Kotlinx.Serialization: first level serialization is broken

How to reproduce:

1. Push a class with an instant to the Mongo collection
2. Find from the mongo collection
3. It should fail to deserialize

Expected Behavior

It should deserialize to the correct class

Actual Behavior

It fails to deserialize

Additional case specific information:

Java 11, OpenJDK. KMongo 4.2.1

Mongo Collection State:

db.schedulerJob.find()
{ "_id" : ObjectId("5fb7d2bfac8e4033d69c163f"), "startTime" : ISODate("2020-11-20T14:29:19.502Z"), "duration" : NumberLong(900000), "targetUserId" : "110433910835404800", "guildId" : "383210853270552578", "reason" : "Time's up!", "action" : "UNMUTE" }

Interface:

interface SchedulerJob {
    @Contextual @SerialName("_id") val id: Id<SchedulerJob>
    val action: Action
    @Contextual val startTime: Instant
    val duration: Long
    val targetUserId: String
    val guildId: String

    fun shouldBePersisted(): Boolean {
        return duration >= Duration.ofMinutes(15).toMillis()
    }

    suspend fun processJob()
}

Effective class:

@Serializable
class UnmuteSchedulerJob(
    @Contextual @SerialName("_id") override val id: Id<SchedulerJob> = newId(),
    @Contextual override val startTime: Instant = Instant.now(),
    override val duration: Long,
    override val targetUserId: String,
    override val guildId: String,
    val reason: String?
) : SchedulerJob {
    override val action: Action = Action.UNMUTE

    override suspend fun processJob() {
        val jda = KoinContextHandler.get().get<JDA>()
        val guild = jda.getGuildById(guildId) ?: return

        val moderationService = KoinContextHandler.get().get<IModerationService>()

        moderationService.unmute(targetUserId, guild, reason)
    }
}

Exception Log:

