KTOR-9606 Fix KotlinxSerializationConverter deserialization of empty channel

ktor-shared/ktor-serialization/ktor-serialization-gson/jvm/test/ConverterTest.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2014-2026 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+import io.ktor.serialization.gson.GsonConverter
+import io.ktor.util.reflect.typeInfo
+import io.ktor.utils.io.ByteChannel
+import io.ktor.utils.io.charsets.Charsets
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.runTest
+import kotlinx.serialization.Serializable
+import kotlin.test.Test
+import kotlin.test.assertNull
+import kotlin.time.Duration.Companion.milliseconds
+
+@Serializable
+data class Payload(val value: String)
+
+class ConverterTest {
+    @Test
+    fun returnsNullForEmptyChannelWithDelayedClose() = runTest {
+        val channel = ByteChannel()
+        launch {
+            delay(200.milliseconds)
+            channel.close()
+        }
+
+        val converter = GsonConverter()
+        val result = converter.deserialize(Charsets.UTF_8, typeInfo<Payload>(), channel)
+        assertNull(result)
+    }
+}

ktor-shared/ktor-serialization/ktor-serialization-jackson/jvm/test/ConverterTest.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2014-2026 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+import com.fasterxml.jackson.databind.exc.MismatchedInputException
+import io.ktor.serialization.JsonConvertException
+import io.ktor.serialization.jackson.JacksonConverter
+import io.ktor.util.reflect.typeInfo
+import io.ktor.utils.io.ByteChannel
+import io.ktor.utils.io.charsets.Charsets
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.runTest
+import kotlinx.serialization.Serializable
+import kotlin.test.Test
+import kotlin.test.assertFailsWith
+import kotlin.test.assertIs
+import kotlin.time.Duration.Companion.milliseconds
+
+@Serializable
+data class Payload(val value: String)
+
+class ConverterTest {
+    @Test
+    fun throwsExceptionForEmptyChannel() = runTest {
+        val channel = ByteChannel()
+        launch {
+            delay(200.milliseconds)
+            channel.close()
+        }
+
+        val converter = JacksonConverter()
+        val cause = assertFailsWith<JsonConvertException> {
+            converter.deserialize(Charsets.UTF_8, typeInfo<Payload>(), channel)
+        }
+
+        assertIs<MismatchedInputException>(cause.cause)
+    }
+}

ktor-shared/ktor-serialization/ktor-serialization-jackson3/jvm/test/ConverterTest.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2014-2026 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+import io.ktor.serialization.JsonConvertException
+import io.ktor.serialization.jackson3.JacksonConverter
+import io.ktor.util.reflect.typeInfo
+import io.ktor.utils.io.ByteChannel
+import io.ktor.utils.io.charsets.Charsets
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.runTest
+import kotlinx.serialization.Serializable
+import tools.jackson.databind.exc.MismatchedInputException
+import kotlin.test.Test
+import kotlin.test.assertFailsWith
+import kotlin.test.assertIs
+import kotlin.time.Duration.Companion.milliseconds
+
+@Serializable
+data class Payload(val value: String)
+
+class ConverterTest {
+    @Test
+    fun throwsExceptionForEmptyChannel() = runTest {
+        val channel = ByteChannel()
+        launch {
+            delay(200.milliseconds)
+            channel.close()
+        }
+
+        val converter = JacksonConverter()
+        val cause = assertFailsWith<JsonConvertException> {
+            converter.deserialize(Charsets.UTF_8, typeInfo<Payload>(), channel)
+        }
+
+        assertIs<MismatchedInputException>(cause.cause)
+    }
+}

ktor-shared/ktor-serialization/ktor-serialization-kotlinx/build.gradle.kts

@@ -15,5 +15,9 @@ kotlin {
             api(projects.ktorSerialization)
             api(libs.kotlinx.serialization.core)
         }
+        commonTest.dependencies {
+            implementation(projects.ktorSerializationKotlinxJson)
+            implementation(libs.kotlinx.coroutines.test)
+        }
     }
 }

ktor-shared/ktor-serialization/ktor-serialization-kotlinx/common/src/io/ktor/serialization/kotlinx/KotlinxSerializationConverter.kt

@@ -7,16 +7,13 @@ package io.ktor.serialization.kotlinx
 import io.ktor.http.*
 import io.ktor.http.content.*
 import io.ktor.serialization.*
-import io.ktor.util.*
-import io.ktor.util.pipeline.*
 import io.ktor.util.reflect.*
 import io.ktor.utils.io.*
 import io.ktor.utils.io.charsets.*
 import io.ktor.utils.io.core.*
 import kotlinx.coroutines.flow.*
 import kotlinx.io.*
 import kotlinx.serialization.*
-import kotlin.jvm.*
 
 /**
  * Creates a converter serializing with the specified string [format]
@@ -58,13 +55,14 @@ public class KotlinxSerializationConverter(
     }
 
     override suspend fun deserialize(charset: Charset, typeInfo: TypeInfo, content: ByteReadChannel): Any? {
-        val fromExtension = extensions.asFlow()
-            .map { it.deserialize(charset, typeInfo, content) }
-            .firstOrNull { it != null || content.isClosedForRead }
-        if (extensions.isNotEmpty() && (fromExtension != null || content.isClosedForRead)) return fromExtension
+        val contentPacket = content.readRemaining()
+
+        for (ext in extensions) {
+            if (contentPacket.exhausted()) return null
+            return ext.deserialize(charset, typeInfo, ByteReadChannel(contentPacket)) ?: continue
+        }
 
         val serializer = format.serializersModule.serializerForTypeInfo(typeInfo)
-        val contentPacket = content.readRemaining()
 
         try {
             return when (format) {

ktor-shared/ktor-serialization/ktor-serialization-kotlinx/common/test/io/ktor/serialization/kotlinx/ConverterTest.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2014-2026 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package io.ktor.serialization.kotlinx
+
+import io.ktor.serialization.kotlinx.json.DefaultJson
+import io.ktor.util.reflect.typeInfo
+import io.ktor.utils.io.ByteChannel
+import io.ktor.utils.io.charsets.Charsets
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.runTest
+import kotlinx.serialization.Serializable
+import kotlin.test.Test
+import kotlin.test.assertNull
+import kotlin.time.Duration.Companion.milliseconds
+
+@Serializable
+data class Payload(val value: String)
+
+class ConverterTest {
+    @Test
+    fun returnsNullForEmptyChannelWithDelayedClose() = runTest {
+        val channel = ByteChannel()
+        launch {
+            delay(200.milliseconds)
+            channel.close()
+        }
+
+        val converter = KotlinxSerializationConverter(DefaultJson)
+        val result = converter.deserialize(Charsets.UTF_8, typeInfo<Payload>(), channel)
+        assertNull(result)
+    }
+}