Implemented skipping of filed with proto Id=0 if it missed in the des…

…criptor

Also optimized skipping of size delimited fields - removed the creation of an byte array in case of skipping

Fixes #2649

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

@@ -45,8 +45,12 @@ internal open class ProtobufDecoder(
              * If we have reasonably small count of elements, try to build sequential
              * array for the fast-path. Fast-path implies that elements are not marked with @ProtoId
              * explicitly or are monotonic and incremental (maybe, 1-indexed)
+             *
+             * Since the library allows the use of fields with proto ID 0,
+             * it is necessary to initialize all elements, because there will always be one extra element
+             * in the fast path array, which misses in the descriptor
              */
-            val cache = IntArray(elements + 1)
+            val cache = IntArray(elements + 1) { -1 }
             for (i in 0 until elements) {
                 val protoId = extractProtoId(descriptor, i, false)
                 // If any element is marked as ProtoOneOf,

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

@@ -59,7 +59,7 @@ internal class ProtobufReader(private val input: ByteArrayInput) {
         when (currentType) {
             ProtoWireType.VARINT -> readInt(ProtoIntegerType.DEFAULT)
             ProtoWireType.i64 -> readLong(ProtoIntegerType.FIXED)
-            ProtoWireType.SIZE_DELIMITED -> readByteArray()
+            ProtoWireType.SIZE_DELIMITED -> skipSizeDelimited()
             ProtoWireType.i32 -> readInt(ProtoIntegerType.FIXED)
             else -> throw ProtobufDecodingException("Unsupported start group or end group wire type: $currentType")
         }
@@ -75,6 +75,13 @@ internal class ProtobufReader(private val input: ByteArrayInput) {
         return readByteArrayNoTag()
     }
 
+    fun skipSizeDelimited() {
+        assertWireType(SIZE_DELIMITED)
+        val length = decode32()
+        checkLength(length)
+        input.scipExactNBytes(length)
+    }
+
     fun readByteArrayNoTag(): ByteArray {
         val length = decode32()
         checkLength(length)

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

@@ -33,6 +33,12 @@ internal class ByteArrayInput(private var array: ByteArray, private val endIndex
         return b
     }
 
+
+    fun scipExactNBytes(bytesCount: Int) {
+        ensureEnoughBytes(bytesCount)
+        position += bytesCount
+    }
+
     private fun ensureEnoughBytes(bytesCount: Int) {
         if (bytesCount > availableBytes) {
             throw SerializationException("Unexpected EOF, available $availableBytes bytes, requested: $bytesCount")

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

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2017-2024 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.protobuf
+
+
+import kotlinx.serialization.*
+import kotlin.test.*
+
+class SkipFieldsTest {
+
+    @Serializable
+    data class Holder(val value: Int)
+
+    @Test
+    fun testSkipZeroId() {
+        // first value with id = 0
+        val hexString = "000f082a"
+        val holder = ProtoBuf.decodeFromHexString<Holder>(hexString)
+        assertEquals(42, holder.value)
+    }
+
+    @Test
+    fun testSkipBigId() {
+        // first value with id = 2047 and takes 2 bytes
+        val hexString = "f87f20082a"
+        val holder = ProtoBuf.decodeFromHexString<Holder>(hexString)
+        assertEquals(42, holder.value)
+    }
+
+    @Test
+    fun testSkipString() {
+        // first value is size delimited (string) with id = 42
+        val hexString = "d2020c48656c6c6f20576f726c6421082a"
+        val holder = ProtoBuf.decodeFromHexString<Holder>(hexString)
+        assertEquals(42, holder.value)
+    }
+}