Add a flag to allow parser to accept trailing commas.

This is one of the popular community requests and one of the main reasons people ask for Json5 support.
Implementing this flag separately will allow alleviating large paint points quickly without a need to wait for full Json5 support.

Fixes #1812
Relates to: #797, #2221

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonErrorMessagesTest.kt

@@ -6,6 +6,7 @@
 package kotlinx.serialization.json
 
 import kotlinx.serialization.*
+import kotlinx.serialization.test.*
 import kotlin.test.*
 
 
@@ -155,11 +156,4 @@ class JsonErrorMessagesTest : JsonTestBase() {
         })
 
     }
-
-    private fun checkSerializationException(action: () -> Unit, assertions: SerializationException.(String) -> Unit) {
-        val e = assertFailsWith(SerializationException::class, action)
-        assertNotNull(e.message)
-        e.assertions(e.message!!)
-    }
-
 }

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonParserTest.kt

@@ -83,7 +83,7 @@ class JsonParserTest : JsonTestBase() {
     }
 
     private fun testTrailingComma(content: String) {
-        assertFailsWithSerialMessage("JsonDecodingException", "Unexpected trailing") {  Json.parseToJsonElement(content) }
+        assertFailsWithSerialMessage("JsonDecodingException", "Trailing comma before the end of JSON object") {  Json.parseToJsonElement(content) }
     }
 
     @Test

formats/json-tests/commonTest/src/kotlinx/serialization/json/TrailingCommaTest.kt

@@ -0,0 +1,128 @@
+/*
+ * Copyright 2017-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.*
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class TrailingCommaTest : JsonTestBase() {
+    val tj = Json { allowTrailingComma = true }
+
+    @Serializable
+    data class Optional(val data: String = "")
+
+    @Serializable
+    data class MultipleFields(val a: String, val b: String, val c: String)
+
+    private val multipleFields = MultipleFields("1", "2", "3")
+
+    @Serializable
+    data class WithMap(val m: Map<String, String>)
+
+    private val withMap = WithMap(mapOf("a" to "1", "b" to "2", "c" to "3"))
+
+    @Serializable
+    data class WithList(val l: List<Int>)
+
+    private val withList = WithList(listOf(1, 2, 3))
+
+    @Test
+    fun basic() = parametrizedTest { mode ->
+        val sd = """{"data":"str",}"""
+        assertEquals(Optional("str"), tj.decodeFromString<Optional>(sd, mode))
+    }
+
+    @Test
+    fun trailingCommaNotAllowedByDefaultForObjects() = parametrizedTest { mode ->
+        val sd = """{"data":"str",}"""
+        checkSerializationException({
+            default.decodeFromString<Optional>(sd, mode)
+        }, { message ->
+            assertContains(
+                message,
+                """Unexpected JSON token at offset 13: Trailing comma before the end of JSON object"""
+            )
+        })
+    }
+
+    @Test
+    fun trailingCommaNotAllowedByDefaultForLists() = parametrizedTest { mode ->
+        val sd = """{"l":[1,]}"""
+        checkSerializationException({
+            default.decodeFromString<WithList>(sd, mode)
+        }, { message ->
+            assertContains(
+                message,
+                """Unexpected JSON token at offset 7: Trailing comma before the end of JSON array"""
+            )
+        })
+    }
+
+    @Test
+    fun trailingCommaNotAllowedByDefaultForMaps() = parametrizedTest { mode ->
+        val sd = """{"m":{"a": "b",}}"""
+        checkSerializationException({
+            default.decodeFromString<WithMap>(sd, mode)
+        }, { message ->
+            assertContains(
+                message,
+                """Unexpected JSON token at offset 14: Trailing comma before the end of JSON object"""
+            )
+        })
+    }
+
+    @Test
+    fun emptyObjectNotAllowed() = parametrizedTest { mode ->
+        assertFailsWithMessage<SerializationException>("Unexpected leading comma") {
+            tj.decodeFromString<Optional>("""{,}""", mode)
+        }
+    }
+
+    @Test
+    fun emptyListNotAllowed() = parametrizedTest { mode ->
+        assertFailsWithMessage<SerializationException>("Unexpected leading comma") {
+            tj.decodeFromString<WithList>("""{"l":[,]}""", mode)
+        }
+    }
+
+    @Test
+    fun emptyMapNotAllowed() = parametrizedTest { mode ->
+        assertFailsWithMessage<SerializationException>("Unexpected leading comma") {
+            tj.decodeFromString<WithMap>("""{"m":{,}}""", mode)
+        }
+    }
+
+    @Test
+    fun testMultipleFields() = parametrizedTest { mode ->
+        val input = """{"a":"1","b":"2","c":"3", }"""
+        assertEquals(multipleFields, tj.decodeFromString(input, mode))
+    }
+
+    @Test
+    fun testWithMap() = parametrizedTest { mode ->
+        val input = """{"m":{"a":"1","b":"2","c":"3", }}"""
+
+        assertEquals(withMap, tj.decodeFromString(input, mode))
+    }
+
+    @Test
+    fun testWithList() = parametrizedTest { mode ->
+        val input = """{"l":[1, 2, 3, ]}"""
+        assertEquals(withList, tj.decodeFromString(input, mode))
+    }
+
+    @Serializable
+    data class Mixed(val mf: MultipleFields, val wm: WithMap, val wl: WithList)
+
+    @Test
+    fun testMixed() = parametrizedTest { mode ->
+        //language=JSON5
+        val input = """{"mf":{"a":"1","b":"2","c":"3",},
+            "wm":{"m":{"a":"1","b":"2","c":"3",},},
+            "wl":{"l":[1, 2, 3,],},}"""
+        assertEquals(Mixed(multipleFields, withMap, withList), tj.decodeFromString(input, mode))
+    }
+}

