Skipped encoding of empty packed collections in protobuf

Resolves #2906

Author: shanshin

formats/protobuf/commonMain/src/kotlinx/serialization/protobuf/internal/PackedArrayEncoder.kt

@@ -25,6 +25,12 @@ internal class PackedArrayEncoder(
         throw SerializationException("Packing only supports primitive number types")
     }
 
+    override fun endEncode(descriptor: SerialDescriptor) {
+        if (stream.size() > 0) {
+            super.endEncode(descriptor)
+        }
+    }
+
     override fun encodeTaggedString(tag: ProtoDesc, value: String) {
         throw SerializationException("Packing only supports primitive number types")
     }

formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/PackedArraySerializerTest.kt

@@ -51,6 +51,12 @@ class PackedArraySerializerTest {
         val s: List<String>
     )
 
+    @Serializable
+    data class PackedIntCarrier(
+        @ProtoPacked
+        val i: List<Int>
+    )
+
     /**
      * Test that when packing is specified the array is encoded as packed
      */
@@ -132,4 +138,30 @@ class PackedArraySerializerTest {
         assertEquals(listData, decoded)
     }
 
+    /**
+     * Test that empty packed repeated field is not presented in a message.
+     */
+    @Test
+    fun testEncodeEmptyPackedList() {
+        val obj = PackedIntCarrier(emptyList())
+        val encoded = ProtoBuf.encodeToHexString(obj)
+        assertEquals("", encoded)
+    }
+
+    /**
+     * Test that absence of packed field and explicit presence with a length 0 are decoded an empty list.
+     */
+    @Test
+    fun testDecodeEmptyPackedList() {
+        val explicitlyEmpty = "0a00"
+
+        val obj = PackedIntCarrier(emptyList())
+
+        val decodedExplicitEmpty = ProtoBuf.decodeFromHexString<PackedIntCarrier>(explicitlyEmpty)
+        val decodedAbsentField = ProtoBuf.decodeFromHexString<PackedIntCarrier>("")
+
+        assertEquals(obj, decodedExplicitEmpty)
+        assertEquals(obj, decodedAbsentField)
+    }
+
 }