Add ability to disallow repeated keys in CBOR

Fixes Kotlin#2662 by adding
a `visitKey` method to `CompositeDecoder`; map and set serializers should
call this so that decoders have an opportunity to throw an error when a
duplicate key is detected.

A new config option `Cbor.allowDuplicateKeys` can be set to false to
enable this new behavior. This can form the basis of a Strict Mode in the
future.

Also fixes a typo in an unrelated method docstring.

core/api/kotlinx-serialization-core.api

@@ -19,6 +19,10 @@ public abstract interface class kotlinx/serialization/DeserializationStrategy {
 	public abstract fun getDescriptor ()Lkotlinx/serialization/descriptors/SerialDescriptor;
 }
 
+public final class kotlinx/serialization/DuplicateMapKeyException : kotlinx/serialization/SerializationException {
+	public fun <init> (Ljava/lang/Object;)V
+}
+
 public abstract interface annotation class kotlinx/serialization/EncodeDefault : java/lang/annotation/Annotation {
 	public abstract fun mode ()Lkotlinx/serialization/EncodeDefault$Mode;
 }
@@ -384,6 +388,7 @@ public abstract class kotlinx/serialization/encoding/AbstractDecoder : kotlinx/s
 	public final fun decodeStringElement (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/String;
 	public fun decodeValue ()Ljava/lang/Object;
 	public fun endStructure (Lkotlinx/serialization/descriptors/SerialDescriptor;)V
+	public fun visitKey (Ljava/lang/Object;)V
 }
 
 public abstract class kotlinx/serialization/encoding/AbstractEncoder : kotlinx/serialization/encoding/CompositeEncoder, kotlinx/serialization/encoding/Encoder {
@@ -448,6 +453,7 @@ public abstract interface class kotlinx/serialization/encoding/CompositeDecoder
 	public abstract fun decodeStringElement (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/String;
 	public abstract fun endStructure (Lkotlinx/serialization/descriptors/SerialDescriptor;)V
 	public abstract fun getSerializersModule ()Lkotlinx/serialization/modules/SerializersModule;
+	public abstract fun visitKey (Ljava/lang/Object;)V
 }
 
 public final class kotlinx/serialization/encoding/CompositeDecoder$Companion {
@@ -460,6 +466,7 @@ public final class kotlinx/serialization/encoding/CompositeDecoder$DefaultImpls
 	public static synthetic fun decodeNullableSerializableElement$default (Lkotlinx/serialization/encoding/CompositeDecoder;Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;Ljava/lang/Object;ILjava/lang/Object;)Ljava/lang/Object;
 	public static fun decodeSequentially (Lkotlinx/serialization/encoding/CompositeDecoder;)Z
 	public static synthetic fun decodeSerializableElement$default (Lkotlinx/serialization/encoding/CompositeDecoder;Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;Ljava/lang/Object;ILjava/lang/Object;)Ljava/lang/Object;
+	public static fun visitKey (Lkotlinx/serialization/encoding/CompositeDecoder;Ljava/lang/Object;)V
 }
 
 public abstract interface class kotlinx/serialization/encoding/CompositeEncoder {
@@ -1119,6 +1126,7 @@ public abstract class kotlinx/serialization/internal/TaggedDecoder : kotlinx/ser
 	protected abstract fun getTag (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/Object;
 	protected final fun popTag ()Ljava/lang/Object;
 	protected final fun pushTag (Ljava/lang/Object;)V
+	public fun visitKey (Ljava/lang/Object;)V
 }
 
 public abstract class kotlinx/serialization/internal/TaggedEncoder : kotlinx/serialization/encoding/CompositeEncoder, kotlinx/serialization/encoding/Encoder {

