Disallow repeated keys in CBOR (WIP, need to add config switches)

Fixes Kotlin#2662

Author: timmc

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

@@ -133,3 +133,9 @@ 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 RepeatedMapKeyException() : SerializationException("Duplicate keys not allowed in maps")

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

@@ -110,6 +110,9 @@ public sealed class MapLikeSerializer<Key, Value, Collection, Builder : MutableM
         } else {
             decoder.decodeSerializableElement(descriptor, vIndex, valueSerializer)
         }
+        if (key in builder) {
+            throw RepeatedMapKeyException()
+        }
         builder[key] = value
     }
 

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.RepeatedMapKeyException
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class CborStrictModeTest {
+
+    /** Duplicate keys are rejected. */
+    @Test
+    fun testDuplicateKeys() {
+        val duplicateKeys = HexConverter.parseHexBinary("A2617805617806")
+        assertFailsWithMessage<RepeatedMapKeyException>("Duplicate keys not allowed in maps") {
+            Cbor.decodeFromByteArray<Map<String, Long>>(duplicateKeys)
+        }
+    }
+}