[Cbor] correctly skip structures with ignoreUnknownKeys setting (#2873)

Author: ardune

formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Decoder.kt

@@ -426,9 +426,11 @@ internal class CborParser(private val input: ByteArrayInput, private val verifyO
             } else {
                 val header = curByte and 0b111_00000
                 val length = elementLength()
-                if (header == HEADER_ARRAY || header == HEADER_MAP) {
+                if (header == HEADER_TAG) {
+                    readNumber()
+                } else if (header == HEADER_ARRAY || header == HEADER_MAP) {
                     if (length > 0) lengthStack.add(length)
-                    processTags(tags)
+                    else prune(lengthStack) // empty map or array automatically completes
                 } else {
                     input.skip(length)
                     prune(lengthStack)

formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborSkipTagAndEmptyTest.kt

@@ -0,0 +1,60 @@
+package kotlinx.serialization.cbor
+
+import kotlinx.serialization.*
+import kotlin.test.*
+
+class CborSkipTagAndEmptyTest {
+
+    /**
+     * A3                                      # map(3)
+     *    67                                   # text(7)
+     *       76657273696F6E                    # ""version""
+     *    63                                   # text(3)
+     *       312E30                            # ""1.0""
+     *    69                                   # text(9)
+     *       646F63756D656E7473                # ""documents""
+     *    81                                   # array(1)
+     *       A1                                # map(1)
+     *          6C                             # text(12)
+     *             6465766963655369676E6564    # ""deviceSigned""
+     *          A2                             # map(2)
+     *             6A                          # text(10)
+     *                6E616D65537061636573     # ""nameSpaces""
+     *             D8 18                       # tag(24) <------------------- Testing this skips properly
+     *                41                       # bytes(1)
+     *                   A0                    # ""\xA0""
+     *             6A                          # text(10)
+     *                64657669636541757468     # ""deviceAuth""
+     *             A1                          # map(1)
+     *                69                       # text(9)
+     *                   6465766963654D6163    # ""deviceMac""
+     *                84                       # array(4)
+     *                   43                    # bytes(3)
+     *                      A10105
+     *                   A0                    # map(0) <------------------- Testing this skips properly
+     *                   F6                    # primitive(22)
+     *                   58 20                 # bytes(32)
+     *                      E99521A85AD7891B806A07F8B5388A332D92C189A7BF293EE1F543405AE6824D
+     *    66                                   # text(6)
+     *       737461747573                      # ""status""
+     *    00                                   # unsigned(0)
+     */
+    private val referenceHexString = "A36776657273696F6E63312E3069646F63756D656E747381A16C6465766963655369676E6564A26A6E616D65537061636573D81841A06A64657669636541757468A1696465766963654D61638443A10105A0F65820E99521A85AD7891B806A07F8B5388A332D92C189A7BF293EE1F543405AE6824D6673746174757300"
+
+    @Test
+    fun deserializesCorrectly() {
+        // Specifically, skipping keys with descendants that contain tags and empty maps
+        val cbor = Cbor{
+            ignoreUnknownKeys = true
+        }
+        // Prior exception:
+        // Field 'status' is required for type with serial name 'kotlinx.serialization.cbor.CborSkipTagAndEmptyTest.DataClass', but it was missing
+        val target = cbor.decodeFromHexString(DataClass.serializer(), referenceHexString)
+        assertEquals(0, target.status)
+    }
+
+    @Serializable
+    data class DataClass(
+        val status: Int,
+    )
+}