core/api/kotlinx-serialization-core.klib.api

@@ -288,6 +288,7 @@ abstract interface kotlinx.serialization.encoding/CompositeDecoder { // kotlinx.
     }
     open fun decodeCollectionSize(kotlinx.serialization.descriptors/SerialDescriptor): kotlin/Int // kotlinx.serialization.encoding/CompositeDecoder.decodeCollectionSize|decodeCollectionSize(kotlinx.serialization.descriptors.SerialDescriptor){}[0]
     open fun decodeSequentially(): kotlin/Boolean // kotlinx.serialization.encoding/CompositeDecoder.decodeSequentially|decodeSequentially(){}[0]
+    open fun visitKey(kotlin/Any?) // kotlinx.serialization.encoding/CompositeDecoder.visitKey|visitKey(kotlin.Any?){}[0]
 }
 abstract interface kotlinx.serialization.encoding/CompositeEncoder { // kotlinx.serialization.encoding/CompositeEncoder|null[0]
     abstract fun <#A1: kotlin/Any> encodeNullableSerializableElement(kotlinx.serialization.descriptors/SerialDescriptor, kotlin/Int, kotlinx.serialization/SerializationStrategy<#A1>, #A1?) // kotlinx.serialization.encoding/CompositeEncoder.encodeNullableSerializableElement|encodeNullableSerializableElement(kotlinx.serialization.descriptors.SerialDescriptor;kotlin.Int;kotlinx.serialization.SerializationStrategy<0:0>;0:0?){0§<kotlin.Any>}[0]
@@ -522,6 +523,9 @@ final class kotlinx.serialization.modules/SerializersModuleBuilder : kotlinx.ser
     final fun build(): kotlinx.serialization.modules/SerializersModule // kotlinx.serialization.modules/SerializersModuleBuilder.build|build(){}[0]
     final fun include(kotlinx.serialization.modules/SerializersModule) // kotlinx.serialization.modules/SerializersModuleBuilder.include|include(kotlinx.serialization.modules.SerializersModule){}[0]
 }
+final class kotlinx.serialization/DuplicateMapKeyException : kotlinx.serialization/SerializationException { // kotlinx.serialization/DuplicateMapKeyException|null[0]
+    constructor <init>(kotlin/Any?) // kotlinx.serialization/DuplicateMapKeyException.<init>|<init>(kotlin.Any?){}[0]
+}
 final class kotlinx.serialization/MissingFieldException : kotlinx.serialization/SerializationException { // kotlinx.serialization/MissingFieldException|null[0]
     constructor <init>(kotlin.collections/List<kotlin/String>, kotlin/String) // kotlinx.serialization/MissingFieldException.<init>|<init>(kotlin.collections.List<kotlin.String>;kotlin.String){}[0]
     constructor <init>(kotlin.collections/List<kotlin/String>, kotlin/String?, kotlin/Throwable?) // kotlinx.serialization/MissingFieldException.<init>|<init>(kotlin.collections.List<kotlin.String>;kotlin.String?;kotlin.Throwable?){}[0]

core/commonMain/src/kotlinx/serialization/SerializationExceptions.kt

@@ -133,3 +133,10 @@ internal constructor(message: String?) : SerializationException(message) {
     // This constructor is used by the generated serializers
     constructor(index: Int) : this("An unknown field for index $index")
 }
+
+/**
+ * Thrown when a map deserializer encounters a repeated map key (and configuration disallows this.)
+ */
+@ExperimentalSerializationApi
+public class DuplicateMapKeyException(key: Any?) :
+    SerializationException("Duplicate keys not allowed in maps. Key appeared twice: $key")

core/commonMain/src/kotlinx/serialization/encoding/Decoding.kt

@@ -275,7 +275,7 @@ internal inline fun <T : Any> Decoder.decodeIfNullable(deserializer: Deserializa
  * [CompositeDecoder] is a part of decoding process that is bound to a particular structured part of
  * the serialized form, described by the serial descriptor passed to [Decoder.beginStructure].
  *
- * Typically, for unordered data, [CompositeDecoder] is used by a serializer withing a [decodeElementIndex]-based
+ * Typically, for unordered data, [CompositeDecoder] is used by a serializer within a [decodeElementIndex]-based
  * loop that decodes all the required data one-by-one in any order and then terminates by calling [endStructure].
  * Please refer to [decodeElementIndex] for example of such loop.
  *
@@ -558,6 +558,18 @@ public interface CompositeDecoder {
         deserializer: DeserializationStrategy<T?>,
         previousValue: T? = null
     ): T?
+
+    /**
+     * Called after a key has been read.
+     *
+     * This could be a map or set key, or anything otherwise intended to be
+     * distinct within the collection under normal circumstances.
+     *
+     * Implementations might use this as a hook for throwing an exception when
+     * duplicate keys are encountered.
+     */
+    @ExperimentalSerializationApi
+    public fun visitKey(key: Any?) { }
 }
 
