KeyValueSerializer: Fix missing call to endStructure() (#2272)

The sequential decoding path was missing it. Use decodeStructure() to
avoid this error.

Co-authored-by: Pavel Vasin <git@vasin.nl>

Author: recursive-rat4

core/commonMain/src/kotlinx/serialization/internal/Tuples.kt

@@ -33,35 +33,33 @@ internal sealed class KeyValueSerializer<K, V, R>(
         structuredEncoder.endStructure(descriptor)
     }
 
-    override fun deserialize(decoder: Decoder): R {
-        val composite = decoder.beginStructure(descriptor)
-        if (composite.decodeSequentially()) {
-            val key = composite.decodeSerializableElement(descriptor, 0, keySerializer)
-            val value = composite.decodeSerializableElement(descriptor, 1, valueSerializer)
-            return toResult(key, value)
+    override fun deserialize(decoder: Decoder): R = decoder.decodeStructure(descriptor) {
+        if (decodeSequentially()) {
+            val key = decodeSerializableElement(descriptor, 0, keySerializer)
+            val value = decodeSerializableElement(descriptor, 1, valueSerializer)
+            return@decodeStructure toResult(key, value)
         }
 
         var key: Any? = NULL
         var value: Any? = NULL
         mainLoop@ while (true) {
-            when (val idx = composite.decodeElementIndex(descriptor)) {
+            when (val idx = decodeElementIndex(descriptor)) {
                 CompositeDecoder.DECODE_DONE -> {
                     break@mainLoop
                 }
                 0 -> {
-                    key = composite.decodeSerializableElement(descriptor, 0, keySerializer)
+                    key = decodeSerializableElement(descriptor, 0, keySerializer)
                 }
                 1 -> {
-                    value = composite.decodeSerializableElement(descriptor, 1, valueSerializer)
+                    value = decodeSerializableElement(descriptor, 1, valueSerializer)
                 }
                 else -> throw SerializationException("Invalid index: $idx")
             }
         }
-        composite.endStructure(descriptor)
         if (key === NULL) throw SerializationException("Element 'key' is missing")
         if (value === NULL) throw SerializationException("Element 'value' is missing")
         @Suppress("UNCHECKED_CAST")
-        return toResult(key as K, value as V)
+        return@decodeStructure toResult(key as K, value as V)
     }
 }
 

core/commonTest/src/kotlinx/serialization/internal/DummySequentialDecoder.kt

@@ -21,8 +21,17 @@ class DummySequentialDecoder(
     override fun decodeSequentially(): Boolean = true
     override fun decodeElementIndex(descriptor: SerialDescriptor): Int = throw Error("This method shouldn't be called in sequential mode")
 
-    override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder = this
-    override fun endStructure(descriptor: SerialDescriptor): Unit = Unit
+    var beginStructureCalled = 0
+    var endStructureCalled = 0
+
+    override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder {
+        ++beginStructureCalled
+        return this
+    }
+    override fun endStructure(descriptor: SerialDescriptor): Unit {
+        ++endStructureCalled
+        return Unit
+    }
 
     override fun decodeInline(descriptor: SerialDescriptor): Decoder = notImplemented()
 
@@ -51,6 +60,6 @@ class DummySequentialDecoder(
     override fun decodeStringElement(descriptor: SerialDescriptor, index: Int): String = notImplemented()
 
     override fun decodeInlineElement(descriptor: SerialDescriptor, index: Int): Decoder = notImplemented()
-    override fun <T : Any?> decodeSerializableElement(descriptor: SerialDescriptor, index: Int, deserializer: DeserializationStrategy<T>, previousValue: T?): T = notImplemented()
+    override fun <T : Any?> decodeSerializableElement(descriptor: SerialDescriptor, index: Int, deserializer: DeserializationStrategy<T>, previousValue: T?): T = decodeSerializableValue(deserializer)
     override fun <T : Any> decodeNullableSerializableElement(descriptor: SerialDescriptor, index: Int, deserializer: DeserializationStrategy<T?>, previousValue: T?): T? = notImplemented()
 }

core/commonTest/src/kotlinx/serialization/internal/TuplesTest.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.internal
+
+import kotlin.test.*
+import kotlinx.serialization.builtins.*
+
+class TuplesTest {
+    @Test
+    fun testSequentialDecodingKeyValue() {
+        val decoder = DummySequentialDecoder()
+        val serializer = MapEntrySerializer(Unit.serializer(), Unit.serializer())
+        serializer.deserialize(decoder)
+        assertEquals(decoder.beginStructureCalled, decoder.endStructureCalled)
+    }
+}