formats/json-tests/commonTest/src/kotlinx/serialization/test/TestingFramework.kt

@@ -92,3 +92,9 @@ inline fun <reified T : Throwable> assertFailsWithMessage(
         "expected:<$message> but was:<${exception.message}>"
     )
 }
+
+inline fun checkSerializationException(action: () -> Unit, assertions: SerializationException.(String) -> Unit) {
+    val e = assertFailsWith(SerializationException::class, action)
+    assertNotNull(e.message)
+    e.assertions(e.message!!)
+}

formats/json/api/kotlinx-serialization-json.api

@@ -87,6 +87,7 @@ public final class kotlinx/serialization/json/JsonArraySerializer : kotlinx/seri
 public final class kotlinx/serialization/json/JsonBuilder {
 	public final fun getAllowSpecialFloatingPointValues ()Z
 	public final fun getAllowStructuredMapKeys ()Z
+	public final fun getAllowTrailingComma ()Z
 	public final fun getClassDiscriminator ()Ljava/lang/String;
 	public final fun getCoerceInputValues ()Z
 	public final fun getDecodeEnumsCaseInsensitive ()Z
@@ -102,6 +103,7 @@ public final class kotlinx/serialization/json/JsonBuilder {
 	public final fun isLenient ()Z
 	public final fun setAllowSpecialFloatingPointValues (Z)V
 	public final fun setAllowStructuredMapKeys (Z)V
+	public final fun setAllowTrailingComma (Z)V
 	public final fun setClassDiscriminator (Ljava/lang/String;)V
 	public final fun setCoerceInputValues (Z)V
 	public final fun setDecodeEnumsCaseInsensitive (Z)V
@@ -130,6 +132,7 @@ public final class kotlinx/serialization/json/JsonConfiguration {
 	public fun <init> ()V
 	public final fun getAllowSpecialFloatingPointValues ()Z
 	public final fun getAllowStructuredMapKeys ()Z
+	public final fun getAllowTrailingComma ()Z
 	public final fun getClassDiscriminator ()Ljava/lang/String;
 	public final fun getCoerceInputValues ()Z
 	public final fun getDecodeEnumsCaseInsensitive ()Z

formats/json/commonMain/src/kotlinx/serialization/json/Json.kt

@@ -364,6 +364,16 @@ public class JsonBuilder internal constructor(json: Json) {
     @ExperimentalSerializationApi
     public var decodeEnumsCaseInsensitive: Boolean = json.configuration.decodeEnumsCaseInsensitive
 
+    /**
+     * Allows parser to accept trailing (ending) commas in JSON objects and arrays,
+     * making inputs like `[1, 2, 3,]` valid.
+     *
+     * Does not affect encoding.
+     * `false` by default.
+     */
+    @ExperimentalSerializationApi
+    public var allowTrailingComma: Boolean = json.configuration.allowTrailingComma
+
     /**
      * Module with contextual and polymorphic serializers to be used in the resulting [Json] instance.
      *
@@ -396,7 +406,7 @@ public class JsonBuilder internal constructor(json: Json) {
             allowStructuredMapKeys, prettyPrint, explicitNulls, prettyPrintIndent,
             coerceInputValues, useArrayPolymorphism,
             classDiscriminator, allowSpecialFloatingPointValues, useAlternativeNames,
-            namingStrategy, decodeEnumsCaseInsensitive
+            namingStrategy, decodeEnumsCaseInsensitive, allowTrailingComma
         )
     }
 }

formats/json/commonMain/src/kotlinx/serialization/json/JsonConfiguration.kt

@@ -32,7 +32,9 @@ public class JsonConfiguration @OptIn(ExperimentalSerializationApi::class) inter
     @ExperimentalSerializationApi
     public val namingStrategy: JsonNamingStrategy? = null,
     @ExperimentalSerializationApi
-    public val decodeEnumsCaseInsensitive: Boolean = false
+    public val decodeEnumsCaseInsensitive: Boolean = false,
+    @ExperimentalSerializationApi
+    public val allowTrailingComma: Boolean = false,
 ) {
 
