Do not try to end structure in encode/decode stucture extensions if a…

core/commonMain/src/kotlinx/serialization/encoding/Decoding.kt

@@ -5,9 +5,9 @@
 package kotlinx.serialization.encoding
 
 import kotlinx.serialization.*
+import kotlinx.serialization.builtins.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.modules.*
-import kotlinx.serialization.builtins.*
 
 /**
  * Decoder is a core deserialization primitive that encapsulates the knowledge of the underlying
@@ -524,10 +524,15 @@ public inline fun <T> Decoder.decodeStructure(
     block: CompositeDecoder.() -> T
 ): T {
     val composite = beginStructure(descriptor)
+    var ex: Throwable? = null
     try {
         return composite.block()
+    } catch (e: Throwable) {
+        ex = e
+        throw e
     } finally {
-        composite.endStructure(descriptor)
+        // End structure only if there is no exception, otherwise it can be swallowed
+        if (ex == null) composite.endStructure(descriptor)
     }
 }
 

core/commonMain/src/kotlinx/serialization/encoding/Encoding.kt

@@ -413,9 +413,14 @@ public interface CompositeEncoder {
  */
 public inline fun Encoder.encodeStructure(descriptor: SerialDescriptor, block: CompositeEncoder.() -> Unit) {
     val composite = beginStructure(descriptor)
+    var ex: Throwable? = null
     try {
         composite.block()
+    } catch (e: Throwable) {
+        ex = e
+        throw e
     } finally {
-        composite.endStructure(descriptor)
+        // End structure only if there is no exception, otherwise it can be swallowed
+        if (ex == null) composite.endStructure(descriptor)
     }
 }

formats/json/commonTest/src/kotlinx/serialization/EncodingExtensionsTest.kt

@@ -0,0 +1,41 @@
+package kotlinx.serialization
+
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.json.*
+import kotlin.test.*
+
+class EncodingExtensionsTest {
+
+    @Serializable(with = BoxSerializer::class)
+    class Box(val i: Int)
+
+    @Serializer(forClass = Box::class)
+    object BoxSerializer : KSerializer<Box> {
+        override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Box") {
+            element<Int>("i")
+        }
+
+        override fun serialize(encoder: Encoder, value: Box) {
+            encoder.encodeStructure(descriptor) {
+                throw ArithmeticException()
+            }
+        }
+
+        override fun deserialize(decoder: Decoder): Box {
+            decoder.decodeStructure(descriptor) {
+                throw ArithmeticException()
+            }
+        }
+    }
+
+    @Test
+    fun testEncodingExceptionNotSwallowed() {
+        assertFailsWith<ArithmeticException> { Json.encodeToString(Box(1)) }
+    }
+
+    @Test
+    fun testDecodingExceptionNotSwallowed() {
+        assertFailsWith<ArithmeticException> { Json.decodeFromString<Box>("""{"i":1}""") }
+    }
+}