Support nullable contextual serializers (#392)

Author: yakivy

krpc/krpc-core/src/commonMain/kotlin/kotlinx/rpc/krpc/internal/SerializationUtils.kt

@@ -10,6 +10,7 @@ import kotlinx.rpc.descriptor.RpcTypeKrpc
 import kotlinx.rpc.internal.rpcInternalKClass
 import kotlinx.rpc.internal.utils.InternalRpcApi
 import kotlinx.serialization.*
+import kotlinx.serialization.builtins.nullable
 import kotlinx.serialization.descriptors.SerialDescriptor
 import kotlinx.serialization.descriptors.buildClassSerialDescriptor
 import kotlinx.serialization.encoding.*
@@ -29,7 +30,7 @@ internal fun SerializersModule.buildContextualInternal(type: KType): KSerializer
     )
 
     @Suppress("UNCHECKED_CAST")
-    return result as? KSerializer<Any?>
+    return if (type.isMarkedNullable) result?.nullable else result as? KSerializer<Any?>
 }
 
 private fun SerializersModule.buildContextual(type: KType): KSerializer<Any?> {

krpc/krpc-test/src/commonMain/kotlin/kotlinx/rpc/krpc/test/KrpcTestService.kt

@@ -68,6 +68,7 @@ interface KrpcTestService {
     suspend fun nonSerializableClassWithSerializer(
         localDateTime: @Serializable(LocalDateTimeSerializer::class) LocalDateTime,
     ): @Serializable(LocalDateTimeSerializer::class) LocalDateTime
+    suspend fun nullableNonSerializableClass(localDate: LocalDate?): LocalDate?
 
     suspend fun incomingStreamSyncCollect(arg1: Flow<String>): Int
     suspend fun incomingStreamSyncCollectMultiple(arg1: Flow<String>, arg2: Flow<String>, arg3: Flow<String>): Int

krpc/krpc-test/src/commonMain/kotlin/kotlinx/rpc/krpc/test/KrpcTestServiceBackend.kt

@@ -129,6 +129,10 @@ class KrpcTestServiceBackend : KrpcTestService {
         )
     }
 
+    override suspend fun nullableNonSerializableClass(localDate: LocalDate?): LocalDate? {
+        return localDate?.let { LocalDate(it.year, it.month, it.day + 1) }
+    }
+
     override suspend fun incomingStreamSyncCollect(arg1: Flow<String>): Int {
         return arg1.count()
     }

krpc/krpc-test/src/commonMain/kotlin/kotlinx/rpc/krpc/test/KrpcTransportTestBase.kt

@@ -176,6 +176,16 @@ abstract class KrpcTransportTestBase {
         )
     }
 
+    @Test
+    open fun nullableNonSerializableParameter() = runTest {
+        val localDate = LocalDate(2001, 8, 23)
+        val resultDate = client.nonSerializableClass(localDate)
+        assertEquals(LocalDate(2001, 8, 24), resultDate)
+
+        val resultNull = client.nullableNonSerializableClass(null)
+        assertNull(resultNull)
+    }
+
     @Test
     fun doubleGenericReturnType() = runTest {
         val result = client.doubleGenericReturnType()

krpc/krpc-test/src/commonTest/kotlin/kotlinx/rpc/krpc/test/LocalTransportTest.kt

@@ -54,6 +54,9 @@ class ProtoBufLocalTransportTest : LocalTransportTest() {
     @Test
     override fun nullableReturn(): TestResult = runTest { }
 
+    @Test
+    override fun nullableNonSerializableParameter(): TestResult = runTest { }
+
     @Test
     override fun testByteArraySerialization(): TestResult = runTest { }