     /** @suppress Dokka **/
@@ -42,6 +44,6 @@ public class JsonConfiguration @OptIn(ExperimentalSerializationApi::class) inter
                 "allowStructuredMapKeys=$allowStructuredMapKeys, prettyPrint=$prettyPrint, explicitNulls=$explicitNulls, " +
                 "prettyPrintIndent='$prettyPrintIndent', coerceInputValues=$coerceInputValues, useArrayPolymorphism=$useArrayPolymorphism, " +
                 "classDiscriminator='$classDiscriminator', allowSpecialFloatingPointValues=$allowSpecialFloatingPointValues, useAlternativeNames=$useAlternativeNames, " +
-                "namingStrategy=$namingStrategy, decodeEnumsCaseInsensitive=$decodeEnumsCaseInsensitive)"
+                "namingStrategy=$namingStrategy, decodeEnumsCaseInsensitive=$decodeEnumsCaseInsensitive, allowTrailingComma=$allowTrailingComma)"
     }
 }

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonExceptions.kt

@@ -46,6 +46,13 @@ internal fun AbstractJsonLexer.throwInvalidFloatingPointDecoded(result: Number):
         hint = specialFlowingValuesHint)
 }
 
+internal fun AbstractJsonLexer.invalidTrailingComma(entity: String = "object"): Nothing {
+    fail("Trailing comma before the end of JSON $entity",
+        position = currentPosition - 1,
+        hint = "Trailing commas are non-complaint JSON and not allowed by default. Use 'allowTrailingCommas = true' in 'Json {}' builder to support them."
+    )
+}
+
 @OptIn(ExperimentalSerializationApi::class)
 internal fun InvalidKeyKindException(keyDescriptor: SerialDescriptor) = JsonEncodingException(
     "Value of type '${keyDescriptor.serialName}' can't be used in JSON as a key in the map. " +

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonTreeReader.kt

@@ -13,6 +13,7 @@ internal class JsonTreeReader(
     private val lexer: AbstractJsonLexer
 ) {
     private val isLenient = configuration.isLenient
+    private val trailingCommaAllowed = configuration.allowTrailingComma
     private var stackDepth = 0
 
     private fun readObject(): JsonElement = readObjectImpl {
@@ -44,8 +45,9 @@ internal class JsonTreeReader(
         if (lastToken == TC_BEGIN_OBJ) { // Case of empty object
             lexer.consumeNextToken(TC_END_OBJ)
         } else if (lastToken == TC_COMMA) { // Trailing comma
-            lexer.fail("Unexpected trailing comma")
-        }
+            if (!trailingCommaAllowed) lexer.invalidTrailingComma()
+            lexer.consumeNextToken(TC_END_OBJ)
+        } // else unexpected token?
         return JsonObject(result)
     }
 
@@ -66,7 +68,8 @@ internal class JsonTreeReader(
         if (lastToken == TC_BEGIN_LIST) { // Case of empty object
             lexer.consumeNextToken(TC_END_LIST)
         } else if (lastToken == TC_COMMA) { // Trailing comma
-            lexer.fail("Unexpected trailing comma")
+            if (!trailingCommaAllowed) lexer.invalidTrailingComma("array")
+            lexer.consumeNextToken(TC_END_LIST)
         }
         return JsonArray(result)
     }

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt

@@ -122,6 +122,7 @@ internal open class StreamingJsonDecoder(
         if (json.configuration.ignoreUnknownKeys && descriptor.elementsCount == 0) {
             skipLeftoverElements(descriptor)
         }
+        if (lexer.tryConsumeComma() && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma("")
         // First consume the object so we know it's correct
         lexer.consumeNextToken(mode.end)
         // Then cleanup the path
@@ -195,12 +196,12 @@ internal open class StreamingJsonDecoder(
 
         return if (lexer.canConsumeValue()) {
             if (decodingKey) {
-                if (currentIndex == -1) lexer.require(!hasComma) { "Unexpected trailing comma" }
+                if (currentIndex == -1) lexer.require(!hasComma) { "Unexpected leading comma" }
                 else lexer.require(hasComma) { "Expected comma after the key-value pair" }
             }
             ++currentIndex
         } else {
-            if (hasComma) lexer.fail("Expected '}', but had ',' instead")
+            if (hasComma && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma()
             CompositeDecoder.DECODE_DONE
         }
     }
@@ -239,7 +240,7 @@ internal open class StreamingJsonDecoder(
                 hasComma = handleUnknown(key)
             }
         }
-        if (hasComma) lexer.fail("Unexpected trailing comma")
+        if (hasComma && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma()
 
         return elementMarker?.nextUnmarkedIndex() ?: CompositeDecoder.DECODE_DONE
     }
@@ -262,7 +263,7 @@ internal open class StreamingJsonDecoder(
             if (currentIndex != -1 && !hasComma) lexer.fail("Expected end of the array or comma")
             ++currentIndex
         } else {
-            if (hasComma) lexer.fail("Unexpected trailing comma")
+            if (hasComma && !json.configuration.allowTrailingComma) lexer.invalidTrailingComma("array")
             CompositeDecoder.DECODE_DONE
         }
     }

formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt

@@ -149,7 +149,7 @@ internal abstract class AbstractJsonLexer {
     protected abstract val source: CharSequence
 
     @JvmField
-    protected var currentPosition: Int = 0 // position in source
+    internal var currentPosition: Int = 0 // position in source
 
     @JvmField
     val path = JsonPath()