KTOR-8654 Fix optionals in config deserialization in HOCON/Map

Author: bjhham

ktor-server/ktor-server-core/common/src/io/ktor/server/config/MapConfigDecoder.kt

@@ -59,6 +59,8 @@ internal class MapConfigDecoder(
         return if (map.containsPrefix(fullPath)) {
             currentPath = fullPath
             newIndex
+        } else if (descriptor.isElementOptional(newIndex)) {
+            decodeElementIndex(descriptor)
         } else {
             CompositeDecoder.DECODE_DONE
         }

ktor-server/ktor-server-core/common/test/io/ktor/server/config/MapDecoderTest.kt

@@ -50,6 +50,14 @@ class MapDecoderTest {
         val enumMap: Map<String, TestEnum>
     )
 
+    @Serializable
+    data class Address(
+        val streetName: String,
+        val postalCode: String,
+        val unitNumber: String? = null,
+        val municipality: String,
+    )
+
     @Test
     fun testSimpleTypes() {
         val map = mapOf(
@@ -206,4 +214,19 @@ class MapDecoderTest {
             SimpleConfig.serializer().deserialize(decoder)
         }
     }
+
+    @Test
+    fun testOptionals() {
+        val map = mapOf(
+            "streetName" to "Test street",
+            "postalCode" to "12345",
+            "municipality" to "Test municipality"
+        )
+        val decoder = MapConfigDecoder(map)
+        val config = Address.serializer().deserialize(decoder)
+        assertEquals("Test street", config.streetName)
+        assertEquals("12345", config.postalCode)
+        assertEquals("Test municipality", config.municipality)
+        assertEquals(null, config.unitNumber)
+    }
 }

ktor-server/ktor-server-core/jvm/src/io/ktor/server/config/HoconDecoder.kt

@@ -30,6 +30,8 @@ internal open class HoconDecoder(
         return if (config.hasPath(fullPath)) {
             currentPath = fullPath
             newIndex
+        } else if (descriptor.isElementOptional(newIndex)) {
+            decodeElementIndex(descriptor)
         } else {
             CompositeDecoder.DECODE_DONE
         }

ktor-server/ktor-server-core/jvm/test/io/ktor/tests/config/HoconDecoderTest.kt

@@ -220,6 +220,26 @@ class HoconDecoderTest {
         }
     }
 
+    @Test
+    fun `default values`() {
+        val content = """
+            address {
+                streetName = "Main Street"
+                postalCode = "12345"
+                municipality = "Test City"
+                # unitNumber is intentionally omitted to test default value
+            }
+        """.trimIndent()
+
+        val address = parseConfig(content)
+            .propertyOrNull("address")?.getAs<Address>()
+
+        assertNotNull(address)
+        assertEquals("Main Street", address.streetName)
+        assertEquals("Test City", address.municipality)
+        assertNull(address.unitNumber)
+    }
+
     private fun parseConfig(content: String): HoconApplicationConfig =
         HoconApplicationConfig(ConfigFactory.parseString(content))
 
@@ -294,4 +314,12 @@ class HoconDecoderTest {
         val enumValue: TestEnum,
         val enumList: List<TestEnum>
     )
+
+    @Serializable
+    data class Address(
+        val streetName: String,
+        val postalCode: String,
+        val unitNumber: String? = null,
+        val municipality: String,
+    )
 }