+    }
 }