Handle EOF in skipElement correctly (#1475)

Improve EOF exception in consumeStringLenient

Fixes #1474

Co-authored-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

Author: sandwwraith

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * 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
@@ -366,6 +366,7 @@ internal class JsonLexer(private val source: String) {
             return takePeeked()
         }
         var current = skipWhitespaces()
+        if (current >= source.length) fail("EOF", current)
         // Skip leading quotation mark
         val token = charToTokenClass(source[current])
         if (token == TC_STRING) {
@@ -445,16 +446,17 @@ internal class JsonLexer(private val source: String) {
                         "found ] instead of }",
                         source
                     )
-                    tokenStack.removeAt(tokenStack.size - 1)
+                    tokenStack.removeLast()
                 }
                 TC_END_OBJ -> {
                     if (tokenStack.last() != TC_BEGIN_OBJ) throw JsonDecodingException(
                         currentPosition,
                         "found } instead of ]",
                         source
                     )
-                    tokenStack.removeAt(tokenStack.size - 1)
+                    tokenStack.removeLast()
                 }
+                TC_EOF -> fail("Unexpected end of input due to malformed JSON during ignoring unknown keys")
             }
             consumeNextToken()
             if (tokenStack.size == 0) return

formats/json/commonTest/src/kotlinx/serialization/json/JsonModesTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.serialization.json
@@ -77,6 +77,20 @@ class JsonModesTest : JsonTestBase() {
         doTest("""{"a": 0, "strangeField": ["imma string with } bracket"]}""")
     }
 
+    @Serializable
+    class Empty
+
+    @Test
+    fun lenientThrowOnMalformedString() {
+        fun doTest(input: String) {
+            assertFailsWith<SerializationException> { lenient.decodeFromString(Empty.serializer(), input) }
+        }
+        doTest("""{"a":[{"b":[{"c":{}d",""e"":"}]}""")
+        doTest("""{"a":[}""")
+        doTest("""{"a":""")
+        lenient.decodeFromString(Empty.serializer(), """{"a":[]}""") // should not throw
+    }
+
     @Test
     fun testSerializeQuotedJson() = parametrizedTest { useStreaming ->
         assertEquals(