 /**

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

@@ -98,6 +98,7 @@ public sealed class MapLikeSerializer<Key, Value, Collection, Builder : MutableM
 
     final override fun readElement(decoder: CompositeDecoder, index: Int, builder: Builder, checkIndex: Boolean) {
         val key: Key = decoder.decodeSerializableElement(descriptor, index, keySerializer)
+        decoder.visitKey(key)
         val vIndex = if (checkIndex) {
             decoder.decodeElementIndex(descriptor).also {
                 require(it == index + 1) { "Value must follow key in a map, index for key: $index, returned index for value: $it" }

formats/cbor/api/kotlinx-serialization-cbor.api

@@ -7,7 +7,7 @@ public synthetic class kotlinx/serialization/cbor/ByteString$Impl : kotlinx/seri
 
 public abstract class kotlinx/serialization/cbor/Cbor : kotlinx/serialization/BinaryFormat {
 	public static final field Default Lkotlinx/serialization/cbor/Cbor$Default;
-	public synthetic fun <init> (ZZLkotlinx/serialization/modules/SerializersModule;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (ZZZLkotlinx/serialization/modules/SerializersModule;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public fun decodeFromByteArray (Lkotlinx/serialization/DeserializationStrategy;[B)Ljava/lang/Object;
 	public fun encodeToByteArray (Lkotlinx/serialization/SerializationStrategy;Ljava/lang/Object;)[B
 	public fun getSerializersModule ()Lkotlinx/serialization/modules/SerializersModule;
@@ -17,9 +17,11 @@ public final class kotlinx/serialization/cbor/Cbor$Default : kotlinx/serializati
 }
 
 public final class kotlinx/serialization/cbor/CborBuilder {
+	public final fun getAllowDuplicateKeys ()Z
 	public final fun getEncodeDefaults ()Z
 	public final fun getIgnoreUnknownKeys ()Z
 	public final fun getSerializersModule ()Lkotlinx/serialization/modules/SerializersModule;
+	public final fun setAllowDuplicateKeys (Z)V
 	public final fun setEncodeDefaults (Z)V
 	public final fun setIgnoreUnknownKeys (Z)V
 	public final fun setSerializersModule (Lkotlinx/serialization/modules/SerializersModule;)V