Exception in thread "pool-3-thread-1" org.bson.BsonInvalidOperationException: readString can only be called when CurrentBSONType is STRING, not when CurrentBSONType is DATE_TIME.
	at org.bson.AbstractBsonReader.verifyBSONType(AbstractBsonReader.java:690)
	at org.bson.AbstractBsonReader.checkPreconditions(AbstractBsonReader.java:722)
	at org.bson.AbstractBsonReader.readString(AbstractBsonReader.java:457)
	at com.github.jershell.kbson.FlexibleDecoder.decodeString(BsonFlexibleDecoder.kt:114)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeStringElement(AbstractDecoder.kt:56)
	at kotlinx.serialization.internal.AbstractPolymorphicSerializer.deserialize(AbstractPolymorphicSerializer.kt:52)
	at kotlinx.serialization.encoding.Decoder$DefaultImpls.decodeSerializableValue(Decoding.kt:234)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:17)
	at org.litote.kmongo.serialization.SerializationCodec.decode(SerializationCodec.kt:69)
	at com.mongodb.internal.operation.CommandResultArrayCodec.decode(CommandResultArrayCodec.java:52)
	at com.mongodb.internal.operation.CommandResultDocumentCodec.readValue(CommandResultDocumentCodec.java:60)
	at org.bson.codecs.BsonDocumentCodec.decode(BsonDocumentCodec.java:87)
	at org.bson.codecs.BsonDocumentCodec.decode(BsonDocumentCodec.java:42)
	at org.bson.internal.LazyCodec.decode(LazyCodec.java:48)
	at org.bson.codecs.BsonDocumentCodec.readValue(BsonDocumentCodec.java:104)
	at com.mongodb.internal.operation.CommandResultDocumentCodec.readValue(CommandResultDocumentCodec.java:63)
	at org.bson.codecs.BsonDocumentCodec.decode(BsonDocumentCodec.java:87)
	at org.bson.codecs.BsonDocumentCodec.decode(BsonDocumentCodec.java:42)
	at com.mongodb.internal.connection.ReplyMessage.<init>(ReplyMessage.java:51)
	at com.mongodb.internal.connection.InternalStreamConnection.getCommandResult(InternalStreamConnection.java:477)
	at com.mongodb.internal.connection.InternalStreamConnection.access$1000(InternalStreamConnection.java:78)
	at com.mongodb.internal.connection.InternalStreamConnection$2$1.onResult(InternalStreamConnection.java:462)
	at com.mongodb.internal.connection.InternalStreamConnection$2$1.onResult(InternalStreamConnection.java:440)
	at com.mongodb.internal.connection.InternalStreamConnection$MessageHeaderCallback$MessageCallback.onResult(InternalStreamConnection.java:745)
	at com.mongodb.internal.connection.InternalStreamConnection$MessageHeaderCallback$MessageCallback.onResult(InternalStreamConnection.java:712)
	at com.mongodb.internal.connection.InternalStreamConnection$5.completed(InternalStreamConnection.java:582)
	at com.mongodb.internal.connection.InternalStreamConnection$5.completed(InternalStreamConnection.java:579)
	at com.mongodb.internal.connection.AsynchronousChannelStream$BasicCompletionHandler.completed(AsynchronousChannelStream.java:250)
	at com.mongodb.internal.connection.AsynchronousChannelStream$BasicCompletionHandler.completed(AsynchronousChannelStream.java:233)
	at java.base/sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:127)
	at java.base/sun.nio.ch.Invoker.invokeDirect(Invoker.java:158)
	at java.base/sun.nio.ch.UnixAsynchronousSocketChannelImpl.implRead(UnixAsynchronousSocketChannelImpl.java:562)
	at java.base/sun.nio.ch.AsynchronousSocketChannelImpl.read(AsynchronousSocketChannelImpl.java:277)
	at java.base/sun.nio.ch.AsynchronousSocketChannelImpl.read(AsynchronousSocketChannelImpl.java:298)
	at com.mongodb.internal.connection.AsynchronousSocketChannelStream$AsynchronousSocketChannelAdapter.read(AsynchronousSocketChannelStream.java:144)
	at com.mongodb.internal.connection.AsynchronousChannelStream.readAsync(AsynchronousChannelStream.java:118)
	at com.mongodb.internal.connection.AsynchronousChannelStream.readAsync(AsynchronousChannelStream.java:107)
	at com.mongodb.internal.connection.InternalStreamConnection.readAsync(InternalStreamConnection.java:579)
	at com.mongodb.internal.connection.InternalStreamConnection.access$1100(InternalStreamConnection.java:78)
	at com.mongodb.internal.connection.InternalStreamConnection$MessageHeaderCallback.onResult(InternalStreamConnection.java:702)
	at com.mongodb.internal.connection.InternalStreamConnection$MessageHeaderCallback.onResult(InternalStreamConnection.java:687)
	at com.mongodb.internal.connection.InternalStreamConnection$5.completed(InternalStreamConnection.java:582)
	at com.mongodb.internal.connection.InternalStreamConnection$5.completed(InternalStreamConnection.java:579)
	at com.mongodb.internal.connection.AsynchronousChannelStream$BasicCompletionHandler.completed(AsynchronousChannelStream.java:250)
	at com.mongodb.internal.connection.AsynchronousChannelStream$BasicCompletionHandler.completed(AsynchronousChannelStream.java:233)
	at java.base/sun.nio.ch.Invoker.invokeUnchecked(Invoker.java:127)
	at java.base/sun.nio.ch.UnixAsynchronousSocketChannelImpl.finishRead(UnixAsynchronousSocketChannelImpl.java:439)
	at java.base/sun.nio.ch.UnixAsynchronousSocketChannelImpl.finish(UnixAsynchronousSocketChannelImpl.java:191)
	at java.base/sun.nio.ch.UnixAsynchronousSocketChannelImpl.onEvent(UnixAsynchronousSocketChannelImpl.java:213)
	at java.base/sun.nio.ch.EPollPort$EventHandlerTask.run(EPollPort.java:306)
	at java.base/sun.nio.ch.AsynchronousChannelGroupImpl$1.run(AsynchronousChannelGroupImpl.java:112)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

[zigzago]

There is a regression in the kotlinx.serialization KMongo codec: first level polymorphism serialization is broken (it works if the class is embedded in an other class). I’m going to release a 4.2.2 version.

[zigzago]

By the way I found an other annoying bug (see #249). Expect a new bug fix release in a few days (a fix on third-party library kbson is required)