Skip null fields in ProtoBuf serialization

runtime/commonMain/src/kotlinx/serialization/Tagged.kt

@@ -64,17 +64,27 @@ abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
 
     // ---- Implementation of low-level API ----
 
-    fun encodeElement(desc: SerialDescriptor, index: Int): Boolean {
+    fun <T: Any?> encodeElement(desc: SerialDescriptor, index: Int, value: T): Boolean {
         val tag = desc.getTag(index)
-        val shouldWriteElement = shouldWriteElement(desc, tag, index)
+        val shouldWriteElement = shouldWriteElement(desc, tag, index, value)
+        if (shouldWriteElement) {
+            pushTag(tag)
+        }
+        return shouldWriteElement
+    }
+
+    fun encodeNullElement(desc: SerialDescriptor, index: Int): Boolean {
+        val tag = desc.getTag(index)
+        val shouldWriteElement = shouldWriteNullElement(desc, tag, index)
         if (shouldWriteElement) {
             pushTag(tag)
         }
         return shouldWriteElement
     }
 
     // For format-specific behaviour, invoked only on
-    open fun shouldWriteElement(desc: SerialDescriptor, tag: Tag, index: Int) = true
+    open fun <T: Any?> shouldWriteElement(desc: SerialDescriptor, tag: Tag, index: Int, value: T) = true
+    open fun shouldWriteNullElement(desc: SerialDescriptor, tag: Tag, index: Int) = true
 
     final override fun encodeNotNullMark() = encodeTaggedNotNullMark(currentTag)
     final override fun encodeNull() = encodeTaggedNull(popTag())
@@ -122,12 +132,13 @@ abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
     final override fun encodeStringElement(desc: SerialDescriptor, index: Int, value: String) = encodeTaggedString(desc.getTag(index), value)
 
     final override fun <T : Any?> encodeSerializableElement(desc: SerialDescriptor, index: Int, serializer: SerializationStrategy<T>, value: T) {
-        if (encodeElement(desc, index))
+        if (encodeElement(desc, index, value))
             encodeSerializableValue(serializer, value)
     }
 
     final override fun <T : Any> encodeNullableSerializableElement(desc: SerialDescriptor, index: Int, serializer: SerializationStrategy<T>, value: T?) {
-        if (encodeElement(desc, index))
+        if (value == null && encodeNullElement(desc, index)
+            || value != null && encodeElement(desc, index, value))
             encodeNullableSerializableValue(serializer, value)
     }
 

runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonOutput.kt

@@ -166,15 +166,17 @@ private class JsonTreeMapOutput(json: Json, nodeConsumer: (JsonElement) -> Unit)
         return JsonObject(content)
     }
 
-    override fun shouldWriteElement(desc: SerialDescriptor, tag: String, index: Int): Boolean = true
+    override fun <T: Any?> shouldWriteElement(desc: SerialDescriptor, tag: String, index: Int, value: T): Boolean = true
+    override fun shouldWriteNullElement(desc: SerialDescriptor, tag: String, index: Int): Boolean = true
 }
 
 private class JsonTreeListOutput(json: Json, nodeConsumer: (JsonElement) -> Unit) :
     AbstractJsonTreeOutput(json, nodeConsumer) {
     private val array: ArrayList<JsonElement> = arrayListOf()
     override fun elementName(desc: SerialDescriptor, index: Int): String = index.toString()
 
-    override fun shouldWriteElement(desc: SerialDescriptor, tag: String, index: Int): Boolean = true
+    override fun <T: Any?> shouldWriteElement(desc: SerialDescriptor, tag: String, index: Int, value: T): Boolean = true
+    override fun shouldWriteNullElement(desc: SerialDescriptor, tag: String, index: Int): Boolean = true
 
     override fun putElement(key: String, element: JsonElement) {
         val idx = key.toInt()

runtime/commonMain/src/kotlinx/serialization/protobuf/ProtoBuf.kt

@@ -40,6 +40,9 @@ class ProtoBuf(context: SerialModule = EmptyModule) : AbstractSerialFormat(conte
             else -> throw SerializationException("Primitives are not supported at top-level")
         }
 
+        override fun shouldWriteNullElement(desc: SerialDescriptor, tag: ProtoDesc, index: Int): Boolean =
+            desc.kind !is StructureKind.CLASS
+
         override fun encodeTaggedInt(tag: ProtoDesc, value: Int) = encoder.writeInt(value, tag.first, tag.second)
         override fun encodeTaggedByte(tag: ProtoDesc, value: Byte) = encoder.writeInt(value.toInt(), tag.first, tag.second)
         override fun encodeTaggedShort(tag: ProtoDesc, value: Short) = encoder.writeInt(value.toInt(), tag.first, tag.second)

runtime/commonTest/src/kotlinx/serialization/protobuf/ProtobufReaderTest.kt

@@ -59,4 +59,9 @@ class ProtobufReaderTest {
         if (isNative()) return // todo: support update on Native
         ProtoBuf.loads(TestIntWithList.serializer(), "500308960150045005") shouldBe TestIntWithList(150, listOf(3, 4, 5))
     }
+
+    @Test
+    fun readObjectWithNullableMissingFields() {
+        ProtoBuf.loads(TestInnerNullable.serializer(), "1a0308ab02") shouldBe t7
+    }
 }

runtime/commonTest/src/kotlinx/serialization/protobuf/ProtobufWriterTest.kt

@@ -58,4 +58,9 @@ class ProtobufWriterTest {
     fun writeNumbers() {
         ProtoBuf.dumps(TestNumbers.serializer(), t6).toLowerCase() shouldBe "0d9488010010ffffffffffffffff7f"
     }
+
+    @Test
+    fun writeNullableFields() {
+        ProtoBuf.dumps(TestInnerNullable.serializer(), t7).toLowerCase() shouldBe "1a0308ab02"
+    }
 }

runtime/commonTest/src/kotlinx/serialization/protobuf/SampleClasses.kt

@@ -32,6 +32,9 @@ data class TestString(@SerialId(2) val b: String)
 @Serializable
 data class TestInner(@SerialId(3) val a: TestInt)
 
+@Serializable
+data class TestInnerNullable(@SerialId(3) val a: TestInt, @SerialId(4) val b: TestInt? = null)
+
 @Serializable
 data class TestComplex(@SerialId(42) val b: Int, @SerialId(2) val c: String)
 
@@ -53,3 +56,4 @@ val t3e = TestString("")
 val t4 = TestInner(t1)
 val t5 = TestComplex(42, "testing")
 val t6 = TestNumbers(100500, Long.MAX_VALUE)
+val t7 = TestInnerNullable(t1, null)