formats/cbor/api/kotlinx-serialization-cbor.klib.api

@@ -7,6 +7,9 @@
 
 // Library unique name: <org.jetbrains.kotlinx:kotlinx-serialization-cbor>
 final class kotlinx.serialization.cbor/CborBuilder { // kotlinx.serialization.cbor/CborBuilder|null[0]
+    final var allowDuplicateKeys // kotlinx.serialization.cbor/CborBuilder.allowDuplicateKeys|<get-allowDuplicateKeys>(){}[0]
+        final fun <get-allowDuplicateKeys>(): kotlin/Boolean // kotlinx.serialization.cbor/CborBuilder.allowDuplicateKeys.<get-allowDuplicateKeys>|<get-allowDuplicateKeys>(){}[0]
+        final fun <set-allowDuplicateKeys>(kotlin/Boolean) // kotlinx.serialization.cbor/CborBuilder.allowDuplicateKeys.<set-allowDuplicateKeys>|<set-allowDuplicateKeys>(kotlin.Boolean){}[0]
     final var encodeDefaults // kotlinx.serialization.cbor/CborBuilder.encodeDefaults|<get-encodeDefaults>(){}[0]
         final fun <get-encodeDefaults>(): kotlin/Boolean // kotlinx.serialization.cbor/CborBuilder.encodeDefaults.<get-encodeDefaults>|<get-encodeDefaults>(){}[0]
         final fun <set-encodeDefaults>(kotlin/Boolean) // kotlinx.serialization.cbor/CborBuilder.encodeDefaults.<set-encodeDefaults>|<set-encodeDefaults>(kotlin.Boolean){}[0]
@@ -22,7 +25,7 @@ open annotation class kotlinx.serialization.cbor/ByteString : kotlin/Annotation
     constructor <init>() // kotlinx.serialization.cbor/ByteString.<init>|<init>(){}[0]
 }
 sealed class kotlinx.serialization.cbor/Cbor : kotlinx.serialization/BinaryFormat { // kotlinx.serialization.cbor/Cbor|null[0]
-    constructor <init>(kotlin/Boolean, kotlin/Boolean, kotlinx.serialization.modules/SerializersModule) // kotlinx.serialization.cbor/Cbor.<init>|<init>(kotlin.Boolean;kotlin.Boolean;kotlinx.serialization.modules.SerializersModule){}[0]
+    constructor <init>(kotlin/Boolean, kotlin/Boolean, kotlin/Boolean, kotlinx.serialization.modules/SerializersModule) // kotlinx.serialization.cbor/Cbor.<init>|<init>(kotlin.Boolean;kotlin.Boolean;kotlin.Boolean;kotlinx.serialization.modules.SerializersModule){}[0]
     final object Default : kotlinx.serialization.cbor/Cbor // kotlinx.serialization.cbor/Cbor.Default|null[0]
     open fun <#A1: kotlin/Any?> decodeFromByteArray(kotlinx.serialization/DeserializationStrategy<#A1>, kotlin/ByteArray): #A1 // kotlinx.serialization.cbor/Cbor.decodeFromByteArray|decodeFromByteArray(kotlinx.serialization.DeserializationStrategy<0:0>;kotlin.ByteArray){0§<kotlin.Any?>}[0]
     open fun <#A1: kotlin/Any?> encodeToByteArray(kotlinx.serialization/SerializationStrategy<#A1>, #A1): kotlin/ByteArray // kotlinx.serialization.cbor/Cbor.encodeToByteArray|encodeToByteArray(kotlinx.serialization.SerializationStrategy<0:0>;0:0){0§<kotlin.Any?>}[0]

formats/cbor/commonMain/src/kotlinx/serialization/cbor/Cbor.kt

@@ -32,13 +32,14 @@ import kotlinx.serialization.modules.*
 public sealed class Cbor(
     internal val encodeDefaults: Boolean,
     internal val ignoreUnknownKeys: Boolean,
+    internal val allowDuplicateKeys: Boolean,
     override val serializersModule: SerializersModule
 ) : BinaryFormat {
 
     /**
      * The default instance of [Cbor]
      */
-    public companion object Default : Cbor(false, false, EmptySerializersModule())
+    public companion object Default : Cbor(false, false, true, EmptySerializersModule())
 
     override fun <T> encodeToByteArray(serializer: SerializationStrategy<T>, value: T): ByteArray {
         val output = ByteArrayOutput()
@@ -55,8 +56,11 @@ public sealed class Cbor(
 }
 
 @OptIn(ExperimentalSerializationApi::class)
-private class CborImpl(encodeDefaults: Boolean, ignoreUnknownKeys: Boolean, serializersModule: SerializersModule) :
-    Cbor(encodeDefaults, ignoreUnknownKeys, serializersModule)
+private class CborImpl(
+    encodeDefaults: Boolean, ignoreUnknownKeys: Boolean, allowDuplicateKeys: Boolean,
+    serializersModule: SerializersModule,
+) :
+    Cbor(encodeDefaults, ignoreUnknownKeys, allowDuplicateKeys, serializersModule)
 
 /**
  * Creates an instance of [Cbor] configured from the optionally given [Cbor instance][from]
@@ -66,7 +70,7 @@ private class CborImpl(encodeDefaults: Boolean, ignoreUnknownKeys: Boolean, seri
 public fun Cbor(from: Cbor = Cbor, builderAction: CborBuilder.() -> Unit): Cbor {
     val builder = CborBuilder(from)
     builder.builderAction()
-    return CborImpl(builder.encodeDefaults, builder.ignoreUnknownKeys, builder.serializersModule)
+    return CborImpl(builder.encodeDefaults, builder.ignoreUnknownKeys, builder.allowDuplicateKeys, builder.serializersModule)
 }
 
 /**
@@ -87,6 +91,14 @@ public class CborBuilder internal constructor(cbor: Cbor) {
      */
     public var ignoreUnknownKeys: Boolean = cbor.ignoreUnknownKeys
 
+    /**
+     * Specifies whether it is an error to read a map with duplicate keys.
+     *
+     * If this is set to false, decoding a map with two keys that compare as equal
+     * will cause a [DuplicateMapKeyException] error to be thrown.
+     */
+    public var allowDuplicateKeys: Boolean = cbor.allowDuplicateKeys
+
     /**
      * Module with contextual and polymorphic serializers to be used in the resulting [Cbor] instance.
      */

formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Encoding.kt

@@ -197,8 +197,21 @@ internal class CborEncoder(private val output: ByteArrayOutput) {
     }
 }
 
-private class CborMapReader(cbor: Cbor, decoder: CborDecoder) : CborListReader(cbor, decoder) {
+private class CborMapReader(val cbor: Cbor, decoder: CborDecoder) : CborListReader(cbor, decoder) {
+    /** Keys that have been seen so far while reading this map. */
+    private val seenKeys = mutableSetOf<Any?>()
+
     override fun skipBeginToken() = setSize(decoder.startMap() * 2)
+
+    override fun visitKey(key: Any?) {
+        if (cbor.allowDuplicateKeys)
+            return
+
+        val added = seenKeys.add(key)
+        if (!added) {
+            throw DuplicateMapKeyException(key)
+        }
+    }
 }
 
 private open class CborListReader(cbor: Cbor, decoder: CborDecoder) : CborReader(cbor, decoder) {

formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborStrictModeTest.kt

@@ -0,0 +1,20 @@
+package kotlinx.serialization.cbor
+
+import kotlinx.serialization.assertFailsWithMessage
+import kotlinx.serialization.decodeFromByteArray
+import kotlinx.serialization.HexConverter
+import kotlinx.serialization.DuplicateMapKeyException
+import kotlin.test.Test
+
+class CborStrictModeTest {
+    private val strict = Cbor { allowDuplicateKeys = false }
+
+    /** Duplicate keys are rejected in generic maps. */
+    @Test
+    fun testDuplicateKeysInMap() {
+        val duplicateKeys = HexConverter.parseHexBinary("A2617805617806")
+        assertFailsWithMessage<DuplicateMapKeyException>("Duplicate keys not allowed in maps. Key appeared twice: x") {
+            strict.decodeFromByteArray<Map<String, Long>>(duplicateKeys)
+        }
+    }
+}

formats/json/api/kotlinx-serialization-json.api

@@ -182,6 +182,7 @@ public final class kotlinx/serialization/json/JsonDecoder$DefaultImpls {
 	public static fun decodeNullableSerializableValue (Lkotlinx/serialization/json/JsonDecoder;Lkotlinx/serialization/DeserializationStrategy;)Ljava/lang/Object;
 	public static fun decodeSequentially (Lkotlinx/serialization/json/JsonDecoder;)Z
 	public static fun decodeSerializableValue (Lkotlinx/serialization/json/JsonDecoder;Lkotlinx/serialization/DeserializationStrategy;)Ljava/lang/Object;
+	public static fun visitKey (Lkotlinx/serialization/json/JsonDecoder;Ljava/lang/Object;)V
 }
 
 public abstract class kotlinx/serialization/json/JsonElement {